Understanding Antennas For The
Non-Technical Ham A Book By Jim Abercrombie, N4JA (Jim
Abercrombie 4ja@prtcnet.com)
Illustrations by Frank Wamsley,
K4EFW
Edited by Judy Haynes,
KC4NOR
Copyright July 2005. Second
Edition
Edited for the web , N4UJW
Editors Note: This is a book length
web article provided by the author FREE for all
hams. This is copyrighted material and is the
property of Hamuniverse.com and/or the article author and is to
be used only for personal non-profit educational use. You may download
a pdf copy of it here....74 pages!
It
is HUGE! Bookmark this page for future
reading or see more options for saving at bottom of
page! The original book contained 60 pages and
illustrations. They are all here! Many of the antennas
described here are in project form on
this web
site.
Here are some of the main topics in
the book that you will learn more about.
Antenna
systems, antennas, simple antenna formulas, basic antenna theory,
feed-lines, matching units, how antennas work, polarization of
electromagnetic waves, frequency, the ionosphere and modes of
propagation, Ground-Wave Propagation, Direct Wave or Line of Sight
Propagation, Propagation by Refraction,
Skywave
Propagation, Greyline Propagation, Long Path Propagation, ham bands
propagation, antenna myths, standing wave ratio, real antenna
systems, Flat Top Dipole, Inverted-V Dipole, Dipole Shape
Variations, Calculating the Length of a Half-Wave Resonant Dipole,
The Decibel, Resistances and Reactance, Feeding Dipoles
Efficiently,
Cause
of Feed-Line Radiation, Baluns, Other types of dipoles, Shortened
Loaded Dipole,
All
Band Dipoles, Sloping Dipole, Folded Dipole, Double Bazooka Dipole,
Broad-Banded Coax-Fed Fan Dipole, Two-Element Collinear Dipole,
Four-Element Collinear Dipole, Coax-Fed Dipoles Operated on Odd
Harmonic Frequencies, Three Half-wave Dipole, All Band Random Length
Dipole, All Band Center-Fed Random Length
Dipole,
A
Two-Band Fan Dipole, Trapped Dipole for 75 and 40 Meters, The
Extended Double Zepp Dipole, The G5RV Dipole, Off-Center Fed
Dipoles, One wavelength Off-Center Fed Dipole, Carolina Windom,
Windom Dipole (Fritzel Type), End-Fed Antennas, End-Fed Zepp,
Alternate Method of Feeding an End-Fed Zepp, End-Fed Random Length
Antenna, The Half-Sloper antenna, Vertical antennas, Ground Mounted
Trapped Verticals, Disadvantages of Using Quarter-Wave Verticals,
Long and Short Verticals,
Unscientific Observations of
Verticals, The Inverted-L Vertical , Vertical Mobile Antennas, HF
mobile antenna comparisons, One wave-length single loop antennas,
Horizontally Oriented Loop, Vertically Oriented Single Loop for 40
and 80 Meters,
Single-Element Vertical Delta Loop,
Directional beam antennas, Monoband Yagi, Three-Element Yagi,
Trapped Multi-band Yagis, SteppIR Antenna, The Log-Periodic
Array,
Directional Cubical Quad and Delta
Loop Antennas, Single Band Cubical Quad, field-strength meter, The
Quagi, Gain vs front to back radio, Feed lines, Antenna
Safety,
Erecting Antennas on Masts, Tower
Safety, Quarter Wave Matching Sections of 70-ohm Coax chart, and
much more!
The Book
Starts Here! Enjoy!
PREFACE
One reason for writing this book is
to educate you so you can make an informed choice concerning the
best antenna for you. Another reason is to dispel the many antenna
myths that circulate in the amateur community. The third reason is a
desire to teach basic antenna theory to the average ham. Therefore,
to achieve that goal, you should read this book from cover to cover.
It was written primarily for the newcomer and the non-technical
old-timer.
This book is about common medium wave
and high frequency (short wave) antennas, but the theory presented
here relates to antennas of any frequency. It is in a condensed form
and the antenna theory is explained so most hams can understand it.
Realizing many hams are mathematically challenged, only simple
mathematics procedures are used. If you can add, subtract, and
divide using a calculator, you will not have trouble with this
book.
A few principles in here are based on
conclusions drawn from the Laws of Physics. Everything else in this
book can be found scattered through The A.R.R.L. Antenna Book
and nothing in here contradicts what is written
there.
I. WHY ALL
THE FUSS ABOUT ANTENNAS Definition: An antenna is a piece of
metal, a conductor of electricity, to which you connect the radio.
It radiates your signal and receives the signals you want to
hear.
Definition: An antenna system consists of the
antenna, the feed-line, and any matching unit.
Most antennas are made of copper or aluminum, while most mobile
antennas are made of stainless steel. A feed-line consists of two
conductors that carry the signal to and from the radio and to and
from the antenna. A matching unit can be an antenna tuner, a series
matching section, or one of several different kinds of matching
circuits at the feed-point.
Does the
type of antenna make much difference? Here is an example: Once in
1959 two of us were involved in testing two antennas on 15 meters.
The late R. Lynn Kalmbach, W4IW, using one antenna received a 30-dB
better signal report on his antenna from a station in England than
we did on our antenna. (Decibel or dB will be explained later).
Thirty dB means his signal appeared that he was running 1000 times
more transmitter power than we were. At that time, we didnt live
that far apart so we couldnt blame it on propagation. We both were
running about equal power. Both antennas were at 50 feet. The
comparison proved that a good antenna could make a difference. Lynn
used a home-built G4 ZU mini-beam; we were using a 15-meter
2-element Mosely Mini-Beam, which had short loaded elements.
Evidently, it had a lot of loss.
Another
example: Today we hear people breaking in to our ragchews with
signals almost level with the noise. Why is that? The reason is they
are using the wrong antennas. Their signals are twenty to thirty
decibels below everyone elses. They are making contacts, but just
barely. The first question our group asks, "What kind of antenna are
you using?" Experienced amateurs know the antenna can make all the
difference. The guy with the poor signal sometimes will blame his
bad signal report on band conditions or his lack of a linear
amplifier. He is just sticking his head in the
sand.
What we
are trying to prove is next to your radio, the most important part
of your station is the antenna. Many years ago, an old-timer said,
"For every dollar you spend on a radio, you should spend two dollars
on your antenna." That is also true today. You can do more to
improve your signal strength with antennas than you can ever do by
increasing your power. Having the ability to make contacts on a
particular antenna doesnt mean it works well!
Any antenna will make contacts, but your
signals will be stronger on some antennas than on others. In
addition, some antennas hear better than others.
II. HOW
ANTENNAS WORK.
First of
all to work properly the antenna system must be matched to the
transmitter. That is, all modern transmitters have an output
impedance of 50 ohms. Antenna systems range in impedance of a few
ohms to several thousand ohms. There are several ways to match them:
pruning the length of the antenna, using an antenna tuner, matching
the antenna with a length of transmission line called a matching
section, or the use one of several matching systems at the antenna
feed-point. Antenna matching is beyond the scope of the material
found in this book and it is suggested you consult a more
comprehensive antenna manual. Simple half-wave dipoles eliminate the
need for a matching system because a resonant half-wave dipole has
an impedance near 50-ohms.
You must
understand electromagnetism to understand how antennas work. If you
attach the two poles of a direct current (DC) voltage source to the
two ends of a coil of wire, current will flow through the coil of
wire and it will become magnetized. The magnetized coil is known as
an electromagnet. Its magnetism will extend out to infinity becoming
weaker with distance. Remove the voltage and the magnetic field
collapses back into the coil. If an alternating current (AC) is
connected to the coil, the magnetism moves out and collapses into
the coil in step with the frequency of the alternating current
source. The north and south poles of the electromagnet reverse on
each half-cycle of the AC voltage.
If voltage
and current can cause a coil to become magnetized, the reverse is
true: A magnetic field can produce a voltage and a current in a
coil. This is known as Faradays Principle of Magnetic Induction. A
voltage will be produced at the ends of the coil of wire as you move
any permanent magnet close to and parallel to the coil. The
difference in this case is the magnet must be kept moving. Move the
magnet in one direction, and current will flow in one direction.
Reverse the direction the magnet is moving and the current will flow
in the opposite direction. Moving the magnet back and forth produces
alternating current. An AC generator spins a coil of wire between
the two poles of a magnetic field. It doesnt matter which one is
moving. The coil or the magnet can be moving. Any moving magnetic
field can induce current in anther coil. It doesnt have to be a
piece of metal we call a magnet. Imagine a moving magnetic field
produced by AC circulating in and out of a coil. If that moving
magnetic field passes through a second nearby coil, it will induce
an alternating current in the second coil. A transformer uses this
method to work. Transformers have a continuous iron core running
from the inside of one coil through the inside of the second coil to
confine the magnetism inside the iron core. This makes the
transformer nearly 100% efficient since only a little of the
magnetic energy escapes.
A straight
wire that has an AC current flowing through it also has a magnetic
field surrounding it. But it is a weaker field than is produced by a
coil. The magnetic field from the wire radiates out into space and
becomes weaker with distance. The radiating magnetic field from a
wire is known as "electromagnetic radiation" and a radio wave is one
type of it. The wire that radiates becomes the transmitting antenna.
Some distance away, a second wire in the path of these waves has
current induced into it by the passing electromagnetic waves. This
second wire will be the receiving antenna. The voltage in the
receiving antenna is many times weaker than the voltage in the
transmitting antenna. It may be as weak as one-millionth of a volt
or less and still be useful. The receiving antenna feeds that
voltage to the amplifiers in the receiver front-end where it is
amplified many thousands or millions of times.
The dipole
antenna is made of a wire broken in the center and where broken,
each half of the wire connects to an insulator that divides the wire
in two. Two wires from the voltage source, which is the transmitter,
are connected across the insulator. On one side of the dipole, the
current in the form of moving electrons flows first from the voltage
source toward one end of the dipole. At the end, it reflects toward
the voltage source. The same thing occurs on the other half of the
wire on the other half cycle of alternating current. An antenna that
is the right length for the current to reach the far end of the wire
just as the polarity changes is said to be resonant. Because
electricity travels at 95% the speed of light in a wire, the number
of times the polarity changes in one second (frequency) determines
how long the wire has to be in order to be
resonant.
III.
POLARIZATION OF ELECTROMAGNETIC WAVES
Electromagnetic waves travel away
from the wire in horizontal, vertical, slanted, or circular waves.
If the antenna wire runs horizontal or parallel to the earth, the
radiation will be horizontally polarized. A wire or conductor that
runs at right angles to the earth produces vertical radiation. A
slanted wire has components of both horizontal and vertical
radiation. Crossed wires connected by proper phasing lines that
shift the phase from one wire to the other wire by 90 degrees will
produce circular polarization. Amateurs working orbiting satellites
at VHF, UHF, and microwave frequencies use circular
polarization.
When your
high frequency signals are reflecting off the ionosphere, it isnt
important if the other stations antenna has the opposite
polarization from yours (the polarization does matter for line of
sight communication). The reflected polarized waves passing through
the ionosphere are slowly rotated causing fading signals (QSB). The
reason the polarization of antennas is most important is that it
determines the angle of radiation. Horizontally polarized antennas
at ordinary heights used by hams produce mostly high angle radiation
and weaker low angle radiation, but this doesnt mean there is no low
angle radiation. It is there but is weaker than high angle
radiation. However, you must put a horizontally polarized antenna up
more than one-wavelength high to get a strong low angle radiation.
One wavelength is 280 feet on 80 meters, 140 feet on 40 meters, and
70 feet on 20 meters. High angle radiation works nearby stations
best and low angle radiation works distant stations (DX) best. A
vertically polarized antenna produces mostly low angle radiation,
with its high angle radiation being weak. For this reason, vertical
antennas do not work as well as horizontal antennas do at ordinary
heights for working stations less than about 500 miles
away.
FREQUENCY
The number
of times the polarity of an AC voltage changes per second determines
its frequency. Frequency is measured in cycles per second or Hertz
(Hz). A thousand cycles per second is a kilohertz (kHz). One million
hertz is a Megahertz (MHz). The only difference between the 60 Hz
electric power in your house and radio frequencies (RF) is the
frequency, but 60 Hz electricity in a wire also produces
electromagnetic radiation just like radio waves. Useful radio waves
start at 30 kHz and go upward in frequency until you reach the
infrared light waves. Light is the same kind of waves as RF except
light is at a much higher frequency. Light waves are used like radio
waves when they are confined inside fiber optic cable. Above the
frequencies of light are found x-rays and gamma
rays.
The radio
bands: The Long Wave Band (LW) starts at 30 kHz and goes to 300 kHz.
The Medium Wave Band (MW) is from 300 kHz to 3000 kHz or 3 MHz. The
High Frequency Band (HF) is from 3 MHz to 30 MHz. The Very High
Frequency Band (VHF) is from 30 MHz to 300 MHz. The Ultra-High
Frequency Band (UHF) is from 300 MHz to 3000 MHz or 3 GHz. Above
these frequencies are several microwave bands which are defined as
the Super High Frequency Band (SHF).
V. THE IONOSPHERE AND
MODES OF HF PROPAGATION
The
Ionosphere
In the
upper air around fifty miles and higher where the air molecules are
far apart, radiation from the sun strips electrons from oxygen
molecules causing the molecules to become ionized forming the
ionosphere. The ionized oxygen molecules and its free electrons
float in space forming radio-reflecting layers. Ionization of the
ionosphere varies by the time of day, seasons of the year, and the
sunspot cycle. The strength of ionization also varies from day to
day and hour to hour. Since the height of the ionosphere varies, the
higher the ionized layer becomes, the farther the skip will be. We
will define skip in section 5 of part V.
The part
of the earths atmosphere called the ionosphere is divided into three
layers. The three layers are, from lowest to highest, the D layer,
the E layer, and the F layer. Each layer has a different effect on
HF radio propagation.
Being at a
lower altitude, the D layer molecules are squeezed closer together
by gravity than those in higher layers, and the free electrons
reattach to the molecules easily. The D layer requires constant
radiation from the sun to maintain its ionization. Radio waves at
lower frequencies such as the frequencies of the AM broadcast band
cannot penetrate this layer and are absorbed. The higher frequency
signals are able to pass through the D layer. The D layer disappears
at night causing AM broadcast stations to reflect from the higher
layers. This is why AM broadcast signals only propagate by ground
wave in the daytime and they can be received from great distances at
night. Like the broadcast band, the D layer absorbs signals on 160
and to a lesser extent 80 meters during the day making those bands
go dead. During solar flares, the D layer becomes ionized so
strongly that all high frequency radio waves are absorbed, causing a
radio blackout.
E-layer
propagation is not well understood. Being at a lower altitude than F
layer, the E layer is responsible for summertime short skip
propagation on the higher high frequency bands. The skip zone is
around 1000 miles, but at times when the E-cloud covers a wide area
in the summer, double hops can be seen. A double hop occurs when the
signal reflects from the ionosphere, then returns to the ground,
reflects from the ground back to the ionosphere where it is
reflected back to the ground. A double hop can propagate the signal
2000 miles or more. The E-layer forms mostly during the day, and it
has the highest degree of ionization at noon. The E layer like the D
layer disappears at night. Even so, sporadic-E propagation can and
does form at night. There is a minor occurrence of sporadic E
propagation during the wintertime. On rare occasions, sporadic E
propagation can surprise you by occurring anytime regardless of the
sunspot cycle or the season of the year.
The F
layer is the highest layer and it is divided into two levels: F1 and
F2. At night the F1 and F2 merge into one layer. During the day, the
F1 layer doesnt play a part in radio propagation, but F2 does. It is
responsible for most high-frequency long distance propagation on 20
meters and above. However, the F layer makes it possible for you to
work DX on the lower bands at night. Sunspots are responsible for
the ionization layers and in years with high sunspot numbers,
worldwide contacts can be made easily on 10-20 meters by F2 layer
propagation. In years of low sunspot numbers, working distant
stations is difficult on those bands. Consequently, ten and fifteen
meters will be completely dead most days and twenty meters will go
dead at night. In years of low sunspot numbers DX contacts are
easily made at night on 160, 80, and 40 meters. The sunspot numbers
increase and decrease in 11-year average cycles.
Since the
curvature of the earth averages about 16 feet every 5 miles, an
object 5 miles from you on perfectly flat earth will be 16 feet
below the horizon. Because light travels in straight lines, you
cannot see objects beyond the horizon. Radio waves travel in
straight lines, but there are ways to get them beyond the horizon.
This is referred to as propagation.
2.
Ground-Wave Propagation
Ground
wave works only with vertical polarization. One side of the antenna
is the metal vertical radiator and the other side of the antenna is
the earth ground. The surface wave in the air travels faster than
the part of the wave flowing through the ground. The surface of the
earth is curved like the curved part of a racetrack. On the curved
track, a car on the outside of the track has to travel faster than
the car on the inside lane to stay even, and the two cars travel in
a curved path. Although the wave in the air travels faster than the
wave on the ground, the two parts of the wave cannot be separated.
Because of this, the radio wave also travels in a curved path that
follows the curvature of the earth.
The AM
broadcast stations use ground wave propagation during the day and
skywave propagation at night. Since radio waves at lower frequencies
conduct better through the ground, an AM broadcast station on 540
kHz will be many dB stronger than a station on 1600 kHz, if both run
the same power. This fact is important in understanding why ground
mounted verticals do not work as well at high frequencies as they do
on the broadcast band.
3. Direct
Wave or Line of Sight Propagation
Antennas
located on high structures can "look" over the horizon and "see" the
receiving antennas. Because refraction is involved, direct waves
travel 20% farther than light waves due to scattering of radio waves
by the environment. Trees and other foliage are invisible to HF
radio waves. Direct wave propagation is possible at all frequencies,
but this mode of propagation is seldom used on our high frequency
bands, but it is the usual propagation mode used by repeaters and
others on VHF and UHF. If you watch TV on an outside antenna or on a
"rabbit ears antenna," you are receiving the signal by direct wave
propagation.
4.
Propagation by Refraction
Refraction
occurs when the lower part of a wave travels slower than the top
part of the wave because the wave is passing through two media.
These media can be two layers of air at different temperatures or
they can be air and a solid. One form of refraction is caused by a
radio wave passing over a hill or ridge being bent as it passes over
the obstruction. This is known as "knife edge refraction." Another
form of refraction occurs when layers of air of different
temperatures bend the radio waves around the horizon. This is called
tropospheric ducting. This mode of propagation makes long distance
contacts possible at VHF frequencies. Tropospheric ducting does
occur on 10 meters and lower frequencies and is noticeable when
other forms of propagation are absent. On high frequency bands, many
hams mistakenly call tropospheric ducting and direct wave "ground
wave."
5. Skywave
Propagation
Skywave
propagation occurs when radio waves are reflected from the
ionosphere. Practically all HF communication is done by skywave. In
the ionosphere, the waves are really refracted twice, and they just
appear to be reflected. The reflections are frequency sensitive,
meaning each ham band reflects differently from the others. Low
frequencies, such as 80 meters, reflect mainly from the lower levels
of the ionosphere and the reflected signal comes nearly straight
back down. This causes 80 meters to propagate to points from local
out to more than a few hundred miles in the daytime. At night, when
the D layer and E layer are absent, signals striking the ionosphere
at lower angles may propagate many thousands of miles on 80 meters.
On the bands from 20 to 10 meters, high angle signals pass straight
through the ionosphere and do not reflect back down to the nearby
stations. The low angle signals on these higher bands reflect from
the ionosphere near the horizon and return to the Earth some miles
away. The in-between region cannot hear the transmitted signals nor
can you hear signals coming from this region. The in-between region
is called the "skip zone." Only when the ionosphere is weakly
ionized do you have a skip zone on 80 meters.
Another
interesting type of skywave propagation seen on the higher HF bands
is called chordal hop propagation seen frequently in
trans-equatorial (TE) propagation, which is propagation crossing the
equator. When this occurs, signals entering the ionosphere are
trapped inside the F2 layer then they are finally refracted back to
earth across the equator thousands of miles away. There is no
propagation between the signal entry point and the exit point. This
is skip in the extreme. On many occasions, we have worked stations
far away across the equator in the southern part of South America
and stations in between could not be heard. We have frequently
worked VQ9LA in the Chagos Archipelago located in the Indian Ocean.
The path to The Chagos Archipelago is across Europe and the Middle
East and finally across the equator to his location in the Indian
Ocean. One time when he was working Europe and North America at the
same time, we could not hear the European stations because our path
to him was via chordal hop propagation. Another way of describing
chordal hop propagation is to call it ionospheric
ducting.
Skywave
propagation sometimes produces an effect called "backscatter." What
happens is the radio waves that strike the ionosphere, instead of
only reflecting father away from the transmitting station, part of
the signal reflects backwards toward the transmitting station.
Stations that are too close to hear each other by direct wave can
communicate by the backward reflecting waves. Both stations that
communicate by backscatter must point their directional beam
antennas in the same direction although their direction toward each
other may be at some other azimuth. Backscatter will confuse
front-to-back measurements of directional beam antennas. This is
because, when you turn the back of the antenna toward the station
you are hearing, you may be able to hear him on backscatter from a
direction opposite from him. You will be hearing him from the
ionized atmospheric cloud in the opposite direction. During intense
solar magnetic storms, when aurora occurs at high latitudes,
stations are able to communicate by backscatter on VHF and UHF by
both stations pointing their directional beams toward the aurora.
This will be due north for stations in the Northern Hemisphere and
due south for stations in the Southern Hemisphere. Audio from aurora
backscatter will have a "wispy" sound.
6.
Greyline Propagation
Greyline
propagation occurs when the sun is low in the sky near dawn or dusk,
although we have seen greyline propagation occur as early as two
hours before sunset or as late as two hours after sunrise. It is
often used to work stations on the other side of the world on 160
and 80 meters. For example, at certain times of the year when it is
approaching sunset here in the States, the sun will have just risen
in Asia or Australia and vice-versa. At that time, radio waves
propagate along the semidarkness path that encircles the Earth
called the greyline. Both locations must be in the greyline in order
to make 2-way contacts. The tilt of the Earth makes the position of
the greyline change as the seasons change. Greyline propagation
occurs between any two locations for a brief period of a few weeks.
Afterwards, different places fall into the greyline. For several
weeks in the fall of the year, an interesting example of greyline
propagation occurs in the southeastern part of the U.S. On 3915 kHz,
the BBC outlet in Singapore can be heard for about an hour before
sunset coming in by greyline propagation. Stations to the east hear
it before we do. Stations farther to the west can hear the fading
signals after it fades out here because the greyline moves as the
earth rotates. For those hearing it, the signal fades in, it peaks,
and it slowly fades out.
7. Long
Path Propagation
Long path
propagation occurs when signals propagate the long way around the
world. It can occur on any band. It usually occurs from stations on
the opposite side of the world from you. We have worked South Africa
via long path by beaming northwest early in the morning on 20
meters. When this happens, we are working him long path through the
nighttime side of the earth. Since at all times half the Earth has
daytime and half the Earth has night, long path propagation is
determined by whether the signal is propagated through the nighttime
path or daylight path. Sometimes the daylight path will bring in
stations by long path propagation and at other times the darkness
path provides long path propagation. One night on 20 meters, we
heard a station in India coming in short path and long path
simultaneously, but the short path was stronger. At the same time,
California was working India by long path and they could not hear
him short path. They were working him through the daylight path, and
he was stronger here on the East Coast via the nighttime
path.
8.
160-Meter (1.8-2.0 MHz) Propagation
Each
amateur band propagates signals differently. The 160-meter band is
our only MW band and it acts similar to the broadcast band. It is
primarily a nighttime and wintertime band as it suffers from high
summertime static (QRN). Most hams that use this band for nearby
contacts use horizontal dipoles or inverted-V antennas. Some hams
use vertical antennas on this band to work distant stations (DX).
These DX contacts are made in the fall and wintertime at night via F
layer or greyline propagation when the static levels are low.
Dipoles and inverted-V antennas do not work well for DX on this
band.
9.
Eighty-Meter (3.5 4.0 MHz) Propagation
The CW
part of this band is called the 80-meter band and the voice part of
the band is known as 75 meters. Like 160 meters, eighty meters
suffers from the same QRN in the summertime. Working DX on this band
is a popular avocation during the fall and winter. However, 80
meters is used primarily for working nets and ragchewing. Eighty
meters is primarily a nighttime band. This band can vary from being
open most of the day in years with low sunspot numbers to being
closed during the middle of the day in years with many sunspots.
Many DX contacts have been made using dipoles and inverted-V
antennas, but a vertical with many ground radials will be
better.
10.
Forty-Meter (7.0-7.3 MHz) Propagation
The
forty-meter band has propagation that can act like either 80 meters
or 20 meters. It just depends on the stage of the sunspot cycle.
During the years with high sunspot numbers, nearby contacts are
possible all day. At night, the skip lengthens making contacts
possible to those parts of the world where it is still dark. Working
DX on 40 meters is a nighttime or greyline event. When the sunspots
are low, forty meters may have long skip during the day, and nearby
contacts may be impossible or they may be very weak. During the time
when we suffer from low sunspot numbers, many DX contacts are made
during early morning, late afternoon, and at
night.
If your
primary interest on forty meters is SSB, our 40-meter voice band is
a broadcast band in Regions 1 and 3. Region 1 is Europe, North Asia,
and Africa and Region 3 is the Pacific, Southern Asia, and
Australia. The top part of 40 meters is a voice band in Region 2,
which is North and South America. To work SSB on forty meters at
night, you will have to find a frequency between broadcast stations.
Strong broadcast stations heard at night begin to fade out slowly as
the morning sun rises and moves higher in the sky. As the suns angle
declines in the afternoon, the broadcast stations begin to break
through the noise becoming stronger as the sun begins to set. It is
only in the middle of the day when no broadcast stations are heard
on forty meters.
Since DX
stations in region 1 and most of region 3 can only transmit below
7100 kHz, working DX on 40-meter SSB is still possible. Stations in
those regions will have to transmit below 7100 kHz. (Australian and
New Zealand amateurs can operate up to 7200 kHz.) They call CQ and
announce where they are listening in our voice band above 7150 kHz.
This is what is called "working split."
11.
Thirty-Meter (10.1-10.15) Propagation
This band
has such a narrow frequency that the only modes allowed here are CW
and digital modes. That means no SSB. Propagation here is much like
40 and 20 meters. Unlike 20 meters, this band stays open longer at
night during years with low sunspot numbers. During the daylight
hours, it has much shorter skip than 20 meters. In the United
States, we are allowed only 250 Watts.
12.
Twenty-Meter (14.0-14.35 MHz) Propagation
The
twenty-meter band is the best DX band because it is open for
long-skip for more hours than any other band and it does not suffer
from QRN as the lower bands. In years of high sunspot numbers,
short-skip and long-distance DX can be worked at the same time
during daylight hours. Although DX is there most of the time, most
of the DX worked is at sunrise, sunset, and all night during peak
sunspot years. During the years of low sunspots, it is common to
work into Europe and Africa during the day and into Asia and the
South Pacific during the evening hours and early at night. Low
sunspot numbers cause 20 meters to go dead for east to west contacts
at night an hour or so after sunset, but there is some TE
propagation. During periods of moderate sunspot numbers, the
propagation on this band is a blend of propagation of low and high
sunspot years.
13.
Seventeen-Meter (18.067-18.167 MHz) Propagation
The
17-meter band propagation acts much like 20 meters except it is
affected more by low sunspot numbers than 20 meters. In periods of
low sunspot numbers, this band does not stay open as late as 20
meters, fading out as the sun begins to set. Yet, the 17-meter band
does stay open all night when the sunspot numbers are high. The
propagation on this band is like a blend of 20 meters and 15 meters,
but it is closer to 20 meters. Most users of this band use dipoles
and other simple antennas since triband beam antennas wont work
here.
14.
Fifteen-Meter (21.0-21.45 MHz) Propagation
Fifteen
meters is a fantastic DX band during the high sunspot years. This
band may be open for 24 hours, and it is common to work more than
100 countries during a contest weekend on this band. Many have
worked more than 300 different countries on 15 meters. In years of
low sunspot numbers, 15 meters may be completely dead for several
days in a row. When it opens during those years, you may hear only
the Caribbean, South America, and on rare occasions the extreme
southern part of Africa via TE propagation.
15.
Twelve-Meter (24.89-24.99 MHz) Propagation
The
12-meter band is much like 15 meters, but it is affected more by
sunspot numbers. Because this band is little used, many hours can
pass without hearing any amateur signals. Occasionally you will hear
South American Citizen Band "pirates" on lower sideband. It is
mostly a daytime band but openings to Asia and the South Pacific are
common early at night during peak sunspot years. The reason this
band is little used is that triband beam antennas dont cover this
band.
16.
Ten-Meter (28.0-29.7 MHz) Propagation
The band
that is most affected by the sunspot numbers is 10 meters. You may
have noticed in this discussion, the higher the frequency, the more
it is affected by sunspots. During peak sunspot years, 10 meters can
be open some days for 24 hours. Mostly it is a daytime band. When
they are at the peak, the sunspots enable you to work worldwide with
power as low as 5 Watts. A 10-meter confirmed country total of over
250 is common. In the low sunspot years, the band can be closed for
days. Ten meters can open for very short skip by sporadic E
propagation during the summer months. Very short skip means contacts
as close as 200 miles out to 1000 miles. Sporadic E propagation can
suddenly occur without regard to the sunspot
numbers.
VI.
STANDING WAVE RATIO
A standing
wave ratio bridge is used to measure the standing wave ratio, or
SWR. SWR is an indication of how well the radiating part of an
antenna is matched to its feed-line or how well the tuner is
matching the antenna system. Most amateurs pay far too much
attention to SWR. An SWR reading below 2:1 is acceptable, because
the mismatch is so small that the feed-line loss can be ignored. If
you are using a modern transceiver, its power may fold back to a
lower power output above this SWR level.
When you
have mismatch between the feed-line and the antenna, part of the
power feeding the antenna system reflects back toward the tuner and
the transmitter. The part of the power going toward the radiating
part of the antenna system is called forward power. The part
reflected back down the feed-line is called reflected power. The
larger the mismatch the larger the reflected power will
be.
If the
feed-line and antenna are not matched, waves traveling toward the
radiating part of the antenna system meet the waves being reflected
back down the feed-line. The waves interfere with each other, and at
certain points along the feed-line, the amplitudes of both waves
combine. This will result in a current maximum to be found at that
point; and at that point, the current will appear to be standing
still. The length of feed-line and the frequency will determine
where this point occurs. At another point, the forward and reflected
waves interfere, and they subtract from each other. At that point,
there will be a current minimum. If you could visualize this
phenomenon, you would see a series of current maximums and minimums
standing still along the feed-line. This is why we refer to them as
standing waves. At different points along the feed-line, where you
have high current, you will have low voltage, and where you have low
current, you will have high voltage. At any point along the
feed-line, multiplying the voltage times the current will equal the
power in Watts. When the feed-line is matched to the antenna,
current and voltage remain the same all along the feed-line because
there is no reflected current to interfere with the forward
current.
As happens
with the current, the voltage will also appear to be standing still.
The voltage maximums and voltage minimums will not be at the same
locations as the current maximums and minimums. SWR is the ratio of
the maximum voltage to the minimum voltage on the line. It is called
"Voltage Standing Wave Ratio" or VSWR, but we shorten it to just
SWR. There is also a current SWR or ISWR, and it is the same value
as the VSWR. For example, if the standing wave voltage maximum is
200 volts and the minimum voltage is 100 volts, the VSWR will be
2:1. If the voltage maximum and voltage minimum are equal, the SWR
will be 1:1. If the voltage minimum is zero, the SWR is
infinite.
In
measuring SWR at the transmitter, you need to realize that feed-line
losses affect the SWR readings. If the feed-line losses are high,
much of the power reflecting back from the antenna will be lost, and
the SWR reading on the meter will indicate it is lower than it
actually is. If a feed-line is so lossy that it consumes all forward
and reflected power, it will measure an SWR of
1:1.
When
measuring SWR on an antenna having a small amount of reflected
power, the length of the feed-line between the bridge and the
antenna may affect your SWR reading. An example of this is a 70-ohm
antenna being fed with 50-ohm coax. Different lengths of feed-line
will give you small differences in SWR readings because at certain
lengths, the mismatched feed-line starts to act like a series
matching section. In the case of a 70-ohm antenna fed with 50-ohm
coax, if the feed-line is a half wave long, the SWR will measure
1.4:1. At some particular length of feed-line and on one frequency,
the SWR will measure 1:1 because that length of that feed-line
transforms the impedance to make a match. Some hams have adjusted
their feed line length to get a perfect match. This is called
"tuning your antenna by tuning your feed-line." With other feed-line
lengths, you will measure something different. Suppose the impedance
of the feed-line and the antenna are perfectly matched. Then there
is no reflected power. You will get a 1:1 reading on the SWR-bridge
with any length of feed-line.
There
is a myth that reflected power is burned up as heat in the
transmitter. The reflected power coming back
down the feed-line sees an impedance mismatch at the transmitter or
tuner and it reflects back up again. The reflected power does not
get back into the transmitter. Because the reflected power reflects
back and forth, the radiating part of the antenna system absorbs
most of the power being reflected back up each time. All of it
eventually is radiated except for the power lost in the feed-line.
The losses in a real feed-line will burn up some of the power on
each pass. This is why the feed-line loss increases with
SWR.
Built-in
tuners are found in most modern transceivers. If yours doesnt have
one, then you can use an outboard tuner to give the transceiver a
proper load. The place you want a 1:1 SWR is between the output of a
transceiver and antenna or between the transceiver and the input of
a tuner in order for the transmitter to deliver its maximum power.
Because built-in tuners are in most modern transceivers, many hams
use them to match antenna systems having high
loss.
VII. REAL
ANTENNA SYSTEMS
In this
book, we will be talking about the losses that rob an antenna of its
maximum performance. The ideal antenna system will radiate 100% of
your transmitter power on all bands without a tuner and in the
direction you want to work. Such an antenna system does not exist.
Many new hams succumb to antenna advertisements making claims that
are exaggerated. No antenna will have low SWR, work all bands
without a tuner, and radiate efficiently at the same time. A dummy
load has a low SWR and will load up on all bands, but it will not
radiate a signal. A resonant coax-fed dipole antenna will have a low
SWR and will radiate efficiently on the band for which it is
resonant, but it will not work well on all bands. For example, if
the tuning range of your tuner has a sufficient range, you will be
able to load up any antenna with it, but it will not necessarily
radiate a signal efficiently. It may have high tuner and feed-line
losses.
When you
choose an antenna, you must decide how much loss you can accept.
DXers and hams that work weak signals at VHF frequencies try to
eliminate as much loss as possible. If your contacts are going to be
made under good band conditions and without much interference, you
can get by with high losses. In that case, coax-fed antennas used on
bands where they are not resonant will allow you to make contacts.
You can be greatly surprised by how little radiated power can be
used to make contacts under ideal conditions. If you want to make
contacts regularly under changing band conditions, you will want to
eliminate as much loss as possible and use antennas with gain. Lower
loss will enable you to hear weaker signals.
Nothing
will take the place of resonant half-wave dipoles, not because they
radiate more efficiently, but because they dont require lossy tuners
and dont have high coax losses. Remember that all antenna systems
have compromises
VIII.
HALF-WAVE RESONANT DIPOLE ANTENNAS
1. The
Half-Wave Flat-Top Dipole
Most
dipoles consist of two pieces of wire of equal lengths with one of
the two ends connected together through an insulator. The far ends
of the wires are also connected to insulators. The two conductors of
a feed-line are separated and connected across the gap at the center
insulator. The antenna is held up by rope that connects the
insulated ends of the antenna to two supports. It is a "balanced"
antenna, because equal currents flow on both halves of the antenna.
Coax is an unbalanced feed-line. (The possible effect of using an
unbalanced feed-line on a balanced antenna like a dipole will be
discussed later.) The dipole that is stretched between two high
supports is called a flattop dipole, distinguishing it from other
configurations.
The
simplest antenna system of all is the half-wave resonant dipole fed
with coax and no tuner. The only reason for using a half-wave
resonant dipole antenna is to eliminate the need for a matching
device such as a tuner. The feed-point impedance will be near 50
ohms at ordinary heights and they can be fed directly with 50-ohm
coax from the output of todays modern radios. The two halves of a
dipole are fed 180 degrees out of phase, meaning when one side is
fed positively, the other side is fed negatively. That is why a
feed-line has two conductors. Of course, the sides swap polarity on
each half cycle.
If you
could visualize the current flowing on the half-wave dipole, the
current will appear to be standing still. The maximum current will
be seen at the center of the wire and no current will be at the
ends. This occurs because the electrons flowing out to the ends
reflect back toward the center where they meet the next wave and the
current is reinforced there. The minimum voltage occurs at the
center and the maximum voltage occurs at the ends of the half-wave
resonant dipole. If you were to measure the voltage and the current
at any point on the dipole wire, the voltage times the current will
equal the power in Watts.
Figure 1.
Flat Top Dipole

2.
Inverted-V Dipole
Another
configuration for the half wave resonant dipole is one having one
support in the center and the ends stretched down toward the ground.
The single support can be a tree, mast, or tower. The ends of a
dipole have high RF voltages on them, and need to be at least 10
feet above ground for safety. This antenna is called an
"inverted-V," because the shape of the dipole looks like a "V"
turned upside down. Most dipoles illustrated in this book can be put
up in the inverted-V configuration. This configuration works well
because the current is concentrated on the middle two-thirds of the
antenna at the apex. The current in an antenna is what is
responsible for the radiation. The ends of the antenna have very
little current in them and it doesnt matter if the ends are close to
the ground. The middle of the antenna is up high where the radiation
is taking place and that is the place you want the radiation to be.
An inverted-V has an advantage that the horizontal space required
for it is less than what is needed for a flattop dipole. The angle
between the wires on an inverted-V needs to be greater than 90
degrees. The gain of an inverted -V is 0.2 dBd and it has a
radiation pattern nearly omni-directional. Since it is easy to
construct and works so well, the inverted-V is the most commonly
used dipole. An explanation of the decibel will come
later.
Figure 2.
The Inverted-V Dipole

Figure 3.
Radiation Pattern of Inverted-V for 80-Meters at 65
Feet

In figure
3 above, the top graph shows how the radiation would appear to you,
if you were situated above the dipole and you were looking down on
it. The plane of the antenna runs from side to side on the top
graph, and that graph demonstrates only a 5-dB null off the ends of
the antenna. Therefore, it is essentially omnidirectional. The
bottom graph shows how the radiation would appear if you were
looking at the antenna from the end of the wire. As you can see, the
pattern shows no radiation at the horizon and its maximum radiation
is at about 40 degrees above the horizon, and the radiation straight
up is only down 3 dB from its maximum. This antenna was modeled on
80 meters with the apex at 65 feet above ground and the ends at 35
feet.
It is a
myth that a horizontal antenna orientation makes a difference on 80
meters at heights used by most amateurs. I have heard many amateurs
say on 80 meters, "The reason my signal is weak to you is because
you are off the end of my dipole." The radiation pattern from a
dipole is essentially non-directional until the dipole is elevated
more than a half wave, that is about 125 feet on 80 meters, and it
is 65 feet on 40 meters. The main reason it makes no
difference regarding orientation is because propagation for signals
closer than 500 miles (the distance of most 80 meter contacts) is
essentially by high angle radiation nearly straight up and down.
Only signals radiated and received at low angles make a difference
in antenna orientation even at low heights above ground. At low
heights, there are nulls about 3 to 4 dB off the dipole
ends.
3. Dipole
Shape Variations
The wire
of a dipole doesnt have to be run in a straight line. A dipole does
not have to be perfectly horizontal. Thats the way it is usually
depicted in books and magazines, but you can bend the legs of the
antenna up, down or sideways.
Figure 4.
Two Dipole Shape Variations

If you
make either wire one-half wavelength long and carefully prune it to
resonance, you can use it without a tuner on and near its resonant
frequency. Both antennas have the current part at the top where most
of the radiation takes place. The vertical parts of these antennas
radiate a weak vertically polarized wave. The only reason these
dipoles are contorted this way is to make them full-sized and to fit
in the available space. Other shapes are possible, and you can be
creative at your location.
There are
many more dipoles than the ones just described. We will explore the
other kinds of dipoles in section "X" of this
book.
4.
Calculating the Length of a Half-Wave Resonant
Dipole
The
approximate length in feet of a half-wave resonant dipole is found
by dividing 468 by the frequency in MHz. The actual length of it
will be determined by several factors. Using larger diameter wire
will make the dipole resonate lower in frequency. Therefore, to make
it resonant at the higher desired frequency,
It must be
shortened. Raising a dipole higher above ground will make it
resonate higher in frequency. An insulated wire will make the dipole
resonate lower in frequency than a bare wire.
Using the
above formula, cut the antenna a little longer than the calculations
say. If the SWR is best at a lower frequency than you desire, the
antenna will have to be made shorter by pulling the excess wire
through the end insulators, folding the ends of the extra wire back
on itself. Then wrap the ends of the overlapped wire on itself so it
wont come loose. This causes the excess wire to "short" itself to
the rest of the antenna. If you are using insulated wire, you will
need to cut off the excess wire. The reverse is true if the antenna
resonates too high in frequency. The extra wire can be let out to
make it resonate on a lower frequency. This is why you originally
cut the wire a little longer.
5. The
Decibel
The
decibel (dB) is a unit of measurement for comparisons of the ratio
of power, current, and voltage and is the term we will use in
comparing antennas in this book. At one time, antenna comparisons
were made using a dipole as a standard, but today most comparisons
use the isotropic radiator as a reference. An isotropic radiator is
an imaginary antenna that radiates equally well in all directions.
It has no gain. The terms "dBi" and "dBd" are used to label which
reference is being used. In this book, we will use the dipole as a
standard for the most part.
How do you
derive decibels from power ratios? The formula for power ratios is
dB = 10 log P1/P2. For voltage and current, the values are doubled.
Formulas of this type are beyond the scope of this book. Doubling
the power will produce a 3 dB stronger signal. Double the power and
double it again will equal a 4 times power increase and that gives 3
dB plus 3 dB or 6 dB. Double 4 and that is a power increase of 8 and
that adds 3 more dB for a total of 9 dB. Increasing the power from 1
Watt to 10 watts or increasing it 10 times will give a 10-dB
increase. Multiply 10-Watts times 10 give us 100 watts, which adds
another 10 dB above 1 Watt for 20 dB. Therefore, increasing the
power another 10 times to 1000 Watts will produce a signal 30 dB
stronger than 1 Watt.
Your
receiver, if modern, will have a signal strength meter or "S Meter."
That meter is calibrated in "S-Units" from one to nine and decibels
over S-9. S-9 is usually calibrated using 50 microvolts ( uV) from a
signal generator. Each S-unit is approximately a difference of 5 or
6 dB. Therefore, a reading of S-9 is about 6 dB stronger than S-8.
Therefore, from S-0 to S-9 is 54 dB. On some low cost transceivers,
the S-units and dB above S-9 are only relative signal readings and
actually have nothing to do with decibels.
IX.
ANTENNA BASICS
1.
Resistances and Reactance
Two
factors measurable in antenna impedance are resistance and
reactance. When we refer to antenna resistance, we are referring to
its radiation resistance. It is neither a resistance like the
electronic component called a "resistor," nor is it the same as the
resistance found in all conductors. Those types of resistances,
called "loss resistances," change electrical energy into heat
energy. Heat energy disappears by radiating out into its
surroundings and it dissipates away to infinity. When we feed RF
into the antenna, the energy put into the radiation resistance
disappears from the antenna by radiation of electromagnetic waves,
and that makes an antenna appear to have a resistor in it. Loss
resistance robs power from the radiation resistance and lowers the
efficiency of an antenna system, but the loss resistance in dipoles
is very low if the feed-line loss is low. The efficiency of any
antenna system is found from a ratio of radiation resistance and
loss resistance. We can either calculate the loss resistance by the
loss in the feed-line from published tables and by estimating the
loss in tuning units. Feed-line loss and tuning unit loss can be
measured, but that is beyond the scope of this
book.
Antenna
systems having reactance prevent the transmitter from delivering its
full power and the reactance needs to be tuned out. There are two
kinds of reactance: capacitive and inductive. Antennas have both. In
antennas, reactance is a virtual reactance meaning the antenna acts
as if there were a capacitor or an inductor in the antenna, but
neither is there. You can only measure the sum of both reactances
but not a value for either one. Using an antenna analyzer, you can
determine whether the sum of the reactance is inductive or
capacitive. Inductive reactance is a negative number and capacitive
reactance is a positive number.
The
reactance of an antenna forms the "J" factor in antenna impedance
measurements. The "J" factor is measured in ohms and the reactance
is expressed as + or "J" ohms depending on whether it is capacitive
or inductive reactance. Capacitive reactance is expressed as +J ohms
and inductive reactance is expressed as -J ohms. Capacitive and
inductive reactance are opposite factors and one can cancel the
other. An antenna having 6 ohms capacitive reactance or + J 6 ohms
and an inductive reactance of J 5 ohms will result in an antenna
with a reactance of 1 ohm capacitive or + J 1. Since one term is
positive and the other term is negative, you subtract smaller value
from the larger. The answer has the sign of the larger one. In
antennas, the reactance and resistance together determine the
overall impedance of the antenna. The J factor is mentioned here
only because you may see it in other books and on the extra class
examination, but it will not be used further
here.
A resonant
antenna has equal amounts of inductive and capacitive reactance, and
the sum of the reactance equals zero. As an example, when the
inductive reactance equals J 5 and the capacitive reactance equals
+J 5, their sum equals zero. When the sum of the total reactance of
an antenna is tuned to zero, its impedance is totally resistive. The
use of an antenna analyzer will tell you if the antenna is too long
or too short for resonance. The simplest way to tune out antenna
reactance is to change its length. The sum of the reactance of a
long antenna will be inductive, and the sum of the reactance of a
short antenna will be capacitive. If an antenna is short because it
wont fit your property, it can be tuned to resonance by putting an
inductor (coil of wire) in each leg. These coils are called "loading
coils." An equal amount of inductive reactance will cancel the
excessive amount of capacitive reactance. An antenna with loading
coils is described in section "X." When an antenna is too long, the
sum of its reactance will be inductive, and a variable capacitor can
be inserted in each leg to tune out the inductive reactance. This is
seldom done because it is easier to shorten the
antenna.
A resonant
antenna may still have SWR if its radiation resistance is not
exactly 50 ohms. Not many resonant antennas have a radiation
resistance of exactly 50 ohms, and most real antennas have a small
amount of SWR. An antenna is resonant only at one frequency per
band. It will also be resonant on its harmonic frequencies, where
its radiation resistance will range from high to very high. Hams
talk about using resonant antennas. What is meant by this is they
use an antenna on its fundamental frequency close to resonance, the
resistance is near 50 ohms, and the SWR without a tuner is near
1:1.
To
calculate the impedance of an antenna with both resistance and
reactance requires a mathematical procedure called the Pythagorean
Theorem. That type of math is beyond the scope of this book.
However, you should know how to use the Pythagorean Theorem to solve
impedance problems on the Extra-Class test. Otherwise, you will have
to memorize the answers from the question pool.
2. Feeding
Dipoles Efficiently
For
maximum power transfer from transmitter to the antenna, the antenna
system must be resonant, and the resistance of the load (antenna
system) has to be equal to the internal resistance of the source
(transmitter). Notice we said an antenna system, not the antenna,
must be resonant. As mentioned previously, an antenna system
consists of the antenna, the feed-line, and any matching networks
(tuners). A tuner at the input end of the feed-line can make a
non-resonant antenna system resonant, and have a resistance of 50
ohms, and that matches the internal resistance of the transmitter. A
tuner will not change the SWR between the tuner and the dipole part
of an antenna system, and will not remove the reactance from the
dipole.
When the
load of an antenna system does not match the source and the
impedance is high, the load will not draw power from the source and
high RF voltages will be present at the output of the final
transistors. In this case, high RF voltages can damage the output
transistors of the transmitter. When the impedance of the load is
low, too much of the power may be dissipated across the internal
resistance of the transmitter possibly destroying the output
transistors. These are the two reasons why transceivers "fold back"
their power when the SWR is high.
It is a
myth that the dipole part of an antenna has to be resonant to be
efficient. When power reaches the radiating part
of the antenna system, it obeys the "The Law of Conservation of
Energy." The Law of Conservation of Energy states, "Energy can
neither be created nor destroyed. Only its form can be changed."
(What is important is to get the power to the dipole itself, because
in some systems power is lost in the feed-line, especially when
using coax with high SWR) The miniscule amount of power in the
dipole that does not radiate is changed into heat, another form of
energy. Because the dipole part of an antenna system is made of
conductors with low loss resistance, 99% or more of the power
reaching it will radiate regardless of its length if that length is
reasonable. The loss resistance of the conductors of the radiating
part of most antenna system is so low it can be ignored. (Short
mobile HF antennas are an exception because they may be lossy
because of the very high current flowing in
them.)
Not all
the energy fed into an antenna system will reach the antenna itself.
If the system has a tuner, part of the power is lost in the inductor
of the tuner and part is lost in the feed-line. When properly tuned,
tuners using T-networks lose about 10% of the power and L-network
tuners lose about 5% of the power being fed to them. Notice we said
properly tuned. However, improper tuning of the antenna tuner may
cause you to believe the feed-line is matched, but when this happens
there is a very high circulating current in the inductor causing it
to get hot. This causes extremely high losses, and very little power
reaches the radiating part of the antenna. In addition, so much heat
is produced in the inductor that it can be damaged. We melted the
plastic insulation that forms the inductor on one tuner this way.
For this reason, some hams dont like tuners, preferring to use
resonant antennas. Read the instructions for your tuner for proper
tuning or you may wind up with a poor signal and a damaged tuner.
The resistive losses in the conductors of the feed-line and the
dielectric losses in the feed-line also rob power from the system.
These are the reasons for you to use the best tuners and feed-lines
possible.
Another
loss to be considered is feed-line radiation. Any energy that
radiates from the feed-line does not reach the radiating part of the
antenna, and it may be absorbed by near-by objects and may not
radiate in the desired direction. When coax radiates, it is called
common-mode radiation. If the feed line can radiate, it can also
receive signals. This can be detrimental because the coax can then
pick up noise from near-by power lines, etc. Feed-line radiation
will also destroy the directional pattern of a beam antenna. The
causes of feed-line radiation will be described in the next
section.
As we
pointed out earlier, when you are using a half-wave resonant dipole
fed with low-loss coax without using a tuner, almost all of the
power coming out of the transmitter will radiate. On its resonant
frequency, the dipole is one of the most efficient antenna systems a
ham can use. However, a half-wave resonant dipole has a finite
bandwidth. Why use a tuner with resonant antennas? On 160 and 80
meters the bands are wide compared to the percentage of frequency.
The width of 80 meters is 500 kHz and its frequency is 3500 kHz. The
width of 80 meters is 14% of the frequency. The 350 kHz of 40 meters
is 5% of the frequency and most of the band can be covered without a
tuner. The 350 kHz width of the 20-meter band is 350 divided by
14000 kHz, or 2.5 % of the frequency, etc. The percentage of
frequency for a band will determine if a resonant dipole will work
the whole band without a tuner. If you are planning to move around
on 160 or 80 meter bands, it makes sense to have a tuner, because
the bandwidth of resonant dipoles on those two bands is narrow. For
example, the normal 2:1 SWR bandwidth of an 80-meter dipole is less
than 200 kHz and the band is 500 kHz wide. However, if you have an
antenna resonant for the voice portion of the band, you can still
use a tuner to work the CW part of the band without inducing more
than a dB of loss. Except for 40 and 10 meters, full-sized resonant
dipoles on the rest of the HF bands will have enough bandwidth for
them to cover the whole band.
The best
place to insert a tuner is up at the antenna feed-point. However, if
it is placed there, you wont be able to reach the tuners controls.
Therefore, it is more practical to place it between the transceiver
and the shack-end of the antenna feed-line. A piece of 50-ohm coax
connects the radio to the tuner. With the tuner located in the
shack, adjustments can be made. Remote automatic antenna tuners can
be placed at the antennas feed-point, but the disadvantage of them
is that the ones available today will not handle high
power.
A coax-fed
dipole and a tuner should not be used to feed an antenna on its even
harmonically related bands. The even harmonics are 2, 4, 6, etc,
times the fundamental resonant frequency. If an 80-meter antenna
being fed with coax through a tuner is used on 40 meters, it will
put out a weak signal because the SWR will be around a hundred to
one. Coax has a tremendous loss with SWR this high. Only a few Watts
from a hundred-Watt transmitter will reach the antenna. However, you
will be able to make contacts with those few Watts. If you want to
use any antenna having high SWR, ladder-line has much less loss than
coax. If you feed an 80-meter dipole on 40 meters using ladder-line
and a tuner, it will only be slightly less efficient than a
half-wave 40-meter coax-fed resonant dipole. However, the SWR will
still be high between the tuner and the antenna, but this doesnt
matter since ladder-line has an insignificant loss. Since the
feed-point impedance will be high, the SWR will only be about 9:1 in
the ladder-line because ladder-line is a high impedance
feed-line.
Extremely
short antennas may not work at all because of the above mentioned
reasons. To reiterate, the extremely high capacitive reactance may
make it impossible for its reactance to be tuned out and reactance
prevents a transmitter from delivering power to the antenna. Even if
you are able to tune out the capacitive reactance, tuning it out
requires an inductor and most of the power will be lost in the
inductor. Do not take the statement about the Conservation of Energy
to mean you can put up any piece of wire and it will radiate your
entire signal.
3. The
Cause of Feed-Line Radiation
Contrary to popular myth, SWR in a
feed-line will not cause it to radiate.The cause of feed-line
radiation is unequal current in the two conductors of the feed-line.
What are the causes of unbalanced current in a feed-line? They are
an unbalanced feed-line feeding a balanced antenna; the feed-line
being brought away from and parallel to one leg of the antenna; the
antenna not being fed in its center; and one leg of the antenna
being close to metal objects. In coax, unbalance causes RF to travel
on the outside surface of the coax shield, and the shield radiates.
When everything is balanced, coax normally has current flowing on
its center conductor and on the inside of its shield. The shield
prevents it from radiating.
Ladder-line will also radiate when it
is fed from the output of a tuner not having a balun. Baluns are
discussed in the next section. Since the output of a transceivers
tuner is unbalanced and feeding ladder-line directly from your
transceivers tuner, the currents in the ladder-line will not be
balanced. When balanced, ladder-line has equal currents with a
180-degree phase difference, which produce waves that null each
other out, and no radiation takes place. Hams mistakenly refuse to
bring ladder-line into the shack because of a fear of feed-line
radiation, but ladder-line does not radiate when balanced. The
simple cure for feed-line radiation is to use a balun at the antenna
feed-point for coax and a balun at the output of the tuner when
using ladder-line.
4.
Baluns
The word
"Balun" is a contraction of " balanced to unbalanced." It is
pronounced "bal un" like "bal" in "balanced and like "un" in
"unbalanced". Many hams mistakenly pronounce an "M" at the end of
the word making it "balum." A balun transforms the unbalanced
transmitter output to a balance feed-line such as ladder-line. It is
also used to connect an unbalanced feed-line such as coax to a
balanced dipole. In the latter case, the balun is located at the
antenna feed-point and is constructed so the balun takes the place
of the center insulator.
There are
two kinds of baluns: voltage baluns and current baluns. They both
accomplish the same thing. The difference in baluns is in the way
they are wound. A voltage balun produces equal voltage with opposite
polarity at its output. As its name implies, a current balun
provides equal currents with opposite polarity at its
output.
Running
the coax through ferrite beads can make a 1 to 1 current balun. In
addition, you can build a 1 to 1choke current balun by winding 8 to
10 turns of coax around a two-liter soda bottle and placing the
coiled coax at the antenna feed-point. Any balun is designed to
"divorce" your antenna from the feed line. It is used to prevent
common mode radiation of coax, which makes the coax to be part of
your antenna. You want it to be able to deliver all your power to
the radiator itself. A choke balun does this perfectly, without
using any ferrite beads or toroids. In most cases common mode coax
radiation does not occur when a balun is not used, but it is
preferable to use one to be safe.
Other
baluns provide a step-up or step-down impedance transformation. A
4-to-1 balun steps up the impedance four times. It will transform a
50-ohm impedance to 200 ohms. This type of balun transformer is used
at the output of tuners to increase the tuning range of a tuner 4
times. If a tuner without a balun can match 500 ohms, a 4-to-1 balun
will increase the range of impedances it can match to 2000 ohms.
Many hams think the 4-to-1 balun is used to match 50 ohms to 450-ohm
ladder-line but it is not. It would take a 9-to-1 balun to match 50
ohms to 450 ohms, and it is not important to match the impedance to
ladder-line.
A balun
should always be placed at the input end of ladder-line or open wire
feeders to prevent feed-line radiation. When using ladder-line a
step up balun is commonly used although a 1:1 balun will
work.
X. OTHER
TYPES OF DIPOLES
1. A
Shortened Dipole Using Loading Coils
If you are
unable to put up a full-sized dipole on your property, putting
loading coils into the dipole could shorten the antenna. See section
IX, part 1. A short antenna has capacitive reactance and the
capacitive reactance can be tuned out with a coil. The overall
length of the shortened antenna will be determined by the amount of
inductance in the coil. Pre-tuned antennas of this type are
available from at least one manufacturer. The main problem with
loaded antennas is they are very narrow banded. If the loading coils
are wound with small diameter wire, the coils may introduce unwanted
loss into the antenna. Loading coils can also be found in shortened
vertical antennas for high frequency (HF) mobile
use.
Figure 5.
A Shortened Loaded Dipole

2. All
Band Dipole
In the
figure below, a dipole is cut to a half wave on the lowest band you
want to operate. Feeding it with ladder-line and a tuner makes it
possible for you to work all the other higher bands. The only losses
in this antenna system are the loss in the tuner and the very small
loss in the ladder-line. This system is more than 90% efficient. As
mentioned above the balun in the tuner will be used, or if your
tuner doesnt have a balun, an external balun can be connected
between the tuner and ladder-line with a short run of coax.
Four-to-one baluns are the most commonly used ones for this
arrangement.
Figure 6.
All Band Dipole

3. The
Sloping Dipole
A lower
angle of radiation can be achieved by tying one end of a half-wave
dipole to a high support and the other end near the ground. It is
fed with or without a balun with 50-ohm coax. The sloping dipole
will show some directivity and have low angle gain in the direction
of the slope. More directivity can be gained if the dipole is strung
from a tower, and the tower is acting as a passive reflector. The
sloping dipole is mostly a vertically polarized radiator and it
works well for DX. Since the sloping dipole is fed in its center, it
does not need to be grounded to the earth as a quarter-wave vertical
does. Make sure the bottom end of a sloping dipole is at least 10
feet above ground because like all dipoles there is high RF voltage
on its ends.
Figure 7.
Half-Wave Resonant Sloping Dipole

In the
picture above, the field of maximum radiation is in the direction of
the slope or toward the right side of the picture. The formula for
the length of a sloping dipole is the same for any half-wave
resonant dipole.
4. The
Folded Dipole
The
B&W Company makes a folded dipole that claims to have a good
match on all bands and it does. However, on the low bands much of
the power is burned up in the resistor that connects the two ends
together. The power going toward the ends encounter the resistor and
is consumed as heat. All that power is lost and does not radiate,
and no power is reflected back to the feed point making the antenna
have low SWR. On the higher bands, a large part of the power
radiates before it reaches the resistor and the antenna is
moderately efficient on those bands. On 80 meters the 90 foot-long
dipole model will produce a signal at least 10 dB lower than that
from a resonant dipole.
If you
remember the single channel TV antennas used years ago, the driven
element was a folded dipole. Folded dipoles are very broad-banded.
That is the reason they were used for TV antennas since a TV channel
is 4 MHz wide.
When
constructing a folded dipole, the formula for calculating the length
of it is the same as for any dipole. The folded dipole consists of
two parallel conductors with the ends tied together. The conductors
can be spaced from less than an inch to more than two inches apart
when made from TV ribbon or ladder-line. At the ends, strip the
insulation back several inches, Twist the bare wires together,
solder them, and run them through insulators. The feed-point is in
the center of only one of the two parallel
conductors.
The
feed-point impedance of a folded dipole at resonance is close to 300
ohms resistive and can be fed directly with 300-ohm TV twin-lead or
a tuner with its balun. This antenna was very popular years ago when
coax was expensive and 300-ohm TV twin-lead was relatively cheap. A
length of 450-ohm can be substituted for the twin-lead. An alternate
feed method is placing a 6:1 balun at the feed-point and then
feeding it with 50-ohm coax. The folded dipole will not radiate its
second harmonic, so it is not good for a multi-band tuner-fed
antenna.
Another
folded dipole type is the three wire folded dipole. We have seen
this dipole only in books and do not know anyone who uses one. The
feed-point impedance is 600 ohms resistive and is fed with
home-built 600 ohm open wire feeders.
Figure 8.
Folded Dipole

5. The
Double Bazooka Dipole
The double
bazooka is claimed by its users to be broad-banded, a quality
especially interesting for those hams operating on 75/80 meters.
Tests done at the A.R.R.L. have shown the double bazooka is only
slightly more broad-banded than a regular dipole, probably due to
the use of a large conductor (coax) for the center part of the
antenna. The double bazooka will not transmit its second harmonic,
and its users say it does not need a balun. Other users say it is
quieter than a regular dipole.
The center
of the antenna is made from RG-58 coax. To find the length of coax
needed, divide 325 by the frequency in MHz. The coax forms the
center part of the double bazooka and a piece of number 12 wire on
each end completes the antenna. The length of each of the end wires
is found by dividing 67.5 by the frequency in MHz. To increase the
bandwidth some builders use shorted ladder-line in place of the
number 12 wire, which makes the end pieces to be electrically
larger.
The
feed-point of the double bazooka is unique. At the center of the
coax dipole, remove about 3 inches of the plastic covering, exposing
the shield. Cut the shield in the center and separate it into two
parts. Do not cut the dielectric or the center conductor. Leave the
center conductor with its insulation exposed. On the feed-line strip
off about 3 inches of outer insulation, separate the shield from the
center conductor, and strip about 1 inches of the insulation from
the center conductor. To attach the feed-line, solder the two
exposed feed-line conductors to the two pieces of the separated
exposed shield of the dipole center. It goes without saying: seal
the feed-point to prevent water from getting in. At each of the two
ends of the coax forming the center of the antenna, the coax is
stripped back and the center conductor and shield are shorted
together and soldered. The end wires are soldered to the shorted
coax ends, run to insulators at the end of the antenna, and the
soldered joints are sealed against the weather.
Figure 9.
Double Bazooka Dipole

6.
Broad-Banded Coax-Fed Fan Dipole
A
broad-banded dipole for 75/80 meters can be constructed by attaching
two equal length dipoles to the center feed-point and spreading the
ends about 3 feet apart using PVC water pipe to separate them. The
completed dipole looks like a bow tie. This makes the antenna to
appear electrically to have that of a large diameter conductor.
Because of this, the overall length will need to be shorter than a
single wire alone. When we used the antenna, we found a length of
110 feet would cover most of the 75/80-meter band without a tuner.
It is fed with 50-ohm coax. The use of a balun is optional. The
antennas for most of the higher bands have enough bandwidth so they
do not need broad banding.
Figure 10.
Broad-Banded Fan Dipole for 80 Meters

7.
Two-Element Collinear Dipole
The
two-element collinear dipole is an antenna that is a full-wavelength
antenna having a two-dBd gain. It can be fed with ladder-line and a
tuner and used as a multiband antenna, or it can be fed with a
quarter-wave-matching stub with 50-ohm coax cable to make it a
single band array. In the stub matching system, a quarter wavelength
of ladder-line is connected across the center insulator, and the
opposite end of the ladder-line is shorted. A shorted quarter-wave
piece of feed-line acts like an open circuit. Going from the shorted
end of the ladder-line toward the dipole, there will be a point
where a piece of 50-ohm cable will find a perfect match. The 50-ohm
feed-point will have to be found empirically (trial and
error).
Figure 11.
Two Element Collinear Dipole

8.
Four-Element Collinear Dipole
The
four-element collinear dipole array consists of four half-wave
segments connected end-to-end with an insulator between each two
adjoining segments. The feed-point is at the center of the array.
The antenna is fed with ladder-line through a tuner. A quarter wave
shorted ladder-line stub hangs down vertically from the insulators
between the inside and the outside half-wave segments. This stub
provides a 180-degree phase shift so that all half-wave segments are
fed in phase. This antenna has a 6-dBd gain and it radiates
bi-directionally at an angle perpendicular or broadside to the plane
of the wires.
This
antenna is too long for most hams to use on 80 and 40 meters, and
the stubs hanging vertically will be too close to the ground. For 20
meters, the four-element collinear array will be 97 feet long and
the stubs will be 18 feet. To find the length of each half-wave
segment, divide 468 by the frequency in MHz, and for the
quarter-wave stubs, divide 246 by the frequency in
MHz.
MFJ has
begun marketing the four-element collinear monoband array. They have
them for 20, 17, and 15 meters. This antenna is so easy to build
that you can do it yourself. All you need is 5 insulators, antenna
wire, and some ladder-line.
It will
have no gain if you use it on bands for which it is not designed
because the stubs are used as phasing lines. It is definitely not a
multiband antenna.
It is
possible to add more half-wave segments to the ends of this array to
make it have 6, 8, 10, etc half wave segments. Adding more segments
will add more gain and make the lobes narrower.
Figure 12.
Four-Element Collinear Dipole

9.
Coax-Fed Dipoles Operated on Odd Harmonic
Frequencies
Antennas
fed with 50-ohm coax can be used on other bands for which they are
not cut. An 80-meter dipole will have a relatively low SWR and will
be resonant at a single frequency on 10 meters and
not
much power
will be lost in the coax even if operated off resonance. A 40-meter
dipole will work the same way on 15 meters. Using coax, a dipole
will work on its fundamental frequency and on odd-harmonic
frequencies and it is not necessary to use ladder-line. The
fundamental frequency is the frequency for which the antenna is a
half-wavelength long, and the odd harmonics are 3 times, 5 times, 7
times, etc. the fundamental resonant frequency. A frequency of 21
MHz is 3 times or the third harmonic of 7 MHz, and 28 MHz is the
seventh harmonic of 4 MHz.
Antennas
operated on their odd harmonics will be resonant a little higher in
frequency than exact multiples of their fundamental frequencies.
Since the odd harmonic antennas input impedance is higher than it is
on its fundamental frequency, many amateurs use a series
quarter-wave matching section of 70-ohm coax to give it a better
match. The 80 meter inverted-V dipole in use here has a 2:1 SWR on
10 meters indicating it has an impedance of around 100 ohms.
However, modeling the antenna for 10 meters shows the resonance to
be below 28 MHz, probably because the antennas fundamental resonant
frequency is 3920 instead of 4000 kHz. A quarter wave 70-ohm
matching section should bring the SWR down to a much lower
level.
As said
earlier, if you try to use coax with a dipole on its even harmonic
frequencies, the feed-point impedance will be very high, the SWR
will be extremely high, and the coax will absorb most of the power.
In addition, when operating a coax-fed antenna on its even
harmonics, the tuner may not be able to provide a match. Operating
any antenna on any of its harmonic frequencies, odd or even, will
work better if it is fed with ladder-line and a
tuner.
Figure 13.
Three Half-wave Dipole

This
antenna is matched by a quarter-wave 70-ohm series matching section.
Three half waves will resonate higher than you would expect because
the center half wave doesnt have to contend with end effects. To
calculate the length of a three half-wave dipole, divide 1380.6 by
the frequency in MHz. Five half waves is found by dividing 2316.6 by
the frequency.
To use a 3
half-wave antenna on 15 meters, the 70-ohm matching section needs to
be 7 feet 7 inches and the antenna needs to be 64 feet long for a
good match. It will be just a little long on 40 meters. When using a
40-meter dipole with a 15-meter quarter-wave matching section, it
will still have acceptable SWR on 40 meters.
Figure 14.
Radiation Pattern of a 15- Meter Three Half-Wave Dipole at 65
Feet

The
pattern shows 6 lobes, 4 major lobes and 2 minor lobes. The vertical
radiation pattern shows low angle radiation.
10. All
Band Random Length Dipole
A random
length of wire cut into two pieces can be used as a dipole, and it
will radiate efficiently. It has to be at least a half-wave length
on the lowest band you want to work. It looks the same as the
all-band dipole and is the same, except it is not resonant on any
band. The random length dipole is being described here to emphasize
that the radiating part of an antenna doesnt have to be resonant.
Because it will have a feed-point impedance that is unusual, it must
be fed with ladder-line a tuner, and a balun. Since you are using a
tuner, it can be used on multiple bands. If you make it very long,
it can have gain over a dipole. For example, if it is four
wavelengths long, it will have 3-dBd gain. As you move to higher
bands, the electrical wavelength of the antenna increases, and each
higher band will have more gain.
A
half-wave antenna radiates perpendicularly to the plane of the wire.
As you move to higher bands, this antenna begins to show some gain,
and instead of two lobes of radiation, the two lobes split into four
lobes and the pattern resembles a 4-leaf clover. As you make the
antenna longer, the four lobes move nearer the to the ends, the gain
increases, and there are minor lobes of radiation between the major
lobes. These minor lobes make it possible to work in all directions.
The longer the wire, the closer the antennas major lobes radiate
bi-directionally toward its ends
Figure 15.
All Band Center-Fed Random Length Dipole

The
problem with using a random length of wire for this antenna is you
may find that because of limitations of your tuner, you may not be
able to tune a particular length of antenna on some bands. Certain
lengths will tune all bands and one of those lengths is 135 feet.
That particular length will be nearly resonant on all bands of 80-10
meters. Resonance only makes it easier to tune, but it has no effect
on efficiency. A length of 260 feet will tune from 160-10 meters.
Lengths of 260 and 135 feet have been used here successfully. Some
hams use random lengths of wire without problems. Then some hams
have had problems with other random lengths. The ones having the
problems solved the tuner problems by changing the length of the
dipole wire. If you plan to put up this antenna using a random
length of wire, you will need to experiment with various lengths
until you find a combination that works.
Tests were
performed here using two towers of equal height and spaced 100 feet
apart. On one tower, was an 80-meter inverted-V 120 feet long fed
directly with coax, and running parallel to it on the other tower
was a 135-foot long inverted-V fed with ladder-line and a tuner. At
the resonant point of the coax-fed dipole and having tuned the
ladder-line fed antenna, it was possible to switch antennas
instantly and many hams were asked to look at their "S-meters" while
the antennas were switched. All hams that participated in the test
said the signals from both antennas were equal. The signals were
measured on analog S-meters, not on segmented LCD meters found on
most of todays transceivers.
11. A
Two-Band Fan Dipole
A two-band
dipole can be constructed by connecting together the feed point two
dipoles for even harmonically related bands. It is fed with 50-ohm
coax with or without a balun. The best example of this is 80 and
40-meter dipoles connected together. Both dipoles are cut for
half-wave resonance on each of the two bands. They are fed together
and the ends of the wires are spread apart. If the ends are close
together, there will be interaction between the dipoles. In such an
antenna system, both dipoles must be carefully pruned for lowest SWR
one band at a time. The lower band will be tuned first since the
shorter dipole will not interact with the longer one. Each dipole
has a low antenna resistance on the band for which it is resonant.
RF energy follows the path of least resistance, and it automatically
selects which dipole will receive power. The remaining antenna will
have a high impedance. High impedance will block RF. Such an antenna
will have a narrower bandwidth than a single band dipole, but close
to the resonant frequency of each dipole, a tuner will not be
needed. To connect many dipoles for multiple bands is possible, but
it is not recommended because multiple wires are prone to interact
and it will be impossible to achieve a low SWR on some bands.
However, on the two band model, the 40-meter dipole will resonate
close to 15 meters, the 80-meter dipole will resonate close to 10
meters, and working four bands with this set-up is possible. Some
hams are using this antenna successfully with a tuner on all bands,
although the signal on 20 meters suffers somewhat because of high
SWR.
Figure 16.
Two-Band Fan Dipole for 40 and 75 Meters

12.
Trapped Dipole for 75 and 40 Meters
A trap is
constructed from a capacitor and an inductor connected in parallel.
It acts as an open switch on the frequency for which it is resonant.
A trap is placed on each side of the dipole. For a 75 and 40 meter
trapped dipole, the traps must be resonant on 40 meters, and each
trap should be placed a quarter wave from the center insulator. The
center section between the traps is electrically isolated from the
ends of the dipole by the traps on 40 meters, and the center section
of the antenna becomes a full-sized half wave resonant dipole for
that band. This antenna is fed with 50-ohm coax and an optional
balun. Wires connected to the outside of the traps are run to the
end insulators and are tuned so the entire antenna resonates on 75
meters. The 75 and 40 meter trapped dipole will be shorter than a
75-meter dipole because the inductor in the 40-meter trap acts as a
loading coil on 75 meters. In addition, the ends of the antenna can
be tuned to operate on the 80-meter CW band instead of the 75-meter
voice band. Several sets of traps can be inserted at the correct
points in the dipole to make a multi-band dipole. Multi-band trapped
dipoles are being sold, but in many cases they will require the use
of a tuner. If a good match is found at a frequency on some bands,
the bandwidth without a tuner will be very
narrow.
Figure 17.
Trapped 75 and 40-Meter Dipole

The
antenna is only 108 feet long instead of 120 feet because of the
loading effect of the traps on 75 meters. These dimensions are for
antennas using the traps made by W2AU. If you use other brands of
traps, the length of the end wires will have to be adjusted. What
you do in that case is make the wire long, measure its resonant
frequency on 75 meters, and prune the ends to resonance at your
favorite frequency.
13. The
Extended Double Zepp Dipole
An
extended double zepp is a long dipole with 3-dBd gain. It is the
longest dipole antenna, which will radiate at right angles to the
plane of the antenna. To find the overall length of an extended
double zepp, divide 1197 by the frequency in MHz. Each leg of the
antenna is 0.64 wavelength long and the total length is 1.28
wavelengths. An extended double zepp for 75-meters at 3.8 MHz is 315
feet. Not many amateurs have space for that antenna. The extended
double zepp is mostly fed with ladder-line. Another method of
matching an extended double zepp is to use tuned lengths of 450-ohm
ladder-line as a series matching transformer connected between the
50-ohm coax and the dipole. The length of the matching section of
450-ohm ladder-line can be found by dividing 135 by the frequency in
MHz.
Figure 18. Extended Double Zepp
Dipole

14. The
G5RV Dipole
An
interesting antenna you can buy that will work somewhat on all
high-frequency bands is the so-called G5RV antenna. It is named
after the call letters of Louis Varney (SK) who designed it. It is a
102-foot long or three half-wavelength dipole antenna on 20 meters
(14.150 MHz), and can be used with a tuner on other bands as well.
In his original design, Varney calculated the length to be 102.57
feet, but chose to make it an even 102 feet since a tuner was going
to be used with it anyway. It was originally fed through a 34-foot
500-ohm homebrew open wire matching section from a 70-ohm coax or
parallel conductor feed-line. The 34-foot open wire line is a half
wavelength on 20 meters and at the end of a half-wave feed-line, you
will see the antennas impedance repeated regardless of the feed-line
impedance. The ladder-line helps partly to match the antenna on the
other bands.
The G5RV
antenna is around 20 feet short of being a half-wave on 80 meters,
and on bands on 20 meters and up, it has theoretical gain. We
believe that gain is negated by losses in the coax of the feed
system, except for 20 meters. At the frequency of the best match,
commercially made models of the G5RV are said to have a 1.8:1 SWR on
80 meters. Where the coax joins the open wire, Varney recommended
using a choke made of 8 to 10 turns of coax. He advised against
using a balun, because, as he says SWR of 2:1 or higher may cause
the balun to heat and possibly burn out. The SWR will be moderately
high or high on bands other than 20 meters. Varney recommends using
the lowest loss coax available and as short a run as practical
because of feed-line losses caused by high SWR. This recommendation
is very important today, as it was when Varney designed it. Some
G5RV antennas put out decent signals and some others have relatively
weak signals. Without further investigating, the only way to explain
this is that some are using lossy coax and baluns while others are
not, and the height above ground may play a part in how well it
works.
The G5RV
antennas being made today use small diameter 50-ohm coax, 450-ohm
ladder-line, and a balun between the ladder-line and the coax,
contrary to Varneys suggestions. There are several variations of the
G5RV antenna being sold today because many believe they can improve
the original design. If you use a G5RV antenna, a tuner will be
required.
The G5RV
shown below is close to the original version of the antenna. This
one pictured below is from an old article that K4EFW found
somewhere. It is like the one he used. As you can see, it uses
300-ohm TV ribbon. The length of the parallel TV ribbon is 36 feet,
but modern designs of this antenna use 34 feet of 450-ohm
ladder-line. All these variations work equally well when they are
used with a tuner. It is shown in the inverted-V configuration but
it could be put up in the flattop configuration as is, with no
modification.
Figure 19.
G5RV Dipole

Jeff,
AI8H, in Oxford, Georgia, had a pair of G5RV dipoles oriented in
different directions. Recently he put up a 75-meter half-wave
inverted-V. Being able to switch antennas, he ran A-B tests on 3902
kHz and the inverted-V was 10 dB stronger than the first G5RV and 15
dB stronger than the other one. Now if we are saying the stronger
signal is 40 dB over S-9 and the weaker signal is 25-30 dB over S-9,
no one will notice the difference. Only under marginal band
conditions will the difference be important. In addition, the G5RV
antenna will work better on the other bands.
15.
Off-Center Fed Dipoles
A long
dipole consisting of multiples of equal half-wave segments is
normally fed in the center using ladder-line. Dipoles do not
necessarily have to be fed in the center. They can be fed in the
center of any one of these half-wave segments, even fed off-center.
A fair match will occur if coax is used.
Figure 20. One
wavelength Off-Center Fed Dipole

The dipole
shown above is a one-wavelength dipole. It is nothing but two half
waves end to end. It is being fed in the center of one half-wave
segment or a quarter wave from one end. It is possible to make it
any number of half waves, and if it is fed a quarter wave from one
end, it will have a fair match. The way it is shown above is an
example of how to feed an antenna with even multiples of a half wave
using coax. A 2:1 or 4:1 balun will improve the match on longer
versions.
The windom
antenna is another example of an off-center fed antenna. The
original windom was fed off center with a single wire. The other
side of the transmitter was connected to ground. The feed-point
impedance at the transmitter was reported to be 500 ohms on all
bands. The antenna was designed by William L. Everett and J.F Byrne
at Ohio State University. W8GZ, whose last name was Windom,
described the antenna in the September 1929 issue of
QST.
A lot of
research concerning the modern variations of the Windom antennas has
been done, including the ones described by Fritzel, K4ABT, W4RNL,
The Carolina Windom, and ON4BAA. The main differences in these
variations are the slight differences in the position of the
feed-point and the impedance of the baluns used for matching. The
Windoms are sensitive to the height over ground, meaning the height
above ground affects the SWR. The offset position of the feed-point
will also determine the feed-point impedance. The one sold by K4ABT
is a variation of the Fritzel antenna, and the one sold by Radio
Works, The Carolina Widom , claims it has a vertical
radiator.
There are
two variations of Windoms, both claiming they have vertical
radiators, The Carolina Windom, and the one previously marketed by
W4COX have two pieces of transmission line in series. The upper
piece is connected to the dipole, and the lower piece is connected
to the transmitter. The feed-point of the dipole is placed off
center. In The Carolina Windom being marketed today, the upper
transmission line is coax. The one made by W4COX had the upper piece
made from ladder-line, but in either case, the principle is the
same. The two pieces are connected together through a line isolator,
a type of balun. The line isolator keeps the lower piece of
transmission line from radiating. Because the antenna is fed
off-center, the marketers of The Carolina Windom claims it causes an
unbalance of current in the upper piece of transmission line. This
is doubtful because there is a balun at the feed-point, which should
prevent the feed-line attached there from radiating. The main
difference between The Carolina Windom and the one sold by W4COX is
that a 4:1 transformer is between the coax and the ladder-line, and
a 1:1 line isolator is between the upper and lower coax cables. Both
variations of this antenna show low SWR on several bands, but a
tuner is used to match it.
Figure 21.
Carolina Windom

Another
unique variation of the Windom dipole is the Fritzel antenna, named
after its inventor and manufacturer, Dr. Fritz Spillenger (SK), a
German ham, call sign DJ2KY. Alpha Delta is now selling an almost
exact duplicate of the original Fritzel antenna. Alpha Delta calls
it an OCF antenna and it is made by Buckmaster Antennas. There are
two models of the Alpha Delta antenna: one for low power and one for
high power, the power rating of the balun being the limiting factor.
The Fritzels short side is 0.18 wavelength long and its long side is
0.32 wavelength long. It is fed with coax and a 6:1 balun.
Theoretically, the feed point impedance is 300 ohms, and the balun
provides a 50 to 300 ohm impedance transformation. Modeling the
antenna on its lowest resonant frequency at 35 feet, it shows about
120 ohms impedance. The original Fritzel antenna being used by K4LMS
reportedly will work all bands with a tuner, but it will work 40,
20, 17, 12, and 10 meters without a tuner with an acceptable SWR.
The Windom being sold by K4ABT uses a 4:1 balun and the feed-point
is at a slightly different location. That one is shown
below.
Figure 22.
Windom Dipole (Fritzel Type)

The
difference between the Windom antenna sold by K4ABT and the original
Fritzel is the difference in the offset of the feed-point. Since the
K4ABTversion uses a 4:1 balun, it appears his is fed at the 200-ohm
point, and the original Fritzel is fed at the 300-ohm point. On any
resonant dipole, the lowest feed-point impedance is found at the
center. As you place the feed-point offset toward either end, the
impedance gets higher. The highest feed-point impedance occurs at
the end of the dipole.
XI.
END-FED ANTENNAS
1. End-Fed
Zepp
A
half-wave resonant antenna can be fed from its end. When fed this
way, it is also known as an end-fed zepp. An end-fed zepp will work
on its fundamental frequency and on odd and even harmonic
frequencies. The name "Zepp" goes back to the days of dirigibles or
Zeppelins, which used trailing wire antennas that had to be fed at
one end. The end of a half-wave antenna has very high impedance, and
an antenna fed this way is said to be voltage fed. Feeding a
half-wave resonant dipole in the center means it is current fed. The
normal way of feeding the end-fed antenna is with ladder-line. One
side of the ladder-line is connected to one end of the antenna and
the other side of the ladder-line is connected to nothing. To secure
the unconnected side of the ladder-line, it is connected to a short
wire running between two insulators. Since the antenna is connected
at its high impedance point, no current flows into an antenna, but
there will be a large current in the center of this antenna. No
current flows from the open side of the feed-line because it is at a
zero current point.
Figure 23.
End-Fed Zepp

The
end-fed zepp can be matched by cutting the ladder-line to a quarter
wavelength with the bottom end of the ladder-line shorted. A certain
distance above the short is a 50-ohm feet-point and it can be fed
directly with coax. MFJ is marketing antennas of this type made for
single bands, and they are selling the parts separately so you can
build your own. You will have to find the 50-ohm point by trial and
error. This method of feed makes it a single band
antenna.
Figure 24.
Alternate Method of Feeding an End-Fed Zepp

2. End-Fed
Random Length Antenna
Below is
another end-fed antenna made from a random length of wire connected
to the back of the tuner. The wire then exits the shack and goes to
a high support where it then runs horizontally to another high
support. The tuners groundside must be connected to a good RF
ground, since a poor ground causes high losses. This antenna is
commonly called a "long wire." Since the end of the antenna comes in
the shack, you will be exposed to high levels of RF. In addition,
this type of installation may cause RF to be picked up in the
microphone, noted by distortion. The feed-point of the long wire
being connected directly at the output of the tuner can have an
impedance of a few ohms to a thousand ohms depending on the antennas
length. If the wire is cut to a multiple of a half wave at the
lowest frequency, the system will be efficient since it is fed at a
voltage point and very little current flows into the ground. This
antenna is really a variation of an inverted-L fed directly without
a feed-line from the tuner.
Figure 25.
End-Fed Random length or Long Wire Antenna

XII THE
HALF SLOPER
1. The
Half-Sloper
The half
sloper is an antenna that is hard to categorize, since it is not a
sloping dipole and it is not a vertical. The half sloper is half of
a sloping dipole. To make one of these antennas, cut a quarter-wave
radiator by dividing 234 by the frequency in MHz and tie an
insulator to both ends. One insulator is tied near the top of a
tower and the radiator wire is run down toward the ground. Coax is
split into its center conductor and shield, and it connects across
the insulator at the tower. The center conductor of the coax is tied
to the quarter wave radiator and the shield is grounded to the
tower. This means the tower is acting as the missing half of the
dipole. It is a difficult antenna to get a good match because the
height above ground of the feed-point and the angle of the slope
affect the impedance. Some users of this antenna say to mount the
feed-point at 45 feet up on the tower and have a beam antenna on the
tower above the feed-point to use as a counterpoise. Other users say
you must find the 50-ohm point on the tower, which is a tedious
task. It has also been said, "Some installations work super, while
others do not work well at all." The half-sloper is used almost
exclusively on 80 and 160 meters. The Alpha-Delta half sloper was
tried here and its performance was disappointing. The signal from it
was down a least 10 dB below a dipole and the SWR wasnt low enough.
The half sloper is mostly vertically polarized and it is directional
toward the slope.
Figure 26.
Half-Sloper

XIII
VERTICAL ANTENNAS
1. Why
Verticals Are Used
Vertical
antennas have the radiator mounted at right angles to the earth. The
vertical is used whenever you desire to radiate your signals in all
directions at a low angle. Low angle radiation is needed to work DX
effectively. Radio waves traveling to the ionosphere where they are
reflected need to hit the ionosphere at a point near the horizon in
order to reflect farther around the curvature of the earth. In order
to get a dipole to radiate a strong signal at low angles, it has to
be more than a wavelength above ground. A low dipole is not
particularly a good DX antenna for 80 and 160 meters. However, the
average dipole at modest heights will outperform any ground-mounted
vertical having a poor ground system. Vertical antennas work very
well at low frequencies such as the broadcast band, but the ground
losses increase as we move higher and higher in frequency (Refer to
section V concerning ground-wave propagation). It is very difficult
to get a good ground for a ground-mounted vertical unless you live
next to salt water. Vertical antennas, because they are unbalanced
antennas, do not need baluns. They are normally fed with
coax.
If a
ground mounted, quarter-wave vertical is all you can put up at your
location (QTH), then use it. However, it will be a mistake to put up
this antenna if you are not be able to have a ground radial system
and are able to put up a dipole. Most ground mounted quarter-wave
verticals manufactured today are trapped in order to work multiple
bands.
The
ground-mounted vertical also needs to be put out in the clear away
from RF absorbing objects. These facts do not apply to half-wave
verticals, which are in themselves different animals, nor do they
apply to high quarter-wave verticals using elevated
radials.
The
approximate length of a full-sized resonant quarter wave vertical
can be found by dividing 234 by the frequency in MHz. Note: 234 is
half of 468, the number we used to calculate the length of a
half-wave antenna. The actual length for resonance may be a little
different from what you calculate, because of the diameter of the
vertical element. Trapped verticals are physically short of a
quarter wave in length because the traps load them. The vertical is
fed at one end at the bottom where it is insulated from the ground.
The center conductor of the coax connects to the vertical element
and the shield is connected to the ground
system.
Figure 27.
Ground Mounted Trapped Vertical

2.
Disadvantages of Using Quarter-Wave Verticals
The most
obvious disadvantage of using any vertical antenna is on 80 meters
it has less than optimum high-angle radiation needed to work
stations within a few hundred miles. Ground-mounted quarter-wave
verticals use a ground system for the other half of the antenna and
the ground system losses can be very high. The ground wave signal
should eventually radiate in space at angles at the horizon, but
since there are very high losses in the ground wave at amateur
frequencies, a ground-mounted vertical has almost no signal down
near the horizon. At angles below 10 degrees, the signal will be
greatly attenuated.
A
ground-mounted quarter-wave vertical with an ideal ground should
have an impedance of 35 ohms resistive. If you were to measure its
impedance, and it measures 60 ohms resistive on an antenna analyzer,
it means it has a loss resistance of 25 ohms. Moreover, that loss
resistance is mostly in the ground system. Under these conditions,
only 58% of the power will radiate as RF, although you will have a
1.3:1 SWR. Forty-two percent of the power will be turned into heat
by the loss resistance. With the feed-point being at ground level,
some more loss comes from the radiated wave being absorbed by power
lines, trees, and buildings with its associated wiring. That loss
does not show up in antenna analyzer
measurements.
The best
ground system for a ground-mounted vertical is 120 wires, called
"radials," radiating from the feed-point like the spokes of a wheel.
These radials need to be a quarter wave long. At the feed-point, the
radials are bonded together and are fed from the shield side of a
coax cable. Not many amateurs have the resources to build such a
ground system. Many short radials will be more effective than a few
long ones. When using a ground mounted vertical, many hams drive an
8-ft. ground rod into the earth for their ground system. The ground
losses are very high in that case. Using a ground rod for the ground
system of a vertical antenna confirms the old adage: "Verticals
radiate poorly in all directions."
To
eliminate ground losses, you can use an elevated quarter-wave
vertical with an elevated ground system called a "ground plane." The
ground plane vertical, as it is called, needs to be mounted high
enough to prevent the return path from coming back through the earth
ground. Ground plane verticals need to be mounted above nearby
objects that absorb RF. They will be nearly 100% efficient if they
are high enough. The ground plane consists of two or more radials,
but most ground planes have three or four. The ground plane radials
do not have to be resonant, but should be at least a quarter wave
long. An elevated ground-plane vertical will be more effective for
working DX than a dipole.
3. Long
and Short Verticals
Verticals
can be less than a quarter wave in length. They can be loaded by
coils or linear loading sections or a short vertical can be fed
directly with a tuning unit at the feed-point. The loss resistance
in a short vertical may be appreciable. Since the radiation
resistance is very low at the feed-point of a short vertical, the
current at the feed-point will be very high. The more current that
flows into loss resistance, the higher the loss will be. Any coils
used in the tuning unit and for loading should be made of as heavy a
conductor as possible, since these can cause appreciable loss when
the current is high. This is also true for the ground system. The
loss described here is called "I squared R loss", which means the
loss in watts is found by multiplying the current times itself and
then multiplying that answer by the loss resistance. That means if
the current into a lossy antenna system is doubled, the power lost
in watts is increased four times. Making a vertical very short and
tuning it to resonance with an inductor will also result in an
antenna with a very narrow bandwidth.
A more
subtle loss of energy in very short vertical antennas is coronal
discharge from the tip end of the vertical. Corona occurs when the
voltage is very high at the end and electrons flow out into the air.
This can be visible at night if the transmitter power is high and
you are at a high altitude. Power is lost from the antenna when
corona is produced because corona is a form of light and light is
another form of energy.
In 1973
while we were working for radio station WWNC in Asheville, North
Carolina, a trapped vertical for 10 through 80 meters was erected.
The length of the antenna was only about 25 feet. A loading coil
near the top made it resonant on 75 meters. The ground system was
the metal body of a 75-foot long mobile home. Fair reports were
received from this set-up. The reports were not bad because of our
having a good ground. One night, while working 75 meter SSB, one of
the neighbors came over and said, "Youre tearing up my TV." Checking
all of the inside connections proved they were tight. Our wife keyed
up the transmitter while we made a trip to the antenna to check the
connection there. Before arriving there, looking up, we saw blue
fire coming off the end of the vertical. The corona was responsible
for the television interference ( TVI ). It was visible because
Asheville is at a relatively high altitude and the transmitter was
running 700 watts. An inverted-V was put up, the TVI disappeared,
and better signal reports were received.
You can
realize up to a 1.5 dBd gain from a vertical antenna by making it
longer than a quarter wave, but there is a limit to how long to make
it and still get low angle radiation. That limit is 5/8th-wave. To
find the length of a 5/8th-wave vertical, divide 585 by the
frequency in MHz. For example, to calculate the length of a
5/8th-wave vertical for 20 meters (14.000 MHz) divide 585 by 14.0.
It equals 41.786 feet or approximately 41 feet 9.5 inches. A tuning
unit will be needed at the feed-point of this antenna, as the
impedance of a 5/8th-wave antenna is low and high current will flow
into it. A tuning unit will usually have enough bandwidth to cover
the entire band on each band of 20 meters and higher. A tuning unit
is also called a matching network. It is similar to an antenna
tuner, but has fixed inductors and capacitors. Tuning units for
80-and 160-meter verticals will cover only a portion of the bands.
Outside the bandwidth limits of the tuning unit, you can use the
tuner at the transmitter end. Radials or ground planes are needed
for a 5/8th-wave vertical and they need to be a quarter wave
long.
The
impedance of a half-wave antenna is high if fed at its end. An
end-fed half-wave vertical will have a small amount of gain over a
quarter-wave vertical. This antenna does not have the ground losses
a quarter-wave vertical has because it is fed at a high impedance
point and the current flowing into the ground is negligible.
Commercially made resonant half-wave trapped verticals now on the
market are end fed at the bottom. A built-in matching network is
found at the base, and several very short radials are mounted below
the feed-point to de-couple RF from the feed-line. These antennas
should be mounted as high as possible away from RF absorbing
objects. Because the ground losses are lower, the half-wave vertical
will outperform a quarter-wave vertical by several dB and in many
cases many dB.
4.
Unscientific Observations of Verticals
At our
home, an old Hy-Gain trapped quarter-wave vertical for 40-10 meters
was erected in 1961. It was mounted on the roof and had two
quarter-wave radials for each band. It worked, but it was never
compared to another antenna. It gave the impression it was a
mediocre antenna. Other antennas replaced it.
One time
in 1964, a grounded 60-ft tower was shunt fed as a vertical on 75
meters. Without having any radials, the transmitted signal was 10 dB
weaker on this vertical 650 miles away in New York than on the
inverted-V.
In 1969, a
4-band trapped vertical was put up on the top of a 60-ft tower. A
15-meter 4-element yagi under it was used for the ground plane. It
was probably the best vertical installation we ever tried. It was
good because it was high and in the clear and the 15-meter yagi made
a good ground plane.
While we
are on 80 meters, a ham 200 miles away frequently joins in the
roundtable. He uses a trapped quarter wave vertical with a chain
link fence as the ground. Several of the others are also 200 miles
away run the same power. His signal is 10 to 20 dB below everyone
elses on the frequency. It is good there are no interfering signals
or noise or he will not be copied.
Another
ham uses a Hy-Gain Hy Tower vertical with 3 ground rods as the
ground system. According to our S-meter, his signal is 40 dB down
below those of the other guys.
Charlie,
AD5TH, works 40 meters using a Hustler 5-BTV vertical ground mounted
with 72 quarter-wave radials. He has an outstanding signal for a
ground-mounted vertical. His installation is out in the clear away
from RF absorbing objects. He says, because of antenna restrictions
at his location, it is the only antenna he can put
up.
Another
ham friend, N2HGL, has both a dipole and a half-wave trapped
vertical on 40 meters. At a location 160 miles away, he is 10 dB
stronger on the dipole, but he is equal in strength on both antennas
in Indiana 600 miles away. This comparison shows the superiority of
the half wave vertical over the quarter-wave one because his signal
with the half-wave vertical was equal to his signal from the dipole.
If he were using a quarter wave vertical, we would expect his signal
would be better on the dipole in Indiana. It also demonstrates the
superiority of a dipole over a vertical for working short
distances.
Bill,
W4ZQL, runs a ground-mounted SteppIR vertical. He lives beside a
salt-water river in Florida that he uses for a ground. He puts out a
very good signal on 40 meters. No ground losses!
5. The
Inverted-L Vertical
The
inverted-L antenna is a wire vertical antenna with part of the top
end bent horizontally. It resembles an "L" turned upside down. The
inverted-L is used to reduce the height required by a vertical and
still keep the antenna resonant and full sized. It is fed at the end
at ground level the same way a ground mounted vertical is fed, and
all the losses we described for a ground-mounted vertical apply
here. Some current flows in the horizontal part of the inverted-L
and for that reason, it has both strong vertical and weaker
horizontal polarization. If you make it a half-wave antenna, you
wont need a good ground because negligible current flows into the
ground. A half-wave inverted-L antenna needs to be fed with 50-ohm
coax and a tuning unit.
An
inverted L for 160-meters is usually made of wire one-quarter
wavelength long or about 127 feet. It runs vertically from near
ground level to the top of a support, perhaps 60 or 70 feet. Then
the end runs horizontally and is tied to a nearby support. The
antenna is coax fed at ground level between the vertical section and
ground system across some type of insulator. A matching network at
the feed-poimt will be required to match it if the impedance is not
equal to 50 ohms.
Figure 28.
Inverted-L

The
picture shows an inverted-L running up the side of a tower. The
feed-point is at ground level with the center conductor of the coax
attached to the bottom end of the wire. The coax shield connects to
a ground system of radials. The total length of wire used in this
antenna is half of what is needed for a dipole since the other half
of the antenna is the radial ground system. The inverted-L is used
mostly on 160 meters, but some have built them for 80 meters. The
inverted-L antenna can also be cut for a half-wave to reduce ground
losses.
6.
Vertical Mobile Antennas
We have
heard many good signals from mobiles, many being stronger than those
from hams using ground mounted quarter-wave verticals. The mobile
antenna, being so short, has a large capacitive reactance. A coil is
inserted in the antenna to provide an equal amount of inductive
reactance to make it resonant. As we said in the paragraph on short
verticals, a coil of this type, carrying a large amount of antenna
current, causes some loss resistance in the system. To reduce losses
in the coil, wind it with a conductor as large as practical. (Thats
exactly what some mobile antenna manufacturers have done.) The
sources of loss in mobile antennas are in the coil losses, losses in
the conductors making up the radiating part of the antenna, corona
discharge, and the ground loss from the vehicle on which its
mounted. However, because of the large amount of metal in the body
of the vehicle, the ground losses are not as high as the losses from
ordinary ground mounted verticals. Matching transformers are now
available that step down the impedance of 50-ohm coax to the very
low impedance of the loaded vertical. Good advice is to use the
transformers rather than to rely on the internal tuner of the
transceiver.
Some low
priced single-band mobile antennas are constructed by using a
polymer shaft and a small gauge wire encapsulated in polymer
material running beside the shaft. The loading coil made of the same
wire is also encapsulated in the polymer. The small wire, because of
its size and because it carries a large RF current, will lose a lot
of power by becoming hot. This type of mobile antenna is rated for
200 watts. If the wire didnt get hot, there would be no power
limit.
All mobile
antennas have corona loss and for such, there is no remedy. Most
amateurs, because they cant see it, dont believe its there. Corona
will not be visible unless you run high power and it is
dark.
Ground
losses from the vehicles body diminish with increasing vehicle size.
This is why 18-wheeler hams have such big mobile signals. To
diminish the ground losses on any mobile installation, you should
use as large as a conductor as possible to bond the coax shield to
the vehicle body. All metal parts of the vehicles body, fame, and
drive system need to be bonded together with heavy ground straps. To
make the mobile antenna system more efficient, use an antenna with
an adjustable inductor and use as long a "stinger" as practical
above the coil. You will increase the radiation resistance by using
a longer stinger, and then the loss will be less because you will
require less coil inductance. The ratio of radiation resistance to
loss resistance becomes larger by raising the radiation resistance
and reducing the loss resistance. As we said earlier, the efficiency
of any antenna system is found from the ratio of radiation
resistance to total resistance, or radiation resistance divided by
total resistance times 100%. The total resistance is equal to all
the loss resistances plus the radiation
resistance.
The latest
development in HF mobile antennas is motor driven variable
inductors. These antennas are known as "screwdriver antennas." The
name refers to the electric screwdriver motors used to vary the
inductance. A control cable is run from the motor to a switch at the
operators position so it can be tuned from the operators seat in the
front of the vehicle. Because a mobile antenna has a very narrow
bandwidth, you will have to tune it often as you move frequency
(QSY). It hasnt been many years since we had to get out of the
vehicle to make inductor changes or make changes in the length of
the stinger when the frequency was changed.
Mobile
antennas for 20 through 10 meters do not require the care in
installation that is needed for 160, 80, and 40 meters because the
length of a mobile antenna becomes closer to a quarter wave as you
move to higher bands. The radiation resistance increases on each
higher band. While moving to higher bands, less inductance is needed
to tune the antenna, and that lowers the loss resistance. A 96-inch
mobile whip is just a couple of inches short of being a quarter
wavelength on 10 meters and a loading coil is not needed there. The
band that has the least mobile antenna efficiency is 160 meters. If
you reach a radiation efficiency of 2% on 160 meters on your mobile
installation, you will be doing well.
Below is
some information concerning mobile antennas, which was received in
an email. There was a 75-meter mobile "shoot-out" in California. (A
shoot-out is an event where a group of hams gets together and
compares signals radiated from various antennas.) Supposedly, equal
power was applied to each antenna under test. Apparently, some type
of field strength meter was used. A screwdriver antenna and a bug
catcher, both with top hats, were used as the standard by which
other antennas were compared because they put out equal signals. The
other antennas are measured in how many dB they were below the
standard. Here are the results of that test, and because it is
hearsay, the accuracy of these figures is not guaranteed, but they
do compare to what we have observed.
Screwdriver/bug catcher with top hats
0 dB reference
Screwdriver/bugcatcher without top
hats -3dB, -50%
Hustler -7
dB, -80%
Outbacker
-9 dB, -88%
Hamstick
-12 dB, -94%
Whip with
autotuner -14 dB, -96%.
The
efficiency of the best 75-meter mobile antenna is from 5% to 10%. In
using the best mobile antenna on 75 meters, a 100-Watt mobile rig
will radiate 10 Watts at most. This means that a Hamstick being fed
with 100 Watts will radiate only 0.6 watts, which is 6% of 10 Watts.
Ninety-nine and four tenths Watts will be converted to heat. The
person sending this information said it was published on the
Internet in some news group. Again with good band conditions, it is
amazing how little signal can be used to
communicate.
The things
that increase the efficiency of mobile antennas
are
Place the
loading coil about half way from the feed-point to the antenna tip.
Efficiency decreases if you put the coil above or below this
point.
Mount the
antenna as high up on the vehicle as possible. This reduces the
ground losses because it reduces the capacitance of the antenna to
ground.
Use a
loading coil with a Q as high as possible. See the ARRL
Handbook for a discussion of coil Q.
Make the
antenna as long as possible. Note: long antennas are prone to strike
tree limbs and bridge overpasses.
Increase
the size of the mast between the loading coil and
feed-point.
Put a
capacity hat above the loading coil. The capacity hat reduces the
number of coil turns needed to resonate the
antenna.
Make the
coil with as large a diameter wire possible. This decreases the coil
loss, which is a large part of the total loss of a mobile
antenna.
Any
changes made in the antenna system that raises the radiation
resistance will increase the efficiency.
XIV.
ONE-WAVELENGTH SINGLE LOOP ANTENNAS
1. The
Horizontally Oriented Loop
To
calculate the length in feet of any one-wavelength loop, divide 1005
by the frequency in MHz. Horizontally oriented one-wavelength loop
antennas have become very popular on 160, 80, and 40 meters and it
is one type of NVIS antenna. (NVIS stands for "near vertical
incidence skywave" because of its high angle radiation pattern.) It
is claimed by its users that the loop antenna is quieter than other
antennas. This is because it doesnt pick up the noise from power
lines, thunderstorms, etc., coming in at low angles. These antennas
radiate on their fundamental frequencies with a broad pattern
straight up to put a strong signal for nearby contacts. Recently
published articles on this type of antenna have called them "cloud
warmers." There are other types of antennas called NVIS antennas
other than loops. They are dipoles at low heights or dipoles with
parasitic reflectors placed under them to cause the signal to
radiate mostly straight up. The NVIS antennas have an advantage in
working nearby stations because you dont get the static noise and
interference from far distances. They are definitely not DX
antennas. An article on NVIS antennas appears in the December 2005
QST.
On their
fundamental frequencies, horizontally oriented loops take up half
the horizontal distance as a half wave antenna for that band. Loops
are two-dimensional antennas having depth as well as breadth. There
are two loop configurations: The square loop and the triangle loop.
Some hams have pulled the loops out in irregular shapes to fit where
the supports are located. The only advantage in using a rectangular
loop instead of a square loop is to take up less horizontal space.
This is true because the gain of a rectangular loop is diminished
below a square loop. The area enclosed by the perimeter of the loop
determines the gain of a loop. A circular loop has the most enclosed
area, but it requires an infinite number of supports. The gain of a
loop comes from the loop having two maximum current points separated
by a distance of one-quarter wavelength. From here on we will call a
horizontally oriented loop a horizontal loop.
We also
modeled the gain of the horizontal loop for the 80-meter band over
real ground. The maximum gain occurs with the loop at 7 meters or
about 25 feet above ground. Mind you, this gain is straight up from
the loop. At that height, its gain is about 9.25 dBi and that
equates to about 7 dBd in free space. The gain of the loop
diminishes slightly as the antenna is raised. The feed-point
radiation resistance at 7 meters height is 35 ohms resistive and 0.0
ohms reactance and you do not need a matching section of 70-ohm
coax. At a height of 10 meters or about 33 feet, the radiation
resistance rises to 63.5 ohms. There the SWR will be 1.27:1, if it
is fed directly with 50-ohm coax. At 15 meters or about 50 feet, the
radiation resistance rises to 118 ohms and a 70-ohm matching section
will be in order. The gain drops to a little less than 7 dBi at that
height. These figures may or may not be applicable to your QTH,
because your soil conductivity may be different from the soil we
used to model it. As you can see from the above numbers, the
feed-point resistance rises as the loop is
raised.
The
horizontal loops also are used on their harmonic frequencies. The
loop with more gain and a superior pattern is a two-wavelength loop.
An 80-meter loop is a two-wavelength loop on 40 meters. The
two-wavelength loop has a lower angle of radiation, but is a very
large antenna for 80 meters. At 3800 kHz it has a perimeter of about
530 feet. A two-wavelength loop is not an NVIS antenna. Using coax
with a tuner is not an ideal way for working a loop on its harmonic
frequencies. This is because of the high SWR in the coax on some
bands will cause high loss. For example, an 80-meter loop fed on 40
meters will have an SWR of 8:1 and the SWR on 20 meters will be
49.5:1. There will be some hams who will say they get satisfactory
results this way, however theory suggests they will have a stronger
signal if they use a ladder-line because ladder-line has less loss.
Feeding a loop antenna with ladder-line makes more sense when
working a loop on harmonic frequencies.
Figure 29.
One Wavelength Horizontal Loop

To realize
maximum gain, make the square and triangle have equal sides. When
the sides are equal, the loop has maximum enclosed area for whatever
configuration you use. Other shapes will work, but the gain will
suffer.
To support
a square loop, you will need four supports, one for each corner. We
hope you will have trees or masts in the right places. A triangular
loop will need three supports. Once you have cut the single piece of
wire to the right length, run the wire through as many insulators as
you have corners. At each corner of the loop, put an insulator and
tie the corner to a support with a rope from the insulator. To make
the feed-point, connect both ends of the loop to an insulator. Strip
the insulation from the outer part of the coax. Separate the shield
from the center conductor. The ends of the coax are connected to the
ends of the loop across the insulator. Most hams do not feed loops
with a balun at the feed-point.
2. The
Vertically Oriented Single Loop for 40 and 80
Meters
Vertically
oriented loops radiate broadside to the plane of the loop. A
horizontally polarized vertically oriented loop has both vertical
and horizontal wires. From here on out, we will refer to a
vertically oriented loop as just a vertical loop. When using this
term, we are not referring to its polarization. If the feed-point is
on one of the horizontal wires, the loop radiates horizontally
polarized waves. The vertical wires radiate weaker vertically
polarized waves. If the feed-point is on one of the vertical wires,
vertically polarized waves will be radiated. The radiation from a
one-wavelength vertical loop has both high-angle and low-angle
radiation. It is a good antenna for both nearby stations and for DX
contacts. It is better than a dipole for DX because the vertical
loop puts out a stronger low angle signal than a dipole
does.
The gain
of a vertical delta loop is 4.55 dBi or about 2.4 dBd. Its
feed-point impedance is about 120.5 ohms. The square vertical loop
has 5-dBi gain and about 2.85 dBd and the feed-point resistance is
143 ohms. They both need to be fed with a series quarter-wave
matching section of 70-ohm coax.
Figure 30.
Single-Element Vertical Delta Loop

Square
vertical loops need two supports. The square vertical loop needs
less vertical space than the delta loop. The vertical space needed
for a square vertical loop for 80 meters is 92 feet. For 40 meters
the vertical space is half that. It is rare to find someone using
the square vertical loop these days. The vertical delta loop is more
common because it needs only one high support. The apex of a delta
loop for 3500 kHz needs to be 102 feet high and on 40 meters, it
needs to be 62 feet. This assumes the bottom horizontal wire will be
20 feet off the ground. In order to make a vertical loop fit on a
shorter support, the sides of the loop can be reduced in length
while making the horizontal wires longer. This will put the two
maximum current points closer together, which has the effect of
reducing the gain.
Like the
horizontal loop, the formulas for finding the length in feet of
these loops are the same: 1005 divided by frequency in MHz. In
addition, because the feed-point resistance is nearly the same as
horizontal loops, quarter-wave matching sections and other methods
can be used to feed the vertical loops. The vertical loop is not as
sensitive to height as the horizontal loop. Both vertical square
loops and vertical delta loops can be operated on harmonically
related bands. (See Figure 32).
Figure 31.
Radiation Pattern of a 30-Meter Delta Loop on 30 Meters. The
Bottom Wire is at 18 Meters above Ground.

The
horizontal pattern shown above demonstrates that the 30-meter delta
loop has a bi-lobal pattern broadside to the plane of the loop. The
vertical pattern below the horizontal pattern shows both high angle
and low angle radiation. The angle of maximum radiation is at 35
degrees above the horizon. The angle of radiation straight up is
only down about 1.5 dB. This is pattern demonstrates the vertical
delta loop is good for both nearby stations as well as
DX.
Figure 32.
Radiation Pattern of a 30-meter Delta Loop on 15
Meters

XV.
DIRECTIONAL BEAM ANTENNAS
1. The
Monoband Yagi
Between
1926 to 1929, Shintaro Uda and Hidetsugu Yagi developed a beam
antenna that had sharp directivity and high gain. Later, work was
done primarily by Mr. Yagi and yagi was the name given to the
antenna until finally recognition was given to Mr. Uda. Its proper
name is the Yagi-Uda Array. Most hams call it a
beam.
A monoband
yagi is the name given to a yagi for a single band. The performance
of any commercially made monoband yagi is touted to have its
dimensions tuned for maximum performance. As you will see later,
this is not always the case. Monoband yagis being sold today are
much improved over older designs because of computer modeling
programs available.
The yagi
is made of two or more aluminum elements mounted on and
perpendicular to a boom. Hams use antenna rotors to turn the antenna
in the direction of the station they want to work. However, there
are wire beams, fixed in one direction, mainly on 80 meters,
suspended between trees or other supports. Most high frequency beam
antennas used by hams are in the horizontally polarized
configuration, which means the elements are parallel to the ground.
CB beam antennas and some two-meter beams are vertically polarized
with the elements at right angles to the ground (See Section
III).
A
2-element yagi has a gain around 3 to 4 dBd. A two-element yagi will
have a driven element with either a reflector or a director. The
driven element is the only element receiving power directly from the
transmitter. The reflector and directors are called parasitic
elements because they receive power from the driven element by
inductive coupling.
The
3-element yagi will have a gain of approximately 5 to 7 dBd or 7 to
9 dBi depending on its boom length. A three-element yagi has one
reflector, one driven element, and one director. Because the yagi
has a low radiation resistance, a matching system is located at the
driven element feed-point. The ratio of the radiation off the front
compared to the radiation off the back is called front-to-back
ratio. Front-to-back ratio and forward gain are factors to be
considered in choosing a yagi design. Both measurements are given in
dB. All yagis have a good front-to-side ratio, with the signal off
the side being 50 dB below the front.
Figure 33.
Three-Element Yagi

Figure 34.
3-Element Yagi Radiation Pattern

The
reflector of a yagi is about 5% longer than the driven element. The
reflector, being longer, will have inductive reactance. The
inductive reactance shifts the phase of the re-radiated wave, which
radiates and combines with the driven elements wave and reinforces
it in the direction away from the reflector toward the driven
element. A director is about 5% shorter than the driven element. The
director, being shorter, has capacitive reactance, and this changes
the phase of the reradiated wave to reinforce the wave away from the
driven element opposite the reflector.
The gain
of a yagi is derived from radiation being concentrated in one
direction at the expense of the other directions. One hundred watts
fed into a yagi with a gain of 6 dBd will have an apparent power of
400 Watts in the main lobe. Because one hundred watts put into a
yagi radiates only one hundred Watts, and because that one hundred
Watts of power is concentrated in the main lobe, it is equal to the
power from a dipole being fed with 400 Watts. This is referred to as
effective radiated power or ERP, but a yagi is not any more
efficient than other antennas. Because of the Principal of
Reciprocity, an antenna having a 6 dBd gain on transmitting will
also have a 6 dBd gain on receiving.
Adding
more directors and increasing the boom length will increase the gain
of a yagi. The front-to-back ratio ranges from 18 dB for a
2-element yagi to over 25 dB for a multi-element yagi, provided the
parasitic elements are carefully tuned. The gain of a yagi is
generally proportional to the boom length and not necessarily the
number of elements. Doubling the boom length, while keeping the
proper number of elements for that boom length, will add about three
more dB of gain.
Tuning the
yagi for maximum gain makes the bandwidth very narrow, and it will
have a poor front-to-back ratio. For these reasons, we dont
recommend tuning a yagi for maximum gain, because you will only
increase the gain by a fraction of a dB at the expense of
front-to-back and feed-point impedance. Tuning the yagi for maximum
front-to-back will help eliminate interference coming from the rear
of the antenna. The building of any yagi involves compromise spacing
and element tuning.
As you
make the yagi larger by adding directors, the main radiation lobe
becomes narrower increasing the gain and ERP. The gain of a yagi
with four elements is about 7 to 8 dBd. You used to see 3 or 4
element yagis advertised claiming a gain of more than 10 dB, but
they never said if that gain was referenced to an isotropic or a
dipole. That gain also involves the gain derived from signals
reflected from the ground adding to the direct wave. A more
realistic gain figure is the "free space gain." Some companies, who
sell monoband yagis, inflate their gain figures. Beware! Increased
spacing of the elements will increase the gain of a yagi up to a
point. Increasing the spacing past that point will reduce the gain.
The spacing of a reflector or director needs to be in a range of 0.1
to 0.3 wavelengths. With a 3-element yagi maximum gain occurs with
both parasitic elements spaced at about a quarter wavelength. Second
and third directors can have wider spacing.
Most hams
do not build yagis but buy them from the many companies who sell
them. Ham catalogs are full of pre-cut and tuned yagis that come in
boxes ready to be assembled in the back yard. Many of these are very
good. However, there is a lot of satisfaction to be gained from
building your own.
In 1971,
we purchased, a 15-meter monobander being sold by a reputable
company. Its performance was disappointing. It had only a 10-dB
front-to-back ratio. That design is no longer being sold. After
reading some books, we readjusted the antenna elements to some new
dimensions and it performed much better. This was the beginning of
our yagi building.
During the
last nearly 50 years, we built many yagis. During the period of 1979
until 1986, many multi-element yagis were constructed, gain
measured, formulas derived for spacing and element length, and the
radiation patterns plotted on graphs. In 1986, a computer program
titled "Yagi" by Dean Straw, N6BV, was bought. From that point on,
that program was used to design and set the element lengths to their
proper values. Not much difference in performance of the new designs
was seen over what was previously used, but tuning parasitic
elements and running back and forth to the field strength meter was
eliminated. There are many better computer programs available today
for designing yagis and other antennas.
The
largest yagis we built were a 4 element 20-meter yagi on a 38-foot
boom, a 5 element 15 meter one on a 27-foot boom, and a 5 element 10
meter beam on a 24-foot boom. These are modest designs compared to
some of the big antennas used by contest stations. All these yagis
were stacked one above the other on a 20-foot mast coming out of the
top of the tower. The 20 meter one was on the bottom, next came the
15-meter, and the 10-meter yagi was on top. This method of stacking
yagis for different bands one above the other makes what is called a
"Christmas tree array." These antennas worked well. Since retiring
and moving back home, we use pre-tuned directional antennas because
of the lack of a good place for an antenna range. Climbing is not
now an option because of age and infirmity.
If you
make the reflector 5% longer than the driven element and the
director 5% shorter than the driven element, you will be pretty much
in the ballpark. The beautiful part about a yagi is it will work
reasonably well with the element lengths only in the ballpark. By
carefully tuning, you will get a fraction of a dB more gain or a few
more dB front-to-back, because the spacing and diameter of parasitic
elements affect the length required for those elements. A yagi can
be tuned for maximum forward gain, maximum front-to-back ratio, or
best impedance, but you can achieve only one of these conditions at
a time. Element tuning, at best, is a
compromise.
Most hams
who are yagi builders do not tune their antennas at all, but use
published dimensions for building them. Yagi builders who do tune,
tune for either gain or front-to-back and then match the driven
element with a gamma match, hairpin match, a series-resonant coax
matching section, or a step down balun. The feed-point of a properly
tuned yagi is close to 25 ohms.
Formulas
for calculating yagi element lengths will not be given in this book.
Because yagi elements are made from telescoping aluminum tubing, the
elements will be tapered. The diameter of the elements and the taper
determine the lengths required for tuning of the elements. A tapered
element will resonate higher in frequency than one not tapered. The
formula to calculate the length of the tapered elements is
complicated, but there are computer programs to do
that.
2. Trapped
Multi-band Yagis
Some yagis
have traps in the elements to make them into a multi-band beam. Many
of these commercially made antennas are available at ham radio
stores or directly from the manufacturers. In a 3-element, 3-band
design, the spacing on the booms is a compromise. A 3-band beam is
known as a "tribander." The spacing is close on 20 meters, optimum
on 15 meters, and wide on 10 meters. You cannot tune the trapped
elements for maximum performance on three bands simultaneously and
have a good match on all those bands. Since a good match is
important to most hams, gain and front-to-back ratio are sacrificed
for a good match on triband beams
The
inductors in the traps load the elements in triband beams.
Therefore, the elements are shorter than the elements of a 20-meter
monobander. Regardless of the compromised design, a triband-trapped
beam is much better for working DX than a dipole. Many hams have
achieved working over 300 entities with tribanders having short
booms.
The
radiation pattern from a yagi is at a lower angle than a dipole.
This gives the impression a yagi has much more gain than it does. A
dipole has unity gain, but that gain will be at a higher angle. The
dipole puts out a weaker signal at the low angles needed to work DX,
and a yagi puts a strong signal at low angles. In comparing a dipole
to a yagi, the yagi may only have a 4 dBd gain in its major lobe.
The gain of the yagi at a low angle may be 10 dB or so better than a
dipole at that same lower angle. The gain of any antenna is always
measured in its major lobe, irrespective of where the angle ar which
the maximum radiation lobe occurs.
Figure 35.
Trapped 3-element Yagi

The above
picture shows two sets of traps in two of the elements and one set
in the rear element. The front element is the director with traps
for 10, 15, and 20 meters (it takes two sets of traps to make the
elements work three bands). Directly behind it is the driven element
with traps also for 10, 15, and 20 meters. The rear element is
trapped for 15 and 20 meters (a single set of traps makes it work
two bands). The entire lengths of the three longest elements are
resonant on 20 meters. The short element is a reflector for 10
meters. Only the part of the antenna between the 10-meter reflector
and the front director is used on 10 meters. The maximum signal is
radiated in a direction coming out of the page toward you. Mosely
builds trapped antennas that have two traps in one enclosure and you
can not determine the bands from the traps as you can on Hy-Gain and
Cushcraft beams.
Some
triband beam models as the one above are built with longer booms so
they would have more gain on 20 meters, a good match on all bands,
and optimum 3-band performance. They achieve this by interlacing
extra monoband reflectors and directors on the boom placed between
the 20-meter elements as is done with the antenna in figure 34. The
extra elements have no effect on 20 meters or any band for which
they are not resonant. Some amateurs mistakenly think the extra
elements work on all bands, but they dont. The Cushcraft A-4 shown
above is not a beam with four working elements on any band. The old
Hy-Gain TH6DXX and Mosley Classic 36 had six elements on the boom.
They both had three trapped elements and three monoband elements.
They had three working elements on 20 meters, three on 15 meters,
and four elements on 10 meters. The trapped reflector worked on 15
and 20-meters. The trapped driven element worked on all three bands.
The trapped director worked on 10 and 20 meters. On the boom was a
resonant reflector for 10 meters and one each resonant directors for
10 and 15-meters. When using one of them, we have often heard
amateurs saying they were using a six-element beam. This gave the
other station the mistaken idea they were working someone with an
antenna with six working elements. Other beam antennas interlace
additional elements of different lengths to make the tribander into
a 5-bander covering 20, 17, 15, 12, and 10 meters. Hy-Gain makes a
5-band yagi for 20 through 10 meters that has 11 trapped and
monoband elements. It is the Hy-Gain TH-11. Mosely makes a 6-bander
that includes two elements for 40 meters. It is the
Pro-67.
In order
to achieve better SWR curves over a wide bandwidth, some triband
yagis have two driven elements spaced 3 to 5 feet apart. The front
driven element is shorter than the rear driven element. Both driven
elements are trapped. This double driven element scheme is called a
log-cell. A log cell, by itself, has a small gain and may slightly
increase the overall gain of the tribander. The KLM KT-34 and the
HY-Gain TH-7 are examples of this kind of
antenna.
Is a
monobander better than a tribander? We dont know if our tests can be
duplicated and no one else has ever said he has actually compared
the two antennas. It is "common knowledge" that traps have loss.
Therefore, the ham fraternity believes a monobander has to be
better. From the tests we performed here, we believe it is a myth
a monobander is significantly better than a tribander having an
equal boom-length. We believe the traps do not have enough loss
to make enough difference to matter. However, monobanders having
very long booms and many directors will outperform any
tribander.
Having two
towers, both having the same height and being 100 feet apart, made
it possible for us to do the experiment described here. The result
is useful information because it was made in a real world situation
that would be comparable to the average hams location. Both antenna
element lengths were set to Hy- Gain specifications. The constants
were terrain, antenna height, antenna boom length, frequency, coax
length, and power level. The only variable in the tests was the two
antennas being tested. The test was performed to see how much loss
antenna traps have. Had there been more than one variable, the tests
would not have been valid, because in any scientific experiment, the
test is valid only when one variable is being tested. In addition,
more than one test has to be made in order to average out the
collected data errors. In this case, many tests were
made.
On one
tower was a 20-meter four-element Hy-Gain 204-BA monobander with a
boom-length of 26 feet. This antenna is arguably not one of the best
monobanders made, but it is what we had and it was about the same
size as our tribander. On the other tower was a trapped 6-element
Hy-Gain TH-6 DXX tribander having a 24-foot boom. The entire
tribander boom-length was used on 20 meters, so both boom-lengths
were comparable.
The
transmitted signal strength of the two antennas was compared on
20-meters. This test involved many DX stations and one local amateur
5 miles away. With both antennas pointing toward the receiving
station, a carrier power of 10 watts was fed from the transmitter,
and held constant while the antennas were "hot" switched several
times. (The power level was unimportant as long as it was held
constant on both antennas). None of the many DX stations involved in
this test could see any difference in either antenna, and, yes,
their analog meters could discern a difference of one dB. These
tests by themselves were not conclusive because of the possibility
of fading signals (QSB). A second series of tests was performed with
a local ham when 20 meters was dead. Testing with him was done to
eliminate QSB from spoiling the results. He could also measure no
difference on his S-meter. He could also see a one-dB difference on
his analog S-meter. As a third series of tests, the antennas were
switched while we looked at the signals on the S-meter from distant
stations and the local station. No differences in received signals
were noted. Maybe the difference was a monobander has only a few
tenths of a dB less loss, such a small amount of difference no one
was able to see it on receiver S-meters. Certainly, the difference
in the two antennas was less than one dB.
Conclusion: The Hy Gain TH6DXX and
the 204-BA antennas perform equally well on 20 meters at a height of
56 feet.
3. The
SteppIR Antenna
The latest
developments in yagi designs are found in the ones being sold by
SteppIR Antennas. There are two, three, and four element versions.
All these versions are frequency agile and cover continuously from
13.5 to 54 MHz. The MonstIR adds three very long elements for 6.9 to
13.5 MHz. The elements are made of fiberglass tubes with
beryllium-copper ribbons inside. Each element has stepping motors to
wind and unwind the copper ribbons to change their lengths inside
the tubes. A multi-wire control cable connecting the control box to
the stepping motors accomplishes this. The proper element lengths
for all frequencies in its range have been calculated by a computer
and stored in the control boxs computer. As you move from frequency
to frequency, the control box in the shack readjusts each element
length. Thus, the antenna is configured into a properly tuned
monobander for any frequency in its range. These antennas are
expensive, but the hams who own them say they are worth the
money.
4. The
Log-Periodic Array
Another
beam antenna that looks like a yagi is the log-periodic antenna. It
is configured using many elements with each element being shorter
than the one behind it. This means the longest element is at the
rear of the array and the shortest element is at the front. All
elements are divided in the center and insulated from the boom, and
all elements are driven. On both sides of the insulator at the
center of each element, wires run from the front element of the
array to the rear element. Each wire criss-crosses the other ones
but they do not touch. That makes a 180-degree phase reversal from
one element to the next one behind it. The feed-point is across the
insulator at the shortest element. The feed-point impedance is about
200 ohms and a 4:1 balun is used to feed it.
The
advantage of the log-periodic antenna is, it that it is very broad
banded and it can cover all frequencies with an SWR below 2:1in its
design frequency range. The disadvantage is the gain of a
log-periodic antenna is lower than a yagi with an equal boom length.
There are designs being sold today that cover continuously from 14
to 30 MHz. In Fort Gordon, Georgia, there used to be a monster
log-periodic at the MARS station that covered from 2 to 30 MHz. The
boom length was 120 feet and the antenna was
rotatable.
5.
Directional Cubical Quad and Delta Loop Antennas
We built a
number of quads at various times and with them on the test stand and
with the bottom wire a foot or so above the ground, worked many DX
stations. When we built yagis and they were on the same test stand
nine feet above the ground, we could hardly get a signal out of the
back yard. Since the vertical beam-width of a quad is narrower than
a yagi and the radiation angle is lower, the quad will work better
at low heights. Because of its lower angle radiation, many quad
users claim a quad "opens and closes" the band.
The
two-element cubical quad is a square-or diamond-shaped loop antenna
that has a second loop acting as a parasitic element. The quad
configuration has all loops in the vertically oriented plane as
figure 35 demonstrates. Feeding it in one of the horizontal wires
results in horizontal polarization, and feeding it in one of the
vertical wires makes it vertically polarized. Every two-element quad
being sold today uses a reflector for the parasitic element,
although it is possible for it to have a director. The theory of
operation is the same as that of a yagi.
Some quad
builders believe a diamond-shaped quad has more gain than a
square-shaped one. Their logic is that since the maximum current
points of both wires are spaced farther apart than with a square
quad, the increased spacing of the current points should produce
higher gain. To find out if this was true, we built both a
diamond-shaped and a square-shaped quad for two meters. Using a
commercial field-strength meter connected to a receiving antenna, we
fed equal amounts of power to both antennas and measured the
radiated field in each ones major lobe. Field strength measurements
were made a few wavelengths away and many wavelengths away from the
quads. No difference in the radiated field of either could be
found.
According
to Bill Orr in his book about cubical quads, a two-element cubical
quad is equal to a pair of 2-element beams; one is stacked over the
other a quarter-wavelength. The ends of the beams bottom driven
element are bent up and the top element has its ends bent down where
the ends of the top and bottom elements are joined together on the
side. When they are joined, this forms the square we call a quad.
The bottom element is then feeding the top element from its ends.
The parasitic elements have the same configuration except the wire
loop has the ends bonded together to form a continuous
square.
There are
multi-element quad designs that use one or more parasitic directors
in addition to the reflector. Adding a director will lower the feed
point impedance. The wire of the reflector is about 3% longer than
the driven element, and each director has about 3% less wire than
the driven element. Adding directors to a two-element quad makes the
horizontal beam width narrower, producing more
gain.
Another
quad design "the delta loop" uses triangular-shaped driven elements.
One or more triangular-shaped parasitic elements make the antenna
complete. Theoretically, the delta loop antenna will have slightly
less gain than the cubical quad, because there is less enclosed area
in the triangular loop. We believe that there are no instruments
available to hams to be able to measure the
difference.
Figure
36A. Single Band Cubical Quad

Most
cubical quad and delta loop antennas that can be rotated are used on
20 meters and higher. A few ambitious hams have built rotatable
quads for 40 meters. Others have made 80-meter quads, supported
between trees, fixed in one direction.
In order
to make the quad smaller, adding loading coils or linear loading
sections in its wires has been suggested, but that will defeat the
purpose of using a quad. Because the quads gain is produced by the
enclosed area inside the loop, reducing the enclosed area will
result in less gain.
The wires
for quads for 20 through 10 meters are strung around the perimeter
of an "X shaped frame made of fiberglass poles or bamboo. Each
element has its own X-shaped frame. A smaller X-shaped metal
structure, called a "spider," attaches the poles to the boom. The
poles are referred to as "spreaders." The four spreaders attached to
the spider form the "X." The "X" can be rotated 45 degrees on the
boom to form a diamond-shaped quad instead of a square quad. A few
have tried with limited success to make the spreaders out of PVC or
aluminum.
A wire is
attached near the ends of the spreaders to form a loop around them.
The two ends of the wire are connected to an insulator to attach the
feed-line as is done on a dipole. The quad loop has a theoretical
feed-point impedance of 100 ohms. To match it, you can use a
quarter-wave matching section of 70-ohm coax, a gamma-match, or a
2:1 balun. More on this is in another paragraph. The delta loop is
made much the same way, but it requires only three spreaders to form
an equilateral triangular loop. It is matched the same way since the
feed-point impedance is about the same.
The
reflector and director are formed the same way as the driven element
except the two ends are shorted together to form a continuous loop.
In order to get maximum performance from a quad you need to tune the
reflector for either maximum gain or best front-to-back ratio. For
tuning purposes, the wire of the reflector is cut a little shorter
than calculated and the ends of the loop are connected to an
insulator. A shorted stub, consisting of two parallel wires, is
connected to the loop ends and hangs down from the insulator.
Another wire is shorted across the two parallel stub wires. The
shorting wire is moved up and down the stub to tune the reflector.
The stub is a means of adjusting the total length of the reflector.
See Figure 36A. A field strength meter is needed to do this and you
need a large area and two people. The field strength meter needs to
be placed several wavelengths away from the antenna. Low power is
fed into the antenna while it is tuned. One person tunes the
reflector while the other person reads the field strength meter.
Tuning the reflector involves tuning the stub for minimum signal off
the back. Once the shorting wire has found its proper position, it
is soldered in place. Quads made by the formulas work satisfactorily
without tuning. Tuning for maximum front-to-back ratio instead of
maximum gain will do more for the performance of the
quad.
Figure 36.
Radiation pattern of a Two Element Cubical Quad at 65
Feet

Some
believe you cannot stack another antenna above a quad. They assume
that because the quad has both vertical parts of the loop, a metal
vertical mast will couple to the vertical part and detune the quad.
We believe a vertical mast will have to be resonant at the operating
frequency to detune a quad. Using MMANA, the quad was modeled with a
metal vertical mast going through the plane of the quad. The only
difference observed was that the resonant frequency was changed by a
couple of kHz. The gain and front-to-back remained the
same.
The gain
of a two-element quad is nearly the same as an average 3-element
yagi. The best part of the gain of a quad is the vertical beam
width, or H-plane of the major lobe logically should be narrower due
to it being equal to two stacked beams. For this reason, the
2-element quad has a lower angle of radiation. A horizontally
polarized quad should have a slight advantage over a yagi. A lower
angle is better for working DX. While operating using both quads and
yagis, we have noticed that the horizontal beam width or E-plane of
a quads pattern is wider than a 3-element yagi. We believe the
horizontal beam-width of the quad is the same as a two-element yagi.
This is why the 2-element quad is not as directional as a 3-element
yagi.
Modeling
our 2-element quad in free space on 20 meters, we found its gain to
be 5.49 dBd. The boom-length of the quad is 8 feet. A three-element
yagi with a boom-length of 16 feet will have 6.4-dBd free- space
gain on 20 meters. The free-space gain of a 20-meter optimum spaced
monoband yagi on a 25-foot boom will only have slightly more
gain.
The
compromise spacing for a 2-element multi-band quad for 20 through 10
meters is 8 feet. This spacing is 0.115 wavelength on 20 meters,
0.175 wavelengths on 15 meters, and 0.23 wavelengths on 10 meters.
These spacings are within acceptable limits. For a single-band
20-meter quad, space the elements 12 feet apart. If you want to
build a 12 and 17-meter dual band quad, the spacing will be 8 feet,
the same as it should be for 15 meters. Eight feet is also a
satisfactory spacing for a 10-meter quad, but it can be as close as
four feet.
With smaller perimeter requirements, loops for the
higher bands can be strung inside and parallel to the lower band
loops to make a multi-band quad. It is easier to make a multi-band
quad than a multi-band yagi. Quad kits for triband and 5-band quads
are available. These kits cost less than a multi-band
beam.
In the
construction of most quads, an insulator is put in the bottom
horizontal part of the wire on the driven element so it can be fed
like a dipole. A 2-to-1-balun transformer will match the feed-point
to 50-ohms, then you can tie all the feed-points of a multi-band
quad together. The Lightning Bolt Antennas 32MCQ/WB quad feeds five
loops this way and the SWR is 1.4:1 or less on all five bands. The
person manufacturing the Lightning Bolt quad went out of business on
December 12, 2005.
With other
more complicated schemes, each quad loop is fed separately, and each
loop uses a 70-ohm odd multiple of a quarter-wave series matching
section placed between the 50-ohm coax and the feed-point. Used this
way, the quarter-wave matching section will match 50 ohms to 100
ohms. A remote antenna switch will have to be mounted close to the
feed-point to select the desired loop. Other builders use a separate
gamma-match on each driven element to get a perfect match to a
50-ohm coax but this method would also require a remote antenna
switch. Without the switch, several pieces of coax, one for each
band, would have to be run into the shack.
If you are
going to build a monoband quad, you need to use the following
formulas to cut the wire loops to these approximate
lengths:
For the
driven element, you divide 1005 by the frequency in
MHz.
For the
reflector, you divide 1030 by the frequency in
MHz.
For the
director, you divide 968 by the frequency in
MHz.
Make any
additional directors the same length as the first
one.
These
formulas were derived experimentally from tests run here. The exact
measurements will be determined by the element spacing, but the
lengths cut by these formulas will be very close for any reasonable
spacing.
After
giving you the advantages of a quad, here are the disadvantages: The
two-element quad for 20 meters is large vertically and horizontally.
When the 20-meter quad is on the ground, the boom is 8 feet high and
most people arent tall enough to maneuver it by holding the boom.
Some quads, which are made from lightweight materials, are flimsy,
and they will suffer during wind and ice storms. The
best-constructed quads have their spreaders made of heavy
fiberglass. Those quads, although they are heavier, stand up well
under adverse weather conditions. The Lightning Bolt quad used here
has stood up very well during three ice storms in the past two
winters.
Here is
some information we discovered after originally writing this book.
The MMANA antenna-modeling program does not perform very well when
modeling a quad. On 20 meters, the modeling program says the
front-to-back ratio of our quad is at a maximum at 14525 kHz, but
actually, it occurs at 14050 kHz. The measurements of actual
front-to-back were made using a field strength meter. We reduced the
power levels off the front to give the same field strength reading
we got off the back. The front-to-back ratio in dB was calculated
from the two power levels. What was interesting was the maximum
front-to-back ratio occurs at a single discrete frequency and the
front-to back deteriorates somewhat on either side of that
frequency. While looking at the radiated field off the front, the
field strength does not vary one dB across the whole band. Maximum
gain and maximum front-to-back was very close to the same frequency.
Not having tested them in this way, we believe yagi beams perform
the same way regarding front-to-back ratio and gain. In working
stations, the gain is the important parameter. Front to-back ratio
is important in reducing interfering signals from behind the
antenna. We decided to tune our quad for maximum front-to-back
rather than for maximum gain. The next step is to lower the quad and
carefully tune the reflector for each band. After running tests to
determine the frequency where maximum front-to-back occurs, we found
the maximum measured front-to-back ratio was 22 to 23
dB.
Good news!
After writing the above paragraph, we lowered the multi-band quad
and reduced the reflector element lengths on the two-element
Lightning-Bolt Quad. The original reflector lengths were too long
according to the field strength readings we made. The formulas that
were originally used to cut the reflector lengths were anywhere from
1029 to 1036 divided by the frequency in MHz. We derived from
field-strength measurements that the maximum front-to-back ratio
occurred when the reflector length was cut by dividing 1022 by the
frequency in MHz. While searching the Internet, we discovered EI7BA
in Ireland used 1019 divided by the frequency on his multi-band
quad. We decided to use his formula and we could lengthen the
reflector by adjusting the stubs if necessary. After cutting the
reflectors to the new dimensions, we made new field strength
readings. The front-to-back ratio occurred near the frequency of our
calculations. In addition, the frequency of maximum field strength
from the front also occurred inside each band. As an example, today
we were listening to GD4PTV on the Isle of Man on 17 meters. On the
front of the quad, he was S7. With the quad 180 degrees from him, he
was inaudible. We also found that other stations were down by at
least six S-units off the back. Originally, stations off the back of
the quad were down only two S-units.
Several
months later it was discovered that a multi-band quad tunes
differently from a single-band quad because the interlaced elements
react to detune each other. That may be the reason we found the
reflectors of a multi-band quad needed to be different lengths than
the 1030/frequency formula.
Here are
some words of wisdom about using a field-strength meter in trying to
tune a quad:
Tuning the
quad reflector stubs can give you misleading data. If you look only
for the minimum signal from the back of the quad, that may not be
the point of best front-to-back because you may have detuned the
quad so that the signal from the front may have also
deteriorated.
Using a
field strength meter, keep the receiving antenna as short as
possible to prevent the receiving antenna from being nearly
resonant. Certain receiving antenna lengths seem to be frequency
sensitive, that is, as you change frequency toward the receiving
antennas resonant point, the field strength meter will give a false
higher reading. The only problem with using a very short receiving
antenna is the meter may not have enough sensitivity to make
measurements from the back of the antenna.
It will be
impossible to achieve a high front-to-back ratio on certain bands on
a multiband-quad because the reflector wires for adjacent bands
affect the tuning by interacting with each other. When the Lightning
Bolt Quad was designed, we are not certain which parameter was used
in its design (gain, SWR, or front-to-back). From the field strength
readings made with that design, it was impossible to draw any
conclusions. If you are going to build a quad, what we said about
designing a yagi is also is true for the quad: you can tune for best
gain, best front-to-back, or best impedance match. You cannot tune
for more than one of these parameters at a time. To tune a quad for
maximum gain is relatively easy using a short antenna on the
field-strength meter.
While we
were trying to measure the frequency of the highest forward gain on
17 meters, we found the maximum field strength occurred at the high
end of the band on one receiving antenna. Subsequently, it was
strongest on the low end of the band on another receiving antenna
having a different length. No changes had been made to the
dimensions of the quad in either case. Trying to move the frequency
of the maximum field to the middle of the band, we adjusted the
length of the reflector stubs and it made no difference to the
frequency where the maximum field occurred. What caused the error
was we were trying to measure the field strength 100 feet in front
of the quad. The long receiving antenna connected to the field
strength meter was acting like a parasitic element and was not
accurately measuring the signal being radiated from the
quad.
6. The
Quagi
A
variation of the quad and the yagi is a marriage of the quad and the
yagi called the quagi. The quagi has a quad driven element, quad
reflector, and yagi directors. Hams who have built the quagi report
the yagi directors work better than quad directors, but we have
never compared the two types of directors.
At one
time we converted a two-meter yagi to a quagi and compared the field
strength readings from both configurations. By changing the driven
element and reflector to quad loops, we measured a signal increase
of 1.8 dB. We also experimented to see what effect the quad
reflector had on the signal. While using the quad driven element we
changed the reflector back to a yagi reflector. What was surprising
to us was the configuration of the reflector had no affect on the
radiated signal. Only by changing the driven element from a yagi
element to a quad element made any change in the field strength. All
these field strength readings were made using a commercially
manufactured field strength meter. To insure our readings were
valid, the power being fed to the antenna was measured and kept
constant.
XVI. GAIN
VERSUS FRONT-TO-BACK
As we have
said before the front-to-back ratio of a multi-band cubical quad can
be maximized by careful tuning to achieve about eighteen to twenty
decibels front-to-back ratio. A properly designed yagi can achieve a
front-to back ratio of better than thirty decibels. A two-element
quad has about the same gain as a three-element yagi. You can tune a
yagi or quad to either maximum front-to-back or maximum gain. You
can also tune them to compromise settings somewhere in between. The
question arises as to which maximum should either antenna be tuned?
It is our opinion that either antenna should be tuned for maximum
front-to-back ratio. In that case the maximum gain will be
deteriorated by only a fraction of a decibel Let us explain why we
reached that conclusion with an example.
Today we
were on 17 meters to work VP8TD on Pitcairn Island in the South
Pacific Ocean. He is a visitor to the island and will be there for
about two more months as of this writing. A resident of the island,
VP6TC, Tom Christian hasnt been heard from in months. I suspect he
is getting elderly and doesnt get on much anymore. Anyway VP6TD had
an enormous pileup going. We were using a three-element SteppIR yagi
up sixty-five feet on our tower. Also, the amplifier puts out about
1490 watts on 17 meters. We make up for a lack of antenna forward
gain with the amplifier. We worked him with one call through the
pileup. The SteppIR replaced the two-element Lightning Bolt Quad
about 10 months ago. When VP6TD answered us I could hear him over
the pileup. From the rear of the antenna were several very loud
Italian hams calling him, one of which continued to call even when
VP6TD answered someone. The Italians were 180 degrees from the front
of our antenna, or directly off the back of where we were beaming.
Because of the superior front-to-back of the yagi, I could hear the
Pitcairn Island station over the Italians. Had we been using the
quad, the Italians would have been at least 10 dB louder and we
could have found it impossible to make the
contact.
Today, we
were in contact with N4XPZ, Joe, on 75 meters while several more
hams were talking about the VP6TD on 17 meters last evening. Joe
said he tried to work the VP6TD station using a single wire antenna.
He complained he could not copy the VP6 because of the Italians who
continued to call even when the VP6 answered someone. That
illustrated the point we are making in this section. The old adage
is true: "You cant work em if you cant hear em!"
XVII.
FEED-LINES COMMONLY CALLED TRANSMISSION LINES
Always use
the best feed-line you can afford. Resist the urge to be penny wise
and pound-foolish. This is particularly true of coax. Better (less
lossy) coax will cost more. This cable is carrying your precious RF
signal to and from your antenna.
The most
common feed lines used by amateurs are 50-ohm coaxial cables. There
are many types of 50-ohm coax such as RG-174, RG-58, RG8-X, RG-8,
RG-213, RG-8 foam, and 9913. In this book we will only discuss these
types. A suffix letter such as an "A or "U" may be attached to the
"RG" numbers such as RG-8U or RG-58A. All these cables have a center
conductor surrounded by a plastic insulating material, called the
dielectric, and a copper braided shield covering it. There is a
plastic covering on the outside of the shield to protect the
conductors from water. The center conductor and the shield carry RF
currents.
These are
the common 50-ohm cables:
RG-174 has
a very small diameter, 0.101 inches. This cable is used to carry
small amounts of RF between circuits in equipment. RG-174 has the
highest loss and the least power handling capability of any coax. It
is useless as an antenna feed-line because of its loss and low power
handling ability.
RG-58 is
larger coax having a diameter of 0.195 inches. It can handle low
power and can be used on the lower bands to feed antennas a one
hundred feet or so away. It is not recommended to use RG-58 on 10
meters because it has a loss of 3dB per hundred feet on that band
and half your power will be lost in the coax.
The next
larger cable is RG-8X, sometimes referred to as mini-8. Its diameter
is 0.242 inches. The dielectric surrounding the center conductor is
foam rather than the solid dielectric used in the most coax. Making
cables with foam insulation can reduce the loss. Some hams are
successfully feeding a kilowatt of power into RG8-X on the lower
bands. You will lose 2 dB of power by using one hundred feet of
RG-8X on 10 meters. On 80-meters the loss of this cable is
negligible.
You will
want to use RG-8 or RG-213 if you are planning to use a kilowatt or
more of power from 160 to 10 meters or for short runs on VHF and
UHF. RG-213 is RG-8 made to military standards. Both have diameters
of 0.405 inches. This cable has lower loss and can handle higher
power because it has larger conductors and a larger diameter
dielectric. RG-8 can handle 4000 watts peak envelope power on the
broadcast band. RG-8 has only about 1dB loss on 10 meters per 100
feet. The loss becomes greater and the power handling rating of any
coax decreases as the frequency of RF is
increased.
There is a
lower-loss version of RG-8. It is called RG-8 foam. Beldens number
for this product is 8214. Because of the dielectric being foam, a
larger center conductor has to be used to keep the impedance 50
ohms. The loss resistance of the larger conductor is less than the
smaller conductor used in regular RG-8. In addition, the foam having
many air pockets has less dielectric losses. Other manufacturers
also make RG-8 foam. One hundred feet of RG-8 foam has a loss of 0.9
dB on 10 meters. Many amateurs will not use RG-8 foam because they
mistakenly believe the foam will soak up water. Cut off a piece of
this foam material and put it into a container of water. It will
continue to float ad infinitum, because it doesnt soak up
water. Most of the water seen in coax gets between the dielectric
and the plastic outer covering and within the braid shield. Water
has also gotten into the strands of the center conductor. Water will
get into any coax if the ends are not properly
sealed.
Solid
conductors have less loss at radio frequencies compared to stranded
conductors. Braid has more loss than a solid conductor used for the
coax shield. A much lower loss coax, especially for higher
frequencies, is available. The Belden 9913 is this product. This
coax has a solid center conductor and the shield consists of a
coating of aluminum foil covered with braid. The aluminum foil is a
solid conductor. The braid over the foil is used to make a good
solder connection because you cant solder aluminum. The mostly air
dielectric material used in this product requires the center
conductor to be larger to make the impedance 50 ohms. Air dielectric
also has less dielectric loss than solid. There are a few
manufacturers making 9913 look-alike products. One hundred feet of
9913 will have a loss of about 0.66 dB on 10 meters. There is a coax
that looks like 9913 but has a stranded center conductor to make it
flexible. It has a little more loss. If you are going to use 9913 on
an antenna that rotates, flexing the cable as the antenna turns will
cause the center conductor to break. Run the 9913 to the top of the
mast, and using a barrel connector, connect the 9913 to a short run
of RG-8. Run the RG-8 across the rotor to the
antenna.
Coax
cables of other impedances are available such as 70-ohm cable. RG-59
and RG-11 are common 70 ohm cable. Hams, except to make quarter wave
matching sections, do not use these cables much anymore. There are
many other types of cable other than the ones described
here.
Open wire
feeders, ladder-line, or window-line have much lower loss than coax.
The three types are essentially the same except for the method of
insulating the two wires from each other. When making open wire
feed-line, you should use solid conductors, as large a conductor as
possible, and as little dielectric as possible. These factors make
open wire have less loss. There is so-called ladder-line for sale,
which is really window line, which is made with 16 gauge solid
conductors. The solid conductors make for low loss. There is another
grade of the same feed-line that has 14 gauge-stranded
conductors.
XVIII.
ANTENNA SAFETY
1.
Erecting Antennas on Masts
Erecting
antennas pose some danger especially if they encounter power lines.
Never erect an antenna near a power line. Make sure to leave enough
clearance so if the antenna supporting structures fall they will
clear the power lines. There are many cases of metal masts being
raised accidentally encountering power lines, electrocuting the
person or persons raising the masts. To raise a mast can expose you
to a large force called leverage, which appears to increase the
weight of the mast. Exerting oneself to raise a heavy mast can
result in painful muscle and back injuries. Never try to raise a
mast without sufficient help.
2. Tower
Safety
A tower is
a wonderful device for supporting wire and beam antennas, but a
person who has never put up one should seek advice of people who
have experience in erecting towers. The obvious danger is falling
off the tower. It should never be climbed without a climbing belt.
Most people falling off a tower do so because of some kind of
equipment failure or the tower collapses because of
overload.
In
erecting a tower, a gin pole strong enough to support the weight of
the tower section being raised should be used. Do not use improvised
gin poles, as the strength of them may not support the weight of the
tower section and the force from the other end of the rope being
pulled by the ground crew. To hold a 50-pound tower section
stationary requires a hundred pounds of force, which is the weight
of the tower section and 50 pounds of force of the ground crew. The
ground crew must exert more than 50 pounds of force to cause the
section to be raised. There would be no greater tragedy than the gin
pole breaking dropping the tower section on the ground crew. Then
there is the possibility of the person on the tower being knocked
loose by the falling, broken gin pole.
Another
problem can arise if under-sized guy cables and clamps are used to
support the tower. We have seen tower failure when guy cables broke
in a windstorm, or an insufficient number of clamps holding the guy
cable allowed the cable to pull through the clamps. Professional
tower people do not use cable clamps. They use "preformed tie-wraps"
that grip the guy cables tighter as the force in the guy cables
increases. Preformed tie-wraps are available from Texas Towers.
Never tie the ground end of a guy cable to a tree. A tree swaying in
a heavy windstorm can put enough force on the cable to cause it to
break or to pull the tower over. Screw-in anchors available from
mobile home suppliers make adequate anchors. Do not anchor a guy
cable where a tree can fall across the guy cable. This could break
the guy cable and cause the tower to fall. Never place a tower near
a house, where if it falls, it could hit the house. Remember Safety
First!
TABLE
1
Quarter
Wave Matching Sections of 70-ohm Coax
These
lengths are for coax having a solid dielectric with a velocity
factor of 0.66 and foam dielectric with a velocity factor of 0.78. ,
You can use odd multiples like 3, 5, 7, etc. of the lengths below if
those lengths are too short for your
installation
Solid dielectric cable Foam dielectric
cable
160 meters
85ft 6in 106ft 4in
80 Meters
43ft 4in 51ft 9 in
40 Meters
22ft 6in 27ft 6 in
30 Meters
16ft 0in 19ft 6 in
20 Meters
11ft 5in 13ft 10 in
17 Meters
8ft 11 in 10ft 9 in
15 Meters
7ft 7 in 8ft 4 in
12 Meters
6ft 6in 8 ft 0in
10 Meters
5ft 8 3/8 in 6ft 11 in
Questions? Email Jim
Abercrombie 4ja@prtcnet.com
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