Antenna Myths

Of all areas of electronics, antennas seem to be the one which produces the most confusion (although 'high end' audio is not far behind). I suspect this is because electromagnetism can be counter-intuitive, especially for those who prefer simple 'mechanical' explanations. People use 'intuition' and so end up going astray. You can find examples of antenna myths not only on the web, but also in published books and popular magazines.

Small antennas

A short antenna has very low gain

Not true. Even an infinitesimally small dipole still has gain of 1.5dBi, versus the gain of 2.16dBi of a half-wave dipole. Shrinking from full size down to nearly nothing loses just 0.66dB of gain. A short dipole will have very narrow bandwidth and/or very low efficiency, so it will be difficult to match and inconvenient to use. If you made a short dipole with a superconductor and use a superconducting matching network, then it would work fine (although with low bandwidth).

High bandwidth is good in a small antenna

Not necessarily. A small antenna has to adopt a compromise between bandwidth and efficiency. The best compromise depends on the application, but a small antenna with high bandwidth always has low efficiency. The two are linked, in that it is the same resistive losses which both broaden the bandwidth and reduce the efficiency. In some cases the resistance may be in the antenna structure; in other cases it may arise from coupling to its surroundings (which means that a lossy antenna will not necessarily heat itself). A larger antenna can combine high bandwidth and high efficiency because it uses radiation resistance to achieve both.

Small magnetic loops make good antennas

Perhaps partly true. Excessive claims have been made for small loops, which seem to me to be based on poor measurements and confusion (e.g. it is not heating up, so it must be high efficiency). However, there is a kernel of truth. Any small antenna has two problems, due to its necessarily small radiation resistance and high reactance: losses within the structure, and losses in the matching network. Losses within the structure can be reduced by using thick enough metal, good joints etc. This leaves the matching network. A short dipole is capacitive, and so needs inductance to be added. The converse is true for a small loop. It just happens to be true that high quality low loss capacitors are much easier to make than high quality low loss inductors. Therefore a small loop resonated by a capacitor will have much higher Q than a short dipole resonated by an inductor. Having started from a better position, we then have more freedom to balance off bandwidth versus efficiency.

In addition, it may be that a small loop is better at inducing currents into its surroundings. These currents act as secondary radiators which, although probably lossy, enhance radiation more than they reduce efficiency. The net result is that performance is reasonably good, but not necessarily as good as sometimes claimed and not for the claimed reasons.

Small antennas work best without the losses of a balun

Not true. This is a classic sign that an antenna actually operates via feeder radiation. Any decent balun has quite low losses, so cannot turn a good antenna into a poor one. However, a good balun can kill off feeder radiation so if that is how the 'antenna' actually works then a balun will stop it working.

Antenna position

The higher the better

Generally true, with exceptions. Raising an antenna will reduce ground losses and obstruction by nearby objects. It may also reduce local interference. Horizontal dipoles don't like being too near ground, as the ground imposes a lossy short. However, raising an antenna will introduce more feeder loss - this will only be an issue at VHF and above.

An exception is if it is desired to do local working via NVIS, for which a lower position may be better. Any antenna which needs a ground connection may need to be near the ground.

Some antennas work best near the ground

Generally untrue, with exceptions. Horizontally polarised antennas work badly near the ground. Vertically polarised antennas can cope with being near the ground, so it may be better to say that if you have to be near the ground then use vertical polarisation. Some applications involve near-field operation (e.g. mobile micro-cells) so the usual conditions for good far-field radiation do not apply. An antenna which relies on ground coupling (e.g. small magnetic loop?) will work best near the ground.

Transmission lines

It is normal to adjust SWR by changing the length of the coax feeder

Not true. This is a classic sign of feeder radiation, due to outer screen currents. The SWR as measured at the transmitter end should vary very slowly with length, due to extra attenuation. Large or cyclical variation means either the SWR meter is faulty, or there are outer currents (probably caused by the lack of a balun). Changing the length of the cable changes the impedance presented to outer currents at the far end. The antenna couples the unwanted outer current to the wanted inner currents, so changing one affects the other.

You don't need a balun with a ground plane

Perhaps. In order to stop feeder radiation from coax you need one of the following to be true: there is a good ground connection (real or virtual) at the antenna, or there is a choke balun to insert a high impedance for outer screen currents. A ground plane antenna provides an approximation to a virtual ground. How good an approximation can be judged by whether the feeder is indeed not radiating. If the SWR varies with feeder length, then you need a balun.

Low SWR shows the antenna is efficient

Rarely true. Low SWR means a good match. The quickest way to achieve this is to use cheap lossy coax, or replace the antenna partly or wholly by a resistor. Unless the antenna is intended to give a good broadband match, a low SWR across a wide frequency range is a sign of serious efficiency losses somewhere in the antenna/feeder system.

Other stuff

Antennas emit photons

True, in a sense, but completely irrelevant. Anyone who talks about antennas in terms of photons is probably trying to impress you (or, possibly, amuse you) but by so doing he shows that he probably does not understand wave-particle duality, probably does not understand physics (although he has learnt some words), and so probably does not understand antennas. An antenna is surrounded by photons. There is the teeming vacuum, plus the virtual photons (i.e. off mass-shell) which make up the local induction field and the real photons which form the radiation field. Completely irrelevant, and potentially misleading, when trying to understand or explain an antenna. RF antennas are firmly in the wave part of wave-particle duality. The frequency region where photons start to become a useful concept is around terahertz or infra-red.


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created 16 May 2011