Understanding Antennas: Dipole, Vertical, and Yagi

Antennas are where amateur radio stops being a consumer activity and starts being an engineering one. The good news is that the underlying principles are few, and three designs cover the overwhelming majority of practical situations.

The most useful way to think about an antenna is not in terms of gain but in terms of pattern — the shape of the field it produces. Gain is just a consequence of concentrating the available power into a smaller part of the sphere.

The dipole: the reference against which everything is measured

A half-wave dipole is two equal conductors fed at the centre. At resonance it presents an impedance near 73 ohms and radiates a figure-of-eight pattern broadside to the wire. Nothing about it is difficult, and nothing else in amateur radio is as well understood.

Its practical strengths are that it needs no ground system, it is cheap to build, and it can be made to work on several bands by feeding it with open wire and a tuner. Its weakness is that it needs space, and at low heights it becomes a cloud warmer — useful for regional contacts, poor for distance.

The vertical: a low angle and a hidden half

A vertical is a quarter-wave radiator worked against a ground plane or a set of radials. Its pattern is a doughnut with the strongest radiation at a low angle to the horizon, which is precisely what you want for long-distance contacts.

The catch is that the radial system is half the antenna, and it is the half most often done badly. A vertical with three short radials will underperform a dipole at the same height. The same vertical with sixteen quarter-wave radials will outperform it on the low angles that matter for DX.

The Yagi: trading coverage for direction

A Yagi adds a reflector behind the driven element and directors in front of it. The extra elements re-radiate the field so that energy to the rear cancels and energy to the front adds. The result is a single forward lobe with real gain and a deep null behind it.

Choosing between them

If you have space and want a simple station that works on several bands, build a dipole and feed it properly. If you have restricted space or want the low angles that favour distance, put up a vertical and spend the effort on radials instead of on the radiator. If you have a mast, a rotator and a specific interest in working distant stations, a Yagi is the logical endpoint.