Handbook, chapter 9. The antenna: the most important thing in the shack
Between a hundred-watt amplifier and an antenna twice as good, a beginner almost always picks the amplifier. That is a mistake, and it costs money.
An antenna works both ways. An amplifier makes only you louder: you are heard better at the far end and nothing changes at yours. An antenna that gives those same three decibels makes both you and everyone you listen to louder. And a contact takes two: a deaf receiving end stops a conversation just as reliably as a weak transmitter.
Height decides more than the type does
The question «which antenna should I buy» is almost always the wrong one. The right one is «how high can I get it», and the answer to that changes the outcome more than the choice between a dipole and a vertical.
Move the slider and see where a dipole actually points.
Below a quarter wave above ground a dipole radiates upwards. That is not a fault of the design but reflection from the ground: the direct and reflected waves add so that the low angles cancel and the zenith is reinforced. For contacts at 300–600 kilometres that is excellent and even has a name of its own — NVIS. For DX it is death.
At half a wave a moderate angle appears; at a full wave, the low lobe — the one you need for distant contacts. The same three metres of wire, raised to a different height, behave like different antennas.
⚠️ In the drawing the ground is taken as perfectly conducting and the dipole's own pattern as flat. Real soil eats into the low angles, so the picture shows the trend rather than measured decibels.
The dipole: the one to start with
A half-wave dipole is a piece of wire cut in the middle with the feeder in the gap. Each leg is a quarter wave long, half a wave together.
Three reasons to start with it:
- it is cheap — a hundred hryvnias of wire and two insulators;
- the feedpoint impedance is about 70 Ω, which is close enough to 50: a transceiver takes it without a tuner;
- it is impossible to build it so badly that it does not work.
The main practical point: cut it long. The wave travels more slowly in wire than in air, so a real leg comes out a few per cent shorter than the calculated quarter wave. You can always shorten it; you cannot lengthen it.
Ground planes and verticals: when there are no two supports
A dipole needs two support points tens of metres apart. If there are none, a vertical is what is left: a quarter-wave whip with radials.
What to understand in advance:
- the radials are not decoration. A whip without them works against whatever is around: against the plumbing, against the cable braid, against you. Three or four quarter-wave radials is the minimum;
- the low radiation angle is both an advantage and a drawback. Good for DX and bad for noise: a vertical picks up everything humming in the town;
- on VHF it is the standard choice, because radials fifty centimetres long bother nobody.
Beams: when there is something to turn
Yagis, quads and log-periodics give gain by not spending power on the sky and on the neighbour behind you. But they need a mast, a rotator and space — and on the low bands, dimensions that not every roof will carry.
The rule is simple: a beam is not a first antenna. First a dipole as high as you can get it, then experience, then a mast.
Polarisation: getting it wrong costs twenty decibels
An antenna radiates a wave oriented the way its elements are. A horizontal dipole gives horizontal polarisation, a vertical whip vertical.
Two antennas of different polarisation hear each other 20 dB worse. Hence the convention: on HF mostly horizontal; on VHF vertical for FM and horizontal for SSB. That is not a whim but an agreement, outside which you simply hear nobody.
Six calculators for this chapter: Flower Pot, Ground Plane, J-pole, Yagi (DL6WU), Kharchenko, log-periodic.
Next — chapter 10, the feeder and SWR: why a cable is not a wire and why a tuner does not cure a bad antenna.