Antennas and feed lines: size, pattern, match

This section asks what an antenna does rather than what it is made of, and what the cable between radio and sky costs you. Three things settle almost every question: size in wavelengths, shape of the pattern, and match.

Wavelengths, patterns and matches

An antenna is measured in wavelengths, not in inches: wavelength in metres is about 300 divided by frequency in MHz, and a metre is roughly 39 inches. Two lengths set the scale, a half-wave element fed in the middle and a quarter-wave whip against a ground plane, so trimming an element moves its resonance up the band and adding length, real or electrical, moves it down. Loading buys resonance, not size: the coil holds energy the shortened element never radiates.

Gain is not extra power. An antenna radiates what the transmitter gives it, and gain says how much of that is pushed into one chosen direction, measured against a reference antenna fed the same power, which is why the ideal reference, favouring no direction, has none to offer. Concentration is bought with structure, and the more elements that work together the tighter the lobe: elements set in front of and behind a driven one take from every other direction to pay for a single forward lobe. A plain element radiates least along its own axis, and its polarization is the plane of the electric field, set by the element's own orientation.

The feed line always takes a share. Coax is standard because it carries its own shield and can be run through walls, strapped to a mast and coiled behind the desk without upsetting anything: convenience, not superiority, since open-wire line loses less and rigid air-spaced line less still. Loss climbs with frequency, as current crowds into the surface of the conductors, and falls as a cable grows fatter with more air in its insulation. Above a few hundred megahertz a connector must hold a steady impedance right through its own body, and outdoors it must keep water out.

SWR reports one relationship only: how completely the far end absorbs what the line brings it. A perfect match reflects nothing and reads 1:1; whatever is reflected travels back down the cable to be spent there. A reading that jumps about on its own points at something mechanical, a loose or corroding joint moving in and out of contact. A tuner ahead of the radio changes what the radio sees, presenting the impedance the transceiver is built to expect; past it the antenna behaves exactly as before.

Where the answers are lost

• Reading gain as extra power or as coverage everywhere. Distractors add it to the transmitter output, call it a rise in impedance, hand the prize to the zero-gain reference or to a single vertical over an array, and have a plain element radiating equally in every direction or best off its tips.
• Answering with the hardware instead of the behaviour. An antenna is named for what it does with the energy, not for the metal it is built from, an inventor's surname or the shape of its driven element, and polarization is not the direction it points.
• Moving resonance the wrong way. Extra length, coils in series with the radiating wires and capacitance at the ends all lower the resonant frequency; a resistor in the radiating part only burns power, and a spring or a stouter tube is mechanics, not loading. A short loaded whip pays for its size in efficiency, not in polarization, desensitization or a refusal to carry digital signals.
• Making SWR the explanation for everything. It gets offered as transmitter efficiency, amplifier gain or a verdict on the station ground; erratic readings are blamed on thunderstorms, over-modulation or a strong neighbour; a tuner is credited with pulling in weak stations, picking antennas or letting one antenna both transmit and receive; and a shielded position is turned into a reflected-power problem.
• Praising coax and its connectors for the wrong virtues. Distractors make it the lowest-loss, highest-power or cheapest line rather than the most convenient, promote the traditional screw-on amateur connector to microwave duty or call it watertight or a bayonet type, seal only some outdoor connectors, and quote 8, 12 or 600 ohms where the service is built around 50.
• Treating line loss as a fixed property. It grows with frequency while the characteristic impedance stays where the cable's geometry put it; a slim flexible cable is not the equal of a fat one or of rigid air-spaced line, carries no more power and is not marked out by a second shield; and water in a connector, a corroded joint, an extra mating surface and a high SWR each add loss, so no single one is the whole answer.

Try it yourself