Antennas: length, polarization, gain, and pattern

Concept

An antenna works best when its physical size matches the wavelength it is handling — engineers call that being resonant. The rule of thumb: higher frequency means shorter wavelength, so a resonant antenna gets shorter as you go up in frequency. On the 2-meter band a common vertical is about 19 inches, which happens to be one-fourth of a wavelength there — a quarter-wave. Trim a wire element shorter and it resonates higher up the band; let it out longer and it resonates lower. When there is no room for a full-length element, you can splice a coil into it partway along, so it behaves electrically like a longer wire while staying physically stubby. That trick is called loading.

Concept

Polarization is just the way an antenna's radio wave is tilted in space — set by the orientation of the wave's electric field. Stand a whip straight up and it launches a vertical wave; lay the elements flat and the wave is horizontal. Two stations hear each other best when they share the same polarization, because a mismatch quietly throws signal away. FM voice on VHF and UHF is almost always vertical, which is exactly why handheld and mobile whips are meant to stand upright.

Concept

Gain is a measure of how much stronger your signal is in one favored direction, judged against a plain reference antenna that sprays energy everywhere at once. An antenna cannot manufacture power — gain comes entirely from focusing the energy you already have, the way a flashlight reflector turns one bulb into a bright beam. Antennas that squeeze the signal one way, like a directional beam of the Yagi family, show the most gain; an idealized point-source radiator is the zero-gain yardstick everything is measured against. Among simple verticals, a five-eighths-wavelength whip wrings out more gain than a quarter-wave by pressing its energy down toward the horizon, where far-off stations live. A half-wave dipole is not the same in every direction either: it is loudest out to its sides and weakest straight off the tips.

Concept

Real hardware trades size for convenience, and every trade costs something. The short rubber antenna on a handheld is far shorter than a full quarter-wave, so much of your transmit power never radiates — fine across the room, frustrating across town. Climb into a car with no outside antenna and the metal shell wraps around your handheld and blocks much of what little escapes. That is why a proper antenna mounted outside the vehicle, standing in the clear, beats any rubber stub used from the driver's seat.