Frequency Bands & Their Aviation Uses

Part II (ii) Radio frequency bands · principal aviation frequencies · polarization

The radio spectrum is divided into bands, each a factor of ten in frequency above the one below. This matters because the dominant way a wave travels — and therefore what it is useful for — changes with frequency. Low frequencies have long wavelengths and bend around the Earth; high frequencies have short wavelengths and travel in straight lines. Aviation lives mainly in the HF, VHF and UHF bands.

118.0 – 136.975 MHz, using Amplitude Modulation (AM), vertically polarized, propagating by line-of-sight (space wave). Range is limited to roughly the radio horizon — higher you fly, farther you reach.

VHF gives clear, static-free, short-range communication ideal for an airport's busy frequencies. AM is used so that two overlapping transmissions produce a tell-tale heterodyne squeal — alerting everyone that two stations transmitted at once, rather than one silently capturing the channel as FM would (the "capture effect", Chapter 10). AM's "you'll hear the clash" behaviour is a safety feature.

3 – 30 MHz, using Single Side Band (SSB) for efficiency, propagating by sky wave (refracted off the ionosphere). This bounces signals thousands of miles — essential over oceans and polar routes where VHF line-of-sight fails. Crews change HF frequency between day and night (Chapter 9).

The VHF band is a fixed width, so the number of channels it holds depends on how closely they are spaced. Traditionally channels were 25 kHz apart; to fit more in the crowded European band, 8.33 kHz spacing was introduced — tripling the number of channels.

The VHF band is 118.0 to 136.975 MHz, a width of 18.975 MHz = 18,975 kHz.
At 25 kHz spacing: 18,975 ÷ 25 ≈ 760 channels. At 8.33 kHz spacing: 18,975 ÷ 8.33 ≈ 2,280 channels — roughly three times as many.
This is why 8.33-spaced frequencies read like 120.275 or 118.905 rather than ending in tidy tenths.

Polarization is the plane of the wave's electric field — vertical or horizontal. The antenna must match it: a vertical wave needs a vertical antenna. Aviation VHF communication is vertically polarized, which is why the COM antenna (the blade) is mounted vertically.

An efficient antenna is sized to the wavelength — commonly a half-wave or quarter-wave. Since λ shrinks as frequency rises, VHF aerials are short blades while HF aerials are long wires or probes. From Chapter 7, a 120 MHz λ is 2.5 m, so a quarter-wave whip is about 0.6 m — a practical blade. An HF λ of 100 m needs a long-wire or a tuned coupler.

The little vertical blade on the fuselage is your VHF COM antenna, matched to the vertical polarization of the signal. Mismatch the polarization and received signal strength drops sharply — one reason antennas are mounted exactly as they are.