Long and Short Range Communication
Air-to-ground communications can be achieved through the use of many types of radio equipment. High frequency bands (HF, VHF, UHF) are relatively static-free and are less susceptible to outside interference than lower frequencies. It must be remembered, however, that the higher the frequency, the more nearly the transmission follows an LOS path. As frequency increases, therefore, communication range decreases.
Systems used for long-range radio communications between aircraft and ground stations may be either amplitude modulation (AM) or single sideband (SSB) transmissions. Single sideband transmitters concentrate all available power into one sideband; therefore, SSB is much more efficient and has greater range than an AM transmitter of the same power. Although HF ground waves attenuate rapidly, sky waves at these frequencies are capable of transmitting at distances up to 12,000 miles or more, depending on ionospheric conditions. HF equipment is used mostly in remote areas where VHF or UHF communication is not possible because of the great distance that must be spanned.
Short Range Air-to-Air and Air-to-Ground
Short range air-to-air and air-to-ground communications are confined to the VHF and UHF bands. VHF channels are spaced at 25 kHz intervals from 116 to 151.975 mHz, and UHF channels are spaced at 50 kHz intervals from 225.0 to 399.9 mHz. Most UHF and HF transceivers have a manual frequency selection capability in addition to a number of preset channels. Transmission and reception are accomplished with a single antenna.