Short-range airborne communications sets operate in the frequency range from about 30 MHz to 3 GHz. The lower portion of this band is the very-high-frequency (VHF) band; the higher portion is the ultra-high-frequency (UHF) band. The VHF/UHF frequency bands have transmission characteristics that differ from those frequencies in the HF band. Radio waves transmitted at these frequencies travel in a straight line. This limits the transmission to line-of-sight. VHF/UHF communications sets are called line-of-sight communications sets. Radio waves at these frequencies normally do not return to earth. Therefore, VHF/UHF transceivers are mainly used for air-to-air and air-to-ground contact in close range operations. Landings and takeoffs are typical situations using air-to-ground VHF/UHF transmissions. Special situations exist where VHF/UHF equipment is involved in long-distance communications. An example of this is the network of remote-controlled transceivers installed along the airways. Pilots of aircraft can talk directly to controllers in distant aviation activities. A system of telephone lines and relay stations connect the remote transceiver sites. The radio part of the transmission takes place over a relatively short distance.
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