UHF omnidirectional antenna systems for large aircraft

This paper discusses the problem of obtaining omnidirectional coverage from antennas operating between 1,000 and 3,000 megacycles on large aircraft. Electromagnetic modeling was used to determine the limitations of several single antenna sites on typical commercial aircraft. Considering all azimuth angles and in elevation to be equally important, the best coverage obtainable from a single radiator is equivalent to the radiation from a free-space dipole for 50 per cent of the time. To improve this, dual antenna systems must be used. Dual antenna requirements depend on whether or not the airborne equipments know when they should be receiving a signal. The distance-measuring equipment (DME) is a typical system that knows when it should be receiving a signal, while radar safety beacon equipment does not know when or from what direction it is being interrogated. Direct parallel feed, the least complicated method of operating dual antennas, allows simple hybrid multiplexing to be used. With this type of operation interference occurs where the individual patterns overlap. Performance in this region is investigated on a probability basis for beacon operation and found favorable; for DME this region is uncertain. In addition, performance is predicted when the RF voltage in one of the dual antennas is (a) shifted periodically in phase, (b) delayed, and (c) interrupted periodically. Considerations involved in an antenna system common to DME and beacon are discussed.