Abstract :
Ever since the early 1940s, resonant arrays have gradually been replaced by rhombic aerials in the h.f. fixed services, for operational and economic reasons. Over the past decade, a growing awareness of the importance of the lower-order propagation modes has led to the erection of larger and higher rhombic aerials, with encouraging results. An assessment is made of the performance limits of large rhombic aerials, and it is concluded that there is scope for further improvement. The use of rhombic-type arrays, giving enhanced gain and lower subsidiary responses, is advocated. Some special considerations which apply to rhombic receiving aerials are discussed, and performance is compared to that of the sloping-V aerial. The relative merits of open-wire and coaxial transmission lines are examined, and the conclusion is reached that, from the economic view point, the former are more suitable for conveying h.f. power over the longer distances. There are, however, several reasons for preferring coaxial feeders for power distribution within the station, and the newly-developed wideband transformers permit the desirable features of both types to be utilized. Some of the switching systems which are now being used to provide the flexibility of interconnection between transmitters and aerials are also described. The aerial distribution arrangements at receiving stations and the design considerations which determine the choice between passive and active schemes are discussed. At most sites in the United Kingdom, aerial distribution amplifiers need not be used if only a few outlets are required from each aerial.
