There are two main reasons for the existence of corrugated horns as feeds for reflector antennas. First, they exhibit radiation pattern symmetry, which offers the potential for producing antennas with high gain and low spillover; secondly, they radiate with very low crosspolarisation, which is essential in dual-polarisation systems. The former property provided the motivation for Kay [88, 89] who conceived the wide-angle corrugated horn in 1962 while working at TRG in the USA. It is said that he was studying the effect of quarter-wavelength chokes at the horn aperture and found that, by adding more than one, the pattern symmetry improved. He noted invariance of the chokes to field orientation and thus coined the term scalar feed to describe his horn. At almost the same time, and quite independently, Minnett and Thomas [101], at CSIRO in Australia, pursued the radiation properties of corrugated waveguide feeds for use in radiotelescopes. They were motivated by efficiency and polarisation considerations. In this latter respect, Rumsey [134] contributed during a stay at CSIRO in 1966, by observing general properties of hybrid modes in respect of polarisation purity.

In this chapter we are concerned with feeds in the class of Fig. 3.1 (a), i.e. the cylindrical corrugated waveguide with a radiating aperture. Feeds of this kind are used at the prime focus of a reflector for, as they can be made with aperture diameters as small as one wavelength or less, they provide for efficient illumination. The feed of Fig. 3.1 (a) also represents a good first approximation to the feed of Fig. 3.1 (b) provided that the horn flare angle is small and that a correction is made for the phase curvature of the aperture fields. Feeds of this kind are frequently used at the secondary focus of a Cassegrain antenna or in beam-waveguide fed antennas. Fig. 3.1 (c) shows a feed that combines features of both of the preceding types and draws on knowledge relevant to both. It is used when a narrow beamwidth radiation pattern is desired in a feed of short length.

This chapter is concerned with the factors which influence the design of conical corrugated horns. We start by surveying these factors, then study the electrical specifications and the procedures needed to design the horn to satisfy the specifications. A large number of design curves are included. The information needed to design various types of corrugated horns is summarised. Finally computer prediction is discussed.

In this chapter, corrugated horns and feeds with non-circular cross-sections have received relatively little attention in the past. Out of nearly 180 papers cited in the bibliography only a few have dealt with horns of rectangular or elliptical cross-section. There are good reasons for this bias, for it turns out that unless a shaped beam is required, circular corrugated horns are superior in almost all respects to non-circular corrugated horns. Furthermore, rectangular and elliptical corrugated horns are difficult to analyse. The simple theoretical models are only an approximation to the true electromagnetic behaviour of the horns and do not give good predictions of cross polar performance. The electromagnetic fields in the horns are more complicated and the fields in the slots are dispersive. The throat region is difficult to design so as to suppress unwanted mode excitation. Finally, the horns are difficult to manufacture and thereby expensive to produce. But, as stated, their one significant advantage compared to circular corrugated horns in certain applications, is their ability to generate a beam with a non-circular symmetry. This is desired in some spacecraft antennas and also some radar antennas. In this chapter we shall discuss their properties but the approach will be limited to a study of the basic characteristics, without the detail which has been devoted to circular corrugated horns.