Corrugated horns are widely used and highly efficient, especially in their use as feeds for microwave reflector antennas. This book is devoted to the theory and design of corrugated horns and scalar feeds and explains why hybrid mode feeds are ideal feeds for reflectors.
Inspec keywords: horn antennas; electromagnetic wave propagation; microwave antennas; circular waveguides; antenna radiation patterns
Other keywords: conical corrugated horns; propagation characteristics; radiation characteristics; elliptical corrugated horns; cylindrical corrugated waveguides; conical corrugated waveguides; microwave antennas; rectangular corrugated horns; cylindrical corrugated horns
Subjects: Electromagnetic wave propagation; Antenna accessories; Single antennas; Waveguides and microwave transmission lines
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.
The most important use in antennas for a hybrid-mode waveguide or horn is as a feed for a reflector, so we begin this chapter by considering, in general terms, how the feed influences antenna performance. Fig. 2.1 shows a photograph of a horn feed in a Cassegrain antenna forming part of a satellite-communication earth station while Fig. 2.2 shows a dielectric-cone feed in an antenna for a similar application. Although different in appearance and construction, the corrugated horn and dielectric-cone feed support nearly identical hybrid fields and in this chapter we shall use the term hybrid-mode feed to cover both types.
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.
In this chapter we shall explore these methods, concentrating especially on the spherical wave expansion method because of its wide applicability. We shall also have need to explore mode conversion along the horn for in many cases the cross polar radiation pattern will be dominated by an undesired higher-order mode rather than the dominant HE11 spherical hybrid mode.
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.
Corrugated horns are not easy, nor cheap, to manufacture. Considerable manpower needs to be devoted to the production of a horn. Changes in the design after manufacture are hard to implement and there is an obvious desire to produce a design, which, when made and tested, will agree with the theoretical predictions. The corrugations are designed for operation over a specified frequency band. It thus immediately follows that all dimensions and tolerances on dimensions need to be considered in relation to wavelength rather than absolute distance. We will deal with tolerances in more detail later in this section. To start with we shall discuss some of the methods which have been used to manufacture corrugated horns.
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.
In this appendix the exact mathematical analysis including space harmonics is presented for cylindrical corrugated waveguides.