This book is concerned with feeds for reflector antennas and as such is believed to be the first general treatment of its kind. The aim is to describe the design principles and methods of analysis so that readers will be able to understand and develop feeds for reflector antennas. In the past 40 years a number of excellent texts on microwave antennas have been published and one of the earliest, 'Microwave antenna theory and design', edited by the late Samuel Silver appears as a reprint in the IEE Electromagnetic series to which this text belongs. Within that classic work, rather little attention is given to feeds per se as the waveguide and horn were the only forms readily available to the antenna designer of the 1940s other than waveguides terminated in dipoles. Much of the design of that period was empirical, although many of the theories we currently use were first developed then. In the nearly halfcentury following the Second World War, antenna technology has burgeoned, stimulated by communications, radar, radio astronomy and remote sensing. There has been a continual requirement to develop improved feeds and reflectors and for an improved understanding of the factors influencing their performance. Today the antenna engineer has available a wide variety of feeds ranging from high performance corrugated horns to simple waveguide-fed dipoles.

Horn and feed antennas can be divided into two categories for the purpose of the analysis of the radiation characteristics: (i) antennas which consist of a well defined aperture with imposed electric and magnetic fields over the aperture and which are a few wavelengths in cross section. This category includes most of the horns and open-ended waveguides used as feeds. The electromagnetic process of determining the fields across an aperture can be separated from the process of determining the radiation characteristics of the fields in the aperture; (ii) antennas which are small in size and in which the currents on the antenna structure determine the radiation characteristics directly.

This chapter deals with the formulation of the problem. Initially, integral equations are used to generate a suitable set of field equations, which are then reduced to a matrix equation to determine surface currents. Far-field patterns are finally determined from the computed surface currents.

Prime-focus waveguide feeds have seen several stages of development. The early units were simple open-ended waveguides, with small flares to control the radiation patterns [1]. Their performance in reflector efficiency was, however, low. The need to improve the aperture efficiency and polarisation characteris tics resulted in the development of corrugated feeds [2, 3]. With a small size and wide flare angles, these horns are designed for an excellent pattern symmetry, low crosspolarisation and low spillover.

Corrugated horns have become the preferred choice of feed antenna for use in reflector antennas for communications, radar and remote sensing where high performance is required. This is because of their superior radiation performance and in particular their high copolar pattern symmetry and low crosspolarisation. The history, principles, design philosophy, analysis techniques and types of horn are reviewed in this chapter. This chapter concentrates on the general principles of design of corrugated horns and then describes developments since that book was published in 1984.

Horn antennas suffer from the disadvantage of poor aperture efficiency and an inability to control the copolar-radiation characteristics except by altering the diameter of the aperture. This is the case for most of the horns described in the preceding chapters, particularly with short wide-angle horns.

The dipole antenna is the most fundamental and simplest antenna and was, historically, the first to be used as a reflector feed. To illuminate the reflector efficiently, a small circular disc was used as a subreflector to direct the dipole radiation toward the reflector. An oversized dipole reflector has also been used in place of the disc reflector. While these designs are simple and are used extensively as low-cost or low-frequency feeds, the asymmetric dipole radiation in its Eand H-planes does not illuminate the reflector efficiently.

This chapter is concerned with the use of more than one element to form the feed for a reflector antenna. In Chapter 2, the design of shaped-coverage antennas using array feeds is described, but one of the fundamental design problems which arises when elements are combined, namely mutual coupling, is not addressed there.