Microwave Horns and Feeds

2: Department of Electrical Engineering, University of Mississippi, USA
3: Department of Electrical Engineering, University of Manitoba, Canada
This book is devoted to describing the theory, design, performance and application of microwave horns and feeds for reflector. The first general treatment of feeds for reflector antennas, it describes design principles and methods of analysis.
Inspec keywords: antenna feeds; dielectric-loaded antennas; reflector antennas; horn antennas; waveguides; microstrip lines; numerical analysis
Other keywords: numerical analysis; microstrip feeds; horn antennas; multimode horns; spherical waves; dielectric feeds; corrugated horns; array feeds; prime-focus waveguide feeds; cylindrical waves; small axisymmetric feeds; reflector antennas; dipole feeds; aperture radiation; resonator type feeds; dielectric-loaded horns; pure-mode horns; lens-corrected horns
Subjects: Waveguides and microwave transmission lines; Antenna accessories; Numerical analysis
- Book DOI: 10.1049/PBEW039E
- Chapter DOI: 10.1049/PBEW039E
- ISBN: 9780852968093
- e-ISBN: 9781849193955
- Page count: 500
- Format: PDF
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Front Matter
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1 Introduction
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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.
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2 Reflector antennas
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This book is concerned primarily with the characteristics of feeds, since feeds are the source of illumination for reflector antennas in the transmit mode and conversely on receive, it is important to understand the main characteristics of reflector antennas.
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3 Radiation from apertures
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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.
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4 Numerical analysis using cylindrical and spherical waves in horns
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This chapter now deals with the internal region of a horn where the transverse electric and magnetic fields across the aperture plane are formed (Figure 4.1). The results of the internal analysis predict the aperture fields and possibly also the reflection coefficient at the input terminals of the horn. If the internal fields are known accurately, the radiation characteristics and the impedance characteristics can be computed more accurately. Thus the aim should be to analyse the internal characteristics of the horn as accurately as possible.
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5 Numerical analysis of small axisymmetric feeds
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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.
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6 Pure-mode horns
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Pure-mode horns radiate from an aperture which contains a single mode and the radiation patterns are almost entirely determined by the transverse fields of the mode and not by the surrounding structure. The most common examples are pyramidal horns and conical horns. Although simple in concept, pure-mode horns do not have good electrical characteristics and for this reason their use as feeds for reflectors is largely restricted to array elements.
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7 Prime-focus waveguide feeds
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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.
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8 Multimode horns
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This chapter is concerned with horns which are designed to radiate or receive more than one mode. The properties of the different modes are used to obtain radiation patterns which either enhance the patterns of the dominant mode or provide extra information at the horn terminals. The aim of this chapter is to explain the fundamental operation of multimode horns and to describe how the modes can be generated. Most of the design effort in multimode horns is concerned with the waveguiding properties of the horn. It is relatively straightforward to understand the radiation patterns needed for a given application, but more difficult to ensure that the multiple modes are generated and propagate correctly along the horn.
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9 Corrugated horns
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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.
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10 Dielectric-loaded horns
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This chapter studies conical horns partially loaded with a dielectric cone. These support hybrid modes similar to those for a conical corrugated horn. The consequence is that they can have the same desirable radiation characteristics as corrugated horns. The dielectric-cone-loaded horns are, in principle, simple to manufacture and can be used as high-performance feeds for reflector antennas, particularly at millimetre wavelength where the conventional corrugated horn is difficult to construct. Developments in plastics technology hold the promise of producing high-quality dielectrics, some of which will be space-qualified at reasonable cost.
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11 Lens-corrected horns
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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.
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12 Dielectric feeds
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Dielectric materials to the design of reflector feeds adds an additional parameter for optimisation. Several advantages can be gained, the most important of which is the fact that dielectric rods can support the HE11 mode and consequently improve the pattern symmetry. To benefit from this property of dielectrics, the feed horn must have a reasonable size, in aperture or length, to support the excitation of the HE11 mode efficiently.
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13 Dipole feeds
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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.
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14 Microstrip and resonator type feeds
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One of the difficulties with microstrip antennas is the resistive losses due to the conductor and its substrate, which become significant at high frequencies. In an attempt to reduce these losses, Kishk et al. have studied the use of dielectric resonators as a candidate for reflector feeds and in lieu of microstrip patches. Feed performances similar to those of microstrip antennas are obtained. In this chapter we provide computed and experimentally verified design results for both microstrip and resonator-type feeds. Their performance with reflector antennas is also discussed.
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15 Array feeds
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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.
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Back Matter
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