The paper provides a review of current research on microwave reflector antennas with particular regard to those antennas which produce pencil-beam radiation patterns. After a preliminary examination of microwave antennas of different types attention is focused on circularly symmetric antennas with axially symmetric feed systems. This class of antenna accounts for the largest number of applications which include microwave point-to-point communication, satellite communication, and radio astronomy. The choice of optimum feed is considered in more detail and it is demonstrated that metallic feeds with corrugated walls represent the best choice in most cases. Brief reference is also made to dielectric cone feeds which may find a place in future systems. The design of dual-reflector systems is considered in depth and different computer optimisation techniques are reviewed. These include the latest methods of diffraction optimization in which more than one of the antenna specifications is involved. An example is given where efficiency and VSWR are simultaneously optimized.

High-efficiency microwave reflector antennas&#8212;A review

Reflector antennas have been used since the radio pioneering era of Lodge, Hertz, and Marconi, but it took the exigent demands of radar in World War II to stimulate a real development in the reflector art. Subsequent interest in the science of radio astronomy and the inception of microwave ground communication links were responsible for a burgeoning growth in the field, so that in the 1940s and 1950s the design principles and requirements for prime focus fed systems were well established. Cassegrain, or secondary focus systems, and horn reflectors came into prominence in the early 1960s with the advent of satellite tracking and communication networks. The desire to maximize the gain, or the gain-temperature ratio, then led to development of sophisticated techniques for properly shaping the illumination over the reflector aperture in order to maximize efficiency and minimize spillover, among them being the shaping of the sub-reflector in Cassegrain systems and the use of multimode and hybrid mode feed horns. Not all reflector antennas utilize paraboloidal surfaces. Some recent developments in line source feeds make the spherical reflector attractive for scanning applications and the conical reflector for deployable, space-borne antennas. The large 1000-foot diameter reflector at Arecibo is a well known example of the former. Although some extremely large spaceborne reflector antennas have been proposed and studied, the largest now in use appears to be the unfurlable 30-foot reflector carried by ATS-6. Finally, some gain comparisons are given for a few of the (electrically) largest reflectors that have been built both for radio astronomy and for space communications. If some milestones in reflector development have been overlooked it is due to the limitations inherent in a review paper.

Some highlights in reflector antenna development

Microwave antennas are often required to carry signals simultaneously over a broad range of frequencies–e.g., the combined td-2 and th common carrier bands encompass a total frequency ratio of about 1.8 to 1 as do the combined 18- and 30-GHz bands. To achieve these bandwidths, an efficient broadband feed horn is required. The corrugated (hybrid-mode) horn is a leading candidate, but it is not immune to some cross-polarization coupling, input reflection, and pattern asymmetry. These problems are introduced mainly by two phenomena: variation of the dominant mode shape with frequency and mode conversion along the horn taper and at waveguide transitions at the horn input. Simple formulas for computing the magnitude of these phenomena and their effects on return loss and radiation patterns are given.

Reflection, transmission, and mode conversion in a corrugated feed

A corrugated feed with nearly ideal radiation characteristics from 17 GHz to 29 GHz has been built using a novel fabrication technique. The bandwidth of single-mode operation was maximized by properly choosing the input parameters of the feed. As a result, only the fundamental mode can propagate at the input from 19 GHz to 28.8 GHz. In this frequency range, the far field is essentially polarized in one direction. At frequencies higher than 28.8 GHz, there is a cross-polarized component caused by an unwanted mode. An approximate calculation of the power in this mode is given. A simple formula for the input reflection coefficient is provided. Results are included that show how to compute mode conversion in a conical taper, cross polarization from a corrugated horn, including contributions from spurious modes, and the reflection coefficient from the smooth-guide to corrugated-guide transition. Comparison of theory and experiment shows good agreement.

Characteristics of a broadband microwave corrugated feed: A comparison between theory and experiment

Properties of cutoff corrugated surfaces for corrugated horn design. [corrugation shape and density effects on scattering

A simpler solution for spherical hybrid modes in corrugated conical horns has been shown to have a deviation from the rigorous solution of less than 0.7 dB for the case considered by Clarricoats. Expressions for the radiation pattern and gain of such a horn with small flare angle have been obtained under balanced hybrid conditions.

Hybrid modes in corrugated conical horns

Expressions for the diffracted far field of a wide-flare-angle corrugated conical horn (operating on the HE11 mode) have been obtained making use of a simpler expression for the aperture field. Expressions for the radiation pattern and gain reduce to a closed form when the flare (semivertical) angle is less than 30°. The theoretical results are in close agreement with experiment.

Diffraction by wide-flare-angle corrugated conical horns

This letter describes a systematic procedure for calculating the half flare angle of a conical scalar horn, illuminating a paraboloidal reflector of known f/D ratio, with a view to realising a prescribed sidelobe level for the secondary radiation.

M.S. Narasimhan

Papers

Hybrid modes in corrugated conical horns

Diffraction by wide-flare-angle corrugated conical horns

Paraboloidal-reflector illumination with conical scalar horns