P.J.B. Clarricoats

Dual-Depth Corrugated Horn Design

The dual-depth corrugated horn gives low cross-polarisation at two separate operating frequencies. A space harmonic formulation has been used to study the influence of the corrugation geometry on the cross-polarisation characteristics. Results show that about seven slots per wavelength are needed for low cross-polarisation and that operation with frequency bands separated by two to one is practical.

Detction of Buried Objects by Microwave Means

A system is described for the detection of non-metallic buried objects by microwave means. It utilises an FMCw radar operating in the range 2-4GHz and incorporates a forward-looking focussed beam antenna. This gives 26dB improvement in signal/clutter over a downward-looking antenna sysem. Experimental results have been obtained with different targets which include buried plastic boxes and plastic pipes. The design of an optimum antenna configuration is discussed.

Optimum Design of Corrugated Feeds for Low Cross-Polar Radiation

The paper describes the prediction and measurement of the cross-polar radiation from corrugated feeds. The influence of the flange, periodicity of the corrugations and corrugation depth is identified and design recommendations are made. Satisfactory agreement between experimental measurements and theoretical predictions are demonstrated for feeds which are suitable for use at the prime focus or secondary focus of a reflector antenna.

Corrugated Waveguide Feeders for Microwave Antennas

The use of corrugated waveguide as feeders for microwave antennas is discussed. They offer the advantage of a potentially lower attenuation than smooth wall circular waveguides. As circular corrugated waveguide feeders are slightly overmoded, we describe results showing that the mode conversion due to bends has negligible affect on the dominant mode radiation pattern. We also discuss a means to provide continual mode suppression. Finally, the cross-polarisation properties of corrugated waveguides are found to be intrinsically superior to those of smooth wall waveguides when the waveguide wall suffers elliptical deformation.

A Dielectric Cone Feed for Broadband Low Crosspolarisation Applications

The paper describes the crosspolar radiation characteristics of dielectric cone feeds. It is shown that they have a potential of much broader bandwidths than the corrugated horns. However, to achieve this potential it is necessary to reduce the crosspolar radiation from the launcher and a novel method of achieving this objective is suggested.

Antennas for Satellite Systems

The paper begins with a review of satellite systems for communications and broadcast and briefly alludes to the significant scientific space programmes which act as a technology driver for the industry. Progress in satellite antennas is considered under the headings array-fed reflectors and shaped-reflectors. It is in the second area that progress is most rapid with the potential for some national beams achieved by this means in the foreseeable future. The problem of reconfigurability is addressed for both classes and very recent work holds promise for making even reflectors reconfigurable. The paper ends with a review of progress in DBS receiving antennas where considerable ingenuity has led to a variety of solutions; those offering the highest efficiency at lowest cost will ultimately prevail. Overall, the trend is towards increased use of computer-aided design and computer-aided-manufacture in the antenna industry. These techniques offer exciting new prospects and the author concludes that there is much to challenge the satellite antenna designer well into the 21st century.

Low Cross-Polar Radiation from Wide Flare-Angle Conical Corrugated Horns

The prediction of the radiation pattern of conical corrugated horns is described using the spherical-mode expansion method. In particular, cross-polar patterns are obtained for several horns and excellent agreement with measured patterns is demonstrated. Three mechanisms of mode conversion are identified and one in particular investigated in detail. It is shown that mode conversion along the horn is not a significant effect and that the peak cross-polar radiation from a 30° corrugated horn lies below - 45 dB over more than a 10% bandwidth.