A note on reflector-feed element realizability and optimality

Reflector-antenna design has reached a mature state of development. Sophisticated techniques, such as the use of array feeds for the correction of surface distortion, are now available. With sophistication, there is the danger that designers will lose sight of fundamental principles. In reviewing a manuscript submitted to the IEEE Transactions on Antennas and Propagation, on the subject of array feeds for reflector antennas, the author found that the numerical results presented in the manuscript were based on an element-pattern/spacing relationship that was unrealizable. The present note may help to avoid such difficulties, by presenting simple formulas that relate the realizability and optimality of feed antennas to reflector f/D, i.e. the ratio of focal length to diameter. The author discusses the optimality of feed elements in terms of a maximum-gain criteria for the reflector antenna, and considers the question of the realizability of an array-feed element in terms of its minimum aperture (i.e. spacing) requirement