Analytical Prediction of Feed Efficiency in Offset Gregorian Reflector Antennas with Non Planar Log-periodic Type Feeds

摘要

Non-planar log-periodic antennas are finding increasing application in wide band reflector antenna systems due to their excellent wide band performance in terms of input impedance and pattern stability. However, the frequency dependent phase center position will cause defo-cussing and therefore a loss in aperture efficiency when they are used in a wide band reflector antenna system. A popular reflector system in high performance systems, such as radio telescopes, is the offset Gregorian reflector antenna due to the high levels of efficiency that can be obtained by eliminating feed or subreflector blockage from the system. Unfortunately, subreflec-tor edge diffraction effects still cause a loss in efficiency, and these should be included to obtain accurate performance predictions. This paper presents a technique to get an initial estimate of the performance of an offset Gregorian reflector antenna fed by a non planar log-periodic antenna by using closed form expressions and simple one dimensional integration of analytical functions. An analytical function is used to approximate the radiation pattern of the feed antenna, and this approximated pattern is used to calculate the efficiency of the reflector antenna system. The frequency dependent phase center position of the feed antenna as well as subreflector edge diffraction is included in the calculations. The results compare favorably with physical optics simulations of the structure, and some simple design guidelines are derived.