Geometrical optical characteristics of the Scwarzschild scanning antenna. Comparison with the Cassegrain antenna

Possible geometrical configurations of Schwarzschild antenna systems are investigated. Input parameters to a rapid design procedure are the subreflector subtended angle and the blocking ratio Ds/D. To allow comparison with Cassegrain antennas, simple formulas are developed for the aperture field, the gain factor, co-and crosspolar components of the radiation pattern and the polarisation efficiency of the antenna using geometrical optics (GO). The scan properties are investigated for both lateral feed displacement (as assumed by Schwarzschild) and displacement of the feed over the special locus minimising the weighted phase error in the aperture plane. Comparison with the focused and defocused properties of the Cassegrain antenna leads to the conclusion that, even without leaving the framework of geometrical optics, there appears to be no reason to prefer the more complicated Schwarzschild antenna system.