The geometric optics condition for zero crosspolarisation in offset Cassegrain antennas is discussed. It is shown that with a feed having perfect pattern symmetry the crosspolar performance of the offset Cassegrain is limited by diffraction. As a result minimum crosspolarisation is only achieved at the geometric optics condition when the subreflector diameter is at least 40 wavelengths.
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Trevor S. Bird (1981) studied this question.
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