The determination of the fields reflected from a twist reflector following illumination by a plane wave

In polarization-twisting Cassegrain antennas, there is a direct interaction between the feed and the twist reflector that should be taken into account in any analysis of the antenna system. The purpose of this investigation is to show how this may be achieved. In general, the twist reflector can be considered in the far field of the feed, and this aspect of the antenna analysis is therefore in essence one of determining the response of the twist reflector to illumination by an arbitrarily polarized plane wave at arbitrary angles of incidence. In particular, within the context of a ray theory analysis, the response of the twist reflector must be determined for relatively large angles of incidence. Computations are carried out to determine the amounts of co- and cross-polar energy generated by a typical twist reflector operating over a specified frequency range at various angles of incidence. Good agreement is obtained with measured values in a variety of situations. In all cases, the electric vector in the incident field is assumed to have unit amplitude and the co-polar energy is taken to be that proportion of the incident energy which, on reflection, is polarized in the direction of perfect twist.