Asymptotic methods for the prediction of dual-reflector-antenna efficiency

Evaluation of the image-region field from dual-reflector antennas using the physical-optics approximation is impractical when the main reflector is large. Because of this, efficiency calculations have previously relied on geometrical optics to provide the scattered field from the main reflector. A more realistic assessment of this scattered field is obtained by the inclusion of edge diffraction from the main reflector. This can be derived either from the geometrical theory of diffraction or by an asymptotic expansion of the physical-optics integral for the main reflector. For the present application, where the scattered field is required on the subreflector surface, these two methods are shown to be comparable. It is shown that the calculated efficiency for a dual-reflector antenna capable of beam scanning can be significantly modified by the inclusion of this edge-diffracted field. Evaluation of the image-region field from dual-reflector antennas capable of beam scanning can be significantly modified by the inclusion of this edge-diffracted field.