The authors have calculated the off-axis receiving pattern of a Cassegrain reflector antenna with a prescribed geometry as function of antenna azimuth in the absence as well as in the presence of a half-plane obstacle. The model used is based on GO and UTD and includes 21 wave contributions to calculate the total field at the feed. These contributions are individually traced through the antenna system and are spatially weighted by the feed pattern assumed. The ray analysis indicates that some of the individual contributions can be neglected to simplify the model. The model proposed analyses the combined problem of obstacle and antenna diffraction since in many cases the obstacle is in the near field of the earth-station antenna. Results of the additional attenuation caused by the presence of the obstacle on the propagation path of an interference source are given.
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