Modeling of blockage effects in axi-symmetric dual reflector systems

The simplicity of axi-symmetric reflector systems has led to their use in many terrestrial and satellite communications, especially in pencil beam applications. The major short-comings of a symmetric reflector are the degradation from the blockage and the uncertainty in predicting the degradation very accurately. This uncertainty can be more pronounced in shaped beams used in satellite applications. In a single reflector system with a prime-focus feed, the blockage effects are very small due to the small size of the feed horn. However, in a dual reflector system with a subreflector and the primary feed, the blockage effects can be appreciable due to the relatively large size of the subreflector. The three primary components to the blockage are: a) the blockage of the subreflector radiation by the feed. b) The blockage of the main reflector radiation by the subreflector. c) The blockage of the main reflector radiation by the supporting structure for the subreflector (usually 3 or 4 struts). An accurate and simple technique for determining the blockage effects is presented in this paper. The method does not require any calculation of shadow areas and so is a uniform technique which is valid over all regions of the radiation pattern.