Anomalous results from PO applied to reflector antennas: the importance of near field computations

In the design of reflector antennas the physical optics (PO) analysis technique is the most popular numerical technique. Powerful computer codes are available for the analysis of single or multi reflector antenna systems. Additionally, ever-increasingly demand on the antenna performance necessitates the computation of antenna far field patterns under various situations. For example, in using multi-reflector antennas such as Gregorian or Cassegrain, it may become necessary to determine the total fields including the feed radiation pattern, subreflector scattered pattern and the main reflector scattered pattern. In these situations, the common practice is to sum up various scattered fields and the incident field contributions to obtain the desired total field. It is the purpose of this paper to demonstrate that the typical approach based on the far field pattern of the feed would result in an erroneous result and special care must be exercised to obtain the correct result. This is demonstrated through a detailed investigation of a representative test case.