The high-frequency limit of reflector antennas is usually governed by the magnitude of the surface error. Whereas little can be done for the high-spatial frequency portion of this error, there are various techniques that can be employed to compensate for large-scale surface errors due to gravity induced distortions for ground antennas or thermally induced distortions for spacecraft antennas. This somewhat tutorial paper describes various techniques that can be utilized to compensate for the gain-loss due to these distortions. The techniques described are 1) Main reflector compensation, 2) Subreflector compensation, 3) Use of a deformable flat plate in the optics path and 4) an Array feed compensation system. Examples are give for the use of each technique along with an estimate of the expected performance improvement.
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