Comparison of spillover loss of offset Gregorian and Cassegrain antennas

The author discusses comparison of Gregorian and Cassegrain antennas with respect to spillover loss and selection of subreflector optics. It was shown that the Cassegrain antenna with the bottom arm has spillover noise lower by at least 5 K at elevation angles 30 degrees and lower as compared to the Gregorian antenna with the arm at the top. Also, from the viewpoint of maintenance, accessibility to the receivers, the feeds, and the subreflector is easy with the arm at the bottom, with the telescope stowed at low elevation angles. However, the National Radio Astronomy Observatory has chosen the Gregorian antenna for the following reasons: first, with the arm at the top, the elevation bearings of the telescope need to be located at a lower height from the ground and, hence, the elevation tower is shorter, saving $3 M to $4 M in steel. Secondly, with the Gregorian antenna, the subreflector can stay installed permanently on the arm, even when prime focus feeds are used. The excess spillover noise of the Gregorian antenna can be reduced by about 2 K by resorting to a feed taper of -15 dB.<>