Reflector surface error compensation in cassegrain antennas

Two focal plane parabolic reflectors 2.8 m in diameter were used to test the feasibility of compensating for reflector surface profile errors by figuring the surface of a Cassegrain mirror. The paraboloids had been designed originally for frequencies less than 7 GHz, and therefore the surface errors had a significant effect on the antenna performance in the 27-40 GHz range. For these frequencies a template could be used to measure the surface profile. The first reflector had a prominent axially symmetrical surface error component. Compensating for this alone simplified the construction of the subreflector and improved the overall aperture efficiency at 34 GHz from 12 percent to 24 percent. The second reflector had a more random distribution of surface errors. Its efficiency was improved from 35 percent to 48 percent at 34 GHz, the improvement in gain varying from 1.1 dB at 28 GHz to 2.3 dB at 40 GHz, the shortfall being within 1 dB of the possible improvement predicted from the measured surface errors. For both reflectors the illumination efficiency set by the feed horn was about 50 percent. The linear cross polarization discrimination on axis increased from 29 dB to 38 dB at 34 GHz. The sidelobe levels and gain improvement obtained suggest that effective rms errors of about 0.45 mm with correlation distances of about 30 cm remained on the reflector surface after compensation, consistent with the estimated accuracy of the measuring and construction techniques adopted.