Measured and predicted performance of a compensating subreflector

The intensity and phase scattering patterns of a profile error-compensating mirror, 30 cm in diameter, have been computed at 34 and 68 GHz. The mirror had been used previously between 28 and 40 GHz to compensate for errors on the surface of a paraboloid 2.8 m in diameter. The results are compared with measured patterns and also the patterns needed to achieve maximum phase compensation. Good agreement was obtained between the measured and computed patterns. Generally the mirror compensated successfully for profile errors with correlation intervals greater than D/10. The residual phase differences between the required and computed patterns were consistent with those deduced previously from the performance of the complete compensated antenna. At 34 GHz they amounted to about 20° r.m.s., giving a shortfall in gain of (0.5 ± 0.3) dB. Results at 68 GHz suggest that the reflector could be operated efficiently at much higher frequencies.