Discussion of the Transmission Efficiency of Quasi-optical Reflector Antenna Across Various Levels of Feeder Performance

For microwave radiometer calibration, higher main beam efficiency reduces interference from environmental brightness temperature (BT). A key challenge for high-frequency microwave sounders is achieving high main beam efficiency with limited mirror size. Although efficiency has improved to around 95-98%, there is still room for improvement. This study investigates the main beam efficiency of ground-based radiometer antenna systems, considering feeders with different radiating qualities and mirror conditions. A quasi-ellipsoidal mirror with feeders operating at multiple discrete frequencies is designed to optimize the optical path for microwave calibration. The key focus is transmission efficiency, which affects BT leakage. Numerical results for various feeders are compared - straight wall corrugated horns with 98.2% Gaussian content, dual-mode horns with 98.5%, curved wall corrugated horns with 99.8%, and ideal Gaussian beams - highlighting achievable transmission levels in different polarizations with different feeders. Design curves for the high main beam efficiency quasi-optical reflector antenna system are provided. Such information is beneficial and of direct referencing value for practical radiometer reflector antenna designs.