Reciprocity-Based Modeling of Microwave Brightness Temperature Transfer in Near Field, From Calibration Target to Antenna

Brightness temperature (BT) transfer in near field region is the typical physical process in the microwave radiometer calibration procedure. In this work, the authors report the numerical framework for calculating BT transfer from the calibration target to antenna inversely based on near field concepts. Specifically, the near field reciprocity is derived between incoherent thermal radiation from the calibration target and the coherent radiation from the receiver antenna, based on the fundamental mode property at the antenna port, which is common in real-world applications. Then, the reciprocity-based calculation routine can be established in the near field, including both the antenna and calibration target in the same computation region. The reciprocity-based calculation is then verified and discussed versus direct calculation results, in the cases of homothermic and heterothermic, close/open scenarios. The reciprocity-based calculation routine has wide potential applications in analyzing realistic calibration problems.