Completed Near-Field Reciprocity Relationship for Brightness Temperature Transfer and 3-D Modeling Investigation

This work proposes a method for modeling brightness temperature (BT) transfer in the near-range calibration procedure of microwave radiometers. The approach allows 3-D analysis of the BT at the antenna that received from the calibration target. Unlike classic far-field methods, the proposed model allows direct evaluation of near-field characteristics in the calibration links. The theoretical foundation of the model relies on the derived near-field reciprocity relationships between the incoherent radiation of lossy materials and the coherent radiation of the antenna. To deal with the realistic absorbing materials, both electric and magnetic losses are incorporated into the model. The method is first validated and analyzed at the simulation level by verifying reciprocity and comparing results with analytical solutions. Subsequently, a fundamental issue in the calibration process is investigated: the optimal form matching between the typical calibration targets and the antenna. A comparative study is conducted within a 3-D full-wave simulation scenario, on the near-field radiation and far-field Lambertian radiation characteristics of pyramid array and coated cavity, along with their BT transfer performance under different receiving antenna configurations. A validation experiment is designed to observe the concerned near-field effect in practice. Results demonstrate that the cavity exhibits reduced transferred BT when received by a diffuse antenna at close range. The proposed model provides a new approach for analyzing near-field characteristics in calibration links. It is expected to open new avenues for optimizing calibration procedures in practical radiometric applications.