Complex Permittivity Estimation From Millimeter-Wave Radiometry

Millimeter-wave (MMW) radiation characteristic has been used in object classification and recognition. Complex permittivity is a key factor affecting the radiation characteristic. Previous study has shown that estimation values of complex permittivity based on multipolarization measurements spread around a special curve (called "C-curve"). In this letter, we analyzed the influence of complex permittivity on MMW radiation. We find that "C-curve" is the manifestation of the solution instability and can be avoided by our new estimation method based on the measurements of multiple incident angles. The simulation and experiment have proved the validity of our method. During research, we also find that objects whose complex permittivities are near the "C-curve" are hard to distinguish by MMW radiometry, unless we measure their radiation of vertical polarization near the Brewster angle. This finding has the guiding significance for object classification, recognition, and information acquisition.