A Free-Space Complex Permittivity Test System Based on Gaussian Beam Model in 110-170 GHz Range

In this letter, a free-space method complex permittivity test system using a Gaussian beam model in 110-170 GHz is proposed. In a free-space method test system, accurate extraction of transmission and reflection coefficients is the key to obtaining the complex permittivity of materials. The traditional free-space method approximates the focused Gaussian beam as the plane-wave to invert the complex permittivity. This plane-wave assumption will introduce a test error. In this study, the transmission and reflection coefficients of the Gaussian beam model were derived based on the plane-wave spectrum technology (PWST) and plane-wave scattering matrix (PWSM) theory. Based on this, a material complex permittivity test system and a beam waist radius test system were built at 110-170 GHz. The test results based on the Gaussian beam model and plane-wave model were compared. Meanwhile, the results were compared with the test results of the quasi-optical cavity method. The results prove that the complex permittivity inversion using the Gaussian beam model has higher accuracy. It provides a more accurate theoretical model for the measurement of complex permittivity of materials by the free-space method in the terahertz range.