Complex Permittivity Characterization of Low-Loss Dielectric Slabs at Sub-THz

This paper presents a novel low-complexity method to characterize the permittivities of low-loss dielectric slabs at millimeter-wave and sub-terahertz bands. The method assumes plane-wave illumination of the material under test and utilizes reflected fields on two air-material interfaces, resolved through a sufficiently wide-band measurement. Unlike conventional methods, it does not need a reference measurement using a metal plate and a back metal plate. The applicability of the method is discussed. The misalignment of a reference plane and the effects of the gap between the back metal and the material under test are analyzed for published reflected-field based methods to show the advantages of the method. Finally, the proposed method is applied to estimate the permittivities of both plexiglass with 30 mm thickness and nylon with 21 mm thickness across 140-210 GHz and compared against the free-space transmission method, showing good agreement. The proposed method is suitable for permittivity characterization of low-loss materials when a wide-band plane-wave measurement is possible.