Complex permittivity measurement of solid and non-solid materials at millimeter wavelengths

This paper describes a free-space measurement method to determine the complex permittivity of dielectrics. A dielectric container comprising two plane-parallel slabs with known properties surrounds a central slab with unknown dielectric properties. The dielectric stack rotates in a free-space planewave measurement system, and we record the transmittance as a function of incidence angle. The best-fit of theoretical transmittance to measured transmittance yields the properties of the unknown dielectric. Experiments are performed at 218 GHz to validate the theory. An uncertainty analysis for the specific measurement geometry in this paper is described in detail, producing a combined uncertainty of 1 % in both the real and imaginary portions of the complex permittivity.