Fast and Robust Characterization of Lossy Dielectric Slabs Using Rectangular Waveguides

Abstract

Waveguide characterization of dielectric materials is a convenient and broadband approach for measuring dielectric constant. In conventional microwave measurements, material samples are usually mechanically shaped to fit waveguide opening and measured in closed waveguides. This method is not practical for millimeter-wave and sub-millimeter-wave measurements where waveguide openings become tiny, and it is rather difficult to shape the sample to exactly the same dimensions as the waveguide cross section. In this article, we present a method that allows one to measure arbitrarily shaped dielectric slabs that extend outside waveguides. In this method, the measured sample is placed between two waveguide flanges, creating a discontinuity. The measurement system is characterized as an equivalent -circuit, and the circuit elements of the -circuit are extracted from the scattering parameters. We have found that the equivalent shunt impedance of the measured sample is only determined by the material permittivity and is rather insensitive to the sample shape, position, sizes, and other structural details of the discontinuity. This feature can be leveraged for accurate measurements of permittivity. We provide an analytical extraction formula for thin-layer dielectric samples. For thick layers, a numerical optimization method based on a feed-forward neural network is introduced to retrieve the permittivity. The proposed method is very useful for measuring the permittivity of medium-loss and high-loss dielectrics from microwave to subterahertz frequencies.

An improved waveguide method for accurate complex permittivity measurement of medium/high-loss material

“…In addition, the deposited conducting paste is difficult to remove, which does not facilitate repeat measurement of samples. Unlike using a closed waveguide, [22] proposes an approach that allows one to measure the sample outside waveguides to avoid the air gap problem. Due to the effects of radiation, the method is only valid for samples thinner than 1/10 of the wavelength.…”