A Novel Rotation-Based Standardless Calibration and Characterization Technique for Free-Space Measurements of Dielectric Material

This article presents a novel transmission-only calibration technique for free-space quasi-optical material characterization, based on rotating the sample around its axis to vary the angle of incidence under which the sample is illuminated. In contrast to common time domain approaches, each frequency point is evaluated individually. Thus, no minimum bandwidth is required and artifacts due to time gating are prevented. In this article, two methods are presented: the first is based on self-calibration, such that all error terms are obtained by the measured sample itself. The second one, which is tailored for thin samples, requires two known standards. Since plane-wave illumination cannot be assumed for highly-focused beams, an analytical model for the coupling of arbitrary paraxial beams is developed, accounting for the lateral beam shift in case of angled samples. Thus, the presented methods are not restricted to free-space beams with high Gaussicity, allowing to employ a variety of feed antennas. Measurements in the frequency range from 220 GHz to 330 GHz of a well-known alumina sample verify the different calibration methods.