Quasi-optical Measurement for Low Loss Material Characterization in Submillimeter Wave Range

An accurate knowledge of dielectric constant of materials is required in many sub-millimeter wave applications. Free-space measurement of materials has always been one of the first choices due to their non-destructive nature and relatively simple sample preparation. However, free-space measurement systems at sub-millimeter frequency range suffer from two main problems: high loss because of divergent beam pattern and diffraction when the beam waist of the radiated beam is relatively large compared to the size of the sample under tests. In order to mitigate these issues, we set up a quasi-optical system using off-axis parabolic mirrors, which enhance the dynamic range and accuracy of the system. Mirrors are preferred over lenses because of their very low absorption loss. Signals from 75 GHz to 750 GHz are generated by using a vector network analyzer and a frequency extender system in six bands. Our setup consists of a pair of horn antennas operates as feed for two off-axis parabolic mirrors. Once a collimated beam is generated, we measure its characteristics using Gaussian beam mode analysis. The sample under test is then inserted midway between the two mirrors where the beam is collimated. We have measured electric permittivity of material like high-density polyethylene and polypropylene and our results highly agree with already published papers. In the future, we would like to measure some less well-known materials to see if they are suitable for sub-millimeter electronics applications.