Design and study of a free-space quasi-optical measurement system.

This thesis describes a free-space quasi-optical measurement system designed to characterise dielectric materials at Ka band. The measurement system consists of a pair of colinear horn-fed lenses between which the sample under test is inserted. A study is performed in order to determine the sensitivity of the structure to misalignments, which reveals that care must be taken to ensure constant exact positioning of the components of the measurement system. Reflection and transmission coefficients (S-parameters) of the sample are obtained using a two-port vector network analyser connected to the feed horns. Calibration of the system is performed using a free-space through-reflect-match (TRM) technique, which is preferred over other calibration techniques because it does not require the movement of any component of the system during the calibration process. Correction terms must be applied to the reflection and transmission coefficients because of errors due to the misplacement of the sample and the calibration procedure. The sample's dielectric constant is then extracted from the corrected reflection and transmission coefficients using one of two different numerical techniques: a root-finding algorithm and a genetic algorithm. Results obtained with both extraction techniques were found to be similar. Moreover, these results are in good agreement with published manufacturer data and with measurements performed by an independent organisation using a resonance technique. This work demonstrates the feasibility of a low-cost, functional and flexible quasioptical measurement system for characterising dielectric materials at Ka band