A millimetre wave, quasi-optical complex impedance bridge

Abstract

This thesis describes the work undertaken in the design and construction of a millimetre wave, quasi-optical impedance bridge. The instrument operates as a nulling complex reflectometer, with nulling being implemented at the signal frequency by interferometric optical techniques. In principle, the quasi-optical components of the reflectometer are operable in the frequency range 80 GHz to 500 GHz, though the range of continuous coverage is determined by the waveguide components and antennas used to couple sources and detectors into the Gaussian beams. Gaussian beam mode theory has been employed in the design and measurement of the quasi-optical components, which behaved as expected. Phase and polarisation effects within the impedance bridge were modelled and the consequent behaviour of the impedance bridge predicted. A comparison of these predictions with early results led to refinement of the model. This model is based upon a more complete description of the transmissivities and reflectivities of the wire grid polarising beam splitters and accurately predicts the instrument behaviour. A data acquisition and control system has been built to facilitate automated operation of the impedance bridge. This system is interfaced to an Acorn® Archimedes microcomputer and the hardware and software has been developed to enable interrupt driven data acquisition. The reflectivity of a quasi-optical load has been measured at 85 GHz with an accuracy and standard deviation of better than 0.1 % In amplitude and a standard deviation of better than 0.10° in phase. The quasi-optical half cube and computer interface components developed in this work form the basis of a stock of general purpose modules now in everyday use at St. Andrews.