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.
Type
Thesis, PhD Doctor of Philosophy
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