This thesis describes work on the theoretical modelling and experimental measurement
of the complex permittivity of dielectrics. The main focus of research has been into the
characterisation of permittivity of planar and layered samples within the millimetre
wave band. The measurement method is based on the free-space measurement of the
transmission and reflection coefficients of samples. A novel analytical method of
determining the transmission and reflection coefficients as functions of frequency
arising from a generalised structure of planar dielectric layers is also described and
validated. The analytical method is based on signal flow techniques. The measurement
and analytical techniques have been applied in two main areas: firstly, the acquisition of
new data on human skin in the band 57 to 100GHz and secondly, the detection and
location of defects in composite materials for which a band of 90 to 100GHz was used.
Measurements have been made on the complex permittivity of a single sample of
excised human skin fixed in formaldehyde. The experimental results have been
corrected to account for the fixing process in formaldehyde and are projected to body
temperature. This data is, to the best of the author’s knowledge, the first of its kind to be
published. Predicted skin permittivity based on various relaxation models varies widely
and only partially fits the measured data. The experimental results have been used to
determine the parameters of a Cole-Cole function which gives the best fit to the
measured data. The measured skin data has also been used to calculate power deposition
in skin exposed to millimetre wave radiation. This work concludes that a skin surface
temperature rise of only 0.20C results from a thirty second exposure to signals of
100W/m2.
Experimental work with fibreglass composite samples has shown that defects such as
delaminations, voids, matrix cracks and improper cure result in resolvable differences in
the dielectric properties of the samples at 90 – 100GHz. The measurement technique is
particularly sensitive to the detection of cracks and its spatial resolution is 20mm or
better. Whilst confirming the general conclusions of previously published work, the
specific findings of this study are novel.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/251 |
| Date | 03 1900 |
| Creators | Alabaster, C M |
| Contributors | Dahele, J S |
| Publisher | Cranfield University, College of Defence Technology; Department of Aerospace, Power and Sensors |
| Source Sets | CRANFIELD1 |
| Language | en_UK |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
| Format | 1883 bytes, 2587847 bytes, text/plain, application/pdf |
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