Thesis (M. Sc.)--University of Hong Kong, 1968. / Mimeographed.

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Thesis (M.Sc.)--University of Hong Kong, 1968. / Also available in print.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, Dec. 2004. / Thesis Advisor(s): David C. Jenn. Includes bibliographical references (p. 75-76). Also available online.

This thesis describes the design, construction and results of an accurate, 73 GHz, dual-polarized atmospheric propagation experiment conducted over a 1.8 km total length radar path. The millimetre-wave equipment consisted of a switched-polarization transmitter and a two-channel receiving system which included a phase-compensated crosspolar cancellation network and a novel, high-performance microstrip IF/LO diplexer. Meteorological instrumentation consisted of an improved electrostatic disdrometer, a raingauge network with high temporal and spatial resolution and a three-vector anemometer. A comprehensive experimental model was developed to predict the system crosspolar discrimination (XPD) response during a wide variety of conditions. This model was used to analyze, for what is believed to be the first time, the effects of: orthomode transducer port mismatches, the frequency response and error sensitivity of crosspolar cancellation systems and the range of possible cancelled system XPD responses during rain. This model also led to the development of a phase compensation technique used to improve the stability of the crosspolar cancellation network. The application of the experimental model resulted in far more accurate determinations of path XPD than would have been otherwise possible. The cancelled XPD results showed a reasonable correlation to horizontal wind velocities and agreed with model predictions for effective mean canting angles ranging between 0 and 6°. The frequent observation of negative differential attenuations and erratic uncancelled XPDs led to the conclusion that drops along the path often did not have consistent shapes and canting angles. This is believed to be due to extremely variable wind conditions. Copolar attenuations considerably lower and higher than expected from the standard predictions were observed. The higher attenuations are satisfactorily explained as resulting from vertical wind conditions and are correlated to the predictions from a proposed model which includes the effects of constant vertical wind velocities. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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An investigation was conducted of the existence of seismic surface pulses (SSP) on crack faces in near-failure fatigue. An SSP has components of various modes of wave propagation. The component with the largest amplitude is a Rayleigh surface wave pulse. The possibility that these surface modes have much higher amplitudes than bulk modes of acoustic emission (AE) was illustrated by an idealized thought experiment relating an SSP on a half-space to the response of crack faces to crack extension. A number of aspects of AE monitoring in finite objects were investigated. Attributes of surface wave propagation on the edge of a specimen were found to be easier to monitor than other modes of wave propagation. Wavelet analysis was used to compare the characteristics of brittle AE with other sources. A new testing paradigm was developed to reduce interference from secondary sources of AE and enhance the investigation of AE from critical crack behavior. Unique specimen design features were developed, data acquisition features sought and validated, a dead weight load frame was modified, and data analysis procedures were developed. Criteria based on velocity, frequency content, amplitude and shape were devised to determine if an AE event is an SSP. The tests were designed to mimic load conditions on structures such as bridges and hence investigate the difference between AE generated in field conditions and that of typical laboratory conditions. Varieties of steel, from very ductile to very brittle, were tested. It was concluded that plastic zone formation, considered a secondary source of AE, was found not to interfere with the SSP activity. The SSP was found experimentally to have 2-3 times the amplitude of the bulk wave AE. The lack of sufficient AE did not allow for determination of conclusive changes in the AE as the specimens approached failure. However, it was found that brittle crack extension in fatigue and ductile failure can produce wave propagation resembling the SSP. / Ph. D.

published_or_final_version / Physics / Master / Master of Science

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