City Research Online

Abstract

Ideally, a pulsed signal guided through its intended transmission medium, should be subjected to a small attenuation and negligible degradation to its wave shape. In practice, this condition is difficult to achieve, since all transmission mediums will impose some form of interference or distortion on a travelling pulse. Also no practical transmission medium is completely loss-free. The amount of attenuation/distortion imposed on a travelling pulse is a sensitive measure of the transmission performance of the medium.

This thesis reports a novel attempt to investigate the effects of the transient electromagnetic (EM) fields produced by the pulse propagation pattern, and its interactions with single element antenna probes. Also to characterise and measure this distortion accurately, as a pulse travels through an open conductor-plane transmission system. In order to carry out the investigations, new loop probes which act as magnetic- and/or electric-field sensors, were analysed and designed for this purpose. The measurement technique consists of a pulser, purposely built in a coaxial arrangement which was used to generate fast (sub-nanosecond) risetime pulses for injection into the test line. An SF6 insulated hemispherical-electrode pulser was also used as a simulator for an impulse voltage generator. Detection of the pulses as they travelled along the line was by directional couplers employing these B-dot/D-dot loop probes, the directional couplers being placed at strategic points in the earthplane.

Simultaneous electric- and magnetic-field measurements using a single loop antenna element is useful for the near-field electromagnetic measurements where the magnitude and phase of the wave impedance are not known. This type of loop probe, described, is not only useful to measure the polarisation ellipses of the electric- and magnetic-field vectors in the near-field region, but also measurements of the time-dependent Poynting vector so as to describe the energy flow.

By altering the geometry of the loop (as opposed to adding resistors in the loop circuit as reported previously), it was found that the loop exhibited a unique coupling characteristic such that it responded equally to the components of the time derivative electric- and magnetic-fields. This important practical feature is explained.

In addition to electromagnetic pulse (EMP) applications, the versatility of the coupling loop readily lends itself for use in electromagnetic compatibility (EMC) measurements and as a tool for fault detection and location on power systems network. These applications were investigated.

A theoretical concept for pulse propagation on a transmission line, using circuit and field theories, is presented. An attempt is made to link the circuit theory with the field theory, with a view to describe the experimental results. Full mathematical theory of the loop is reviewed in which new equations were developed to satisfy the general loading condition of the loop.

Descriptions and design specifications of the devices used in the experimental studies are given and their performances in transient field measurements have been verified in a number of unique applications. This demonstrates the integrity of the measurement system and the sensitivity characteristics of the directional coupler. With suitable angular adjustments, it was shown that the loop can be made to couple only to a pulse travelling in one direction.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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