Doctoral Degrees (Electrical and Electronic Engineering)

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Analysis and synthesis algorithms for the electric screen Jauman electromagnetic wave absorber
(Stellenbosch : Stellenbosch University, 1993-11) Du Toit, Leendert Johannes; Cloete, J. H.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering
ENGLISH ABSTRACT: An extensive literature study revealed numerous Jauman absorber examples with reasonable absorption properties. Unfortunately, tractable and detailed design techniques were found to be scarce, and often only applicable to absorbers with two or three layers. The research described in this report was therefore aimed at, and culminated in, general design methods for multilayered electric screen J auman absorbers. As a starting point, the synthesis problem is formulated by idealizing the spacers (assumed lossless and commensurate) and resistive sheets (assumed to have zero thickness), and by considering the absorption of a normally incident plane wave. An equivalent circuit model is derived, using the analogy between plane waves in stratified media, and guided waves in TEM transmission lines. The network is analyzed using Richard's frequency surrogate, S = tanh(s = cr +jw), and concise equations and algorithms are presented for symbolic and numerical analysis. Maximum bandwidth synthesis of the classic one-layer absorber, or Salisbury screen, proved to be simple, clearly illustrates the analytic approach, and apparently has not been published before. The two-layer absorber was also found to be algebraically simple enough to be synthesized in closed form, is dealt with comprehensively, and the treatment consolidates and formalizes many of the design techniques available in the literature. Networks comprising commensurate transmission lines and conductances have been investigated by Richardsl , but unfortunately the topology-driven realizability constraints on the input impedance of the Jauman network is only dealt with briefly. Fruitless investigations by the author showed this to be a formidable problem, and as a result the research concentrated on tractable and iterative synthesis algorithms for multilayered absorbers, instead of formal filter synthesis techniques. These algorithms may be summarized as follows: • A key concept in the multilayer zero-placement synthesis methods that will be presented, is the ability to physically realize a given set of reflection coefficientzeros. This involves solving a set of highly non-linear equations, and a gradientmethod iterative algorithm has been developed to achieve this . • The first application of the aforementioned algorithm is to synthesize all reflection zeros at S →∞, thereby obtaining a maximally flat reflection coefficient magnitude response. Stable and rapid convergence was found for up to at least 20 layers, thereby extending the two- and three-layer algebraic solutions available in the literature. It was found that a stringent restriction exists on the maximum dielectric constant (Er) of the spacers, thereby limiting the practical implementation of these solutions . • Through judicious manipulation of reflection zeros at distinct physical frequencies, an equiripple absorption response may be obtained. An elegant algorithm is presented to facilitate this, and it was found that these solutions represent substantial improvements over examples available in the literature. Restrictions still apply to the spacer Er, but these are more relaxed and practical equiripple absorbers are possible. In addition, the spread in sheet resistivities is much smaller than in comparable maximally flat solutions. • Numerical searches indicated that the aforementioned equiripple responses are very close to, but not absolutely optimal, in the sense of maximum bandwidth. The small bandwidth and/or absorption improvements that were found were almost negligible from a practical viewpoint, but the optimal synthesis problem is academically very important. Through use of the general Chebyshev approximation method, an algorithm is developed which finds the local optimal response in the vicinity of such a parent equiripple solution. Although it might be tempting to classify the algorithm as a brute force method, it will be shown that this is not the case, and that its solutions provide the answer to the fundamental and unsolved optimal design problem. These algorithms have been implemented, and tables of resistive sheet values are presented for N up to 8, a range of Er values corresponding to low loss foams, and for various absorption levels.
Borehole radar system analysis in stratified geological systems applied to imaging of platiniferous reefs in the bushveld igneous
(Stellenbosch : University of Stellenbosch, 2003-12) Herselman, Paul Le Roux; Cloete, J. H.; Mason, I. M.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
The imaging of platiniferous reefs in the Bushveld Igneous Complex (BIC) is of great economical and sociological importance. Borehole radar technology has been identified as a viable mapping tool to be used in day-to-day mining operations, but a critical assessment has to be made on the feasibility of this postulation. The system analysis made of the borehole radar deployed in the BIC is presented in this dissertation. The analysis is done using a specific example - the GeoMole borehole radar system. A novel procedure, based on the basic theory of electromagnetic radiation and propagation, is proposed by which the entire physical radar system can be characterized. The power transmitted by an unconventional borehole-deployed transmitter is estimated by a sequence of free space measurements, numerical simulations and theoretic derivations and approximations. Antenna transfer functions (magnitude and phase) are numerically simulated for a variety of deployment configurations. The total system transfer function of the receiver analogue and digital chain is determined. This enables the calculation of the radar's performance figures necessary to determine the applicability of the radar in a specific geological setting. A radar system is only complete when considered in its environment. The BIC is a stratified system of numerous rock layers. An in-depth study is done on the propagation of radiowaves in stratified lossy media. Only the case for non-magnetic media is discussed in this dissertation. The developed theory is used to predict the system response to a typical transmitted radar pulse in the UG1 - UG2 stratigraphy of the BIC, determine the maximum detection range of reef horizons and estimate the reflectivity of the reefs. Resolution is one of the key parameters that determine the performance and accuracy of imaging. An algorithm is proposed, developed and tested by which the resolution of the system is increased and overlapping echoes become resolvable. Even though some of the techniques are developed with a specific system in mind, the applicability of the concepts and algorithms is universal.
A close range baseband radar transceiver for application in borehole radar systems
(Stellenbosch : University of Stellenbosch, 2007-12) Van der Merwe, P.J. (Paulus Jacobus); Cloete, J. H.; Van der Walt, P. W.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: A monostatic baseband radar is required with the capability of detecting close range targets that appear at distances comparable to the system’s resolution, without compromising the radar’s maximum range. The application in borehole radar imposes further constraints associated with the physical limitations and variable electromagnetic environment of different borehole diameters and conditions. This dissertation discusses the complete design process of the analog section of a monostatic radar that successfully addresses these issues. The proposed transceiver employs a series duplexing arrangement consisting of an antenna, transmitter, receiver and an isolation switch. An exponentially decaying tail is observed in the current flowing on a borehole radar antenna when excited by pulse waveforms. The characteristics of this tail depend strongly on the borehole environment. A measurement technique is developed that accurately quantifies this exponential decay by digitizing a logarithmic representation of the antenna current while it is operating in various boreholes. Transmitters are then designed to drive these antennas with waveforms that prevent the formation of current tails. This is achieved through the use of pole-zero networks or alternatively by generating certain asymmetric, bipolar waveforms. The transmitters are simultaneously designed to have an output impedance approximating a short circuit after the transient is generated. In the series configuration proposed here, the duplexing of the antenna between transmitter and receiver is then reduced to simply isolating the receiver during transmit-mode. The switch responsible for this isolation disconnects the receiver and presents a short circuit between antenna and transmitter during transmit-mode, while connecting the receiver terminals between the antenna and the short circuited transmitter terminals in receive-mode. The required close-in performance of the transceiver dictates that the transition between these two states of the isolation switch occur in a time similar to the duration of the transmitter waveform. The switching artefacts generated by the switch are consequently similar to the radar data signal. The isolation switch employs an innovative configuration (using both transistors and diodes) which accepts a single control signal and causes the switching artefacts to be generated as a common mode signal, while a differential path is created for the radar data signal which is being switched. This leads to effective suppression of the switching signal in the signal passed to the receiver. Dissipative filtering is advocated as a fundamental design principle for high fidelity receivers and it is shown how it can be applied by using constant impedance equalizers and diplexers as basic building blocks. This principle is used as the basis for the design of this transceiver's receivers, which incorporate both standard gain blocks and operational amplifiers. A complete borehole radar system, based on the transceiver developed here, was built and tested; resulting in the first known practical monostatic borehole radar system. Data obtained in field trials are presented and suggest that the monostatic system compares well with current state of the art bi-static systems.
An experimental study of artificial isotropic chiral media at microwave frequencies
(Stellenbosch : Stellenbosch University, 1994-12) Smith, Anthonie Gronum; Cloete, J. H.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: An artificial chiral medium can be made by embedding metal helices (chiral or handed structures) with random orientation inside au achiral hnst medium. The electromagnetic behaviour of such artificial chiral media can be explained by an additional (to p. and t') constitutive parameter, the chirality parameter e. Chiral media have certain special properties of which optical activity is the most prominent. Optical activity is the term used to describe the rotation of the polarization plane of a linearly polarized wave as it travels through a chiral medium. The chirality parameter is directly linked with this rotation. The reflection and the transmission coefficients from a chiral slab are well known in terms of the constitutive parameters and the thickness of the sample. In the thesis a set of .inversion equations are derived that can be used to determine the constitutive parameters (p., £, e) of a chiral medium from the measured S-parameters 811, S21x and S2l'jJ (Le. the reflection and co- and cross-polarized transmission coefficients respectively). An accuracy analysis of the measurement method is made. This is done by using the first order partial derivatives of the inversion equations. A sensitivity analysis is done on the inversion equations and the results are in an analytical form. This makes it possible to determine the contribution of each of the measurement errors to the total expected error. A statistical root mean square method is used to predict the expected error in the measured constitutive parameters. Several artificial chiral samples are measured and an accuracy analysis done on the measurements. The inaccuracy of the method of inversion for low loss samples that are multiples of half a wavelength thick is illustrated by example and explained by the sensitivity analysis. The free-space system (11-17 GHz) used to measure the S-parameters is described in detail. The radiation patterns of the focused lens antennas are measured and compared to those obtained from two theoretical models. A free-space calibration procedure is developed and the dispersion in the focal region of the antennas incorporated into it. The results from different combinations of calibration standards are compared and related to the idea of minimum sensitivity calibration. The possible use of an artificial chiral medium as a microwave absorber is also discussed.
The measurement of radio frequency complex permeability of thin round wires
(Stellenbosch : Stellenbosch University, 2003-04) Stuart, Thomas (Thomas Edward Walter); Reader, H. C.; Cloete, J. H.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: This thesis is concerned with the measurement of the complex permeability of thin round wires at radio frequencies. This is of interest as such wires are used in various applications, such as absorbing chaff. Iron and nickel alloys are also used for their good tensile properties but have an undesired electromagnetic effect which needs to be characterised. Although little work has been done in this field in recent decades it remains a relevant problem. In this thesis the advantages of accurate wide-band measurements performed by automatic network analysers are applied to the field. The measurement system is a closed coaxial transmission line with a short circuit termination. The centre conductor is the wire of interest. The surface impedance of the wire is related to complex permeability and is measured using low-loss transmission line approximations applied to half-wavelength resonances. The loss associated with complex permeability is separated from conductivity by a D.C. conductivity measurement. A full wave analysis of the coaxial mode was performed and compared to measured values. The maximum error of the propagation constant was found to be 31% at the highest frequencies and was primarily due to length uncertainties. By varying parameters expected error bands around the measured permeability were found. These bands are of the order 1 and demonstrate that the system is sufficiently robust. The measurement of the permeability of two non-magnetic wires was performed and a relative permeability of 1 was found, demonstrating the correct working of the system. A steel wire was measured and compared to measurements found in literature. The permeability dropped as frequency rose as was expected, and an acceptable comparison to other measurements was made as there is no verification standard. Thus a simple measurement system that takes advantage of calibrated automatic network analyser measurements has been developed and demonstrated to work with sufficient accuracy.
The non-destructive measurement of the radio frequency properties of hard rock borehole cores
(Stellenbosch : Stellenbosch University, 2005) Rutschlin, Marc; Cloete, J. H.; Palmer, K. D.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: The effective use of borehole radar in geophysical exploration requires accurate knowledge of the dielectric properties of the geological system in which it is employed. The attenuation and propagation velocity of pulses through rock must be known in order to plan and interpret experimental data. Conventional destructive methods for the measurement of hard rock cores require the careful preparation of samples. This firstly necessitates the selection of sampling position and the resulting estimation of rock properties based on sparse measurements, and secondly results in the loss of material and thus data. The ready availability of cylindrical borehole core samples invites the use of a nondestructive means of measuring their properties. A novel design for a flexible guarded capacitor which conforms to a core's cylindrical surface is presented here. T he proposed device has numerous advantages over previous methods. No material is lost to sample preparation and a detailed characterisation of the entire core, including inclusions and transitions between rock types, may be performed. A detailed methodology for the rapid construction of a robust capacitor is given. Guidelines for its operation to achieve repeatable and accurate measurements of the complex dielectric constant of samples of varying homogeneity in the 1- 25 MHz frequency range are presented. The increased amount of data collected from complete core samples is analysed statistically, and amongst other things allows the estimation of the rock's homogeneity. Comparisons of the dielectric properties measured in the laboratory to propagation velocity estimates obtained from crosshole borehole shoots show that a more homogeneous sample is a better predictor of bulk propagating conditions. Detailed studies of the dielectric properties of economically important diamondiferous and platiniferous geological systems show that borehole radar is a feasible tool for the high resolution delineation of ore bodies and other geological targets.
Parallel algorithms for electromagnetic moment method formulations
(Stellenbosch : Stellenbosch University, 1991-12) Davidson, David Bruce; Cloete, J. H.; McNamara, D. A.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: This dissertation investigates the moment method solution of electromagnetic radiation and scattering problems using parallel computers. In particular, electromagnetically large problems with arbitrary geometries are considered. Such problems require a large number of unknowns to obtain adequate approximate solutions, and make great computational demands. This dissertation considers in detail the efficient exploitation of the potential offered by parallel computers for solving such problems, and in particular the class of local memory Multiple Instruction, Multiple Data systems. A brief history of parallel computing is presented. Methods for quantifying the efficiency of parallel algorithms are reviewed. The use of pseudo-code for documenting algorithms is discussed and a pseudo-code notation is defined that is used in later chapters. A new parallel conjugate gradient algorithm, suitable for the solution of general systems of linear equations with complex values, is presented. A method is described to handle efficiently the Hermitian transpose of the matrix required by the algorithm. Careful attention is paid to the theoretical analysis of the algorithm's parallel properties (in particular, speed-up and efficiency). Pseudo-code is presented for the algorithms. Timing results for a moment method code, running on a transputer array and using this conjugate gradient solver, are presented and compared to the theoretical predictions. A parallel LU algorithm is described and documented in pseudo-code. A new graphical description of the algorithm is presented that simplifies the identification of the parallelism and the analysis of the algorithm. The use of formal methods for extracting parallelism via the use of invariants is presented and new examples given. The speed-up and efficiency of the algorithm are analyzed theoretically, using new methods that are simpler than those described in the literature. Techniques for optimizing the efficiency of parallel algorithms are introduced, and illustrated with pseudo-code. New parallel forward and backward substitution algorithms using the data distribution required for the parallel LV algorithm are described, and documented with pseudo-code. Results obtained with a Occam 2 moment method code running on a transputer array using these parallel LU solver and substitution algorithms are presented and compared with the theoretical predictions. PARNEC, a new Occam 2 implementation of the thin-wire core of NEC2, is discussed. The basic 'theory of NEC2 is reviewed. Problems with early attempts at combining Occam and FORTRAN are reported. Methodologies for re-coding an old code written in an unstructured language in a. modern structured language are discussed. Methods of parallelizing the matrix generation are discussed. The accuracy of large moment method formulations is investigated, as is the effect of machine precision on the solutions. The use of the biconjugate gradient method to accelerate convergence is briefly considered and rejected. The increased size of problem that can be handled by PARNEC, running on a transputer array, is demonstrated. Conclusions are dra.wn regarding the contributions of this dissertation to the development of efficient parallel electromagnetic moment method algorithms.
Real-time HV OHTL sag monitoring system based on power line carrier signal behaviour
(Stellenbosch : University of Stellenbosch, 2005-12) De Villiers, Wernich; Cloete, J. H.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
ENGLISH ABSTRACT: A new method of measuring the change in the average height of phase conductors above the ground plane of High Voltage (HV) Overhead Transmission Lines (OHTLs) was discovered in 1999, at Stellenbosch University. The new method, called Power Line Carrier-Sag (pLC-SAG), measures average overhead conductor height variations in real-time by exploiting high frequency signal propagation characteristics on the existing PLC system. The novelty of the newly discovered PLC-SAG system naturally led to a thorough testing and investigation of the technique. This thesis explains the methodology used to produce unique experimental data, which has indeed proven that the average height of an OHTL can be tracked very accurately via the PLC-SAG technique for continuous periods. As the experiments on two live 400 kV transmission lines in South Africa were being undertaken, a serious concern regarding the new technique arose. Major HV Station impedance variations seemed to influence the PLC system and clouded the interpretation of PLC-SAG recorded data. Such HV Station impedance variations typically occur only a few times per year. A new Power Line Carrier Impedance (PLC-IMP) technique was then discovered, by which these changes could be monitored. No structural changes to the existing PLC-SAG system were required for this technique. This was seen as a major breakthrough in the presented study. Not only does this newly established technique make it possible to develop a stable PLC-SAG system, but also a potential real-time condition monitor application. Its use on PLC systems has been proposed to the main Power Utility in South Africa.

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by browse.metadata.advisor "Cloete, J. H."

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