The thesis describes a method of' beam steering aimed at producing a useful amount of deflection of an antenna beam from boresight, by a simple and Inexpensive method. For large antennas, It is difficult, as well as expensive, to steer the beam by more than a few beamwidths. The method studied was developed with particular reference to the beam steering requirements of Direct Broadcast Satellite flat plate antennas. The method involves two dielectric wedges, having circular faces, placed In front of the antenna. By adjusting the relative angular position of the wedges, the beam can be steered In any direction, up to a maximum value from the antenna boresight direction. The maximum value of the deflection is determined by the wedge angle and the dielectric constant of the wedge material. This method of beam steering Is Independent of frequency. Frequency limitations are, however, set by the need for a matching layer on the wedge faces to reduce the loss due to reflection and also due to interference of multiple reflection effects on the radiation pattern. Extensive investigations are made (both theoretical and experimental) to predict the performance of the wedges as the beam Is steered. The effect of reflection is studied and a new matching technique is devised. Other practical considerations for example, stepping the wedges for weight reduction, the effect of loss in the dielectric material and cross polar performance are also taken into account. Some effort is given to explore the possibility of using metal loaded artificial dielectrics using polyurethane as the base material. Finally, a number of charts are presented to determine the amount of beam steering for an arbitrary position of the wedges.

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A selection of novel modelling techniques are presented for predicting the behaviour of a thickness mode composite piezoelectric transducer comprising an array of ceramic pillars embedded in a polymer matrix. Finite element analysis is employed to establish the vibrational and electromechanical characteristics of an arbitrary composite structure. Thereafter, the equivalent thickness mode transducer parameters are employed in a modified transducer model to establish the impedance, transmission and reception characteristics. A novel algorithm for calculating the acoustic pressure profile, resulting from an arbitrarily excited, rectangular aperture is employed to determine the field structure of the composite transducer. This algorithm is incorporated into an array model capable of predicting the field characteristics of steered and focused arrays. Theoretical and practical results, for composite, sliced and monolithic ceramic arrays are compared. Finally, low frequency, stacked composite structures are considered. A range of novel characteristics are obtained by using, to full advantage, the versatility of composite materials. It is believed that these modelling techniques, which have been verified experimentally, offer considerable insight into composite transducer behaviour and are conducive to effective, computer based, design.

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Static current-compounding excitation schemes that provide an isolated synchronous generator with a field current that increases both with increasing load and with worsening power factor have been in existence for many years. Such schemes, while capable of maintaining the on-load terminal voltage substantially constant, require brushes to carry the excitation current to the generator field winding and in some practical applications these brushes may present serious maintenance problems. In a recent development, a new brushless frequency-converter type of excitation system was introduced. This system, while using only passive components, can furnish a loaded synchronous generator with a compounded excitation current such that its terminal voltage is maintained to within a reasonably close tolerance, irrespective of the load power factor. The central unit of the system is a special purpose exciter, the rather unconventional stator windings of which make the concept of an automatically-regulated and brushless generator a practical reality.

A complete mathematical model for a generator system consisting of an isolated laminated salient-pole alternator, exciter and prime mover is presented, with emphasis on the inherent electromagnetic nonlinearities in the alternator and its exciter. An equivalent circuit, representing the rotor circuits accurately, has been adopted to model the al ternator in the dqo reference frame. A computer program has been developed to calculate the unsaturated parameters of the model using the machine design data. A new approach has been developed to account for the electromagnetic saturation effects on the model reactances. Consequently new saturation factors, based on the machine design particulars have been deri ved. The advantages of these saturation factors, compared with conventional factors, are that both mutual saturation effects between the main and leakage fluxes, and between the direct- and quadratureaxis fluxes are considered. A mathematical nonlinear model, utilising the new saturation factors, is presented for a system containing an isolated laminated salientpole alternator and a direct thyristor static exciter. A digital computer program has been developed to simulate the system. The predicted results, for some steady state and dynamic candi tions, showgood agreement with test results and clear improvement over those obtained if saturation is either neglected or considered using the conventional saturation factors. At high saturation levels, the conventional method of calculating the machine transformer voltages, using static saturated reactances, gives unacceptable errors. A method for calculating these voltages correctly, in models utilising the currents as state space variables, is presented using new derived dynamic saturated reactances. This dynamic reactance concept is presented in a generalised form so that it can be applied to any machine with different saturation factors. The previous mathematical model of the alternator system has been modified according to the dynamic reactance concept, and the computer program has been developed accordingly. The predicted results confirm the need to apply this concept especially to dynamic conditions characterised by high saturation levels. To extend the analysis to a wider range of loading conditions, the alternator has been modelled in the abc reference frame. The unsaturated, static and dynamic saturated reactances of the machine in this reference frame have been obtained using conventional dqo-abc transformation techniques. Starting from the fundamental machine relations, a new set of equations, in the phase reference frame, has been derived employing the new dynamic reactance concept. A comprehensive system consisting of an isolated laminated salientpole alternator, brushless exciter, thyristor divert automatic voltage regulator and a diesel prime mover has been studied. Both the alternator and the exciter have been modelled in the abc frame to comply with the nature of rectifier loading associated with the exci ter. A complete steady state and dynamic mathematical model is presented where the t~r technique has been applied to the dynamic variable topology of the system electrical circuits. The model presented covers all the possible modes of operation associated with the exciter rotating bridge rectifier circuit. A digital computer program has been developed to simUlate the system. The predicted results obtained using the new set of saturation factors in conjunction with the dynamic reactance concept show good agreement with the test results. The study presented confirms the validity of the mathematical models developed for the alternator systems. Also, it supports the metlxxi by which the electromagnetic nonlinearity has been accounted for.

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