This work is an investigation of leaky ray (mode) attenuation in graded multimode fibres. In contrast to slab or rectangular dielectric waveguides, cylindrical fibres possess leaky modes which greatly modify their propagation properties. The power carried by such modes eventually diminishes to zero, thus adding no useful contribution to the signal transmission over fibre channels. Suitable filtering techniques are therefore proposed.

Predictive coding, which is often associated with differential pulse code modulation (DPCM), can offer an attractive means of data compression of television signals. This thesis describes investigations of various adaptive and non-adaptive DPCM schemes in order to reduce the high bit-rate required for the transmission of broadcast quality colour television signals.

The ever increasing demand for power and the correspondingly greater complexity of power systems is leading to more severe problems of system stability, and considerations of these are now requiring a more detailed modelling of the generator system than has hitherto been undertaken. The object of this thesis is to study the steady-state stability of a synchronous machine during small oscillations in the system and also to investigate the significance of various system parameters on the system stability.

The application of computerised tomography in non-destructive testing, in emission and transmission mode, utilising X- and gamma-rays is the subject of this study. Measurements for tomography using X- and gamma-rays and the operation of radiation detectors are based on photon interactions with matter and the theory governing these interactions is-discussed. The theory of the mathematical reconstruction of a two dimensional distribution from its projections is shown and reconstruction techniques and applications reviewed. The principles of radiation detection and measurement are presented in order that the physical significance of the data recorded can be assessed. Detection characteristics pertinent to imaging applications were measured for a number of detectors (e.g. NaI, BGO, CsF, HPGe, CdTe). The importance of these characteristics in determining the suitability of detectors for imaging applications are discussed. The relative merits of each detector type are compared and their relative suitability predicted. A prototype scanning rig was designed and used to carry out tomography experiments with phantoms in emission and transmission mode and contrast and spatial resolution under various conditions were determined. The usefulness of line scans and contrast measurements in quantitative image analysis is shown. The effects of scattered photons on image quality are studied by means of the selection of different discriminator energy windows. In each scan the counts within the full width at half and tenth maximum of the photopeak were used as the data for the reconstruction. A method for subtracting the scattered photon contribution from the peak window is also developed and its successful application demonstrated. The results obtained from the scanning experiments illustrate the image degradation caused by scattered photons. The effect of scattered photons increase with energy and are more acute in emission mode.

A highly satisfactory method of preparing graphite-loaded epoxy-based coated wire and disc voltammetric electrodes has been developed. The novel technique of multilayer coating and curing of coated surfaces in the atmosphere above a 40% m/m hydrofluoric acid solution presents many advantages: the possibility of obtaining very thin and smooth layers of loaded-epoxy base has been demonstrated to be of great versatility and convenience for the preparation of electrodes in various shapes and sizes. The technique has been extended to the easy and inexpensive fabrication of other types of voltammetric electrodes, namely coated plastic, coated glass and coated fabric electrodes, and to the preparation of ionselective electrodes. Three different types of sulphide ion-selective electrode have been prepared and have been evaluated in standard solutions. The construction adopted here could be a viable alternative to existing methods for the fabrication of inexpensive home-made units. The advantages and versatility of the novel coating and hardening technique in terms of thinness and smoothness of layers, the lack of interference usually presented by common hardeners and catalysts and the resistance shown by the hardened surfaces to common reagents and solventss have been demonstrated. The possibility of polishing and prompt renewal of dirty or damaged surfaces coated by the method presented here has been one of the advantages of the system discussed in the present work. A simple technique of saturating the surfaces of coated plastic electrodes with graphite powder has been introduced. It has been shown to be of great help in the fabrication of coated plastic electrodes and other systems of comparatively low electrical resistance.

This thesis describes an investigation into the application and performance of direct sequence spread spectrum techniques for land mobile radio systems. There is a brief description of the basic principles of operation of direct sequence systems. The multiple user facility is analysed and values obtained for the maximum number of simultaneous system users in terms of system parameters. This clearly illustrates the need for power control. A possible method of providing power control is described. Comparison of user density is made against conventional narrowband modulation methods. There is some discussion of the effect of sequence cross-correlations on the number of system users. The system organisation is mentioned, showing possible application of a calling channel. Consideration is given to the possibilities of bandsharing with narrowband modulation systems. Figures are derived for the resulting interference to existing systems which would be caused by such an arrangement. A brief resume of the pertinent features of the land mobile radio channel is given. The effects of shadowing on the output quality and spectral efficiency of direct sequence systems is discussed. There is an analysis of the effects of shadowing on the user density in small cell schemes. An analysis shows the effects of multipath propagation on direct sequence performance by reference to a simple two path channel. Details are given of a simple experimental direct sequence spread spectrum transmitter and receiver constructed. The measured results for the performance of the system against various forms of interference and channel degredation are compared with their theoretical values. Finally ideas for future work are discussed.

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No description available.