Mobile Data Communication via Leaky Feeders.

This thesis examines the feasibility of transmitting very high speed data, at low error rates, from a mobile vehicle to a fixed locality via a leaky feeder. Possible system layouts and operating parameters are initially discussed; here the feeder height is an important consideration. Attenuation of the single wire mode of propagation is studied and a solution is given to the severe problem caused by reflections, in this transmission mode, at the ends of raised sections of the line. A detailed examination of performance is undertaken for a buried feeder arrangement. Both signal reception and data transmission qualities of this layout are studied for changes in a variety of parameters, which include: car separations and speed, aerial position, frequency (40 and 80MHz), weather conditions, and transmitter power. Various car aerial polar diagrams are measured. Using a simple model the relative coupling loss to the feeder in each case is calculated and compared with practical results. The cause of major signal fluctuations is investigated, which leads to a discussion concerning the coupling mechanism to the feeder and the preferred type of car aerial. The data transmission performance is examined using 100 kbs-1 frequency shift keying and predominantly 1010 data, although a pseudo random sequence is also used. The theoretical performance is derived and compared with practical results, under both fading and non-fading conditions. This enables performance predictions to be given for both expanded systems and the use of lower data rates. Distribution of errors in received data for a range of operating conditions are briefly analysed and possible coding strategies are discussed. Finally, the advantages of using diversity are considered. Two forms are investigated, frequency and directional diversity, and for each type the resulting improvement in performance is shown, together with suitable system layouts for diversity operation