Diversity Techniques for Leaky Feeders.

This thesis examines the use of diversity techniques for improving the performance of a leaky feeder communication link. Tests have been carried out at frequencies of 40 and 80 MHz on an experimental system with the cable buried in soil just beneath ground level along side a road. The system was originally installed to establish the feasibility of transmitting high transmission rate (100 kbs-1) data from a vehicle to a cable for a vehicle manufacturers test track telemetry system. Coupling to both loop and dipole aerials were examined and compared against the coupling to a vehicle mounted loop and monopole. Severe fading, characterised by a Rayleigh distribution, was experienced at the extremes of radial range, regardless of aerial configuration, which could result in severe error bursts on a data link. Close to the cable, within a radial range of half of a freespace wavelength, this work has established that fading can be completely eliminated with the appropriate choice of aerial, despite the scattering of the coupling mode. Various diversity mechanisms and techniques are then considered primarily as a means of mitigating the fading outside this radius. Three mechanisms, frequency, space and field-component were chosen for a detailed study in conjunction with a transmission, frequency offset technique. For each mechanism, the improvements afforded by diversity are found to be substantial and under Rayleigh fading conditions, the correlation coefficient of the diversity signals was found to be an adequate descriptor of the diversity gain. The factors determining the correlation function are also identified