Investigation into the Effects of the Troposphere on Vhf and Uhf Radio Propagation and Interference between Co-Frequency Fixed Links

Previous studies on anomalous, over-sea propagation have been either focused on single links employing space/antenna diversity or on point-to-multipoint links, usually involving single frequency. Measurements on two co-linear, trans-horizon paths (50km and 140km long) over the English Channel have been made over periods in excess of a year in order to investigate the propagation characteristics of VHF and UHF signals propagating over the sea. The setup comprises a transmitter located on Jersey and receivers on Alderney and Portland. Signal strengths, meteorological factors within the lowest 1km as well as their mutual relationships have been studied. Signal strength enhancements have been observed on both paths, primarily in the late afternoon and evening periods, in the spring and summer months. These enhancements occur for different percentages of time between 12% and 21%. It was observed that the enhancements at both receiving sites and both frequencies may/may not be concurrent, leading to a probability of interference. The values of median lapse rate of refractivity in lowest 1km of atmosphere, effective earth radius factor and surface refractivity significantly less than those used by ITU have been observed. Refractivity gradients indicative of super-refraction and ducting are observed between heights of 52m and 84m for considerable amounts of time. Different current propagation models have been used to predict the median propagation loss values, which do not always clearly point out the dominant propagation mechanisms. This study has made available further results regarding enhanced signal strength events, has improved the values of some critical parameters linked to tropospheric propagation and has identified certain trends relating weather to signal level enhancements. These issues bear direct relevance to radio propagation in marine and coastal areas.