The propagation of VHF and UHF radio waves over sea paths
This thesis is concerned with the statistical studies of VHF/UHF radio wave propagation over the sea path at the limits of line-of-sight range. The objective is to provide a set of data that leads to the understanding of the characteristic of VHF/UHF radio wave over the sea path. A series of experiments were conducted using two paths of around 33 and 48 km across the English Channel. These two paths are between fixed land-based locations that provide an unobstructed condition. This allows a prolonged period of data collection under several sea states and atmospheric conditions without the heavy expenses of ship borne trial. The statistic studies showed that the high signal strength variation observed at both receiving sites are the results due to ducting and super-refraction. It occurred around 43 to 76% and 31 to 48% of the total time (percentage of days) during summer 2001 and 2002 respectively. In comparison, the total time was below 10% during winter period. Across the Jersey-Alderney path (48 km), high fading phenomenon was observed which is a result due to interference fading between the diffracted and troposcattered signal. The statistics showed that it occurred at around 35 to 55% of the total times during summer with an average fading range of around 10 and 7 dB during autumn and summer respectively, with an average fading period of around 7 seconds. The results from simulation showed that when the VHF/UHF signal reaches the radio horizon, the dominant propagating mechanism is smooth earth diffraction. Beyond the radio horizon, the attenuation rate increases dramatically and at a certain distance (depending on the frequency, antenna height and seasonal condition), the diffracted signals will be weaken and the troposcatter effect will become the dominant propagating mechanism.