TRANSHORIZON UHF RADIOWAVE PROPAGATION ON OVER-SEA PATHS IN THE BRITISH CHANNEL ISLANDS S. D. Gunashekar (1) , D. R. Siddle (2) , E. M. Warrington (3) (1) Radio Systems Research Group, Department of Engineering, University of Leicester Leicester LE1 7RH, United Kingdom, Email: - sdg10@le.ac.uk (2) As (1) above, but Email: - drs13@le.ac.uk (3) As (1) above, but Email: - emw@le.ac.uk ABSTRACT Three transhorizon, 2 GHz radio paths have been established in the British Channel Islands in order to investigate the characteristics of long-range UHF propagation over the sea. During the measurement period considered in this paper (August 2003 to September 2004), signal strength enhancements occurred about 8% of the time, predominantly in the late afternoons and evenings and in the summer months. The propagation conditions have been modelled using the AREPS radiowave propagation assessment software package, the results of which suggest that low-level evaporation ducting is not the only propagation mechanism responsible for enhanced signal strengths. INTRODUCTION Enhanced signal strengths of UHF radio waves are caused by super refraction and tropospheric ducting. These are important effects since they not only result in lower signal losses than expected but also enable signals to travel to beyond line of sight distances. Furthermore, in addition to improved communications, tropospheric ducting may also result in unwanted interference to/from distant stations to take place. Ducting depends on micrometeorological conditions in the troposphere, in particular the spatial and temporal distributions of temperature and water vapour. These in turn, influence the variation of the refractivity of air with height. Over the years, extensive research has been conducted to explain the propagation of radio waves in the context of tropospheric ducting [1] [2] [3] [4] [5]. In this paper, since we are dealing with over-sea propagation, it is expected that evaporation ducting is the principal propagation mechanism involved [2] [3] [4]. Evaporation ducts occur immediately above the surface of the sea and other large water bodies and exist primarily because the amount of water vapour present in the air decreases rapidly with height in the first few metres above the surface of the sea. Typically, the evaporation duct height is of the order of only a few metres [1] [2] [3] but there may be large variations in this, depending on the conditions in the atmosphere and the state of the sea. By virtue of how they are formed, evaporation ducts are practically permanent features over the sea surface [1] [2] [3] [4]. Preliminary results of experiments undertaken to investigate the occurrence of super refraction/ducting over several over-sea paths in the British Channel Islands are presented here. EXPERIMENTAL ARRANGEMENT Three transhorizon 2 GHz radio paths have been established in the British Channel Islands (Fig.1) in order to investigate the characteristics of long-range propagation of UHF radio waves over the sea. These are from Jersey to Alderney (48.5 km), Jersey to Guernsey (33.5 km) and Jersey to Sark (21 km). This network of purpose-built transmitters and receivers is operated by the Radio Systems Research Group, University of Leicester, as part of a research project sponsored by Ofcom, UK. Meteorological data are available from the Channel Light Vessel anchored in the English Channel, northwest of all three radio paths, and from weather stations at the Alderney, Jersey and Guernsey airports with heights of 71, 84 and 102 metres above mean sea level respectively. The distance of the Channel Light Vessel to the midpoint of the Jersey-Alderney link is approximately 70 km. Each site has two antennas positioned at different heights above the sea, and signal strength measurements are made using alternately the high and the low antenna. All antennas are of the shrouded UMTS Yagi design with a beamwidth of approximately 26° and a gain of 14.5 dBi. The 100 W transmitter at Jersey radiates vertically polarised signals and the system has been synchronised such that transmissions are only from high antenna to high antenna and low antenna to low antenna. One pair of high and low transmit antennas points towards Alderney while the other pair points in a direction that is midway between Guernsey and Sark and is utilised for both these receiving stations. Finally, the effect