Measurements of non line-of-sight VHF propagation over the sea surface

On two occasions in 1994, a set of trials were carried out to measure the signal strength as a function of the range between three vertically-polarised floating transmitters, operating at frequencies in the VHF band, and a ship-mounted receiver. The trials corresponded to sea states from 2 to 5, and ranges up to 30 Nautical miles (1 Nm=1852 m), beyond line of sight of the transmitter. The paper describes the variation of signal strength as a function of the range and sea state. A literature search suggested that very little has been published on this kind of propagation. The paper derives the equations to extrapolate the data from the received power vs. range curves, in order to determine the variation of signal strength with the range, and to study its variability with the sea state.