The deviation of low-frequency ground waves at a coast-line

After consideration of the methods which have been suggested for computing the deviation of ground waves at a coast-line, the phenomenon is re-examined in the light of recent experimental and theoretical work on the phase disturbances at such a boundary. It is shown that the deviation may be calculated from the rate of change of phase with distance along the path of propagation. The changes in this rate which occur at the boundary give rise to a considerable increase in the magnitude of the deviation as the receiving point is brought within a few wavelengths of that boundary. This increase near the coast seems to provide an explanation of the unexpectedly large deviations previously observed at medium frequencies. A series of simultaneous measurements of the phase change and the deviation at 127 kc/s along a number of paths crossing the south coast of England are described. Although general agreement between the measured deviations and those derived from the phase curves was obtained on some paths, there were appreciable discrepancies on others. These discrepancies are attributed to the irregularities in the phase surface which were evident over the area and which the method of derivation did not take into account.