Ground-wave propagation over an inhomogeneous smooth earth. Part 2: Experimental evidence and practical implications

摘要

This paper is a sequel to an earlier one and recapitulates its main points, in particular the recovery in field strength that can occur on crossing a land/sea boundary. Further experimental evidence is reviewed which is in agreement with the theory but is not sufficiently strong to be conclusive. The optimum conditions for obtaining the recovery in practice are discussed, favouring the choice of a wavelength of about 4 m where the distances required are quite small, and of 100 m where the attenuation with distance in the diffraction region is much greater over land than over sea. Experiments on approximately these two wavelengths are then described which give the expected increases in field strength and show that the calculated curves fit the observations as closely as the experimental conditions warrant. In the light of this confirmation of the theory, some of its practical implications are reviewed, in particular with regard to the siting of transmitters near stretches of water in relation to their service areas, the possibilities of frequency-sharing under ground-wave conditions, and the ground-wave coverage of medium- to long-wave navigational aids.