Influence of the ground near transmitting and receiving aerials on the strength of medium-frequency sky waves

The strength of the low-angle sky waves radiated by a medium-frequency aerial depends on the conductivity of the ground extending for many wavelengths in the direction of propagation. The field strength is greatest if the aerial radiates over open sea from the coast, and falls to a limiting value as the distance between the aerial and the sea increases. Measurements confirming the theoretical variation of field strength with distance from the sea are described, and the effects of ground and ionospheric irregularities are discussed.