Radio-Wave Propagation between World Wars I and II

The period between World Wars I and II, 1918 to 1941, is particularly noteworthy for the significant developments that led to a comprehensive understanding of the factors that control the propagation of radio waves through the atmosphere. In slightly over two decades it became possible to identify the basic physical mechanisms with the aid and correlation of theory and experiment, so that by the end of this period it was possible to prepare predictions of future propagation conditions covering a wide variety of frequencies, transmission paths, for day and night, and seasonal variations. During the entire period, numbers of scientists were working toward the calculation of field strengths of radio waves propagated by both the ground wave and the ionosphere. Watson's paper in 1918 initiated the ground wave calculations by considering the propagation over a perfectly conducting sphere. It was soon recognized that the ground could not be treated as perfectly conducting, and the theory of propagation for an imperfectly conducting ground was not produced in usable form until the 1937-1941 period. Watson's predictions for field strengths in the shadow zone proved to be much lower than experimental values. This led to increased interest in the ionosphere as a possible explanation. Effective work on ionospheric problems began in 1924 with the development of the Eccles-Larmor theory of electronic behavior. The effect of the earth's magnetic field extended this theory to include doubly refracting layers and ionospheric layer anistropy.