The Optical Behavior of the Ground for Short Radio Waves

The rôle of the ground in radio transmission is first considered generally. In short-wave propagation taking place via the Kennelly-Heaviside layer only the ground in the vicinity of the antennas is involved, and its effect may be included in antenna directivity. The utility of so ascribing the ground effect exclusively to the terminals of a radio circuit rests on the applicability of simple wave reflection theory in which the distance between the terminals does not appear. For this purpose reflection equations, similar to Fresnel´s equations for a nonconducting dielectric, are employed with a complex index of refraction. The paper describes experiments undertaken to determine the limits of applicability of these optical reflection equations and discusses the results. Particular emphasis is placed on the identification of direct and reflected waves. The existence of a surface wave, foreign to simple reflection theory, is recognized with vertical antennas, when the incident wave is not sufficiently plane. At angles of incidence between grazing and the pseudo-Brewster value the requirements of planeness are severe. The relation of optics to Sommerfeld´s theory is discussed. The experiments include tests made with the aid of an airplane. For short-wave communication via the Kennelly-Heaviside layer, use of the modified Fresnel equations is shown to be justified. These equations fail only at substantially grazing incidence and then merge into the Sommerfeld ground wave solution. The ground effect is always to discriminate against radiation or reception at very low angles.