Radio Wave Reflections from the Earth

Certain of the electrical constants¹⁶ of the surface of the earth of importance in radio transmisison are as follows:

The dielectric constants given are with respect to air as unity. The conductivity values are in electromagnetic units and can be converted to mhos by multiplying by 10⁹. The absorption factor is the ratio of the actual field intensity to the field intensity with no absorption¹⁶ and indicates which terrain is the poorest, sea water being assumed perfect. These constants are determined by studying radio transmission, although it is possible to make direct measurements.¹⁷

If the ground were a perfect conductor, then the reflection of radio waves by the surface of the earth would be somewhat as explained in Chapter 6 for a transmission line. The electric field at the surface of a perfect conducting plane must be zero because there can be no voltage drop over such a surface. The reflected electric component must reverse to cause cancellation. From a study of these phenomena,³ it is concluded, theoretically, that a horizontally polarized wave is reflected from a perfectly conducting surface with a 180° phase shift and that a vertically polarized wave is reflected without a phase shift.

Since the earth is not a perfect conductor, the theoretical situation just described may not exist. Studies of wave reflection under actual conditions have been made.¹⁷ Both the ratio of the amplitude of the incident to reflected wave and the phase shift vary with the conductivity and the dielectric constant of the earth and with the frequency and angle of incidence. These values can be calculated¹⁷ or taken from curves.¹⁸