Propagation Effects Caused by Finitely Conducting Ground on Lightning Return Stroke Electromagnetic Fields

Abstract

Consider a radio antenna located over ground and transmitting at a given frequency. If the ground is perfectly conducting, the amplitude of the radio wave generated by the antenna decreases inversely with distance as one moves away from the antenna, i.e., the signal decreases as 1/r, where r is the distance from the antenna to the point of observation. However, if the conductivity of the ground is finite, then the amplitude of the radio wave decreases much more rapidly than the inverse distance. The higher the frequency of the wave, the higher the rate of decrease of the radio signal with distance. This attenuation of the radio signal or the electromagnetic wave by finitely conducting ground is called propagation effects. Attenuation of the electromagnetic wave results from the absorption of energy from the electromagnetic field by the finitely conducting ground. The higher the frequency of the electromagnetic field, the higher the amount of energy absorbed by the ground. Let us analyze this a bit further. Consider an electromagnetic field generated by a radio antenna tuned to a given frequency. If the ground is perfectly conducting, at any given point on the ground the electric field is perpendicular to the ground surface and the magnetic field is in the azimuthal direction (Fig. 12.1a). The direction of energy flow or the Poynting vector (i.e., E × H) of the electromagnetic field at that point is directed parallel to the ground. Now consider the electromagnetic field of a radio antenna located over finitely conducting ground. In this case, the magnetic field has the same direction as before, but the electric field is inclined to the surface of the ground (Fig. 12.1b). That is, there is a component of the electric field parallel to the ground. This component is called the horizontal electric field. The Poynting vector in this case is directed to the ground, indicating that energy is absorbed from the electromagnetic field by the ground. With increasing frequency or with decreasing ground conductivity the angle between the vertical and the direction of the electric field increases, i.e., the horizontal electric field increases. Thus, the energy absorbed by the ground increases with increasing frequency and with decreasing conductivity.