Wave Tilt Fluctuations near a Vertical Discontinuity in a Flat Ground Plane

Using microwaves (4.8 GHz) and laboratory models, the electric field tilt was measured across a horizontal plane having an abrupt transition from a highly inductive surface to a highly conductive surface. The wave tilt was found to exhibit an interference pattern in front of the boundary. This phenomenon can be explained in terms of the reflection and backscattering from the boundary region. It is predicted that this effect will be observable in wave tilt measurements made on the earth's surface, and may result in the misleading interpretation of field data in addition to obscuring the effect of real changes in local ground conductivity. Surface impedance measurements, however, should not suffer from this problem. The experimental results further support the idea that accurate interpretation of ground parameters from wave tilt and surface impedance measurements cannot be made within a distance of one wavelength (measured in the earth) from any substantial lateral discontinuity. The highly inductive surface consisted of a grounded pin bed (aniso. tropic) structure. Since the subsurface field in such a structure must propagate normal to the surface, it is ideal for synthesizing surface impedances for propagation over the earth which has a large index of refraction.