Analytical Expressions of Critical Distances for Near-Ground Propagation

This paper investigates the near-ground propagation and the condition under which the wave propagation can profit from the proximity of an interface. To this aim, asymptotic approximations of the electric field radiated by an elementary vertical electric dipole over a half-space are introduced. The asymptotic formulations coincide with other approximations previously carried out in the literature. In this comprehensive formulation, in addition to the direct and reflected wave, a third field component is taken into account, physically originated by the diffraction at the interface of the two media. This component, ignored by ray-based techniques, can become predominant and stimulate the near-ground propagation. This paper presents an analytical expression for the condition under which a near-ground propagation can benefit from the special component. Two critical distances associated with the beginning and the end of the preponderance zone of the component are also evaluated. The benefits of the near-ground wave propagation as well as the validity of the three analytical expressions have been verified in the frame of various scenarios.