Modeling and Simulation of Near-Earth Wave Propagation in Presence of a Vegetation Layer

The problem of near-earth wave propagation at VHF in the presence of a vegetation canopy that is either infinite or truncated in extend is considered in this paper. When the vegetation layer is continuous and infinite, a homogeneous two-layer half-space model (air/vegetation/ground) is employed. In order to arrive at a computational efficient solution, a second order asymptotic evaluation for the electric fields of an arbitrarily-oriented, infinitesimal electric dipole-for source and observation points located in the vicinity of the air/vegetation interface-is carried out through the method of steepest descents. The formulations are valid in the far field, with the limitation that the exponentially decaying pole and branch cut contributions have been ignored. It is observed that the Norton wave, though it is highly localized near the air/dielectric interface, is a significant contribution either when the dipole and observation points are both located above the dielectric layer or when one is above and the other is within the layer. For the related problem, when the dipole is embedded inside a truncated vegetation layer, a semi-exact solution for the received field at locations exterior to the vegetation layer is obtained by a surface-field integration technique in which the spatial domain of integration is over a plane containing the truncated face of the vegetation canopy. The numerical results are computed using stationary phase approximations and show improvement over those determined through an existing ray-tracing approach