Propagation modeling of VHF radio channel in forest environments

In previous works, we have described a method, based on a parabolic equation (PE) algorithm, which allows computation of impulse response of stationary determinist channels. We have applied this method in various types of situations, including canonical problems and real radio links over irregular terrain in forest environments. In this previous approach, the radiowave was computed, in a first step, outside the vegetation using boundary conditions on top of the trees and, in a second step, values of path loss inside the forest were deduced using some restrictive hypothesis. In our present model, the electromagnetic field is now directly computed in the entire domain above ground, including the vegetation bulk, using the full-wave PE algorithm. The forest is treated like a lossy atmospheric layer characterized by its permittivity and conductivity. The method can also be used in the case of irregular ground and with sophisticated multi-layered and/or range-dependent forest models. We present results that illustrate the influence of the different parameters (frequency, antenna characteristics, electromagnetic constants, etc.). We also make some comparisons with the well-known Tamir theory. Unlike Tamir, we have experienced noticeable asymmetry in the treatment of upward and downward links when one antenna is inside the forest and the other outside: for instance, computation of the transmitted beam is relatively straightforward when the corresponding antenna is over bare ground and far more difficult when it is inside the vegetation layer. Neither the depolarization effects nor the random nature of propagation media are taken into account at this point in our model but it does not seem to be an issue to do it and we are currently working on this question.