Evaporation duct propagation and near-grazing angle scattering from a rough sea

Evaporation ducts created by the rapid decrease of humidity with height just above the sea can result in greatly enhanced signal levels near the sea surface for over-the-horizon paths at frequencies above approximately 3 GHz. As frequency increases above approximately 10 GHz, these enhanced signal levels can be reduced by scattering at near-grazing incidence from a wind-roughened sea surface. In general, the reduction is greater for strong ducting conditions and high wave heights. There are several radio propagation models in use or proposed that account for rough surface effects in a ducting environment, but experimental validation of these models is lacking. It is the purpose of this paper to describe an effort to validate one such model using existing propagation data from three previous experiments.