Propagation over a valley: asymptotic results

Prediction of radio signal strength for signals propagating over valley to a mobile receiver in terrestrial clutter, such as trees and buildings, is addressed. Starting from first principles, an accurate asymptotic approximation is found that describes the propagation as a combination of the usual two-ray component, and a "whispering gallery" mode that results from guided propagation over the valley, leading to a signal which is over 20 dB higher than would be observed in the case of flat terrain. This extends previously known flat terrain propagation models to include this sort of variable terrain.