Radiofrequency speckle in mountainous terrain

We report the results of several field experiments to characterize broadband UHF radiofrequency propagation through mountainous terrain and into deep shadow zones. Utilizing an existing digital broadcast television station (center frequency 497 MHz with a standard 6 MHz bandwidth) as our signal source, we performed spectrum analyzer surveys to determine both spatial and spectral characteristics of signals present within deep shadow zones. Directional surveys revealed that RF power within shadow zones arrives primarily via adjacent topographic reflectors and that these reflected signals suffer significant frequency selective fading. Translational surveys within shadow zones showed slow changes in the reflected spectrum for longitudinal translations and rapid spectrum changes for transverse translations. The frequency selective fading and translation direction dependence of the reflected spectra suggest a phenomenon analogous to laser speckle, caused by the coherent scattering of the RF signal from the rough surface of adjacent topographic reflectors.