VHF propagation over hilly, forested terrain

Propagation measurements at low altitudes over hilly, forested terrain made with the objective of developing a computer-based propagation model capable of predicting path loss over similar terrain given the terrain profile between transmitter and receiver are reported. The measurements were made at a frequency of 110.6 MHz with the very high frequency omnidirectional range (VOR) station at Gardner, MA, as a transmitter. The received signals were measured at distances between 7 and 15 km by making vertical descents with a helicopter from altitudes of roughly 600 m down to 10 m above ground. Negligible specular ground reflection was observed from this terrain, and no reflection is included in the model. We found good agreement between the measurements and a multiple-diffraction model based on an extension of the Deygout approximation. Use of two knife edges was sufficient to characterize the terrain diffraction. Predictions of the Longley-Rice computer model are included for comparison.