Assessment of Terrain Shadowing and Knife Edge Diffraction Effects on VHF Radio Propagation in Saki, Oyo State, Nigeria

Abstract

Very High Frequency (VHF) radio waves operate within 30–300 MHz frequency range and are widely used for terrestrial communication, particularly Frequency Modulation (FM) broadcasting. They propagate through direct and near Line of Sight (LOS) paths, making them sensitive to terrain between the transmitter and receiver. Obstructions create terrain shadowing along the propagation path, while diffraction allows part of the radio signal to bend around such obstacles and reach locations behind them. This study examined the effects of terrain shadowing and knife edge diffraction on VHF radio propagation using measured Received signal strength measured at 70 locations along seven selected routes using a tinySA Ultra spectrum analyser, while SRTM elevation data were processed in QGIS to examine terrain obstruction, LOS conditions, Fresnel zone clearance and knife edge diffraction. Out of the 70 propagation locations analysed, 51 (72.86%) experienced terrain obstruction, while 19 (27.14%) maintained clear LOS. The calculated knife edge diffraction loss averaged 8.99 dB across all paths and 12.33 dB for obstructed paths, with a maximum value of 22.12 dB. The mean RSS was -73.72 dBm, while the Egli and Free Space Path Loss (FSPL) models predicted -78.30 dBm and -21.22 dBm, respectively. Egli recorded Mean Absolute Error (MAE) of 17.97 dB and Root Mean Square Error (RMSE) of 22.83 dB, compared with 52.50 dB and 55.51 dB for FSPL. Their Coefficients of Determination (R²) were -0.815 and -9.733, respectively. The findings show that terrain plays an important role in VHF signal behaviour in the study area.