VHF Diffraction and Groundwave Propagation Tests Using Ionospheric Sounders

Abstract : Two Granger Associates Model 911 sounders were used at VHF (32 to 64 MHz) to make path-loss measurements and relative antenna efficiency measurements on short ground-wave paths (less than 50 km) in the United States and in Thailand. The first test consisted of measuring the path loss as a function of frequency on a 40-km diffraction path over a low, wooded ridge. These tests indicated that a simple scalar diffraction model did not predict enough loss by about 20 to 40 dB. Tests on 49.2 MHz over a 5-km level, forested path near Ban Mun Chit, Thailand showed that broadside alignment of horizontal dipoles at 10 ft above ground produced signal strengths 15 dB greater than any other combination of horizontal dipoles, and 20 dB greater than when the dipoles were aligned end-on (the worst alignment).