The influence of moisture in the ground, temperature and terrain on ground wave propagation in the VHF-band

Results are given from a general investigation regarding the different factors that govern vhf ground wave propagation at low heights and short distances, i.e., mainly in the Sommerfeld region. In this work the influence on field strength from ground moisture and temperature, snow, topography, and vegetation, and also field strength variations along mixed land-water paths have been studied. The measurements refer to terrain typical for middle Sweden. Suitable formulas for field strength calculations over smooth ground are developed. The moisture in the ground is found to play a principal role and a method is shown for calculation of changes in the path attenuation caused by known or predicted changes in the water content of ground. The influence of temperature is found to be smalls even when the ground freezes, but a covering of snow may be of greater consequence. Characteristics of local variations and exponential attenuation of the field in a forest is demonstrated, the fluctuations being very large and the average exponential attenuation factor mostly very small. Measurements along a mixed land to fresh-water path verifies Millington´s recovery effect. The field over a hill is found to have a maximum value a certain distance below the top towards the transmitter.