VHF Propagation Near the Ground. An Initial Study

This report describes an initial investigation of propagation near the ground in forested terrain at a frequency of 110.6 MHz. The objective was to study propagation effects that influence the illumination of ground-clutter targets by a ground-based radar. We used as a transmitter an aircraft-navigation aid, the VHF omnidirectional range at Gardner, Massachusetts. Ground-based measurements of signal strength at heights from 2 to 15 ft above the ground were made at two locations, 2.2 and 8.8 miles away, and the field strengths corresponding to free-space propagation at each location were measured with a helicopter hovering over the site of the ground-based measurements at sufficient altitude to avoid terrain-diffraction effects. The results of the ground-based measurements are compared with calculations that model the propagation effects on the basis of terrain profiles determined directly from relief maps with corrections for tree height. The model took into account diffraction by masking hills. In addition specular reflection from the open fields immediately in front of the receiving antenna at both sites was found to produce the steep gradients in field strength observed below a height of 10 ft above ground.