Near Earth Propagation in the 900 to 2,300 MHz Band: Modeling, Analysis and Measurements in the Vicinity of Green Bank, WV
Daniel Davis & Gary Brown (Virginia Polytechnic Institute & State University)
During the last four years, the ElectroMagnetic Interactions Laboratory has been involved in the modeling, measurement, and analysis of near earth propagation in the 900 to 2,300 MHz frequency band. The modeling phase comprised the application of methods developed to predict the electromagnetic scattering from rough surfaces for radar clutter and remote sensing studies. Some of the difficulties encountered during this part of the research entailed (1) terrain elevation data base incompatibility with the electromagnetic scattering modeling needs, (2) applying a two dimensional scattering model to a three dimensional problem, and (3) developing some initial results with incorporating foliage effects in the model. The second phase of this research covered the measurement of various modes of near earth propagation including line of sight, shadowed line of sight, diffraction by both smooth and forested hills, and a long (25 km) occluded paths. The measurements were accomplished with a team comprising Wes Sizemore (NRAO-GB), Bruce Naley (NSWC-DD), Earnest Chilton (NSWC-DD), and other helpers from NSWC-DD. All measurements were accomplished by placing a receiver on the Jansky Laboratory roof (near to and above the Control Center) and locating a transmitter at various remote sites representing the path conditions noted above. Signal strength measurements under low and moderate wind conditions and both vertical and horizontal polarizations were acquired when the Green Bank Telescope was down for maintenance. Relatively high gain, narrow beamwidth antennas were employed where possible to minimize the out of plane terrain scattering effects.
The analysis phase comprised (1) incorporating cable loss and antenna gain calibrations in the links equations, (2) comparisons between various forms of the model and data, and (3) limited work with some very approximate foliage propagation characterization. The agreement between the measurements and the various models will be presented and discussed. In general, the agreement was good but there were exceptions especially when dealing with mid-path diffraction by a foliage covered mountain and fairly windy conditions. Finally, some suggestions for further analysis and measurements will be presented.
