Harold T. Dougherty and Evan J. Dutton

Abstract: This report is tutorial in presentation, with emphasis on the application of engineering formulations of the effects of terrain upon terrestrial microwave systems. Many of these formulas have been available for a decade and longer, but they have not been widely used. Although they are well based in theory and experiment, their applications had been considered too detailed and tedious until the present widespread availability of personal computers. Therefore, these formulas have been presented here in a form readily adaptable as computer programs and sUbprograms. The units are carefully specified for use with terrain data bases for incorporation in a system design that would permit tradeoffs between terrain geometry, hardware parameters, and system performance criteria. There are additional formulations similarly well based in theory and experiment; they are not presented here, but they are adequately referenced. The readers should consider them for an increased sophistication of their system design. The first part of the report discusses the basic geometry of great–circle–plane terrain characteristics and the application of appropriate geometrical techniques to determine if a path is line–of–sight (LOS), obstructed by a common (single) obstacle, or by separate transmitter and receiver horizons. The concept of Fresnel zones and Fresnel ellipses is introduced, and the importance of their use in determining whether a path is LOS or diffracted is discussed. The next part of the report discusses LOS paths, and the significance of reflected signals into the direct radio path is examined. Specular reflection from finite reflecting surfaces is discussed primarily with attention directed to the theoretical reflection coefficient for smooth, infinite, planar surfaces and the consequent modifications for Earth curvature, finite surface extent, and roughness of the surface. Then the report discusses diffraction paths with primary reliance on theory, as modified for CCIR purposes, first for isolated terrain obstacles (rounded) and then for irregular terrain, which can block both terminals on a great–circle–plane radio path. Effects of vegetation along a terrestrial–link path are also briefly considered.

Keywords: terrain; diffraction; clearance; Earth's surface; engineering formulation; Fresnel zones; link design; radio horizons; reflection; terrestrial-link propagation

Disclaimer: Certain commercial equipment, components, and software may be identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.

For questions or information on this or any other NTIA scientific publication, contact the ITS Publications Office at ITSinfo@ntia.gov or 303-497-3572.

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