7

TERRAIN REFLECTION AND DIFFRACTION

7.1 INTRODUCTION

In many applications, line-of-sight communications are influenced by reflections from the surface of the Earth. Transmitting and receiving paths near the surface of the Earth will experience reflections from the ground if sufficient antenna directivity is not available to avoid ground illumination. Examples include VHF and UHF broadcast systems where receivers (such as radio and television antennas) are located near the ground surface, preventing its influence from being removed even with large transmitting antenna heights.

In this chapter, the theory of direct plus reflected wave propagation is presented, along with a graphical method for predicting propagation effects in the presence of irregular terrain. This chapter provides a first discussion of “multipath” interference for the simple case of a single multipath (reflection from the ground surface). Multipath propagation effects are further examined from an empirical and statistical point of view in Chapter 8. In addition, a rule of thumb for siting antennas in point-to-point links to avoid obstruction of the line of sight by terrain is presented. The latter is “site specific” in that the focus is on situations where the terrain elevation is known as a function of distance from the transmitter and the receiver. Empirical models for propagation loss in cases where the terrain is not specified (as for a mobile communication system) are dealt with in Chapter 8. Because the ...

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