urvature of the earth's surface between adjacent microwave stations must be taken into account when determining the antenna heights. The formula for calculating earth curvature is as in Equation 50.1 where h is the earth's curvature in True earth radius feet, di and d2 in miles, and K is a constant. h = d1d2 1.5 K (50.1) Figure 50.2 'Normal' 4/3 propagation In this equation d and d. are the distances from the path location The possibility of abnormal propagation must also be considered. in question to either end of the microwave hop. An effect known as earth bulge occurs more often than a system As may be seen from Figure 50.1, earth curvature has its greatest designer would like, especially in humid coastal areas. An atmos- effect upon path clearance at the centre of the path. However, when pheric condition such as an inversion layer, with atmospheric den- calculating the total path clearance required using additional for- sity increasing with elevation will cause a bending of the microwave mulas to be described later, the earth curvature must always be signal opposite to the curvature of the earth and, if the inverse included regardless of the location along the path. bending is severe, the microwave signal will be diverted into the earth (Figure 50.3). The term 'earth bulge arises from the apparent bulging of the earth sufficiently to block the microwave signal. The K factor most often used to protect against outage due to earth bulge is 2/3, although very conservative designs may use factors as low as K=2/3 True earth radius K=1/2 Figure 50.1 Effect of earth curvature on path clearance True earth radius The K factor in Equation 50.1 is a constant whose value depends upon the actual propagation of the microwave energy along the Figure 50.3 Earth bulge effect path. Various values of K are used to describe propagation that differs from a straight line, and an understanding of these 'K factors', as they are called, is important. Other propagation anomalies may occur from time to time. These Microwave energy, due 