h path clearance. Figure Atmospheric duct effect Desired Reflected Earth surface K= 1/2 Figure 50.7 Effect of second Fresnel zone clearance True earth radius The formula for calculating Fresnel zone clearance is as Equation 50.2 where F, is the distance in feet from the path line to the edge of the first Fresnel zone; d, and d, 2 are the distances in miles from the point in question to each end of the path; D is the total path Figure 50.5 Super refractive effect length in miles; and f is the frequency in GHz. (50.2) At this point the reader might be tempted to conclude that as long as earth curvature is taken into account, or as long as line of sight exists between the two ends of a microwave path, that the path clearance is satisfactory. This is not the case. Fresnel zone clearances greater than F 1 may be calculated by As microwave energy propagates from the transmitting antenna, Equation 50.3 where N is the Fresnel zone number. an expanding wavefront is created. Looking at the wavefront on a cross-section basis, it may be seen to consist of a series of concentric FN = 1 (50.3) areas around the path line as shown in Figure 50.6. These areas are numbered beginning with the area immediately around the path line. Propagation anomalies that alter the path line will obviously The signal at the receiving antenna will be affected if a portion of change the clearance above obstructions. It is possible that received the wavefront is blocked, or if a portion of the wavefront is reflected signal variations may occur due to the clearance equalling an even from an obstruction along the path. numbered Fresnel zone clearance. This, in combination with a highly reflective surface, could cause a deep fade in the received signal. It is worth noting that if earth bulge using a K = 2/3 is considered, many path designers allow a zero Fresnel zone clearance (also First Second called grazing). Third Fourth Fresnel Fresnel Fresnel Fresnel 50.1.2 Path profiles The first step in planning a microwave path is to examine topo- graphic maps