obe and the side lobes are shown. The beamwidth is shown at the 70 percent voltage point which corresponds to the half power point. The beamwidth is given in degrees. It will be noted that as the wavefront gets farther from the antenna, the dimensions of the Combined baseband wavefront get larger at the half power point. The effects of this will be shown in an example later. A pictorial of the Fresnel zones is shown in Figure 50.16. Figure Figure 50.14 Space diversity receivers 50.16(a) is a cross section of Figure 50.16(b) taken through the The formulas for U and I SD may be combined to provide an unavailability formula for the space diversity case as in Equation 50.7 where USD is the unavailability in decimal form for space diversity. The other factors are as defined previously. (50.7) (b) ) Notice that for U SD' the frequency does not appear in the formula. Figure 50.15 Antenna beam width This indicates that the old 'rule of thumb' requiring greater antenna spacing at lower frequencies is not the case. Experience indicates that antenna spacing between 30 and 60 feet provide the best compromise between improvement in propagation beam axis. The rings (shaded and non-shaded) are known as the reliability and increase in cost due to increased tower height. It Fresnel zones. The distance from T to a point on the circle to R is should be remembered that both the top-to-top antenna path and the longer by some multiple of an half wavelength than the main beam. top-to-bottom path must meet the necessary path clearance criteria, This difference in length is the cause of the Fresnel interference however. phenomenon. 50/8 Line of sight radio systems The peaks and valleys will depend on the type of obstruction. A highly reflective obstacle will give deep nulls. If the towers are high enough to clear many zones then an obstacle in the right place can cause deep cancellations. Some possible solutions to the cancellation problem are: 1. The Fresnel zones are closely spaced. This indicates that a slight increase or decrease in 