SAMPLE COMPUTATIONS 



159 



1,000 

 700 

 500 



300 

 200 



E 



^ 100 



20 



40 



60 80 100 120 140 



HEIGHT ,h , IN m 



160 



180 200 



Figure 4.51. Height versus distance at Washington, D.C., for minimum midspring 



{May) dxicting conditions. 



Therefore, in this case, 



do = dzQQ = 72 km. 



In either case, as can be seen from figure 4.41, do is greater than 40 km. 

 Therefore, any height of the receiving antenna will be satisfactory at this 

 point, since the first shadow zone has not yet been reached. 



For the 90-km distance, case (b), it can be seen that this distance is 

 beyond the onset point of the first shadow zone, and figures 4.49 to 4.51 

 must be used here. 



Figures 4.49 to 4.51 are used differently depending upon which side of 

 the reflection point B of figure 4.52 one desires to locate an antenna. If 

 on the lefthand side of point B, one must subtract the desired distance 

 from the distance from B to the top of the duct traversed by the ray and 

 use this new distance to determine height, h, in figures 4.49 to 4.51. This 

 procedure must be followed every time the ray heads downward. If on 

 the righthand side of point B of figure 4.52 (or any other reflection point), 

 one simply uses the distance from the reflection point directly. One must 

 keep in mind that any previous half-lengths must be subtracted from the 

 desired distance from the transmitter, as well as subtracting do, in order 

 to obtain the distance used in figures 4.49 to 4.51. 



