PERMANENT ECHOES 



L39 



TARGET 



3T IN DEGREES 

 10 



CENTER OF EARTH 



Figure 37. Line-of-sight geometry. 



These curves are plotted in Figures 38 and 39, and 

 their use is illustrated in Figure 36. The center of 

 the antenna is at 200-ft elevation, and the height 

 of the shielding ridge 4 miles away is 400 ft. For a 



t IN DEGREES 15 



8 IO 



IN MILES 



Figure 38. Line-of-sight curves. 



50 70 90 



RANGE IN MILES 



Figure 39. Line-of-sight curves. 



200-ft rise in 4 miles the angle is found from Figure 

 38 to be 0.5 degree. This curve can then be used to 

 determine the height of the shielded region at other 

 ranges. Thus the range at which the shielded region 

 is 4,000 ft high for the case considered is found from 

 Figure 39 by using h 2 - h = 4,000 - 200 = 3,800 

 ft for height and the ^-degree curve, giving 53 miles. 

 It is desirable to be able to estimate diffraction 

 effects in a simple fashion suited to the approximate 

 nature of this kind of work. As shown in Section 

 15.4.8 the field intensity varies in. a rather compli- 

 cated manner with the diffracting angle d d and the 

 distance of the shield d\ [Figure 7 and equation (10) ]. 

 In Figure 40 is plotted the relative field intensity 

 compared to that obtained without a shield for 

 shields at several distances. This graph is intended 

 for 200 mc but may be used on other frequencies by 

 changing di in proportion to the change in A. It 

 enables one to make an estimate of the effectiveness 

 of a shield. Thus if a shield is 1 mile away it may be 

 neglected for values of B d in excess of +3°. Likewise 



