140 



SITING AND COVERAGE OF GROUND RADARS 



6j IN DEGREES 



Figure 40. Diffraction over a ridge. (200 mc) (For other frequencies change di in proportion to the change in wavelength.) 



objects below —3° in the shadow region would give 

 weak echoes in most cases. For intermediate angles 

 the relative intensity may be read from the curve. 

 For shields closer than 0.1 mile the methods of 

 Section 15.4.8 should be employed. 



Figure 15 shows that the relative intensity of a 

 diffracted wave is virtually constant for a given 

 angle when the distance from the edge is large. 

 Equation (22) may then be written in the form 



RADAR 



\2di 



(47) 



where 6 d is in radians (1 radian = 57.3°) measured 

 from the geometrical shadow line (Figure 7) and 

 di is in the same units as A. This equation is approxi- 

 mate, and the error is of the order of a/b. 



Where the shield consists of several ridges close 

 together, an equivalent shield is used instead of 

 successive shields. The height and distance of the 

 equivalent shield is found by constructing a triangle 

 between the radar and the reflecting object which 

 encloses the shielding ridges. The apex of this triangle 

 is then treated as though it were the diffracting 

 edge. In Figure 41 H and di are the quantities to 

 be used in equations (10) and (47). 



The general procedure to be followed in preparing 



Figure 41. The equivalent shield. 



a prediction of permanent echoes will now be out- 

 lined. By examining a topographical sheet the azi- 

 muths are determined at which profiles should be 

 prepared. This will normally be about every 10 

 degrees. Where the shielding is obviously good the 

 interval may be 20 degrees, but where the terrain 

 is questionable such as a region of low hills the 

 profiles should be taken at 5-degree intervals. The 

 profiles are prepared and the angle of the line of 

 sight determined as described above. 



The next step is to make an overlay of a map of 

 scale 1 to 1,000,000. The principal features as coast- 

 line, towns, and rivers are sketched in to aid in 

 reading the completed chart. On this is drawn a 

 polar coordinate system with azimuths marked every 

 10 degrees and range circles every 10 miles out to 

 the full range of the indicator. 



On the overlay are now drawn the coverage 

 contour lines. These lines represent the limits of 

 the heights of the shielded regions. Targets or 

 mountains below these coverage contours will not 



