158 



SITING AND COVERAGE OF GROUND RADARS 



15.6.8 L D e Diagrams of Medium Height Sites 



In dealing with radars at medium heights, say 

 from 100 to 1,000 ft, a more involved treatment is 

 required, owing to earth curvature effects. The 

 procedure followed in this section is to compute a 

 value of rf max for each lobe from which a sinusoid 

 is constructed at the angle of the lobe. The envelope 

 of the lobes is considered to be of principal interest, 

 the lobe shape being of secondary importance. 



The strength of a wave is measured in miles, that 

 is, the distance at which the standard target must 

 be to give a standard signal response such as a 

 signal-to-noise ratio of one. The distances corres- 

 ponding to the direct and reflected waves are added 

 to get lobe maxima and subtracted to get minima. 

 The direct and reflected waves will therefore be 

 computed separately. The phase shift due to reflec- 

 tion will be taken as 180°, and the phase shift due 

 to other causes than path difference will be considered 

 negligible. This assumption greatly simplifies calcu- 

 lations and is a good approximation for small angles 

 and horizontal polarization. For vertical polariza- 

 tion, especially in the VHF band, it is a poor 

 approximation. 



The direct wave is affected only by the modified 

 antenna pattern. The reflected wave is affected by: 



1. Shoreline diffraction. 



2. The modified antenna pattern. 



3. Earth curvature. 



4. Coefficient of reflection. 



5. Divergence. 



Terrain effects such as reflection areas of limited 

 extent, the shoreline, cliff edges, and obstacles involve 

 diffraction. A simple, flexible method for solving such 

 problems will be developed in the next section. 



15 69 Shoreline Diffraction 



Unfortunately sites of sufficient height are 

 frequently some distance inland, and a considerable 

 portion of the reflection surface is on land. The 

 poor reflecting qualities of land, especially when 

 rough, causes the high angle lobes due to nearby 

 reflection to be reduced as much as 50 per cent in 

 length. This is a common cause of poor high coverage 

 so often experienced in field installations and the 

 inability to detect high-level bombing attacks except 

 at perhaps 10-mile ranges. In this section will be 

 developed a method of computing the vertical 

 coverage pattern for the typical high site with part 

 land and part sea reflecting surfaces. 



In most cases the profile of the land between the 

 transmitter and the shore will be found to be too 

 rough for coherent reflection, as may be determined 

 from equation (16). If substantial regular areas or 

 obstacles occur between the antenna and the shore 

 line they should be treated as described in Section 

 15.6.12, on the modified antenna pattern. 



156 10 Sea Reflection 



with Diffuse Land Reflection 



The problem treated in this section will be that 

 shown in Figure 57. The land in the foreground is 

 so rough as to cause only diffuse reflection, and no 

 regular areas exist which will affect the vertical 

 pattern below 15°. 



The diffuse reflection from the land area has a 

 random phase relation, and the field intensity in a 

 particular direction is relatively small. The effect of 

 the land reflection on the interference pattern is 



RADAR ANTENNA 



SHORE LINE 



Figure 57. Fresnel zones on land and sea areas. 



