166 



SITING AND COVERAGE OF GROUND RADARS 



The product of f T and/. 4 is the modified antenna fac- 

 tor /(t). This is plotted in Figure 64. With a larger 

 reflecting surface, the length of lobes would approach 

 twice the value of /..,. Figures 64 and 65 were drawn 

 for purposes of illustration and would not ordinarily 

 be required. 



Example 18. Cliff Edge Diffraction. A 200-mc radar, 

 Figure 66, with an antenna as described in Example 



h, =50 FEET 



CLIFF EDGE =3000 FEET 



f = 200 MC 



f«) = rf» 



MODIFIED ANTENNA 

 PATTERN FOR SEA 

 LOBES 



ROUGH 

 LAND N 



4000 8000 12,000 



DISTANCE d. IN FEET 



Figure 66. Cliff edge diffraction. (Example 18.) 



16, is 50 ft above a rough land surface, the top of a 

 cliff whose edge is 3,000 ft away. 



The geometrical shadow line makes an angle with 

 the horizontal of 



tan 



50 

 3,000 



= - 0.955 c 



At this angle, the relative intensity, z = 0.5. Other 

 values of z may be read from Figure 27 after con- 

 verting the angle of diffraction to v by means of 

 equation (47). 



a.° 



- = o.6i e d ° . 



4 



4.92 



X 57.3 



1 2 X 3,000 



At 0.377° in the shadow region v = -0.377 X 0.61 

 = -0.23. From Figure 27, z is 0.4. This angle, 

 referred to the horizontal at the antenna, is 



7 = * - 0.955 = -1.332°. 

 Some other values are : 



The modified antenna pattern f(y) is the product 

 of z and f A and is plotted in Figure 66. This pattern 

 gives the factors for both the direct and reflected 

 waves for the sea lobes. 



Example 19. Land Reflection and Diffraction. This 

 site is similar to that of Example 18 except that the 

 cliff top is smooth. This is shown in Figure 67. 



/C3 



ANTENNA 



f=200 MC 



,50 FT 



X£ 



SMOOTH LAND 



600 FT 3000 FT 



550 FT 



5000 FT 



SEA 



Figure 67. Land reflection and diffraction. (Example 

 19.) 



The smooth land causes land lobes to be formed 

 as in Example 17 which furnish high angle coverage. 

 The sea lobes are computed using the method of 

 Example 18 for the direct and reflected rays. If the 

 cliff top were tilted down, the land lobes would be 

 tilted by the angle of the land. Speculation about 

 complex sites yields many unusual patterns, but in 

 practice the results are usually disappointing. Com- 

 plex sites seldom have horizontal symmetry, and 

 gaps in the coverage pattern may be expected. 

 Attempts to reinforce the pattern in a particular 

 direction by siting back from the cliff edge generally 

 cause poor coverage at other angles. Best all-round 

 CHL operation results from siting on cliff edges and 

 exclusive use of the sea as a reflector. 



16615 Earth Curvature Effect 



on Lobe Lengths 



The effect, of earth curvature on lobe angles was 

 described in Section 15.6.4. The angles to be used 

 with the modified antenna pattern of the image 

 antenna are affected by earth curvature, and there- 

 fore the strength of the reflected wave is also affected. 

 In Figure 68 is shown a radar antenna at height h x 

 above the earth's surface, with the center line of the 

 antenna pattern parallel to GH, the horizontal at 



