006 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



presence of hilLs, trees, and bnildings is difficult or impossible to compute, 

 but the order of magnitude of these effects may be obtained from a con- 

 sideration of the other extreme case, which is propagation over a per- 

 fectly absorbing knife edge. 



The diffraction of plane waves over a knife edge or screen causes a 

 shadow loss whose magnitude is shown on Fig. 7. The height of the ob- 

 struction H is measured from the line joining the two antennas to the 

 top of the ridge. It will be noted that the shadow loss approaches (> db 





X/ 



500' 



d,<d2 



LOSS RELATIVE TO FREE SPACE 

 L, -I-L2 +L3=18.5 + 2,5 + 6 =27 DB 



Z 



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50 — 



100 — 



200- 



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-5 



LU 

 U 



Z 



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 < 



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Fig. 6 — Dif't'iactiou loss relative to free space transmission at all locations 

 beyond line-of-sight over a smooth sphere. 



