608 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



LU 

 O 

 < 



a. 

 in 



2 

 o 

 a. 

 u. 



ffl 

 O 



UJ 



o 



2.5 



FIRST FRESNEL ZONE 



Fig. 8 — Transmission loss versus clearance. 



At grazing incidence, the expected loss over a ridge is 6 db (Fig. 7) 

 while over a smooth spherical earth Fig. 6 indicates a loss of about 20 

 db. More accurate results in the vicinity of the horizon can be obtained 

 by expressing radio transmission in terms of path clearance measured 

 in Fresnel zones as shown in Fig. 8. In this representation the plane 

 earth theory and the ridge diffraction can be represented by single lines; 

 but the smooth sphere theory requires a family of curves with a param- 

 eter M that depends primarily on antenna heights and frequency. The 

 big difference in the losses predicted by diffraction around a perfect 

 sphere and by diffraction over a knife edge indicates that diffraction 

 losses depend critically on the assumed type of profile. A suitable solu- 

 tion for the intermediate problem of diffraction over a rough earth has 

 not yet been obtained. 



Experimental Data Far Beyond the Horizon 



Most of the experimental data at points far beyond the horizon fall 

 in between the theoretical curves for diffraction over a smooth sphere 



