112 



DIFFRACTION OF RADIO WAVES OVER HILLS 



The net result is that the total received field is 

 represented more closely by the path ACB than by 

 the path A DEB. The energy received over any given 



Figure 5. Field intensity computation for a profile of 

 two hills by a solid triangle. 



path such as path ADEB decreases rapidly as the 

 number of diffractions in that path increases. How- 

 ever, for any profile there is always at least one 

 path between transmitter and receiver such as path 

 ACB that requires no more than one diffraction, 

 and the field intensity over this path is usually 

 controlling. In other words, the profile consisting of 

 two hills can be approximated for computation pur- 

 poses by a solid triangle which is formed by a line 

 from the base of the transmitting antenna to the base 

 of the receiving antenna and lines from the base of 

 each antenna tangent to the hill that blocks the line 

 of sight. By the same reasoning it appears that a 

 profile which includes any number of hills can be 

 represented approximately by the circumscribing 

 triangle. 



The principal assumptions that are basic to this 

 method of treating radio propagation over hills and 

 other obstructions are as follows: (1) the height of 

 antennas is greater than about one-half wavelength, 

 (2) the size of obstructions is large compared with 

 the wavelength, and (3) the distance between anten- 

 nas is large compared with either the antenna height 

 or the size of the obstructions. These assumptions 



limit the application of this theory to wavelengths 

 shorter than a few meters. 



The principal differences between the diffraction 

 over an irregular earth and the diffraction over a 

 smooth sphere is illustrated in Figure 6 for trans- 



FIELD INTENSITY 

 S BAND 50-FTANT 

 1 WATT RADIATED 

 REFLECTION G0EFFICIENT=-1 



LINE OF SIGHT 

 I L_L 



\ 



in I I 



f 



MAXIMUM VALUES 

 FALL ON THIS LINE 



J I I I I I II 



50 100 

 MILES 



Figure 6. Comparison of diffraction over irregular earth 

 and over a smooth sphere for S-band waves over sea 

 water. 



mission of S-band waves over sea water. The dashed 

 line shows the field intensity versus distance over a 

 perfectly smooth earth. The solid line shows diffrac- 

 tion over a solid triangle which represents what is 

 expected when the sea is rough, that is, when the 

 height of the water waves is large compared with 

 the S-band radio waves. It will be noted that there 

 is little difference between the two methods for 

 distances less than about twice the optical range, 

 but at greater distances the solid triangle theory 

 indicates that some energy will be received at 

 appropriate distances. 



These views on the transmission of meter and 

 centimeter radio waves over multiple obstacles are 

 speculative. There is little experimental evidence to 

 support them, but also there appears to be even less 

 experimental evidence to contradict them. 



