70 ‘ f TECHNICAL SURVEY 
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 (8) 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- 
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. 
“She 
6 
40 
GERDECT: aitaaie ile SPACE FIELD 
le ee CONE a 
FIELD INTENSITY 
S BAND SO-FT ANT 
1 WATT RADIATED 
MAXIMUM VALUES. 
LINE OF SIGHT FALL ON THIS LINE 
-20}REFLECTION COEFFICIENT=-1 
605 7000 
MILES 
20) 
OB ABOVE ipV PER METER 
oo 
-40 
Figure 6. Comparison of diffraction over irregular earth 
and over a smooth sphere for S-band waves over sea 
water. 
