266 RADIO WAVE PROPAGATION EXPERIMENTS 
Ficure 15. Cross section of Porth Gain Bay (Wales) in vertical plane through transmitter and receiver. 
tion was measured as a function of the distance from 
the transmitter. This distance varied between 3,500 
and 42,000 yd. The observed values of the field 
strengths correspond well with calculations based on 
electromagnetic theory. 
In a second series of experiments’ both the trans- 
mitter and receiver were stationary. The topography 
of the location and the experimental setup are rep- 
resented schematically in Figure 15. The main con- 
clusion drawn from these experiments was that even 
for a calm sea (vertical amplitude of the waves less 
than 8 in.) the mean amplitude of the reflected ver- 
tically polarized beam was only. about half the steady 
amplitude of the direct wave. The amplitude of the 
reflected wave, however, occasionally rises to greater 
values than that of the direct wave, but this lasts only 
4 OVER SEA 
GRAZING ANGLE IN DEGREES 
FicurE 18. Sea reflection coefficient p, versus grazing 
angle y at 3.2 cm. Vertical polarization. 
for short lengths of time of the order of 0.5 sec. 
In the case of the reflection of horizontally polar- 
ized radiation at the surface of a smooth sea it is 
known that even for grazing angles as large as 10° 
the amplitude of the reflected wave is very nearly equal 
to that of the direct wave. The irregularities of the 
sea, however, reduce the amplitude of the reflected 
wave. This reduction is due to scattering, i.e., to non- 
specular reflection of the radiation by the irregu- 
larities of the sea surface. It is recalled here that the 
surface irregularities will play an important role as 
soon as they are larger than A/y, A being the wave- 
length and y the grazing angle in radians. 
The Radiation Laboratory workers,*’ used an air- 
plane as the carrier of the receiver flying toward the 
transmitter. For 10- and 3.2-cm waves the latest re- 
4 OVERSEA 
GRAZING ANGLE IN DEGREES 
Ficure 19. Sea reflection coefficient 9, versus grazing 
angle y at 3.2 cm. Horizontal polarization. 
