32 TECHNICAL SURVEY 
I, 
OB BELOW 1 WATT 
70- 
AN FREE SPACE 
i i 
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~ Wray 
mn 
"PINE (57-FT STATION) 
120- 
SEPT I6 
Dieter Bee Solletle 916; a nite 
21 5 10 15 20 6 ut 
(Minn 
SEPT 17 
70 
SEPT 18 
16 21 2 7 12 17 22 
TIME IN HOURS ——S 
Il FREE SPACE 
\ "BEACH" (16-FT STATION) 
SIGNAL LEVEL AS A FUNCTION OF TIME 
SEPT 16 TOIS 
Ficure 10. Variation of signal strength over 3 days. Two receivers on S band, Flathead Lake, Montana. 
well marked as in this graph. Another feature of 
interest is the fact that the maximum signal level 
is fairly close to the free space level. This has been 
found to hold approximately in a number of other 
propagation experiments where, in the presence of 
a duct, the maximum received level seems to occur 
not far from the theoretical free space signal level. 
No explanation for this behavior has been given, and 
it may be purely accidental. 
Figure 11 presents, for part of the same period as 
shown in Figure 10, the value of k as a function of 
time at a point on the transmission path. Here k 
is a measure of the slope of the M curve in the lowest 
strata. Combining equation (17), Chapter 1, and 
equation (4), Chapter 2, we have 1/ka = dM/dh - 
10-5. Thus when k is negative a duct is present. It 
will be seen that the incidence of negative values of k 
correlates well with high signal strength in Figure 10. 
CANADIAN EXPERIMENTS 
The Canadian transmission experiments are being 
undertaken by the Tropospheric Subcommittee of 
