TRANSMISSION EXPERIMENTS 31 
The one-way signal does not seem to be able to 
increase much beyond this value, whereas the range 
of detectable radar targets rises with extreme rapidity. 
SIGNAL STRENGTH OF TARGET AT EASTERN POINT 
DB BELOW 1 WATT 
DB BELOW i WATT 
SIGNAL STRENGTH 
Ficure 8. Correlation between one-way and radar sig- 
nal strengths over the same path. X band, Massachu- 
setts Bay. 
EXPERIMENTS IN NORTHWESTERN 
UNITED STATES AND CANADA 
STATE COLLEGE OF WASHINGTON PROJECT 
During 1943, a series of transmission experiments 
were carried out by a group of workers from the 
State College of Washington under the auspices of 
Division 14, NDRC.134; 187,164, 228 The first series of 
tests were made in the neighborhood of Spokane over 
14- and 52-mile optical paths and over a 112-mile 
nonoptical path. Later in the same year a transmis- 
sion path 20 miles lone with receivers both below 
and above the optical horizon was installed on the 
east side of Flathead Lake, Montana. 
Among the tests carried out by this group was an 
experimental telephone communication on 10-cm 
waves which gave excellent results. The earlier 
experiments demoustrated the necessity of having 
detailed data on the refractive index variation in 
low levels and thus led to the development of the 
State College of Washington wired balloon sonde, 
described in the preceding chapter and of basic 
importance for further propagation work. The first 
model of the sonde was used systematically in con- 
nection with the Flathead Lake transmission path. 
The location of these experiments has a climate of 
a continental type, there being several mountain 
ranges between these spots and the Pacific coast. 
The air is comparatively dry, and the structure of 
the lowest strata is subject to the large variations: 
of temperature and of stability typical of continental 
conditions. 
The general results of these tests are similar in 
many respects to those found at the east coast of 
RANGE IN STATUTE MILES 
60 
DB BELOW 4 WATT 
SIGNAL STRENGTH 
Ficure 9. Correlation between maximum radar ranges 
and one-way signal strength. X band, Massachusetts 
Bay. 
the United States. The signal types are analogous, 
but the times and frequencies of occurrence are often 
quite different. In the Flathead Lake experiments, 
where strong ducts were often present, signal level 
variations of 50 db were observed for the optical 
path, 55 db for the nonoptical paths. The correlation 
between the observed M curves and the received 
signal strength was extremely close, high signal 
levels being observed when the measured M curves 
showed the presence of a duct; and standard signal 
levels, when the M curve was of the standard type. 
Similar observations were later made many times 
over in other experiments such as those at Massa- 
chusetts Bay, already described. 
Figure 10 shows typical signal records in form of 
hourly maxima and minima over a three-day period 
for the 20-mile path on Flathead Lake. Though the 
path itself is entirely over water, the over-water 
trajectory of the air is limited by the dimensions 
of the lake. Both receiving stations are below the 
line of sight, the upper by 91 ft, the lower by 132 ft. 
There is, in this graph, a rather clearcut distinction 
between periods of standard propagation with a 
comparatively limited margin of variability of the 
signal, and periods of sunerrefraction accompanied 
by very deep fades. This behavior is found in most 
propagation experiments but is perhaps rarely as 
