EXPERIMENTS I !N NOKTII \\ KST 



65 



receiver of the long transmission path. The radar 

 target is near the one-way receiver so thai both paths 

 are practically coincident. When the radar signal was 



below the limit of sensitivity, it is indicated on the 

 graph by this limit so that the lower points of the 

 diagram really have little physical significance. If a 

 straight line is drawn, averaging the variation of the 

 higher points, its slope is roughly 2:1 as should be 

 expected. 



100 



80 60 



DB BELOW 1 WATT 

 SIGNAL STRENGTH 



40 



Figure 9. Correlation between maximum radar ranges 

 and one-way signal strength. X band, Massachusetts 

 Bay. 



Figure 9 shows a correlation between the one-way 

 signal strength on the S band and the maximum 

 range of fixed echoes detected by the S-band radar 

 along the coast. It is interesting to note that super- 

 standard radar ranges do not appear until the one- 

 way signal has reached a certain, rather larger value. 

 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. 



s! EXPERIMENTS IN NORTHWESTERN 

 UNITED ST ATI S \M) 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 ' 137 ' 164 ' 22s 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 long 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 demonstrated 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 

 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 



