68 



TRANSMISSION EXPERIMENTS 



1.5 



0.5 

 



20 30 



RANGE IN NAUTICAL MILES 



Figure 12. Signal strength at several elevations as function of distance. (Near San Diego.) 



84 EXPERIMENTS IN 



THE SOUTHWESTERN UNITED STATES 



The Navy Radio and Sound Laboratory at San 

 Diego has performed a considerable number of 

 propagation experiments which have substantially 

 aided our understanding of the phenomena of guided 

 propagation. Moreover the meteorological conditions 

 found in this part of the United States are rather 

 unique; and, while they are not, perhaps, reproduced 

 at many other places of the earth, they are so clear- 

 cut and regular as to facilitate greatly experimental 

 investigations and their interpretations. 



The meteorological conditions at San Diego during 

 most of the year are characterized by the presence 

 of a high-pressure area and high-level subsidence. 

 In more concrete terms, there is a surface stratum 

 of comparatively cool and moist air on top of which 

 there is a layer of very dry, warm air. The transition 

 between the two strata is as sharp as can be found 

 anywhere, and the transitional layer is often no more 

 than a few hundred feet thick. The height of the 

 transition layer above the ground is usually between 

 1,000 and 3,000 ft and sometimes as much as 4,000 ft. 



During the winter of 1942 to 1943, a series of 

 measurements were made on the intensities of arti- 

 ficial fixed echoes of a 700-mc radar located near 

 San Diego, 125,138 and these were compared with 

 measured temperature and humidity gradients in 

 the lower atmosphere. A pronounced correlation 

 between excessive echo ranges and nonstandard 

 M gradients at once appeared. The quantitative 

 aspects of these correlations will not be discussed 

 here since they are very similar to others of this 

 type already reported. 



Another set of observations where the receiver 

 was located in a plane is shown in Figure 12. 3 The 

 receiving antenna was a Yagi, mounted in the nose 

 of the plane, records being made when the plane was 

 flying over the ocean toward the transmitter which 

 was a 500-mc radar. Figure 12 represents the results 

 of flights at various altitudes on two different days, 

 the maxima of the signal strength curves corres- 

 ponding to the "lobes" of the transmitter pattern. 

 On one of these days a duct was present as shown 

 in the inset where M is plotted against height. The 

 dot-and-dash straight line in this diagram represents 

 the condition dh/dM = constant. The most con- 



