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80 MILE LINK SAN PEDRO TO SAN DIEGO 

 TRANSMITTER AND RECEIVER AT 100 FT. ALTITUDE 



BASE Of TEMPERATURE INVERSION 



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Figure 13. Signal strength over 80-mile path, San Diego to San Pedro, correlated with height of temperature inversion. 



spicious feature of Figure 12 is the difference between 

 the signal distribution in the absence and presence of 

 a duct at 500 ft, the lowest level measured, whereas 

 the intensities agree fairly well at the higher levels. 

 This behavior is in full agreement with the general 

 predictions of propagation theory. Nevertheless, the 

 detailed interpretation led to a slightly different 



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Figure 14. Computed number of modes trapped versus 

 observed field strength, San Diego Bay. 



result from that expected, as was brought out by 

 subsequent experimental investigations. 



In 1944 a one-way transmission path was operated 

 between San Pedro and San Diego, an over-water 

 path 10 - 159 80 miles long with both terminals at an 

 elevation of 100 ft, which were thus well below the 

 optical horizon. Three fairly low frequencies, 52, 

 100, and 547 mc, were used. Figure 13 shows a field 

 strength diagram of bihourly means for a period of 

 about six weeks in the early fall of 1944. At the top 

 of these diagrams is shown the height of the base 

 of the temperature inversion, which is a quantita- 

 tive measure of the height of the elevated duct. In 

 order to compare these data with the results of duct 

 theory, Figure 14 shows the number of lowest modes, 

 trapped in the elevated duct, plotted against the 

 signal strength. For each point indicated, the number 

 of trapped modes is calculated by simple waveguide 

 theory from the measured M curves while the field 

 strength is that simultaneously measured on the 

 transmission path. For the lowest frequency, 52 mc, 

 the duct is always beyond cutoff and no trapping 

 should occur; nevertheless, the field strength record 

 shows considerable fluctuation. 



As seen from Figure 14 there is no correlation be- 



