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BELL SYSTEM TECHNICAL JOURNAL 



portion of this curve is for straight-out-to-sea transmission where the 

 attenuation law is seen to be normal. 



In the field strength measurements made during the ship-to-shore 

 development, those of the S. S. America en route across the Atlantic 

 are especially illuminating. The results are given in Fig. 20. The 

 vessel was in-bound so that the curve develops from the right to the 

 left, altho the effect is just the same as if it developed in the reverse 

 direction with the vessel out-bound as was shown by another set of 

 measurements which gave generally similar results. The actual 

 measurement results are indicated by the points and by the con- 



1000 

 S00 



too 

 S700 

 *600 



S50C 



^400 



-300 



•200 



100 



TRANSMISSION 1 lST 

 Col Beach and S. S. Gloucester 

 July 20, 1921 

 South Bound oh Course 3 



320 KlLO-OYCLES 



UA8HE0 Curve Austin-Comen Formi 



100 lib 120 T3T3 TO r?6 T5D" 



Fig. 19 



necting heavy line curve. The light curve A is a plot of the Austin- 

 Cohen formula, the absorption term of which is for daylight trans- 

 mission over water. The light curve B is a plot of the simple inverse- 

 with-distance law without any absorption term. 



This curve shows up the following important factors: 



(1) The enormous variation between day and night in the received 

 field strength which occurs at distances of the order of 1,000 

 miles (1,600 km.) using wave lengths of 350 to 400 meters as 

 now employed in broadcast transmission. The curve shows 

 night to day variations of the order of 100:1 or a power ratio 

 of 10,000:1. This means, for example, that it would require 

 10,000 times more power to "get thru" as well during the day 



