TRANSATLANTIC RADIO TELEPHONY 137 



The curves are plotted between time of day as abscissas and field 

 strength in microvolts per meter as ordinates. The time during which 

 darkness prevailed at Rocky Point and at London is indicated by the 

 block-fills on the time scales. The overlap of these block-fills indicates 

 the time during which darkness extended over the entire transatlantic 

 path. For Fig. 9 the darkness-belt is as of February 1 and for Fig. 10 

 as of March 21. The curves show the mean of the results and also 

 the boundaries of the maximum and minimum values observed. 



Received Signal Strength. The outstanding factors to be noted 

 concerning the signal strength curves are: 



1. The diurnal variations are plainly in evidence. During the first 

 test period covered by Fig. 9, for example, the field strength varied 

 in the ratio of the order of 15 to 1 between day and night conditions, 

 running about 100 microvolts per meter during the night and averag- 

 ing about 6 microvolts per meter during the day. The diurnal varia- 

 tion is also to be seen in Fig. 10 although the variations between night 

 and day transmission are less marked. 



The measured daylight values lend support to the Austin-Cohen 

 absorption coefficient. The average of the observed daylight value 

 for the period of these tests is between 7 and 8 microvolts per meter, 

 while the calculated value is 9.5. Concerning the high field strengths 

 obtaining at night, it should be noted that the maximum observed 

 value of 237 microvolts per meter does not exceed the value of some 

 340 microvolts which it is estimated should obtain at London were 

 no absorption present in the intervening medium, i.e., were the waves 

 attenuated in accordance with the simple inverse-with-distance law. 

 While no definite conclusions can yet be drawn from these results as 

 to the cause of the diurnal variations, this indication that the upper 

 limit of the variation is the no-absorption condition suggests that 

 the diurnal fluctuations are controlled by the absorption conditions 

 of the medium rather than by reflection or refraction effects. 



2. An indication of the seasonal variation which apparently occurs 

 in developing from winter to early spring is found in a comparison of 

 the signal strength curves of Figs. 9 and 10. On the whole the signal 

 strength received in the second test period is considerably less than 

 that received for the first period. This drop in the average of the 24 

 hours is caused by a large decrease in the night-time transmission 

 efficiency. The daylight transmission does not change much, but 

 what little change there is lies in the direction of an increase as the 

 season advances. 



3. A decrease in the transmission efficiency is observed between 

 the time of sundown in London and sundown in New York, that is, 



