TRANSATLANTIC RADIO TELEPHONY 139 



Ratio of Signal to Noise Strength; Words Received. The noise curve 

 of Fig. 9 and that of Fig. 10 can, therefore, be read as "The strength 

 of the signal tone which can just be heard through the noise." It can, 

 therefore, be directly compared with the signal curve itself and the 

 difference between the two curves is a measure of the level of the actual 

 signal strength above that which would just permit of the signals being 

 heard. Actually, the difference between the two curves, as shown in 

 the figures, is proportional to the ratio of the signal to the noise 

 strength, because the curves are plotted to a logarithmic scale. 



This signal to noise ratio is plotted in Fig. 11 for the test period 

 which corresponds to Fig. 9, and Fig. 12 for the test period which 

 corresponds to Fig. 10. These ratio curves are derived by going back 

 to the original data and taking the ratio for each unit measurement 

 period and spotting it upon the chart as shown by the black points. 

 An average is taken of the points for each hour of the 24-hour period 

 as shown by the circle points. The dash line curves of Figs. 11 and 12, 

 therefore, trace the average diurnal variation of signal to noise ratio. 



These curves show: 



1. That the signal to noise ratio reaches its minimum during the 

 time when the sunset period intervenes between London and New 

 York. 



2. During the night in London the ratio increases more or less con- 

 tinuously and reaches a maximum around the time of sunrise in 

 London. 



3. During the course of the daylight period in London the ratio 

 starts out high and drops rather rapidly during the forenoon and 

 assumes a more or less constant intermediate value during the after- 

 noon until sundown. It is during this afternoon period in London 

 that the business hours of the day in London and New York coincide, 

 so that this is the most important period from a telephone com- 

 munication standpoint. 



The drop in the very low ratios obtaining in London in the early 

 evening is due to the fact that an increase in noise occurring at this 

 time is accompanied by a decrease in transmission efficiency from 

 America. This may readily be seen by referring to Fig. 10. The 

 noise increases as the night belt, proceeding westward, envelops Eng- 

 land and improves the transmission of atmospherics, which arise 

 possibly in continental Europe, Asia and Africa. As the shadow wall, 

 proceeding westward, intervenes between England and America, the 

 transmission efficiency of the desired signals from America drops and 

 it is not until the night belt extends as far west as America that the 

 transmission efficiency improves sufficiently to overcome the dis- 



