TRANSATLANTIC RADIO TELEPHONY 



311 



frequency. In Fig. 2 we have shown the calculated field strength 

 at 5,000 kilometers for a power of 85.9 kilowatts supplied to the Rocky 

 Point antenna, using efificiency data of Fig. 1 and the radio transmis- 

 sion formula given by Espenschied, Anderson, and Bailey.'* Since 

 this curve reaches a maximum near 18.5 kilocycles, the reason for the 

 operation of early transatlantic radio-telegraph circuits in the range 

 10 to 30 kilocycles becomes apparent in light of the limitation then 

 placed on the receiving systems. 



30 40 



FREQUENCY 



50 60 



KILOCYCLES 



Fig. 1 — Assumed resistance and efficiency of Rocky Point antenna. 

 (Effective height 75 meters.) 



Systematic measurements of radio noise by the warbler method,^ 

 begun early in 1923, have yielded important information on the 

 variation of noise with frequency.^ From measurements begun by 

 engineers of Bell Telephone Laboratories and continued by engineers 

 of the International Western Electric Company at New Southgate, 

 England, during 1923 and 1924, the average daylight noise curve, in 

 Fig. 2, was obtained. It is seen that the noise decreases with increasing 



^ Lloyd Espenschied, C. N. Anderson, and Austin Bailey, "Transatlantic Radio 

 Telephone Transmission," Bell System Tech. Jour., 4, 459; July, 1925. Proc. I. R. E., 

 14, 7; Feb., 1926. 



^ Ralph Bown, C. R. Englund, and H. T. Friis, "Radio Transmission Measure- 

 ments," Proc. I. R. E., 11, 115; April, 1923. 



