310 BELL SYSTEM TECHNICAL JOURNAL 



1. The signal-to-noise ratio at the receiving location, which in 



turn is dependent upon four factors — 



(a) The efficiency of the transmitting set, 



(b) The efficiency of the transmitting antenna, 



(c) Attenuation in the radio path, 



(d) Variation of radio noise with frequency; 



2. Band width of the transmitting antenna; 



3. Receiving antenna efficiency; 



4. Available space in the frequency spectrum. 



1. Signal-to-Noise Ratio at the Receiving Location. At the time that 

 the transatlantic radio-telephone development was undertaken, engi- 

 neers of the Western Electric Company Engineering Department 

 (now Bell Telephone Laboratories) had developed a form of water- 

 cooled vacuum tube capable of generating efficiently large amounts of 

 power at any frequency up to perhaps several hundred kilocycles. 2 

 Therefore transmitter efficiency, although a major problem in itself, 

 imposed no restriction on the frequency for the telephone circuit. 



For transmission over a given path, utilizing a particular trans- 

 mitting antenna with constant power supplied to it, there will be, in 

 general, a frequency at which the greatest signal-to-noise ratio is 

 obtained. To illustrate this point, we have chosen the problem of 

 transmission from an antenna of the type used at the Rocky Point 

 station of the Radio Corporation of America in U. S. A. to a receiving 

 station in England, a distance of approximately 5,000 kilometers. 

 The approximate variation with frequency of loss resistance, radiation 

 resistance, and efficiency of this antenna is shown in Fig. 1. The loss 

 resistance at 60 kilocycles was determined by engineers of Bell Tele- 

 phone Laboratories, while the data in the lower frequency range were 

 published by Alexanderson, Reoch, and Taylor.^ The radiation re- 

 sistance was calculated from the measured effective height of the 

 antenna. It is seen in Fig. 1 that the antenna efficiency increases 

 with frequency throughout the range we are considering, first rapidly 

 and then more slowly. 



For a constant power radiated, radio attenuation tends to cause a 

 decrease in the average received signal strength as the frequency is 

 increased. This effect is in the opposite direction to the effect of 

 antenna efficiency, so that for a given power supplied to the antenna 

 the field strength at a given distance will be a maximum at a certain 



2 W. Wilson, "A New Type of High Power Vacuum Tube," Bell System Tech. 

 Jour., 1, 4; July, 1922. Elec. Comm., I, 15; August, 1922. 



' E. F. W. Alexanderson, A. E. Reoch, and C. H. Taylor, "The Electrical Plant 

 of Transocean Radio Telegraphy," Trans. A. L E. E., 42, 707; July, 1923. 



