TRANSOCEANIC TELEPHONE SERVICE 259 



interference is moderate. The first transatlantic radiotelephone cir- 

 cuit opened in 1927 was a long- wave circuit (58.5-61.5 kilocycles). 

 In providing the next few channels for the initial growth of the service 

 the opportunity to determine the utility of short waves was embraced. 



The short-wave range is vastly wider in kilocycles but, nevertheless, 

 has its limitations as to the number of communication facilities it 

 afifords. For a given route of a few thousand miles a single frequency 

 gives good transmission for only a part of the day. For example, 

 from the United States to Europe a frequency of about 18,000 to 21,000 

 kilocycles (17 to 14 meters) is good during daylight on the Atlantic. 

 But in the dawn and dusk period a frequency of about 14,000 kilocycles 

 (22 meters) is better. For the dark hours something like 9000 kilo- 

 cycles (33 meters) gives best transmission and for midnight in winter an 

 even lower frequency near 6000 kilocycles (50 meters) is advantageous. 

 Thus, in considering the short-wave range in terms of communication 

 circuits, we must shrink its apparent width materially to take account 

 of the several frequencies required for continuous service. 



At the present time the frequency spaces between channels are 

 much greater than the bands of frequencies actually occupied by useful 

 transmission. This elbow room is to allow for the tendency of many 

 stations not to stay accurately on their nominal frequencies but to 

 wander about somewhat. But in spite of this allowance, cases of 

 interference are common and one of the activities which must be carried 

 on in connection with a commercial system is the monitoring of inter- 

 fering stations and the accurate measurement of transmitting fre- 

 quencies to determine the cause of the conflict. To permit intensive 

 development of the frequency space offered by Nature the greatest 

 possible constancy and accuracy of frequency maintenance in trans- 

 mitting sets will be required. 



The fact that channels have been assigned (within wide bands set 

 aside for a particular service) with little regard to the geographical 

 location of stations may result in neighboring channels having much 

 stronger signals than those in the channel being received. When 

 this is so, a severe requirement is placed on the selectivity of the re- 

 ceiver to prevent interference. 



Interconnecting with Wire Circuit Extensions 



The skeleton of a radiotelephone circuit is in its essentials very 

 simple. It consists merely of a transmitter and a receiver at each end 

 of the route and two oppositely directed, one-way radio channels 

 between them. These two independent channels must be arranged 

 at the terminals to connect with two-wire telephone circuits in which 



