Two-Way Radio Telephone Circuits * 



By S. B. WRIGHT and D. MITCHELL 



This paper deals with the problems of joining long-distance radio tele- 

 phone transmission paths to the ordinary telephone plant. It gives the 

 possibilities and limitations of various methods of two-way operation of such 

 circuits where the radio channels employ either long or short waves. It also 

 describes the special terminal apparatus for switching the transmission paths 

 under control of voice currents and lists the advantages of using voice- 

 operated devices. 



RADIO telephone circuits are now in regular use between New York 

 and London, New York and Buenos Aires, San Francisco and 

 Honolulu, and many other points. At each end of such a circuit there 

 are a transmitting radio station and a receiving radio station, usually 

 geographically separated. The radio provides two one-way trans- 

 mission paths. The circuit is completed by means of one-way wire 

 lines which connect the radio sending and receiving stations to a 

 common point. At this common point some rather intricate appara- 

 tus is called for in order to permit switching of the circuit to the wire 

 telephone plants at the two terminals. This paper explains why this 

 intricate apparatus is necessary even for the comparatively simple 

 case of short-wave radio circuits which use different frequency bands 

 to transmit in opposite directions. It also describes the latest form 

 of this terminal apparatus in which provision is made for certain 

 switching of privacy apparatus by means of which an important saving 

 is made in the amount of privacy apparatus required. The original 

 form of this apparatus is described in an earlier paper. ^ 



Transmission Paths 



Radio telephone circuits may employ the same frequency band for 

 transmission in the two directions or they may employ separate bands. 

 The present long-wave telephone circuit between New York and Lon- 

 don is of the first type, while most existing short-wave circuits are. of 

 the second. 



A short-wave circuit, using separate frequency bands, is shown in 

 its simplest form in Fig. L It is formed of two sets of terminal appara- 



* Presented at I. R. E. Convention, Pittsburgh, Pa., April 7-9, 1932. Published 

 in Proc. I. R. E., July, 1932. 



' S. B. Wright and H. C. Silent, "New York-London Telephone Circuit," Bell 

 5ys. Tech. Jour., 6, 736-749; October, 1927. 



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