OVERSEAS RADIO EXTENSION 251 



had experience with short-wave operation reaHzes that the cost of 

 such radio facihties is proportional to the standard of service and to 

 the degree of reHability and exactitude of operation which is under- 

 taken in the terminal stations. 



JoiNiNCi OF A Radiotelephone Link with Wire Network 



The manner of joining the transoceanic radio links with the wire 

 network to meet the requirements of through two-way transmission is 

 an interesting and important development in itself. In general this 

 technique is an outgrowth of wire telephone practice and is so new as 

 not yet to have been fully applied to all of the radiotelephone links in 

 existence. 



The problem is that of how to form the two oppositely directed 

 speech channels which comprise the radiotelephone link itself into the 

 usual two-w^ay telephone circuit suitable for use as a regular telephone 

 toll line and for termination before long-distance traffic operators at 

 each end. 



The transmission equivalent of the radio paths may be continu- 

 ally changing over a considerable range due to fading. It is undesir- 

 able that noise or speech on the incoming channel be reradiated on the 

 outgoing channel. Any tendency for the system to sing must be 

 avoided. It must be possible to change the amplification looking into 

 the transmitters over a wide range so as to get a fully modulated out- 

 put from them, irrespective of the length of the connected lines or 

 the volumes of the talkers' voices. Furthermore, in some cases, as 

 where the same radio-frequency band is used for transmission in the 

 two directions, the radio transmitter tends to interfere with the re- 

 ceiver at the same end. 



A solution of these conflicting requirements necessitates that only 

 one of the radio paths be connected to the wire network at a time. 

 This fundamental principle at one stroke wipes out singing, reradia- 

 tion or echoes, and permits independent adjustments of amplification 

 in the two radio paths. To apply it, it becomes necessary to employ 

 voice-current-operated switching devices which connect alternately 

 the sending or receiving radio channel to the wire line as the sub- 

 scriber talks or listens, automatically following the conversation and 

 serving the needs of the subscriber without his volition. 



Various mechanisms for carrying out this function have been de- 

 vised. Some employ mechanical relays for switching while others 

 use vacuum tubes, but in principle they are much alike. The broader 

 ideas involved are illustrated in Fig. 6. When the circuit is quiescent, 

 i.e., neither subscriber speaking, the receiving radio channel is con- 



