TWO-WAY RADIO TELEPHONE CIRCUITS 369 



tus connected by two one-way channels, each of which consists of a 

 transmitting wire Une, a radio link and a receiving wire line. The 

 function of the terminal or "combining" apparatus is to tie together 

 these two one-way paths in such a manner that they may be connected 

 at the switching centers to various telephone subscribers via the usual 

 telephone circuits. 



When the United States subscriber, designated as A in Fig. 1, 

 talks, electrical waves set up by his voice pass over a wire line to a toll 

 office. They then divide in a hybrid set. Part of the energy is dissi- 

 pated in the output of a receiving repeater, and part is amplified by a 

 transmitting repeater and passes over a wire line to a radio transmitter, 

 as indicated in the upper transmission path of Fig. 1. 



The waves are then amplified and transformed into radio-frequency 

 energy and radiated. Some energy is picked up by a distant radio 

 receiver, amplified, and transformed back into voice-frequency energy 

 which passes over a wire line to the overseas terminal. The receiving 

 repeater at this point makes up for the loss of the receiving wire line. 

 From its output the waves pass into a hybrid set, part being dissipated in 

 the network and the other part going through the toll-office to the over- 

 seas subscriber B. Due to the imperfect balance between the sub- 

 scriber's line and the network, a portion of this energy will be returned 

 over the lower transmission path to the United States subscriber A as 

 echo. 



The action when the overseas subscriber B talks is substantially the 

 same as that described above except that the useful speech waves pass 

 over the lower transmission path. 



In long-wave radio circuits the scarcity of suitable radio channels 

 makes it highly desirable to use the same frequency band for transmis- 

 sion in both directions. This results in two additional radio paths be- 

 coming important, namely, those between the radio transmitter and 

 the radio receiver at each end of the circuit. By using specially 

 directive antenna arrangements, transmission over these paths may be 

 partly balanced out. In practice, this balance cannot be made very 

 effective in reducing the relative importance of these paths without 

 sacrificing materially the receiving directivity against natural radio 

 noise. The effect of these added transmission paths is to make the 

 transmission problem more difficult, as will be explained. 



Transmission Characteristics 

 Returning to consideration of the simple four-wire set-up involved 

 in short-wave operation, the transmission characteristics of the circuit 

 evidently depend on the sum of the effects of the radio and wire line 



