RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 619 



radio transmitter. Some of the radiated energy from the transmitter 

 impinges on the receiver. If the receiver should be tuned to the same 

 wave-length the signals will then get back onto the telephone line and 

 some will again go to the transmitter, producing by this circular path 

 a singing circuit. A circuit so constructed will be entirely useless due 

 to the singing produced. Now it appears possible to use a hybrid 

 coil to connect a balancing circuit to the telephone line, with conjugate 

 connections to the transmitter and to the receiver so that the incoming 

 signals from the receiver will not go to the transmitter. Such a hybrid 

 circuit will work provided it can be balanced and maintained in balance. 

 However, anyone who has tried balancing such circuits knows that it is 

 generally not practicable to provide a balanced circuit suitable for all 

 wire line connections and for the variable gains in the radio link. 

 Additional means must therefore be used. Now, of course, it is 

 possible to operate the incoming radio circuit and, therefore, the 

 receiver on a different frequency as is usually done, in which case the 

 signals from the local transmitter will be tuned out. However, if a 

 similar system is used at the other end of the radio link the signals 

 from the near end transmitter will come in on the far end receiver, will 

 again go out on the far end transmitter, will come back into the near 

 end receiver whence they get back into the near end transmitter, 

 thereby making a loop circuit again which will produce singing even 

 though the round trip path of such a circuit may be 6,000 miles. 

 It is therefore found necessary, when connecting with telephone lines, 

 to provide a system which will at all times keep the incoming energy 

 of the receiver from going out on one's own transmitter. 



To accomplish the foregoing is the function of the "Vodas" ^ as 

 indicated on the diagram, a device which connects the telephone line 

 to either the transmitter or the receiver but not to both simultaneously. 

 It must, however, connect them at proper and suitable times so that a 

 two-way conversation can take place. A simple system comes immedi- 

 ately to mind to accomplish this purpose. It is that of a voice- 

 operated relay which throws the telephone line from the receiver to 

 the transmitter whenever the speaker on that end speaks, with the 

 relay making the reverse connection when he stops speaking. Such a 

 simple circuit has been used in some cases but has been found not to 

 be adequate for general use. To begin with, the line is not switched 

 until part of what the speaker has said has arrived to actuate the relay. 

 Some clipping, therefore, occurs. To make things worse many words 

 begin with sounds of small energy like f's and s's, which may not be 

 sufficient to actuate the relay. The relay will then not operate until 

 the vowel sound following arrives and when the relay does operate the 



