EXTENSION OF THE TELEPHONE SYSTEM TO SHIPS 145 



tubes then available. Of course, improvements can be made in the 

 system at the present time as a result of the information obtained in 

 the development using the very much larger vacuum tubes now 

 available. 



These decisions, therefore, determined the general type of system 

 to be used, namely, one in which many of the known advantages of 

 single side band transmission were sacrificed in order to secure simple 

 apparatus, to make use of then existing power tubes and to enable the 

 transmission to be received generally. 



The problem of securing the two-way operation necessary aboard 

 ship and for combined radio and wire operation may be attacked in 

 several ways. In general, there are three methods available: 



(1) In which the east and west channels are established alternately 

 and not simultaneously, by switching. The push-button 

 scheme is a familiar example, although unsuitable for tying in 

 with the wire telephone system. Another arrangement is the 

 use of voice-operated relays to throw the terminal apparatus into 

 the sending or reciving condition, depending upon the direction 

 of transmission. 



(2) The use of the principle of balance to separate the outgoing 

 from the received transmission. The radio receiving antenna 

 circuit is balanced with respect to the transmitting antenna 

 circuit. 



(3) Employment of different frequencies for the two directions of 

 the two-way transmission, relying upon frequency-selecting 

 circuits for affecting separation. The first two methods allow of 

 operation on the same or on different carrier frequencies. 



All of these fundamental methods were considered in their several 

 possible embodiments, and compared from the standpoint of the 

 conditions to be met in the radio system itself and in linking it with a 

 public service telephone system. The system finally adopted em- 

 ployed different frequencies for sending and receiving and secured 

 discrimination by frequency selection supplemented at the land 

 station by a moderate degree of special separation and balance. By 

 using sharply selective receiving circuits, a moderate frequency differ- 

 ence between east and west channels sufficed to give the necessary 

 degree of separation. 



Preliminary Tests 



By the time these decisions had been made there was available for 

 experimental purposes a plot of land near Cliffwood, New Jersey. It 



