ULTRA-SHORT-WAVE RADIO TELEPHONE SYSTEM 



537 



this type of circuit, namely, 9 decibels. Transmission from Green 

 Harbor to Provincetown is accomplished on a frequency of 65 mc. and 

 in the reverse direction on 63 mc. This does not represent the minimum 

 possible frequency spacing for this equipment, but was a convenient 

 one for the experiment. 



At Boston and at Provincetown the circuit appears at a jack in the 

 switchboard beside the jacks of wire toll circuits. As far as the oper- 

 ator is concerned, switching and ringing operations are performed in the 

 same manner as for other similar grade toll circuits and there is nothing 

 to designate that this toll circuit has a radio link. The insertion of a 

 cord into the jack starts the radio transmitter at that end of the circuit. 



AUTOMATIC 

 VOLUME CONTROL 



Fig. 3 — Block schematic of ultra-short-wave receiver. 



The receivers at both ends are kept in constant operation while the 

 circuit is available for traffic but are started and stopped by the opera- 

 tion of a key at the local test board. Ringing is accomplished by 

 sending a 1000-cycle tone interrupted at 20 cycles over the circuit. 

 Privacy equipment similar to that used on the transatlantic short-wave 

 radio channels is installed at the terminal offices. 



The transmitters are crystal controlled and are capable of delivering 

 15 watts of carrier power which can be completely modulated. It was 

 estimated that this would give a reasonably satisfactory circuit. A 

 block schematic of the Green Harbor transmitter is shown in Fig. 1. 

 The Provincetown transmitter is of similar construction. 



The receivers are of the double-detection type (see Fig. 3), and to 

 make unattended operation possible and at the same time permit high 

 selectivity at these frequencies, a crystal oscillator is used as the source 



