RADIO TELEPHONE SERVICE TO SHIPS AT SEA 



419 



ing conditions, the carrier frequency stability was made as good as that 

 required for point-to-point service and the transmitter has been de- 

 signed with the object of holding the frequency within 0.01 per cent. 

 This facilitates the establishment of contact between the ship and shore 

 and obviates the necessity for frequent retuning of the shore receiver. 

 The background noise on the unmodulater carrier, due to commutator 

 ripple, etc., is inappreciable and the audio-frequency characteristics 

 from 200 to 2750 cycles is flat to within ± 2 db. 



In addition to these electrical requirements, the mechanical design 

 must be such as to withstand ship's vibration, permit easy access to the 



Fig. 10 — New York traffic positions. 



interior so as to facilitate wave change, and at the same time protect 

 the operators from electrical shock. 



The transmitter consists of a crystal oscillator and associated am- 

 plifiers. The crystal provides the necessary carrier frequency accuracy 

 and stability and the amplifiers step up the power of the carrier to the 

 desired level. Audio-frequency filters are placed in all voltage supply 

 circuits to eliminate background noise. The modulation system with 

 its associated transformers is designed to produce the requisite audio- 

 frequency quality. A diagram of the circuit is shown in Fig. 11. 



Very thorough electrical shielding is necessary between amplifier 

 stages to prevent singing. This shielding makes the changing of coils, 



