44 HILL [chap. 3 



C. The Radio Transmitters 



The radio transmitters operate at preset frequencies between 40 and 47 mc/s 

 and radiate approximately 3 W r.f. F.M. is used as the modulation system and 

 the maximum deviation is + 125 kc/s. Thermionic valves are used throughout. 

 Ami)litude modulation is unsatisfactory because of the movement of the buoy 

 antenna. 



The signals from the hj^drophones are connected directly to the reactance 

 modulator; this alters the oscillator frequency by an amount pro])ortional to 

 the signal voltage. The oscillator frequency is doubled in the anode circuit of 

 the oscillator valve and doubled a second time in the drive valve for the push- 

 I)ull output valves. The output from the transmitter is matched to the half- wave 

 aerial on top of the buoy. 



The battery supply for the transmitters consists of two composite H-V + 

 90- V dry batteries each weighing about 5 lb. The life of these two batteries is 

 approximately 24 h for continuous operation. 



With advances in transistor design, it is expected that the transmitters could 

 in future be transistorized. This would allow increase in the radiated power for a 

 given weight of batteries, or alternatively a longer life. The signal-to-noise 

 ratio could be improved by using jihase modulation by an amplitude-modulated 

 sub-carrier of, say, 10 kc/s. 



D. The Long-Range Buoy-Radio Receivers 



Commercial transistorized receivers are installed in the recording buoys. 

 These receive signals from the ship's transmitter in the frequency range 1.5 to 

 2.5 mc/s and are connected to the transmitting aerial; suitable filtering to 

 remove the 40-45 mc/s signal is required at the input. At the audio output there 

 is a narrow band filter centred at 800 c/s and suitable relays for switching the 

 recorder on and off. 



In order to avoid spurious signals operating the switching system, the 800 c/s 

 amplitude modulated transmission from the ship has to persist for 2 sec before 

 the recorder is switched on. Thereafter the recorder will continue to operate so 

 long as the modulation persists and for 10 sec after it is switched off. Thereby 

 short breaks in the radio transmissions will not affect its operation. The 

 modulation can be keyed in the ship and the pattern is recorded both in the 

 buoy and in the ship. This pattern can be related to the shot instant, which is 

 not directly transmitted to the buoys; the recorders are not switched on until 

 after the shot fires. 



E. The Long-Range Buoy- Recorders 



The miniature 35-mm photographic recorders in the buoys contain 200 ft of 

 film. A recording speed of 2 cm/sec is usually employed; this gives a total 

 recording time of 50 min. There are up to 12 recording channels available. 



