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April, 1951
LAYTON : BUSINESS RADIO
12?
amplifier valves (6V6 GT's). Connection to the amplifier input transformer may be from a 15 ohm moving coil microphone or from a 600 ohm line, whilst the output transformer secondary is connected in series with the anode and screen supply to the 832 P. A. valve.
The transmitter provides a carrier output of 10 to 12 watts, modulated to 100% on peaks, and uses a coaxial feeder (nominal 75 ohm impedance) to carry this output to its dipole aerial.
hand-microphone carries the transmit button which controls the H.T. supply to the transmitter, and is connected to the control box by means of an extending cable whose cover is of spiral moulded rubber. Coaxial cable of nominal impedance 30 ohms is used for connecting a piano-wire type of quarter-wave aerial with the main equipment.
A Practical Case
During the making of the film " Pool of London," Ealing Studios, Ltd., decided to
Fig. 5. The Pye PTC 704 equipment in use.
Pye P.T.C. 113 Mobile Equipment
Electrically, the mobile sets are of similar general design to the fixed equipment, except that they use smaller types of valves and components and derive their H.T. supplies from a compact rotary transformer. This latter operates from a 6 or 12 volt battery supply according to the user's specification. The complete equipment is contained in a steel case measuring 14J ins. x 7J ins. x 9 ins., and is intended for mounting in the boot of a car.
The operator's control box measures 7 ins. x 4 J ins. x 2 J ins. and carries the on-off switch /volume control, and a built-in loudspeaker. It is usually mounted within easy reach of the driving position. The small
make use of radio-telephony on their location work, and experiments which preceded this decision provide interesting material for discussion here.
Initially, it was required that the studio production office should be able to communicate with a vehicle operating in the vicinity of Tower Bridge, and although this amounted to no more than about 12 miles of range, the project was not without its problems. First, the terrain at Ealing is on average only about 50 ft. above sea level, and secondly, the signal had to traverse a vast built-up area in which many thousands of buildings would tend to impede and dissipate the transmitted carrier wave.
For first experiments an aerial mast was