Early in 1941 the Royal Corps of Signals established a HQ at Bachelors Hall, at Hundon in Suffolk where radio "hams" were recruited to design and manufacture a small radio telephony set, simple to use, able to withstand damp conditions and operating on high frequency over a short distance of 10-20 miles. Under G2AW Major Hills and G2QV Captain Shanks, G8CK Bill Bartholomew designed the transceiver; G8JI Tom Higgins and G2RD Ron Dabbs the receiver; G8CK Bill Bartholomew and GM8MQ Jack Millie the transmitter; and G8PP Les Parnell, GM2CQI Jimmy McNab and John Mackie the power supply and metalwork. The resultant set, code named TRD (using the initials of Ron Dabbs) was housed in a metal case about 15 inches long, 9 3/8 inches wide and 9 1/4 inches high. It was powered by a large conventional 6 volt 85 A.H accumulator battery, the voltage being boosted by the vibrator in the set to 240 volts with a power output of approximately 1 1/2 watts. The frequency range was 48-65 mcs, then rarely used but later commonly used by the BBC television.
After extensive tests a sum of £2,000 was allocated for the purchase of parts which were generally obtained from commercial companies.
The metal case was constructed by the Metalbox Company of North London, the back and sides in one piece and access to the components was by removing the front panel with the attached component chassis. The transmitters were TVO/310s or RK34s, twin triodes and were used in a self-excited configuration, having a coil across the two grids, while the anodes were connected to a coil between them, the anodes being fed by a central tap on the anode coil, and an RF choke and capacitor to the HT supply, which was supplied by the vibrator pack, using mainly a 4 pin vibrator and an OZ4 cold cathode rectifier, producing around 250 volts DC. Occasionally the vibrator was a self-syncronous 6 pin type which produced the DC without a rectifier. These were fed by a transformer, all contained in a Masteradio vibrator pack made by the Masteradio factory in Rickmansworth Road, Watford. The self-excited oscillators were anode modulated by a 6V6 fed by a 6C5, both metal valves of the octal variety, as were the OZ4s. The aerial terminals inside the front panel were connected inside via a piece of flat twin feeder, ending in a single-turn loop the same diameter as the coil and pushed in between the turns at the low impedance point at the centre tap at which point the HT was fed via an RF choke with a value of 2.5 milli-henries. Outside the front panel, the aerial terminals were connected to a 72 ohm flat twin feeder leading to the dipole antenna hidden in a tree.
The microphone was an ordinary Post Office telephone handset.
The receiver was a super-regenerative type and although very sensitive, had to be isolated from the radiation caused (and thereby giving away the position to the enemy) by the simple means of using an untuned buffer RF amplifier, which was an EF50, a popular valve at the time with tiny pins. The super-regenerative valve was a Mullard Red 'E' type EF39. The output was via a 6V6 tetrode and could be via either a loudspeaker or headphones.
In 1942 the Royal Signals HQ moved to Coleshill where construction and improvements to the TRD set continued, including the TRM and TRF.
At stand down the number of sets in use numbered 250 TRD, 28 TRM, 36 TRF and 200 Number 17 sets. It is thought all the TRD sets were collected at stand down and thrown down a mine shaft which was then capped.
Written by Arthur Gabbitas.
Description of use of wireless sets and batteries, charging technology coming soon
- Savage Set
- TRD
- No.17
- Murphy receivers
- Generators and batteries
- Aerials
- Codes and Ciphers