The S.O.E 51/1 Miniature Transmitter by John Heys G3BDQ

I enjoyed reading the articles by Rodney King and Phil our Club Chairman which compensated a little for those of us unable to attend the meeting when John Elgar-Whinney gave his talk on Covert Radios. Some recent reading has revealed to me that the otherwise excellent ‘Paraset’ had weaknesses in its receiver section. Selectivity was poor and the regeneration control was noisy when operated. The set being a simple TRF design had to be in the oscillating condition to resolve CW and in this condition the ‘Paraset’ unfortunately radiated harmonics and sub-harmonics. These could disturb all of the broadcast receivers in use within a 100 yard radius. Such spurious radiation could have proved fatal to the operator should any German Detector Vans have been present in the area. For this reason ‘Parasets’ were withdrawn from illicit operations in occupied territory despite their removal when possible to non-built up areas. The Mark XV receivers overcame this problem by having an RF stage which isolated the oscillating circuit’s radiation from the antenna. RF could come in but very little could go out when the set was in receive mode.

g3bdq-miniature-51-transmitter-2 g3bdq-miniature-51-transmitter-1Between 1941 and 1945 the S.0.E reduced the weight and complexity of the clandestine radios in use. An early transceiver often weighed more than 45 lbs but by 1945 the weight had been reduced to under 5 lbs. The 51/1 transmitter for instance weighed just 1.25 lbs and was small enough to be carried in a coat pocket. Its matching receiver weighed about 2.5 lbs.

I have re-drawn (to evade copyright) and made slight changes to the circuit of the 51/1 transmitter. This set is an outstanding example of minimalism in design and uses a minimum of components. In the field normal operation involved the use of a domestic AC supply and had a clever arrangement involving a pair of shorting bridges (not shown on my diagram) which allowed the use of mains supply voltages of between 200 and 260 volts. Unfortunately some parts of Paris at that time still had 110 volt mains supplies and had to be avoided. Unusually the power section of 51/1 transmitter employed an auto-transformer with a single tapped winding and a pair of miniature tetrodes which were arranged to provide full wave rectification. The smoothing was simply a single 8 uF capacitor which had a maximum working voltage of 350v. The valves were type CV136 as was also the crystal oscillator valve. This meant that should this vital valve fail one of the rectifier valves could be pressed into service. Doing this led to some reduction of transmit power and a deterioration of the signal tone but nevertheless still allowed transmission.

The oscillator was a basic tuned anode circuit and it had a small neon bulb to indicate maximum power output when tuning up. Only one anode coil was needed for there was a Plunger consisting of a ferrite rod which when inserted into the centre of the coil from an aperture in the sets metal case lowered the tuning range from the 5.8 to 10.5 MHz to the 3 to 5.8 MHz frequencies. The normal power output was from 3 to 4 watts from an input power of 17 to 19 watts. The coil had nine tapping points which could be used to get a good match to the antenna. The complete 51/1 was housed in a black wrinkle paint metal box that measured just 5.25 X 4.5 X 1.5inches. Such a tiny transmitter could be pocketed and walked away from a transmitting site without having to lug away a heavy suitcase that contained a 40 lb radio.

Initially when parachuted in, the undercover agent had a canvas sack which held, in addition to the transmitter’s length of mains supply cord, 40 feet of antenna wire, a lamp socket / plug,6 feet of earth wire, and an adaptor which allowed the use of crystals in a variety of holders. The set had an integral Morse key, and had a normal range of transmission of at least 600 miles. Included was a pair of crystal ‘ear-plugs’.

The 51/1 transmitter had a companion receiver, the 53 Mark 1 which also used miniature valves, three in number. This receiver tuned from 3 to 12 MHz in three bands with the 3 to 6 MHz band being used with a ferrite ‘Range Plunger’. The set could be used with batteries or on mains voltages between 110 and 220 volts AC. Its dimensions were 4 X 3.5 X 1.5 inches and it weighed under 1 lb not including batteries or power pack. This set also came inside a canvas sack and had accessories such as 15 feet of antenna wire, 6 feet of earth wire, a lamp socket-plug and printed frequency calibration curves. Both miniature units came into service towards the end of the War in Europe.

By John Heys G3BDQ – Vital Spark December 2012.

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