Behind the Lines: a Short History of Spy Radios in WW II; NA
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By RODNEY CHAMPNESS, VK3UG Behind the Lines: A Short History of Spy Radios in WW II; NA Operating a spy radio within occupied quency, the German monitoring serv- ice could easily keep an eye on these Europe during World War 2 was a risky frequencies. When transmissions were again heard they started from the ex- business. In this, the first of three articles, we tremities of the cocked hat triangle take a look at the equipment used and the and took more bearings. By this means, often over a period of several days or techniques employed to avoid getting caught. even weeks, the size of the area that the transmitter was located in would Since World War II, we have seen from the Gestapo had large numbers be reduced to something like a trian- many films which have shown snip- of receivers with panoramic displays gle of about 200 metres a side. If the pets of the French Resistance and the attached to them and could monitor clandestine/spy radio operator kept spy radios that were used for their virtually all high frequencies (HF) at on operating from this location he or clandestine activities. These films, the one time. she was very likely to be caught. although based on fact, were drama- Once a suspect signal was detected, tised and were certainly not always as it was then observed on a normal com- Countermeasures accurate as they could have been. Af- munications receiver and a recording To minimise the chances of being ter all, why should the facts spoil a made on a wire recorder. Radio direc- caught, the operator was told to only good story! tion finders were also immediately transmit for very short periods - about Gathering information about a coun- put into operation and a direction ob- 3 minutes maximum - and to shift try that you are at war with and ar- tained from each station listening to location regularly. One feature of the ranging for people behind the lines to the signal. It was normal to use three early spy radios was the use of a sim- create sabotage was considered a le- of these stations to get the direction to ple two or three-valve regenerative gitimate activity by all sides. In Brit- a clandestine station. The three direc- receiver. However, when being used ain, the organisations that were very tions were plotted onto a map and a for Morse code (CW) reception, the involved in this activity during World small area, called a "cocked hat", was regenerative detector would radiate War II were the Special Operations obtained where the three lines nearly enough signal for it to be picked up by Executive (S.O.E.) and Military Intel- intersected (the direction finding any German radio detection groups in ligence (MI6). equipment wasn't accurate enough to the near vicinity. As a result, superhet How did these radio transmitter have all three lines intersect at the receivers quickly superseded TRF re- units get into the hands of the Resist- one point). ceivers. ance? They were largely flown in by The clandestine radio transmitting Another favourite technique of the the various aircraft such as the station would be within the cocked Gestapo radio detection groups when Westland Lysander and the Lockheed hat, which was a triangle of about closing in on a spy radio installation Hudson. These aircraft were able to 16km per side. A triangle of this size was to remove the mains power from land on makeshift airfields, to drop occurred because the direction find- a building block. If the transmission off materiel and to transport agents in ing stations were usually several hun- immediately ceased, then it was highly and out of the country. Two and four- dred kilometres away from the trans- likely that the clandestine radio was engine bombers were used for larger mitter being traced. The main fixed located in that block. After that, it was airdrops. direction finding stations were at only a matter of time before the opera- The radio operators who used the Brest, Nuremberg and Augsbourg. tor was found and "suitably dealt small radio transceivers were always Because the radio operator usually with". in considerable danger of discovery. only had about three different crys- To overcome this tactic, many of German radio direction finding groups tals to control the transmitter fre- the later sets were fitted with both 76 SILICON CHIP RFC1 0.1 Vi V2 H-0 6SK7 6SK7 HEADPHONES 50pF .01 0.1 50k 1M ),0RECEIVE 150k AERIAL OSJc 3000 EARTH TRANSMIT 50k REGENERATION VR1 20k 10k TUNING/ RECEIVE TICKLER LAMP TRANSMIT +350V V3 OVR1b 6V6 LOADING 10" LAMP • 25k X -...--06.3VAC CRYSTAL 20k 25pF 100pF Fig.1: the 3-valve Paraset featured a regenerative receiver based on detector stage V1 and audio output stage V2, plus a crystal-controlled transmitter stage (V3). The RF power output was 4-5W, while the operating frequency range was from 3-7.6MHz. battery and mains power supplies and ment was available late in the war. reception conditions. could be switched from mains to bat- (2). Usable with both mains (110V (7). Simple in design and easy to tery operation within a second or so. and 230V) and battery supply (usu- use. The break in the transmission was so ally 6V). (8). Fitted with a non-radiating re- short as to be unnoticeable and this (3). Able to change over from mains ceiver, which ultimately ruled out re- could be a real life saver. to battery operation quickly to avoid generative receivers. The receiver did The operators also often had what detection as detailed earlier in the not have to be tremendously sensitive we would call "cockatoos" or look- article. as the transmitters in Britain were outs to warn of suspicious activities, (4). Able to transmit on a variety of reasonably powerful at 250 watts. And so that they could close down quickly. radio frequency bands between about there was access to a 15 kilowatt trans- Although the Germans had a very ex- 3MHz and 8MHz and preferably up to mitter if needed. tensive network of listening sets and around 16MHz. This meant that com- (9). Maintained accurately on fre- radio direction finding equipment, not munications could be maintained at quency so that the operator at the many of the spy radio operators were almost any time of the day or night listening station knew where to look caught. However, the activities of the from anywhere in occupied Europe. for the signal. This was achieved by radio direction finding troops meant (5). Sufficiently powerful to achieve using a small selection of quartz fre- that the agents couldn't go about their the previous requirement. This usu- quency crystals. clandestine work in a careless way. If ally required 3-30 watts of radio fre- (10). Headphone operation only. they did, they soon ended up in a quency (RF) output. The receiving sta- Radios were banned in occupied coun- German prison, which usually had tions in Britain and elsewhere had tries, so no "radio" noise could be fatal results. sensitive receivers and large anten- tolerated. In any case, it was easier to nas. produce sets for headphone opera- Technical history (6). Able to transmit CW (Morse tion. The Third Reich did have a Obviously, the radio transmitting code). Transmitters for Morse code number of approved broadcast band and receiving equipment used by the are simpler to make and have a much only sets which had limited reception resistance radio operators was pur- greater range for the same power than range so that stations outside their pose-built to suit the job in hand. The an AM (amplitude modulated) trans- borders could not easily be tuned. equipment needed to be: mitter. Additionally, the voice of the (11). A quiet Morse key. Some were (1). Small enough and light enough operator would not be recognised by quieter than others and many were to be carried in an inconspicuous suit- those hunting him/her and Morse code enclosed to keep noise down and to case. Small enough to fit in a coat is much more accurately copied than make sure the operator didn't receive pocket was even better and such equip- voice transmissions under difficult an electric shock. SEPTEMBER 1998 77 BAND CHANGE U PLUNGER 8 +■ 350VWI'm • 9HI .001 V1 AERIAL .01 6AM5 240VAC / .01 V2 0 6AM5 NEON HI ik .01 II-OEARTH Fig.2: the 51/1 transmitter used three 6AM5 valves, two to rectify the high voltage from the transformer and the third as the oscillator. The whole circuit operated at half mains supply and this, together with the dangerously high DC voltages that were present at many points, meant that it had to be well insulated from the operator. Did the spy radios achieve all of radiated a handy signal for the Gestapo By contrast, the AP4 was a smaller these ideals? No, but there were a radio detection groups. Because of transceiver which weighed in at about number of really good tries and most this, the Paraset was mainly us€d in 4kg (including the 120/220V AC power of the later sets did incorporate most country areas where the receiver's sig- supply). It operated from 2-8MHz of the criteria listed above. nal would not be detected. using a single valve in the transmitter The transmitter section was based (with 8W output) and a 3-valve super- The sets of the Resistance on a single 6V6 valve, used as a crys- het receiver. The correct tuning of the A number of radios had been devel- tal controlled oscillator cum power transmitter was accomplished by ob- oped prior to World War II and the output stage.