The Whaddon Mk VII - Paraset Clandestine Radio

Reproduced from QRP QUARTELY Fall 2009 by permission from President

A Short History of the Paraset from AE4IC Bob Kellog, QRP QUARTERLY

The remarkable little rig that came to be known as the "Paraset" came into being c. 1941, as a means of providing Allied clandestine field operatives with an ability to transfer intelligence information back to Britain. The set was apparently designed at the main receiving site at Whaddon, and officially known as the Whaddon Mark VII. At least one knowlledgeable source involved in the producction of these sets claims that "Paraset" was never an official name, but instead was a kind of nickname that came about from the practice of issuing the sets to agents that would be parachuted into enemy-controlled territory.

Original technical information on the Paraset is somewhat scarce, as Winston Churchill ordered most documentary eviidence of support for clandestine operations of that period destroyed immediately after WWII. His aim was to prevent it from falling into Soviet government hands. Nevertheless some information survived, most notably from a Belgian ham named Ioe le Suisse, ON5LJ (SK). ON5LJ's information has been preserved by Jo Scholtes, ON9CFI, who later passed it on to Mario Galasso, IK0MOZ. Mario, by the way, has an excellent website (http://www.qsl.net/ik0moz/paraset_eng.htm) for those interested in replicating the Paraset.

Historical information on the developpment of the Paraset comes primarily from the. few personnel who were a part of its development and are still available for comment. Geoffrey Pidgeon, author of The Secret Wireless War [1] has supplied much of the information [2] through his own knowledge gained when he was a part of the development team, and through his contacts with others in the team. Mr. Pidgeon himself was involved in early production and his father was in charge of Stores (Supplies) at Whaddon.

Mr. Pidgeon writes that the R&D team developing the Paraset was led by Dennis Smith, "a brilliant wireless engineer." Mr. Smith later went on to a Mobile Construction section that designed and built the Ascension air-to-ground agent contact system and a number of other transmitters and receivers intended for agents and Resistance fighters. There were a total of approximately nine people involved in the development, which apparently started sometime in 1941. By 1942, Mr. Pidgeon had been assigned to the metal workshop at Whaddon Hall, and states that they were making small runs of about twenty Mark VII sets by the end of that year.

As can be seen from Figure 1, the Paraset consisted of a simple two-valve (tube) receiver and a single-valve transsmitter. [3] The power supply was conntained in a separate enclosure and used batteries as the primary energy source, feeding a vibrator-based power supply to supply the necessary high voltages. The receiver consists of a regenerative detecctor and a single stage of audio amplificaation. This design scheme was quite commmon at the time among amateur radio operators of modest means because it required the fewest number of expensive valves. The transmitter was a high powwered (5-7 watts), crystal controlled oscillator connected directly to a single wire antenna, which was never more than 20M long and often much less. Again, this was a common design scheme of the time. Because of the transmitted power level, the Paraset might be considered the first QRP rig used for official purposes.

The small number of tubes and low power output from the transmitter also contributed to low power consumption, an important factor in clandestine sets. Batteries were often used in these sets as the primary power source for reasons of dependability. Mains power was not always available, and could disappear at any time. In many instances, agents had to operate in very rural settings where elecctrical power was non-existent. Even though the Paraset was relatively frugal in its use of power, it was still necessary to carry along at least one and possibly two automobile batteries. Moreover, the battteries had to be charged relatively freequently and that posed considerable operrational risks. However, the use of batteries also provided a defense against a common detection technique used by German direction finding teams. The teams would routinely cut off power to a section of the city while listening to what they thought might be a clandestine transmitter. If transmissions stopped coincident with the power being turned off, they could narrow their search to a specific part of the city.

The low power output was nevertheeless adequate for the purpose, given the large antennas and sensitive receivers at the listening sites in England. A recent propagation study [4] has shown that an antenna efficiency of just a few percent would probably have resulted in an SNR of 20 dB or so for a path from Calais in France to the Whaddon receiving site. A path from Marseille to Whaddon would have resulted in a similar SNR. However, the designers probably did not do studies of this type in considering requirements for the Paraset. More likely, they would have relied on personal knowledge of communications with low power transsmitters over the distances involved. A seccond factor in favor of the agents was that 1945 corresponded with a solar sunspot maximum, giving the war years very good propagation conditions.

The Paraset was apparently an immeediate success and production seems to have been transferred to a factory at Little Horwood when it opened in early 1943. Mr. Pidgeon was also transferred to the metal workshop at Little Horwood and remained there through most of the remainder of the war. While there, he made portions of the custom keys and other items which were an integral part of the Parasets. The Parasets immediately began production in lots of fifty and a hundred. Unlike the metal box sets which are most popular with replicators, these early sets were issued in slightly larger wooden cabinets. These sets were normallly placed in a small attache case which would accommodate the power supply and had storage for valves and crystals. Some Parasets were apparently also made in workshops at Watford run by the Special Operations Executive (a group somewhat akin to the US OSS).

Figure 2 [5] is an example of the attache case variety of the Paras et. The power supply, shown on the left side of the case, is a mains supply and not the vibrator-based supply which was more common. Note the key knob which appears just below the power switch. The remainder of the key is located below the front panel. This was a feature of all Parasets. This particular set belonged to Oluf Reed Olsen, a Norwegian pilot and member of the British Secret Intelligence Service (SIS). Olsen survived the war, and has written a book concerning his adventures. Olsen was either more skilled or luckier (or both) than many of his collleagues undertaking the dangerous misssion of relaying information back to the Allies during the war. Many of his collleagues fell prey to German direction fInding units, who became quite skilled in finding agents while they were actively sending back information. An interesting point here is that a regenerative receiver of the type used in the Paras et is often capable of transmitting an inadvertent signal of its own, thereby aiding the direction finding units. However, meaasurements made on SM7EQL's replica set showed the receiver radiation at full regeneration to be -38 dBm, which equals 158 nW. This is probably another indication of its excellent design, and a feature that was probably greatly appreciiated by its users.

A second example of the wooden variiety Paraset is shown in Figure 3. [6] This particular set is owned by Dr. E. Evrard, who discovered it in his late father's effects whilst clearing his late parents' home. According to Dr. Evrard, he recolllects playing with the set as a small child and says that it was given to his father by M. Georges Ronval, a member of the Belgian resistance during WWII. M. Ronval worked for Belgian Railways. Very probably, M. Ronval was able to give valuable information concerning schedules for important trains carrying troops, prisoners, war materials, and other valuables. Travel schedules for strategic materials and local weather information were two of the typical subjects of traffic carried by the Parasets. Remember, there were no weather satellites in those days.

The metal-cased sets popular with replicators apparently came somewhat later. These sets are sometimes referred to as "cash box" sets because of the size of the outer case. Figure 4 [7], courtesy of SM7UCZ, is an example. Its origin and original user are unknown. Note that the valves are carried in the top of the case and installed each time the set was to be used. Otherwise, the outer case would have to have been much deeper and thereefore larger. The Paraset schematic also apparently evolved slightly over time, as can be seen by comparing the early schenatic of Figure 1 with the later version in the companion article on replicating the Paraset. Differences are minor and involve power circuitry feeding the receiver and small changes in the transmitter

Acknowledgements

Virtually all of the information preesented here comes from the archives of the Paraset Club (http://www.theparasetclub.co.uk/), a small group of some 70 enthusiasts committed to honouring the memory of the design and manufacturing team who created the Whaddon MkVII "Paraset" radio equipment and the Special Agents who used it for clandestine operations in WWII. Readers interested in more information concerning the Paraset are encouraged to visit the IKOMOZ website mentioned earlier and/or contacting the Paraset Club via its website.

References

1. Pidgeon, Geoffrey. The Secret Wireless War, published by Arundel Books, 2008. (Distributed by ARRL.)

2. Personal communication from Geoffrey Pidgeon to Secretary of the UK Paraset Club.

3. Figure 1 photo supplied courtesy of Ken Gordon, W7EKB.

4. Austin, Brian, G0GSF. "HF Propagation and Clandestine Communiications During the Second World War," Radio Bygones, No. 120, August, September 2009.

5. Figure 2 photo supplied courtesy of Johnny Apell, SM7UCZ.

6. Figure 3 photo supplied courtesy of Dr. Eric Evrard.

7. Figure 4 photo supplied courtesy of Johnny Apell, SM7UCZ.

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All above from Bob Kellogg AE4IC reproduced by permission from QRP QUARTELY