Radio Development

See also: SDS An overview, Hannington Hall (HQ), SDS Staff at Coleshill, SDS Bunker Locations,

Page created 15/11/11

This information has been kindly provided by the Aux Units Signals Team.

Interest in the Auxiliary Units has been intense for some two decades (at least). Much has been discovered about the Operational Patrols – much less is known about its sister organisation, the “Special Duties Section” (SDS). Essentially this was a spy network, consisting of ATS officers and many civilians, supported by some technical bods on loan from Royal Signals. The idea was for ordinary people to report back the movements and strength of enemy units (following an invasion) to the defending allied army via a secret radio network.

It all sounds rather straightforward – until engineers start asking questions about the state of radio technology at the time (1940). A key problem revolves around making the network secure against interception. Those familiar with the escapades of SOE agents in occupied Europe, will know that their radio operators always dreaded being discovered by simple radio location techniques – resulting in them being dragged off by the Gestapo to interrogation and (usually) an unpleasant death. So how could our SDS operators hope to remain undetected with Germans monitoring every beep on the airwaves?
The claim from the original design team is that they designed a special set, known as the “TRD”, whose output could not be easily intercepted using a standard receiver. The TRD signal was, so to speak, “hidden”, even though to the sets’ users it was sending and receiving plain speech. No detailed information about the TRD survives (nor do the sets), but we have the claim of the designers  that they achieved this feat by using a special technique called  “double modulation”, which could not be readily received by the standard AM sets of the time.

Without going into the technicalities, what we can say is that, if true, then this was a quite remarkable achievement for a set with a mere six valves in it, and that was small and light enough to tuck under your arm and walk off with. And even more remarkable, it was simple enough for non-technical civilians to use (it sent plain speech not Morse), in an age when 2-way radios generally required specialist operators. No other set, with similar capabilities, of that time is known to us – and we are specialists in the field.

Despite attempting to build a working TRD for some years, we have not (so far) managed it. Our failure to emulate their cleverness does not prove that their claims are false, of course. But if we can do it, we will be able to put on record another of those tantalising secrets that has almost vanished from the WWII story.

Auxiliary Units Signals Team*
(which currently has 11 members)
14th November 2011

* The team is working to establish exactly what the SDS achieved. We can be contacted via email to