Spy Tech: A Quiet Radio For Spies | Hackaday
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Normally, when you think of a radio transmitter, you want the strongest signal and range. But if your radio operator is secretly operating as a spy, broadcasting their position isn’t a feature; it is a liability. This fact didn’t escape World War II radio designers.
In late 1942, the British realized they needed a way for Special Operation Executive agents, resistance members, and other friendly forces to communicate with an aircraft without attracting undue attention. Two engineers from the Royal Corps of Signals developed a pair of transceivers — the S-Phone — operating around 380 MHz just for this purpose. Frequencies this high were unusual at the time, which further deterred enemy detection.
The output power was below 200 mW, and the ground equipment consisted of a dipole strapped to the operator. No transistors, so with rechargable batteries, the rig weighed about fifteen pounds and reused some parts of a paratrooper radio, Wireless Set Number 37. The other side of the connection was installed in an airplane.
Close Air Support
An S-Phone appearing in “School for Danger,” a 1943 film.<br>" data-large-file="https://hackaday.com/wp-content/uploads/2026/05/sphone.png?w=443" class="size-medium wp-image-1111739" src="https://hackaday.com/wp-content/uploads/2026/05/sphone.png?w=383" alt="" width="383" height="400" srcset="https://hackaday.com/wp-content/uploads/2026/05/sphone.png 443w, https://hackaday.com/wp-content/uploads/2026/05/sphone.png?resize=239,250 239w, https://hackaday.com/wp-content/uploads/2026/05/sphone.png?resize=383,400 383w" sizes="(max-width: 383px) 100vw, 383px" />An S-Phone appears in "School for Danger," a 1943 film.<br>The low power and directional antenna meant that it was nearly impossible to pick up any signal on the ground if you were more than a mile away. The airplane that the operator was facing, on the other hand, could pick up the voice signal up to 30 miles away. Unfortunately, they also had to be under 10,000 feet, exposing the plane to enemy fire.
The highly directional gear could give the pilot a clue that he was closing on the target, and when the signal suddenly went out, it indicated that the aircraft was directly overhead the transmitter.
The Special Operation Executive had a lot of cool gear, and you can learn more about their gadgets and methods in the 1943 film "School for Danger" that you can see below. Look for the S-Phone at around the 7-minute mark. Interestingly, the two main characters are actual Special Operation Executive agents who actually did the things that are fictionalized in the movie.
The CryptoMuseum has a scan of the S-Phone manual. There are many interesting tidbits there. For example, the set came with a lamp that could show if the transmitter was working. These radios used early NiCad batteries. The manual goes to great lengths to explain that you should not try adding sulpheric acid to the batteries.
Inside the S-Phone.<br>" data-large-file="https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png?w=800" class="wp-image-1111754 size-full" src="https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png" alt="" width="1071" height="571" srcset="https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png 1071w, https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png?resize=250,133 250w, https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png?resize=400,213 400w, https://hackaday.com/wp-content/uploads/2026/05/sphoneimage.png?resize=800,427 800w" sizes="auto, (max-width: 1071px) 100vw, 1071px" />Inside the S-Phone<br>Joan-Eleanor
An operator using the Joan transceiver.<br>" data-large-file="https://hackaday.com/wp-content/uploads/2026/05/je.png?w=400" class=" wp-image-1111749" src="https://hackaday.com/wp-content/uploads/2026/05/je.png?w=377" alt="" width="257" height="273" srcset="https://hackaday.com/wp-content/uploads/2026/05/je.png 400w, https://hackaday.com/wp-content/uploads/2026/05/je.png?resize=236,250 236w, https://hackaday.com/wp-content/uploads/2026/05/je.png?resize=377,400 377w" sizes="auto, (max-width: 257px) 100vw, 257px" />An operator using the Joan transceiver.<br>Where the British had the Special Operation Executive, the United States had the Office of Strategic Services. Working at RCA laboratories, OSS engineers along with [Al Gross W8PAL] who would become a pioneer in the development of walkie-talkies, pagers, and cordless telephones, designed the Joan-Elanor, named after the engineer’s wife and a WAC member.
Joan was the field transceiver, technically SSTC-502, while Eleanor, SSTR-6, was mounted in the aircraft. Joan weighed less than four pounds, using a super-regenerative dual triode that doubled as the transmit oscillator. Originally, the radio was set for 250 MHz, but when it was found that the Germans had the ability to receive at that frequency, they pushed Joan-Eleanor to 260 MHz.
The radio had a range of about 20 miles...