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

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.

Joan-Eleanor

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 tranceiver, 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 and, unlike the S-Phone, the aircraft could fly overhead at 30,000 feet. It also took ordinary batteries, so you didn’t need a charger as the S-Phone did.

The system recorded transmissions on a wire recorder in the aircraft. The intent was that agents behind enemy lines could secretly transmit intelligence reports to aircraft flying what appeared to be routine reconnaissance flights.

The radio gear was usually jammed in the rear of the host aircraft, usually a DeHavilland Mosquito, along with an operator aft of the bomb bay. The operator entered the position through a side hatch and remained there the entire flight. You can see an OSS film about the system, which was classified until 1976, in the video below.

Tech

These radios had a few things in common. Both used frequencies that were uncommon at the time, making it less likely the enemy could overhear or even detect conversations. This made it less risky to speak “in the clear” so agents didn’t need incriminating code books and cumbersome encoding and decoding steps.

Similarly, both systems used voice, meaning that agents didn’t need to learn Morse code. They probably needed a little training to use the equipment, but that was far easier than expecting a resistance fighter to study Morse code for weeks.

While the S-Phone depended on directionality, Joan seemed content to rely on being high in frequency. Both had to be lightweight, easy to conceal, and quick to set up and take down.

The Joan radio was critical for agents going behind enemy lines. They’d be brought to an airbase in a car with blacked-out windows to prevent them from knowing where they were leaving from. They’d be given forged papers, an entrenching tool, local money in a belt, a pistol, a food package, a silk map, and, of course, a Joan radio.

We wonder if any Joan radios were captured during the war? A lot of wartime high-tech was highly protected, and we’re sure the agents were instructed on how to destroy the radios. Spies were also famous for using suitcase or even shoe radios.