The S-Phone system was a UHF duplex radiotelephone system developed during World War II for use by Special Operations Executive agents working behind enemy lines to communicate with friendly aircraft and coordinate landings and the dropping of agents and supplies. The system was composed of a "Ground" transceiver, designed by Captain Bert Lane, and an "Air" transceiver designed by Major Hobday, both of the Royal Signals.
The "Ground" set - which was to be used by agents on the ground - weighed about 15 lbs including batteries and was typically worn attached to its operator with two canvas straps. It was a highly directional unit which required the operator to face the path of the aircraft. It had the useful trait that transmitted signals could not be picked up by ground monitoring stations more than one mile distant; however, its signal was only good to 10,000 ft which brought the aircraft within range of flak. It permitted direct two-way voice communication with an aircraft up to a range of 30 miles. While the S-Phone provided directional information to the pilot it gave no range information, although a pilot could tell when he was directly over the "Ground" operator because at that point no communication was possible.
The primary purpose of the S-Phone was not navigational as such; rather, it was to provide a reasonably secure channel for coded conversation between staff officers based in London and agents in the field, allowing for the exchange of orders and information. The security of a circuit was sometimes tested by using an officer who could recognize whether a voice was actually that of the agent who was supposedly speaking to him.
- Frequency: 337 MHz (TX) / 380 MHz (RX)
- Output Power: 0.1 to 0.2 W
- Soerensen, Lars Peter. "SOE Radios". Clandestine Radios and SOE Equipment 1940 - 1945. Retrieved 23 January 2008. Cite has empty unknown parameter:
- Foot, Michael Richard Daniell (2004). SOE in France. Routledge. p. 80. ISBN 0-7146-5528-7. Cite has empty unknown parameter:
- Wireless for the Warrior, Volume 1 Wireless Sets No. 1 - 88, Louis Meulstee, G. C. Arnold Partners 1995, ISBN 1-898805-08-3