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S-Phone   Type 13
UHF duplex ground-to-air radio - wanted item

S-Phone was a two-way full-duplex UHF radio system, developed in 1942 in the UK, for use by the Special Operations Executive (SOE) during WWII. It was used by resistance groups and SOE agents operating behind enemy lines, for communication with intelligence officers in airplanes, flying close to the occupied territory at high altitue. The S-Phone consisted of a ground unit and a matching airborne unit. The ground unit is also known as Type 13, Mk II, Mk III or Mk IV [A].

The airborne transceiver was developed by Major Hobday of the Royal Signals [1]. The transceiver that was used on the ground was developed by Captain Bert Lane, also of the Royal Signals. The systems were built, tested, further improved and installed by Charles Bovill, who also worked on the Rebecca and Eureka navigation systems.

The ground unit was carried on the chest of the operator, with a webbing kit holding it in place. An open dipole antenna, consisting of two short rods, was plugged into the front of the unit and aimed in the direction of the airplane.
S-Phone ground transceiver

The image above shows the ground transceiver that was carried on the chest of the ground operator, who would aim the antenna towards the plane simply by looking at it. This way it was possible to set up a two-way voice link up to a range of 30 miles, with a maximum height of 10,000 ft, permitting the ground operator to guide the airplane to the dropping or landing zone.

The high directivity of the antenna and the low power of the transmitter, ensured that the signal could only be picked up by the plane and not by any distant ground monitoring station. The microphone was encapsulated in rubber, completely shielding off the mouth, so that enemy scouts in close proximity of the dropping zone could not hear the operator speaking.

Apart from guidance during landing or dropping, it was also used for passing 'coded' messages and instructions between field agents and staff officers back in London. The image on the right shows a ground operator with the transmitter on his chest talking into the microphone.

Sticking out at the front of the transceiver is the vertical open dipole antenna that should be aimed at the airplane. The ground set transmitted at 337 MHz, whilst the airborne transmitter operated at 380 MHz, both with an output power of 0.1 to 0.2 Watt. Power was supplied by a series of batteries and a vibrator pack, all of which were carried in the webbing kit.

The image on the right shows the interior of the ground-based S-Phone. At the left are some miniature (midget) valves and at the right is the audio transformer. Further images below.

At the center of the unit is a rectangular piece of pertinax with a two-pin socket for the antenna. The device shown here is probably a Type 13 Mark III.
Ground transceiver interior

S-Phone ground transceiver
S-Phone with its cover removed
Ground transceiver interior
Close-up of the ground transceiver
Another detail of the S-Phone ground transceiver

  • S-Phone Ground
    This is the body-worn transceiver that was used on the ground. It is featured on this page and several versions were released between 1942 and 1944: the Type 13 Mark II, the Mark III and finally the Mark IV (1944). The Mark II and Mark III versions each had 4 valves, whilst the Mark IV was equipped with 5 valves.

  • S-Phone Air
    This is the airborne counterpart of the above Type 13 ground transceiver. It consisted of a transmitter with 2 valves and a receiver with no less than 13 valves in order to pick up the week signals of the ground station at distances of more than 30 miles.

  • S-Phone Ship
    The S-Phone could also be used for secure communication between a ship and a passing airplane in a similar manner to the gound-based S-Phone. In addition, it could also be used for secure communication between two ships. For this an extra transmitter/receiver pair was installed that used the alternate frequencies.

  • S-Phone Suitcase
    Like the S-Phone Ship, this version was equipped with a double transmitter/receiver pair to allow switching between standard and alternate frequencies. It was used for secure point-to-point connections, such as the secret radio link between Sweden and occupied Denmark, known as Minestrone.
Wanted item
S-Phones are very rare these days and Crypto Museum would very much like to add it to its collection. The device featured on this page is part of the collection of Museum Jan Corver in The Netherlands. If you have an S-Phone available, or if you know where we could find one, please contact us. Your help is much appreciated.

  1. S-Phone Type 13/MK IV, Description and Operating instructions
    12 December 1943. Kindly supplied by André Halbeher, August 2014.
  1. Wikipedia, S-Phone
    Retrieved August 2013.

  2. Pathfinder Parachure Group Europe, Image of S-Phone ground operator
    May 2005. Retrieved August 2013. 1

  3. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004
  1. Website no longer available in january 2015.

Further information
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Crypto Museum. Created: Sunday 11 August 2013. Last changed: Tuesday, 31 May 2022 - 12:36 CET.
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