S-Phone Type 13 Mk. IV (1943) – Portable UHF Radio Transmitter-Receiver

Close up of the S-Phone type 13 Mk. IV radio transmitter-receiver

The S-Phone UHF duplex radiotelephone system was a short-range wireless communication technology developed during World War Two for voice transmission over ultra high frequency (UHF) bands. Designed by the British Royal Signals Corps the S-Phone was primarily intended for military use by Special Operations Executive (SOE) agents deployed in enemy territory, where it enabled full-duplex communication, meaning users could speak and listen simultaneously, much like a conventional telephone. The equipment allowed the agents to communicate with friendly aircraft and coordinate landings and the dropping of other SOE agents and supplies.

Variants

The S-Phone on the photographs is a Type 13 Mk. IV “ground” set of 1943. It is on display at the Musée de l’Armée at Hôtel des Invalides. There were also other S-Phone variants; an airborne version designed for air-to-ground communication from an aircraft and a ship version that could be used to communicate with aircraft or other ships/submarines. The S-phone was produced from 1942 to 1944.

Operation

The S-Phone type 13 Mk IV “ground” set was used by the SOE agents and consisted of a transmitter, receiver, antenna, headset and a battery made up of ten rechargeable cells. The field set weighed around 7 kilo’s and was small enough to fit in a suitcase for transportation. The operating time with the batteries fully charged was 4.5 to 5 hours. During its use the operator could carry the set using canvas straps and a battery belt.

1943 S-Phone type 13 Mk. IV radio transmitter-receiver

Operating in the UHF spectrum allowed the system to provide relatively clear audio with reduced interference compared to lower-frequency systems. Because different frequencies were used for reception and transmission, no single receiver could overhear both sides of a conversation. However, its effective range was highly directional and typically limited to line-of-sight distances with a maximum signal range of 48 kms (30 miles) horizontally and 3 kms (10,000 feet) vertically, which required the agent to face the exact flight path of the aircraft. The upside to this was that its signals could not easily be picked up by enemy ground monitoring stations, but the downside was that aircraft had to fly within range of the feared German FlaK anti-aircraft guns.

Schematic drawing of the operational range of the S-Phone – courtesy Musée de l’Armée Les Invalides, Paris

Close up of the S-Phone type 13 Mk. IV radio transmitter-receiver battery pack — Close up of the S-Phone type 13 Mk. IV radio transmitter-receiver vibrator pack

Detail of the S-Phone type 13 Mk. IV radio transmitter-receiver headphones

Specifications

The full S-Phone Type 13 Mk. IV set consisted of the following equipment:

1 S-Phone.
1 Battery Belt with Nickel-Cadmium cells (1.2 volt each) in protective sheaths, a vibrator pack H.T. generator, charging and power sockets.
2 Folding Aerials (one spare)
1 Headset consisting of microphone, earphones and connecting plug.
1 Battery Charging Unit.
1 Aerial Power Indicator.
10 Battery Vents (in sample bag) to replace solid stoppers.
1 3BA Box Spanner in sample bag.

The total pack size was 45.7 cm × 40.6 cm × 20.3 cm (18″ × 16″ × 8″) and weighed 12.7 kg (28 lbs) including handle and spare parts.

See the 1943 S-Phone manual for more details.

References