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SE 88/5
Abwehr spy radio transmitter - wanted item

SE-88/5 was a clandestine 5 Watt short-wave (SW) radio transmitter and receiver, also known as a spy radio set, developed around 1938 at OKW-Aussenstelle Berlin-Stahnsdorf 1 (Germany) for the German Intelligence Service, the Abwehr. It was used during WWII for foreign espionage activities, such as during Unternehmen Seelöwe (Operation Sea Lion) – the preparations for the invasion of Great Britain [3]. In 1942, production of the SE-88/5 was moved to OKW Aussenstelle Wurzen [1].

The radio set consists of an S-88/5 transmitter and an E-75 receiver which was renamed E-88 for this purpose. In some situations, the S-88/5 was also used as a standalone transmitter, which was the case for many agents that were sent to the UK early in the war to gather information about troop movement and military equipment.

Nearly all German spies were captured however, and were either turned or executed. The ones that were turned came under the control of the so-called XX Comittee (say: twenty comittee), better known as the Double-Cross System [2].
Confiscated SE-88/5 radio set, photographed during WWII [3]

The S-88/5 transmitter was built around a single KL2 valve (tube), and could be crystal-operated as well as free running. The transmitter was seriously overpowered in order to obtain the desired 5 Watts output, but it is unlikely that it ever produced more than 3 Watts. When present, the E-88 receiver — basically an E-75 — was built around three KF4 valves. The S-88 transmitter and E-88 receiver were also available in a single enclosure, in which case it was known as the SE-92/3.

Complete kit with batteries more key and leather carrying cases. Crown copyright [3]. Transmitter with morse key, spare valve and antenna wires. Crown copyright [3]. Transmitter interior showing the KL2 valve. Crown copyright [3]. Complete set of batteries. Crown copyright [3].
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Complete kit with batteries more key and leather carrying cases. Crown copyright [3].
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Transmitter with morse key, spare valve and antenna wires. Crown copyright [3].
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Transmitter interior showing the KL2 valve. Crown copyright [3].
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Complete set of batteries. Crown copyright [3].

In the news
Below is a clip from a newsreel that was shown in British cinemas in 1940, after the execution of two German spies that had been captured in London. At their arrest they had been carrying two leather cases – similar to the ones that were used for photographic equipment – holding a 'spy' S-88 transmitter and ancillaries, such as batteries, wire antennas and a special type of morse key.

Circuit diagram
Below is the circuit diagram of the transmitter, which is basically a modified Hartley oscillator. At the right is the tuned circuit and the antenna outputs. A lamp in the ground lead can be used to tune for maximum power output. At the left is the KL2 oscillator valve (tube) which is specified for a maximum anode voltage of 135V, but is used here at 250V to obtain a higher output power.

A crystal is connected in series with a feedback loop between the tuned circuit and the grid (g1) of the valve. By shorting the contacts of the crystal, the oscillator can be used in free-running mode (VFO). The same concept was used with a number of early Abwehr transmitters, such as the SE-92/3, the S-97/3 and the S-98/3, all of which are built around the same KL2 valve. Note that this principle puts a rather high load on the crystal, which may lead to unstable operation. For this reason, the circuit design was changed in a later version of the S-98/3 transmitter.

  1. British Pathé, News reel showing Nazi Spies' Radio Set (1940)
    1940. Retrieved May 2018 from YouTube.

  2. Wikipedia, Double-Cross System
    Retrieved May 2018.

  3. Arthur Bauer, Unternehmen Seelöwe (Operation Sea Lion)
    Image of confiscated SE-88/5 taken from interrogation report KV-2/1700. Page 28.
    5 September 1940. Crown Copyright.
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© Crypto Museum. Created: Monday 14 May 2018. Last changed: Friday, 16 August 2019 - 13:54 CET.
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