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Burst encoders
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SP-20   P-928
Solid-state spy radio set

The SP-20 is a spy radio set, developed around 1970 by Pfitzner/Teletron and AEG Telefunken in Germany, as a replacement for the aging valve-based SP-15. It was used by a number of Stay-Behind Organizations in Europe (Gladio) as well as by the Special Forces of the German Army. The two versions can be determined by their colour, green and grey, but are otherwise identical.
The SP-20 consists of a transmitter, an antenna tuner, a receiver, a synthesizer and accessories. Messages were transmitted in morse code by means of a manual key or a burst encoder. Various burst encoders have been used with it.

The image on the right shows part of the military version of the SP-20 as it was used by German Special Forces (SF). All units are painted in the standard NATO olive green colour. It was often used with the RT-3 burst encoder. The version used by stay-behind (Gladio) was grey and was used with a variety of morse burst encoders.
SP-20 military version

Most SP-20 units have been found without a matching receiver. Although the manufacturer developed a digital receiver for the SP-20, only a limited quantity was built and it was never put into large scale production. Instead the old FE-8 receiver, that was supplied with the SP-15 spy set, was used. All units are housed in a similar case of approx. 155 x 105 x 32 mm, except for the FE-8 receiver. A similar case was later used for the Speicher burst encoder and the PSU.

A revised version of the SP-20 set was planned and partly developed in 1983 [A]. It was known as SP 20 neu (new) and also as P 928. It consisted of the existing transmitter and synthesizer, with a new KE-30 receiver and a KÜG Kurzzeitübertragungsgerät (burst transmitter) with external keyboard and display, but as far as we know, the improved P-928 system was never released.
SP-20 military version S-6800 transmitter ASG-6800 Antenna Tuner KS-30 synthesizer Military variant of the SP-20 Complete military SP-20 spy radio set Cable between synthesizer and transmitter Short SMB cable for connection between transmitter and antenna tuner

  • Military version
    Most common version of the SP-20 in olive green enclosures, initially crystal operated and consisting of the S-6800 transmitter and the ASG-6800 antenna tuner. The crystal was later replaced by the KS-30 synthesizer that was connected via a 1 metre shielded cable.

  • Stay-Behind version
    Special version for the secret Stay-Behind Organisation, sometimes referred to as Gladio, that would activate themselves during the Cold War in the event of a war between Europe and the USSR. This version can be recognised by its grey enclosures.

  • SP-20 neu
    Revised version of the SP-20, also known as Projekt 928 (P-928), that was planned for release in 1983. Although and handfull of prototypes of the KE-30 receiver were manufactured, the complete P-298 was never released as it had been surpassed by the new pan-European FS-5000 by the time it was ready.

Complete military SP-20 setup.

Military version
The most common version of the SP-20 is the military variant, which can be recognised by the olive green colour of its cases. Initially, the military variant consisted of two modules (transmitter and antenna tuner), and was crystal operated. The two units were manufactured by Telefunken.
The two units can be mounted together by means of a metal rail on the top surface of the transmitter and a similar rail on the bottom surface of the antenna tuner. The transmitter is connected to the antenna tuner by means of a short coaxial cable with SMB plugs at either end.

The set was used in combination with the older Wandel & Goltermann FE-8 (BN-48) receiver that was also used with the earlier SP-15 radio set. Initially, the transmitter was crystal-operated, but the crystals were later replaced by a digital frequency synthesizer manufactured by Pfitzner.
Military variant of the SP-20

The image above shows a complete military SP-20 set with KS-30 synthesizer and the older FE-8 receiver, as it was used by German Special Forces (SF) during the Cold War. The unit at the front is an RT-3 burst encoder. Suitable webbing gear was supplied for transport of the radio station.
Stay-Behind version   wanted item
The Stay-Behind version of the SP-20 can be recognised by its grey enclosures. The set is largely the same as the military variant but the units have a mounting rail at either side of their body, so that they can be combined horizontally. The set consisted of a transmitter and an antenna tuner.
A Power Supply Unit (PSU) was available as an option. It was housed in a similar enclosure and could also be used as a power inverter, allowing the set to be powered by an alternative 6-24V DC power source, or the 12V battery of a car. The battery could also be recharged by the PSU.

The set was later complemented by the new KS-30 synthesizer that was developed by Pfitzner. The image on the right shows a complete SP-20 radio station in grey stay-behind enclosures. The rather rare PSU is visible at the left. At the left edge the later MMP burst encoder is visible.
Complete stay-behind version of the SP-20. Copyright Gerhard Reinel [5] WANTED ITEM

Note that with this version, the synthesizer is not connected to the transmitter by means of a 1 metre shielded cable, but by means of a small grey connector block, as shown above. In the image, all units are mounted together by means of the sideways rails, except for the antenna tuner which is always mounted on top of the transmitter. Initially, this set was supplied with the Speicher burst encoder, but this was later replaced by the much faster and more versatile MMP.
Projekt 928   SP-20 neu
In the late 1970s, Pfitzner started the development of the successor to the SP-20 radio set, which was designated P-928. It was also known as SP-20 neu (new). Based on the existing set, with a new receiver and a faster burst encoder, the new system was planned for introduction in 1983.
But by the time the set was completed, orders had already been given for the development of the flexible pan-European FS-5000 system. As a result, Pfitzner's P-928 set was never released.

The image on the right shows a KE-30 receiver with dark front panel that was developed as part of the P-928 project [6]. It is housed in the same enclosure as the KS-30 synthesizer but only has one 20-pin socket, that is located at the left.
KE-30 receiver is grey enclosure with dark front panel [6] WANTED ITEM

The space at the right (on the synthesizer used for the 2nd socket), is taken up by a LED bar which gives an indication of the received signal strength. Although the receiver would probably have been a good replacement for the aging FE-8 (BN-58), it was never rolled out. There are claims that the technical specifications of the KE-30 could not compete with those of the FE-8.

In 1983, the planned P-928 set consisted of the following components [A]:
Model Description (German) Manufacturer Remark
S-6800 Sender SP-20 AEG Telefunken Transmitter S-6800
ASG-6800 Abstimmgerät für SP-20 AEG Telefunken Antenna tuner ASG-6800
KS-30 Steuersender Pfitzner Synthesizer
KE-30 Empfänger Pfitzner Receiver (digital) 1
KÜG Kurzzeitübertragungsgerät Pfitzner Fast burst transmitter 2
KÜG Tastatur und Display Pfitzner Keyboard and display for KÜG 2
- Zubehörsatz - Accessories

  1. A small number of these receivers were manufactured for evaluation.
  2. We have never seen the KÜG device. If you have more information, please contact us.

Transmitter (Sender) S-6800, also known as KS-30 Antenna Tuning Unit (Abstimmgerät) ASG-6800 Synthesizer KS-30 Receiver FE-8, also known as BN-58 (same as used with SP-15) Digital receiver KE-30, the planned replacement for the FE-8 Multi-cable between synthesizer and transmitter Connection block between synthesizer and transmitter kit with tools, spares and supplies
Wire antenna Webbing kit used by Special Forces RT-3 burst encoder Speicher burst encoder MMP high-speed burst encoder

Transmitter S-6800
The S-6800 transmitter was developed by AEG Telefunken and is fully transistorised. It is both crystal and synthesizer operated and is suitable for CW (morse) and FSK (frequency shift keying). The maximum ouput power is 30W.

Note the two rigs on top of the unit, allowing it to be attached to the ASG-6800 antenna tuner (see below). The transmitter is also known as Sender SP-20.

 More information
S-6800 transmitter

Antenna Tuner ASG-6800
The ASG-6800 antenna tuner was designed for use in combination with the S-6800 transmitter. Like the transmitter, it was manufactured by AEG Telefunken in the mid 1970s.

The tuner was connected to the transmitter by means of a short coaxial cable with an SMB-connector at both ends. At the bottom it has two rigs that allows it to be slotted onto the S-6800 transmitter (see above).

 More information
ASG-6800 Antenna Tuner

Synthesizer KS-30
Initially, the S-6800 transmitter was crystal operated. Later, an external synthesizer unit (KS-30) was developed by Pfitzner. This was probably early in 1983. The sythesizer allows a frequency coverage of 2-24MHz in 1kHz steps.

It was added to all existing SP-20 stations and connected to the transmitter via a multi-pin connector on the left. A rather long cable was supplied to connect the KS-30 to the S-6800.

 More information
KS-30 synthesizer

Receiver FE-8   BN-58
The SP-20 did not have a matching receiver. Instead the existing FE-8 receiver of the old SP-15 radio set was re-used, probably because of its excellent performance and reliability.

The FE-8, also known as the BN-58, had two frequency bands and featured permeability tuning. As a result, it had linear frequency scales that were calibrated in MHz rather than units.

 More information

KE-30 Receiver
In the late 1970s, Pfitzner developed a new receiver for the SP-20. It was digital, was housed in the same enclosure as the other units, and was supposed to replace the old FE-8 receiver.

The new receiver was designated KE-30, but for several reasons it was never taken into large-scale production.

 More information

With the military variant of the SP-20, it was common practice to connect the synthesizer to the transmitter by means of a one meter cable.

The image on the right shows a typical multi-cable as it was supplied with the military SP-20 variant. The full wiring of this cable is given on the KS-30 page.
Cable between synthesizer and transmitter

Connection block
With the stay-behind version of the set however, it was more common to place the synthesizer to the left of the transmitter and connect the two units by means of the small connection block shown in the image on the right [1].   
Connection block for fitting the KS-30 synthesizer directly to the S-6800 transmitter. Photograph by Jim Meyer [1].

Each SP-20 unit came with a set of spare parts, tools and accessories. In most cases, these were packed in a sturdy 'lunch box' with a canvas strap to prevent the contents from falling out.

The image on the right shows an example of such a toolbox. It contains a screwdriver, pliers, insulation tape, ceramic antenna insulators, etc.
Tools and accessories for the SP-20

The antenna tuner makes it possible to use virtually any type of antenna with the S-8600 transmitter. In practice, a long antenna wire was used on most occasions. The antenna, and a suitable ground, are connected to the left side of the tuner, via 2 banana-type sockets.

The images below show some of the typical wire antennas that were supplied with the SP-20. Please note that transmitter and receiver each needed a separate antenna.
Antenna wire

Webbing kit
When used by German Special Forces, the olive green SP-20 radio set was usually carried in two plastic cases, attached to the webbing kit of the radio operator. In theory it was possible to operate the radio from within the carrying cases.

The image on the right shows the canvas webbing kit and the two plastic cases.
SP-20 webbing kit

Toolkit Close-up of the contents of the toolkit Wooden reel with wire antenna Simple set of wire antennas Military wire to be used as antenna Antenna reel in original packaging The various components of the webbing kit The S-6800 transmitter and the KS-30 synthesizer in their pockets

Burst Encoders
In order to minimise the risk of interception and radio direction finding (RDF), a burst encoder was often used with the SP-20. It allows a pre-recorded coded message to be played back in morse code at very high speed, in order to keep the transmission as short as possible.
The first burst encoder that was issued with the SP-20 was this electro-mechanical RT-3 unit. A small military-grade metal box that allowed a message of no more than 25 characters to be stored mechanically. Once on-air, the message was played back by operating a hand crank.

Later, more advanced burst encoders were issued, such as the GRA-71, MMP and Speicher.

 More information
The RT-3 burst encoder. Click for additional information.

Speicher Burst Encoder
The Speicher (memory) was an electronic burst encoder for sending numbers at high speed in morse code. It was powered directly from the mains and was housed in a similar case as the units of the SP-20 spy radio set.

The Speicher was probably issued in the 1970s to replace the rather limited RT-3. Eventually it was replaced itself by the more advanced MMP.

 More information
Speicher (memory) burst encoder

MMP Burst Encoder
The MMP was a fully electronic high-speed (1200 baud) burst encoder that was used with both the SP-15 and the SP-20 spy sets.

The MMP replaced older devices, such as the mechanical RT-3, the American AN/GRA-71 and the early electronic Speicher. It could hold more than 1000 letters and numbers in its battery-bakced CMOS memory and could send them at various speeds between 15 and 1200 baud.

 More information
High-speed morse burst encoder MMP-B

Crystal operated set with RT-3 burst encoder With KS-30 frequency synthesiser and RT-3 burst encoder SP-20 with Speicher burst encoder SP-20 with Speicher burst encoder KS-30 synthesiser and Speicher burst encoder SP-20 with MMP burst encoder

Initially, no technical documentation of the SP-20 was available, but over the years, we've received contributions from a number of our visitors. The documentation below, is what we have at the moment. The circuit diagrams of the transmitter are incomplete. If you have more complete documentation, or if you have other information that might be useful, please contact us.

Crypto Museum is still looking for the grey stay-behind variant of the SP-20 and would also like to add the KE-30 receiver to its collection. If you have any of these available, or if you have any other SP-20 related parts that are not yet shown here, please contact us.
  1. SP-20 neu, System
    System overview of SP-20 (German). 5 pages.
    Pfitzner Teletron, 16 March 1983

  2. Gerätebeschreibung KS 30
    Service Manual of the KS-30 synthesizer (German). 77 pages.
    Pfitzner Teletron, 16 March 1983 (first released July 1977).
     Circuit diagrams at DIN A3 size

  3. Sender S-6800, full circuit diagram
    AEG Telefunken, 1971-1976. 11 pages.

  4. Antenna Tuner ASG-6800, full circuit diagram
    AEG Telefunken, 1971-1976. 7 pages.

  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence.

  2. Museum Jan Corver, Exhibition Secret Messages
    Grey stay-behind version of SP-20 and Speicher. December 2008.

  3. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

  4. Winchester Electronics, SME20P connector
    Visited August 2016.

  5. Gerhard Reinel, Image of complete SP-20 set in grey enclosures
    Photograph made in 2001 from an anonymous collection.
    Personal correspondence, August 2016. Reproduced here by kind permission.

  6. Anonymous contributor, Image of KE-30 receiver in grey enclosure
    Personal correspondence, August 2016. Reproduced here by kind permission.

Further information

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© Crypto Museum. Created: Thursday 08 October 2009. Last changed: Wednesday, 14 September 2016 - 06:26 CET.
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