Spy radio
Burst encoders
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KSG   Kurzsignalgeber
Electromechanical burst encoder

The KSG, or Kurzsignalgeber (Burst Transmitter), was an electromechanical morse burst encoder, that was developed by the German Intelligence Service, the Bundesnachrichtendienst (BND), and introduced around 1957. It was intended to be used in combination with the FS-8 transmitter, a predecessor of the FS-7, but could also be used with other spy radio sets, such as the SP-15 [2].
When in storage mode, the device measures approx. 107 x 107 x 27 mm. It consists of a die-cast aluminium body with rounded corners that is generally painted in grey or blue hammerite.

The device has two black lids: one at the top that gives access to a large metal disc, and a smaller one at the bottom that gives access to a storage comparment. Inside the storage compartment are a metal crank, a pair of tweezers, and a large number of curved metal inserts with coded notches at the outer rim. Most inserts have a number engraved on both sides, e.g. 1 and 9.
Operating the KSG

The notches are actually the mechanical representation of the engraved number in morse code. This is also the reason why most insert can be used for two different numbers. For example: the morse 2 ··--- can be used as the 8 ---·· simply by reversing it. The tweezers are used to pick the required numbers from the box and insert them into the metal disc at the top side of the device.
When coding a message, the plain text is first translated to numbers using some kind of conversion scheme. The numbers are then added to the numbers from a so-called One-Time Pad, a list of truely random numbers of which only two copies exist. When properly used, the One-Time Pad code is unbreakable.

Once the message is encrypted, the metal disc is removed from the top side of the KSG and the numbers of which the cipher text consist are inserted into the disc in reverse order. Short messages are padded with spaces or garbage.
Storage compartment at the bottom

There should be no room between any two metal inserts and the last one should be held in place by the metal clip. The disc is then placed back in its bay and fixated with the nut at the centre. The metal crank is now inserted into the small hole at the side of the device and rotated clockwise a number of times until the disc rotates. The disc should be rotated until the two red dots are aligned [3]. The device is now ready to be connected to the KSG transmitter or the FS-7.
Once the KSG is connected to the transmitter and the transmitter is tuned, the transmitter should be turned on and the operator rotates the crank of the KSG with a constant speed between 1 and 2 revolutions per second until the red dots are aligned again. This takes about 7¼ turns.

When rotating the metal disc, a small switch, mounted in one corner of the device, senses the notches of the inserts that represent the morse dots and dashes. Whenever it meets a notch, the switch is closed and the transmitter (CW) is turned on. In between the notches it is off.
Insert 7 placed in the gate

This way, the entire message is transmitted in morse code in less than 8 seconds. The KSG was used by the BND for Stay-Behind operations during the Cold War (sometimes referred to as 'Gladio'). It was initially used in combination with the matching KSG Transmitter. It was also used with the SP-15 spy radio set that was introduced in the early 1960s, before it was replaced by other models, such as the RT-3, the GRA-71, the Speicher and eventually the digital MMP.
Closed KSG burst encoder Bottom view of closed KSG Top side with lid removed Encoder disc in the top section Bottom view with open lid Storage compartment at the bottom Removing the encoder disc Picking a number
Crank Tweezers Close-up of the insert with the morse number 8 The number 2 (reverse side of the 8) Insert 7 placed in the gate Shifting the insert through Gate closed, holding the inserts in place Close-up of the notches at the outer rim of the disc
Encoder disc removed from the KSG Encoder disc next to the KSG Encoder disc with open gate Sensing switch The switch sensing a gap The switch sensing a notch KSG with the crank in place Connecting the transmitter
Fully populated encoder disc Top view of a fully populated encoder disc Inserting the populated encoder disc Mating the disc with the notch Tightening the nut at the centre Aligning the two red dots Operating the KSG FS-7 transmitter with mains power supply and KSG burst encoder. In front: crystals, antenna, frequency tables and code sheets.

FS-8 Transmitter   KSG-Sender
Wanted item

The KSG burst encoder was initially designed for use in combination with a small clandestine 7.5W transmitter, the so-called FS-8 or KSG-Sender (KSG transmitter), which was also developed by the Bundesnachrichtendienst (BND) and built by the German manufacturer Hirschmann [1].

FS-8 (KSG-Sender) - wanted opject, copyright Detlev Vreisleben [1]

The image above shows an example of an original FS-8 from the former collection of Detlev Vreisleben in Koln [1]. The cable at the left is for connection to the KSG burst encoder. The cable at the right goes directly to the AC mains (110 or 220V). At the right side are two banana-type sockets for the antenna and the counterpoise. Crypto Museum are currently looking for a FS-8.
If you know of one that might be available, please contact us.

 More about the FS-8
KSG with SP-15
Although the KSG was developed for the accompanying FS-8 transmitter, it was also used with the later SP-15 spy radio set before it was replaced by other models. Evidence for this can be found in several documents and publications, such as the 1969 book Nicht länger Geheim [2].
This book was was published in East-Germany (DDR) during the Cold War and exposes Western European (BND) and American (CIA) espionage activities against the Warsaw Pact countries.

The book was replublished in 1975 with even more espionage cases. The image on the right shows a page of the 1975 edition, on which a captured SP15 spy radio set is shown. According to the caption it was found an a BND agent.

At the top is the FS-7 transmitter with its mains power supply. At the right is the KSG encoder.
Photograph of BND equipment captured in the DDR

In front of the transmitter is a set of crystals and a roll-up antenna. The paperwork at the front consist of operating instructions frequency tables and a map. At the far left is a metal container with a roll of One-Time Pad cipher material which is, of course, compromised in this case.

FS-7 transmitter with mains power supply and KSG burst encoder. In front: crystals, antenna, frequency tables and code sheets.

 More about the SP-15 spy radio set
SP-15 transmitter (FS-7) and KSG KSG with SP-15 transmitter (FS-7) FS-7 transmitter with mains power supply and KSG burst encoder. In front: crystals, antenna, frequency tables and code sheets.

The electrical circuit diagram of the KSG is very simply. In fact it is nothing more than a small switch (that senses the the gaps and notches on the outer rim of the disc) with two 100 ohm series resistors to reduce the current, and a capacitor that acts as a spark killer. Like this:

Connecting the KSG to the KSG-Sender is straightforward, as the latter already has a fixed cable with a suitable 3-pin DIN plug for direct connection to the KSG. When connecting the KSG to the later FS-7 transmitter, a conversion cable to the 5-pin 270° socket of the FS-7 is needed:

  1. Detlev Vreisleben, Images, documentation and backgrounds of KSG and KSG-Sender
    Personal correspondence. December 2014.

  2. Albrecht Charsius & Julius Mader, Nicht länger Geheim
    Berlin, 1969. 2nd edition, 1975.

  3. Bundesnachrichtendienst (BND), KSG Operating instructions (German)
    Date unknown, probably around 1957. 6 pages diary format. 1

  1. Kindly supplied by Detlev Vreisleben [1].

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Crypto Museum. Last changed: Friday, 28 August 2015 - 15:34 CET.
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