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Short-wave spy radio transmitter

AK-20 was a short-wave spy radio station, developed around 1975 by Mechanikai Laboratórium (ML) in Budapest (Hungary). The modular radio station was usually mounted inside a briefcase and covered 2-32 MHz with an output power of 20W, suitable for medium-range communication in Europe. The radio set was used during the Cold War by the Hungarian Intelligence Agency BAS for communication between agents and Control in Budapest, and possibly by the Stasi as well. 1

The set is suitable for the transmission of high-speed telegraphy in F1 (FSK), A1 (CW or morse code) and A3J (SSB), with a maximum data speed of 1000 baud. Such high-speed transmissions are generally known as bursts, and a cassette player is used here for playing back the data.

In addition, the set can be extended with several interfaces and peripherals to make it suitable for the transmission of 5-level teleprinter signals (baudot). The image on the right shows the basic transmitter that produces an output power of 250 mW, shown here with a manual morse key.
Hungarian AK-20-V spy radio set

The transmitter is powered via a connector at the rear left, which is normally connected to the Power Amplifier (PA), which in turn receives its power from the Mains Power Supply Unit (PSU). The PA acts as a power hub for all modules except for the tape player. The 250 mW output of the transmitter is connected to the PA which increases it to 20 W, adjustable in 10 steps. Note that the AK-20 does not contain a receiver. In practice a separate (commercial) one was used for this.

Developement of the radio was started at the Mechanikai Laboratorium (ML) in Budapest in 1975 with the first sets being delivered early in 1976. The set was mainly used by the Intelligence Agency BAS (also known as Department III of the Ministry of Internal Affairs), but some sets were delivered to the East-German Stasi, for whom the manual was translated into German [2]. 1 It is currently unknown how many AK-20 sets were built, but it is certain that very few have survived.

  1. It is currently unclear whether the Stasi in East-Germany (DDR) actually used the AK-20 in an operational context, or that they had received only a couple of devices from the Hungarians for evaluation. In any case, the operator's manual was (re)written in German by the manfacturer for this purpose, and the device was known in the DDR under the codename Eger [4].

Complete AK-20 in suitcase. Copyright Detlev Vreisleben [3].
Complete AK-20 outside suitcase. Copyright Detlev Vreisleben [3].
Hungarian AK-20-V spy radio set
Rear view
Top view
Frequency setting
Morse key
Suitcase   wanted item
The AK-20 radio station consisted of a number of modules that were joined together by means of cables. The modules were usually mounted inside a briefcase as shown in the drawing below [3]. Some of the modules, such as the tape cassette player, were removable. Furthermore, the features of the set could be enhanced by adding a number of (optional) external peripherals.

Complete AK-20 set in briefcase (wanted item). Photograph copyright Detlev Vreisleben [3].

The top section of the suitcase was used for storing additional materials, such as the antenna wires, power cables, instructions and frequency charts. After connecting a suitable antenna and counterpoise (ground), the automatic antenna tuner matches the antenna in 1 to 3 minutes.

 AK-20 Reference Chart

Modular radio station
The drawing above shows the basic set of modules as they were mounted in the lower half of a briefcase, with space at both sides for cables etc. In addition, the set could be enhanced with some external peripherals. The following standard modules are known:

  • Transmitter (synthesizer and exiter)
  • AK-20-Lvf
    Linear Power Amplifier
  • AK-20-H
    Antenna Tuner with Indicator (removable)
  • AK-20-M
    Tape recorder/player (removable)
  • AK-20-T
    Power Supply Unit (PSU)
  • AK-20-A
    Modulation add-on 1
  • ?
    External tape recorder (AUX)
  • AK-20-Msp
    Tape recorder 1
  • JG-1
    Telegraphy (telex) adapter (FSK)
    papter tape puncher/reader
  • S/P-K
    Serial to Parallel interface 1
  • P/S-K
    parallel to Serial interface 1
  1. These devices were also made by Mechanikai Laboratórium (ML) in Budapest.

At present, the AK-20-V is the only module of the AK-20 spy radio set in our collection. It is a combination of a synthesizer and an exiter that produces an output power of 250 mW, which is amplified to 20 W by the AK-20-Lvf linear amplifier. The rotary switch, to the right of the output socket, allows the transmission power to be adjusted between 100 mW and 20 W in 10 steps.

The transmitter measures 35.5 x 26 x 8.5 cm and weight 4 kg. Compared to other spy radio sets, it has a rather unconventional design and looks almost like a domestic piece of equipment, with a row of DIN sockets at the front panel.

12V DC power is connected to a socket at the rear and is normally supplied by the PSU via the Power Amplifier. The 250 mW output from the exiter is available on a coxial socket at the top left/rear, which is normally connected to the input of the Power Amplifier. It is shown here with an adapter to the common BNC standard.

The transmitter is shown here with the manual morse key mounted on the top cover. Please note that this was not always the case, as it could be mounted elsewhere as well. The DIN sockets at the front are for connection of the peripherals, such as the morse key and the cassette player.

Morse key
Although the AK-20 was generally used with a high-speed morse burst encoder (i.e. the tape cassette player), it was also possible to use a manual morse key in case of an emergency. For this purpose, a small simple key was supplied with the kit, such as the one shown here.

Note that, although the morse key shown here is brown, the set was usually supplied with a grey one. The key has a tapered bottom plate that can be slotted into a tapered socket which might be mounted on top of the device, like on this one.
Morse key

Rear view
Top view
Antenna socket
Frequency setting
Morse key
Morse key
RF output, output selector and morse key socket
The diagam below shows an overview of the controls and connections of the AK-20-V, most of which are located at the recessed front panel. From left to right are: the DC power switch with a power indicator (green), a tuning indicator (red), a switchable compressor/limiter, the MODE selector (modulation) and the input sockets for tape player, external player (aux) and morse key.

At the far right is the frequency selector that consists of five thumb-operated digit switches. It allows the frequency to be set with an accuracy of 1 kHz, between 2 MHz and 32 MHz. The power output of the complete set can be controlled with the Power Selector at the top rear, between 100 mW and 20 W in 10 steps. Please note that the maximum output of the exiter is 250 mW, but that the scale of the selector defines the output power of the Power Amplifier (not that of the exiter).

Despite the fact that Hungary was an Eastern Block country, behind the so-called Iron Curtain, during the time the AK-20 was developed, the interior is surprisingly modern. It is mainly constructed with first class components from manufacturers like Texas Instruments and KGV.

The interior can be accessed by removing two bolts from the top panel, two more bolts from the bottom panel and 4 small brackets at the sides. The top and bottom panel may now be removed. The interior consists of two major building blocks: the HF section in the top half of the case and the driver section at the bottom.

The top half contains the HF sections and is divided into four shielded compartments. It is hinged at the back and can be folded away to expose the solder side (i.e. the bottom) of the driver stages that reside in the bottom half.
Interior (bottom view)

The image above shows the compartimented lower half of the unit, as seen from the bottom. The long section at the front contains the wiring of the front panel. Note that the device contains ICs that were mainly made by western manufacturers like Texas Instruments and Motorola. Some ICs are manufactured by Tesla in Slovakia. Note in particular the crystal filer and the quartz oscillator made by KGV in West-Germany. It is rumoured that these components were obtained via Austria.

Removing the top cover
Removing the brackets at the sides
Bottom view
Lifting the top section
Interior (bottom view)
Interior (bottom view)
Block diagram
Below is the block diagram of the extremely complex transmitter. All frequencies are derived from a single 10 MHz reference clock that drives three Phased Locked Loops (PLLs), each of which has a programmable divider (÷n) that is configured with the digit selectors at the front right of the transmitter. The other modules are shown at the top of the diagram (with blue background).

For a full and detailed description of the above block diagram, please refer to the German technical description [B]. The numbers in the text correspond to the numbers above.

The image below shows the pinout of the 8-pin male socket at the rear of the AK-20-V, when looking into the socket from the rear of the unit. The pins are numbered from left to right: 1, 2, 3 and 4. The upper row is marked 'b', whilst the lower one is marked 'a'. +12V should be connected to pin b1. The minus terminal of the battery (0V) goes to pin b2. Pins a1 and a4 are bridged.

Pinout of the power socket of the AK-20-V when looking into the socket.

  • Frequency
    2-32 MHz
  • Steps
    1 kHz
  • Deviation
    < 80 Hz
  • Modulation
    A1, F1, Φ1, A3J
  • Power
    20 W (max)
Help required
Not much of the history of the AK-20 transmitter is currently known and, apart from a page in Louis Meulstee's excellent book Wireless for the Warrior - Volume 4 [2], there is no information about this radio set in the public domain. If you have any information about this radio, its history and/or its use, please contact us. We are also looking for the missing parts of this station, in particular the PSU, the PA, the Antenna Tuner and the Cassette Player.

  1. AK-20 Technical Manual
    Original circuit diagrams in Hungarian and Russian.
    Mechanikai Laboratórium, June 1975. SECRET. 1

  2. Kurzwellensender AK-20
    German translation of the operating instructions and technical description.
    Mechanikai Laboratórium, date unknown. Streng geheim. 1 2

  3. AK-20 Reference Chart
    Recreation of factory drawing 263-000000-00/02 (page 53 of the manual).
    Crypto Museum, August 2018.
  1. Manual kindly provided by Günter Fietsch [1].
  2. Photograph of complete set kindly provided by Detlev Vreisleben [3].

  1. Günter Fietsch, AK-20 documentation and circuit diagrams
    Received June 2015.

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

  3. Detlev Vreisleben, Photographs of complete AK-20 set in suitcase
    Germany, 12 March 2005. Received August 2015.

  4. BStU, MfS - HA III, Nr. 9581
    Date unknown. Kindly supplied by [3], September 2015.
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Crypto Museum. Created: Friday 14 August 2015. Last changed: Tuesday, 23 July 2019 - 07:49 CET.
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