Click for homepage
← 31211
Covert RF listening device · 375 MHz - under construction

31211-5 is a radio frequency (RF) covert listening device (bug), developed in the late 1960s by PGH Funkmechanik 1 in Freiberg (DDR), for use by department 26 (German: Abteilung 26) of the repressive State Security Service (MfS or Stasi) of the former DDR (East-Germany). It is a second generation transmitter and works on the UHF frequencies between 360 and 400 MHz (Band IV).

The covert device was intended for overhearing a conversation in a room and was often used as part of a concealment, for example hidden inside an object or a piece of furniture. It measures 97 x 26 x 15 mm and weights 72 grams, including a Sennheiser MM-21 dynamic microphone.

The image on the right shows a typical 31211-5 with a Sennheiser MM-21 connected to it. At the right is a (green) wire antenna. The red and blue wires at the left are for connecting a battery or a power supply unit (PSU). The bug is powered by a 7 to 10V DC source, but performs best at 8.5V.
31211-5 transmitter with dynamic microphone

The device featured here, has been adjusted for a nominal operating frequency of ~ 375 MHz, which is subject to the supplied voltage, the ambient temperature and any objects in the vicinity of the transmitter and in particular its antenna. Despite the fact that it is a two-stage transmitter — it consists of an oscillator and an RF amplifier — it suffers from the so-called hand effect, which may cause it to drift as much as 1 MHz at either side of the nominal frequency of 375 MHz.

The device has a very sensitive built-in audio pre-amplifier with a high dynamic range, as a result of which any conversations in a room can be picked up with great ease. The pre-amplifier is built around a Siemens TAA131 integrated circuit (IC) that had been developed especially for miniature hearing aids and that had just become available in countries West of the Iron Curtain in 1966. 2

  1. Source: Prof. Schiffel, via Detlev Vreisleben [1]. PGH = Produktions-Genossenschaft des Handwerks.
  2. For this reason it is assumed that the 31211-5 was developed no earlier that the late 1960s.

31211-5 transmitter 31211-5 transmitter with dynamic microphone DDR bug 31211-5 (375 MHz) Sennheiser MM-21 microphone Transmitter with lid removed Interior of the device, after removing the lid Top view with shields removed Amplifier PCB - component side
1 / 8
31211-5 transmitter
2 / 8
31211-5 transmitter with dynamic microphone
3 / 8
DDR bug 31211-5 (375 MHz)
4 / 8
Sennheiser MM-21 microphone
5 / 8
Transmitter with lid removed
6 / 8
Interior of the device, after removing the lid
7 / 8
Top view with shields removed
8 / 8
Amplifier PCB - component side

The diagram below gives an overview of the characteristics of the transmitter. At the front is the wiring for connection of the battery and an external dynamic microphone. Also at the front are two small holes through which the adjustable capacitors of the RF stages can be reached. At the far end is a 21 cm wire that acts as the antenna. For the best results, it should be straightened along the longitudinal axis of the device, in which case the body itself acts as the counterpoise.

At the side of the enclosure, almost at the far end, is a hole behind which an adjustable capacitor is located. Turning this capacitor 90° to either side, allows the nominal frequency to be altered by ± 2 MHz. This should only be done when necessary, as it reduces the performance somewhat [A].

The 31211-5 was part of a family of miniature covert listening devices, consisting of low-power (short range), medium power (medium range) and high-power (long range) transmitters, either bare or concealed inside an object or a piece of furniture. Most of them were manufactured in the DDR, but some were imported from sister agencies in countries like Czechoslovakia and Bulgaria.

The following variants of the 31211-5 are known:
  1. Standard single-channel version for 2000Ω dynamic microphone (MM-21)
  2. Dual-channel version (the two channels are mixed)
  3. With built-in identifier (complete device known as 31221)
  4. Adapted for external audio masking device (typically a sub-carrier modulator)
  • 31211-5
    Transmitter (bug)
  • 31211-52
    PSU for 31211-5 1
  • 31221
    31211-5 with identifier
Suitable receivers
  • 31210
  • 31220
  1. Source: MfS, Inventurliste 1987 [4 p.50].

Circuit diagram
Block diagram
The 31211-5 is a two-stage transmitter, which consists of three functional blocks, as illustrated in the diagram below. At the center is a free running oscillator that delivers a signal at the desired frequency (375 MHz). This signal is buffered and amplified in the PA stage at the right. At the left is a high-gain audio amplifier with a high dynamic range, that directly modulates the oscillator.

Block diagram of the 31211-5 transmitter

Initial design
Below is the initial circuit diagram of the 31211-5, as found in the original technical description [A]. Note that this is just an intermediate design, that does not reflect the actual circuit of the device. The circuit is built around two RF transistors, of which the first one is the oscillator (T1). It is modulated directly at its base by the audio signal from the electret microphone at the far left.

Circuit diagram as given in the technical documentation [A]

The oscillator has a tuned circuit with a stripline (L1) that is capacitively coupled to the second stage, which is an RF amplifier in grounded base configuration (T2). The antenna is connected directly to a tap on the stripline (L2) of the tuned circuit of T2. In this design, the (+) terminal of the battery is connected to the chassis. The circuit is powered by a voltage between 5.5 and 12V.

Final design
Below is the circuit diagram of the final design of the device, as taken down by Crypto Museum in 2018 [5]. This circuit matches the technical discription in the original documentation [A], but is significantly diffent from the circuit above. It is powered by 9V DC and has (-) on the chassis.

Actual circuit diagram as taken down by Crypto Museum from the 31211-5 in its collection in October 2018 [5]

This design is also based on two RF transistors, of which T1 is the oscillator and T2 is the RF amplifier in grounded base configuration. However, the two stages are transformer-coupled (L1a/L1b) and a trimmer is present to allow fine tuning by approx. ± 2 MHz. Furthermore, an AF amplifier is added at the left. It is based on a very early TAA131 integrated circuit 1 which was introduced by the West-German company Siemens in 1966, especially for use in hearing aids [B].

Note that there are at least four variants of the audio amplifier section. The circuit diagram above reflects the basic variant which had a single input for a 2000Ω microphone. Other variants were available with dual audio input, a built-in identifier, or an interface to an external audio masking module. The unpopulated area of the audio board was used to accomodate these extra features.

  1. This dates the design to the late 1960s.

Top view of the transmitter, with shields removed. Click for a closer look.

The transmitter is housed in a silver-plated brass enclosure that measures 97 x 26 x 15 mm. It is closed at one of the large sides with a silver plated lid that can easily be removed. After lifting off the lid, the contents of the bugs are revealed. The interior is divided into four compartments.

The smallest compartment holds the wiring to the external batteries and the microphone. The wires are connected to the second compartment via a series of feed-through capacitors. This compartment holds a PCB with the power line filters and an audio amplifier, which is built around a very early TAA131 integrated circuit (IC) from the western manufacturer Siemens.

The image on the right shows the amplifier board after removing it from the soldered pins of the feed-through capacitors at both sides. A short wire connects the board to the chassis.
Amplifier PCB

The board is partly unpopulated. This space is used for other variants of the audio modulator. At the right are two long compartments that hold the RF stages. They are normally shielded, but for the image at the top of this section, we have removed them temporarily. The amplifier board is connected to the RF stages via four feed-through capacitors. At the solder side of the amplifier board are two 27k resistors that are directly solderd to the pads of the feed-through capacitors.

Two further 4k7 resistors are soldered directly to two of the feed-through capacitors below the audio board. Together, these resistors provide the bias voltage at the base of the RF transistors. The 27k resistors are probably soldered at the back of the audio board, so that the circuit can be optimised for a particular supply voltage. In the given configuration, the best performance was measured with a power supply of 8.5V DC.

Note the two plastic (red) tubes that run through the amplifier compartment. They can be used to access the tuning capacitors of the striplines.
Audio board removed from the transmitter

The two long compartments hold the RF stages, each of which is built around an Japanese NEC 1 2SC289 RF transistor (marked 289), and a tuned circuit which consists of a stripline (a silver-plated copper wire) with a tuning capacitor at one end. The other end is soldered to the chassis.

The components inside the RF compartments are soldered closely to the feed-through capacitors from the audio board. The image on the right gives a good view of the two white ceramic RF transistors. The one at the rear is the oscillator, whilst the one at the front is the RF amplifier.

At the left, part of the striplines are visible. At the right are the plastic red tubes, through which the adjustable capacitors – mounted at the end of the striplines – can be reached for alignment. The two RF stages are coupled via a stripline transformer mounted to the separation wall.
Oscillator and RF amplifier

About 1/3rd from the ground-end of the oscillator's stripline, is a tap with a trimmer connected to ground. It allows the nominal frequency of the transmitter to be shifted by ± 2 MHz, but this should only be done when it is unavoidable, as it reduces the transmitter's performance. The antenna is galvanically coupled to the RF amplifier's stripline at ~ 1/3rd from the ground-end.

  1. The 2SC289 was manufactured by the Japanese company NEC, but also by the US company American Microsemiconductor. It is an RF transistor with an ft of 1.1 GHz. Datasheet wanted

Transmitter with removed lid. Note the '375' written on the white isolation. This is the transmission frequency. Transmitter with shields removed from the RF stages Top view with shields removed Amplifier PCB Audio board removed from the transmitter Oscillator and RF amplifier Circuits around T1 and T2 Transformer coupling from oscillator to RF amplifier
1 / 8
Transmitter with removed lid. Note the '375' written on the white isolation. This is the transmission frequency.
2 / 8
Transmitter with shields removed from the RF stages
3 / 8
Top view with shields removed
4 / 8
Amplifier PCB
5 / 8
Audio board removed from the transmitter
6 / 8
Oscillator and RF amplifier
7 / 8
Circuits around T1 and T2
8 / 8
Transformer coupling from oscillator to RF amplifier

  • Frequency
    360 — 398 MHz (typically 375 MHz)
  • Tuning
    ± 2 MHz (via trimmer)
  • Modulation
    F3 (FM)
  • Deviation
    ≥ 25 kHz @ -70dB (1000 Hz, 600Ω)
  • Audio
    200 Hz — 60 kHz 1
  • Input
    200Ω (e.g. MM-21)
  • Voltage
    7.2 — 10.5V (9V nominal)
  • Current
    ≤ 16 mA @ 9V
  • Output
    12.5 mW @ 9V (50Ω)
  • Range
    400 m
  • Antenna
    ¼λ (21 cm)
  • Temperature
    -20 — +50°C
  • Δf temp
    40 kHz / °C
  • Δf voltage
    0.5 MHz / V
  • Δf env.
    ± 200 kHz
  • Dimensions
    97 x 26 x 15 mm (including lid)
  • Weight
    70 grams
  • Semiconductors
    2 x 2SC289 (RF), 1 x TAA131 (audio), 1 x BC112 (or BC147) 2
  1. This makes the device suitable for subcarrier audio masking modules.
  2. Only in certain versions of the modulator.

  1. Montage und Bedienungsvorschrift für Gerät 1211-5
    Operating instructions and technical description (German).
    Date unknown. 7 Pages. 1

  2. TAA131 datasheet
    Siemens AG, Analog integrated circuits, Data Book 1976/77. pp. 175-177.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, 1211-5 / 31211-5, technical description and instructions
    Personal correspondence, September — October 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Louis Meulstee, 31211-5 (GDR bugs II)
    Wireless for the Warrier, Volume 4 Supplement, Chapter 124 v1.00.
    Retrieved August 2018.

  4. MfS, Inventurlisten der operativen Technik
    Abteilung 26. Berlin, 10 September 1987. 2

  5. Paul Reuvers, Circuit diagram of 31211-5 in collection
    Crypto Museum, 10 October 2018.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.
  2. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Tuesday 09 October 2018. Last changed: Saturday, 13 October 2018 - 14:29 CET.
Click for homepage