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Covert RF listening device

31218-1 is a radio frequency (RF) covert listening device (bug), developed in 1976 in the former DDR (East-Germany) at Außenstelle Beucha of the Institut für Technische Untersuchungen (ITU) 1 . It operates in the 900 MHz band and was used by the East-German secret intelligence service, the repressive Ministerium fur Staatssicherheit (Stasi), for wireless surveillance of people or objects.

The 31218-1 belangs to the 3rd generation of DDR radio bugs and is based on the 31217-1, which in turn is based on the 31216-1. Like the 31217 it does not have a built-in battery and microphone, but instead features a two-stage audio amplifier. In addition, the 31218 is fitted with an RF power amplifier that raises the output power to ~ 100 mW, making the device suitable for an operational range of ~ 500 metres.

The image on the right shows the basic 31218-1 variant with an external – permanently fitted – dynamic microphone made by Knowles 2 (USA).
31218 bug

With the device shown here, the microphone and battery wires have been taped to the body of the transmitter by means of cellotape. This was the usual practice at the Stasi. It was also possible to fit an (optional) SVM-144 or SVM-145 audio-masking 3 module between the microphone and the bug, to reduce the chance of accidental or deliberate discovery of the device and its intelligence.

The device was commonly operated in combination with the special 31215 or 31225 surveillance receiver. It was part of a family – consisting of the 31216, 31217 and 31218 – and was available in several variants, allowing it to be used with a variety of microphones and audio-masking 3 units. It was mainly used in combination with microphones from Western manufacturers such as Sennheiser and Knowles 2 , commonly ordered under the pretence of hearing aid manufacturing.

  1. Institut für Technische Untersuchungen (ITU) was a covert operation of the OTS, the Operativ-technische Sektor (Technical Operations Sector) of the MfS (Stasi). Not to be confused with the CIA's OTS.
  2. Note that Knowles was an American manufacturer that supplied miniature microphones for hearing aids. Ironically, many of their microphones were developed with funding from the US Central Intelligence Agency (CIA) for use with CIA bugs.
  3. In Stasi terminology, audio masking was known as Sprachverschleierung (speech concealment), or SV.

31218 high-power bug 31218 bug Dynamic Knowles microphone 31218 bug compared to the size of a hand 31218-1 with original envelope
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31218 high-power bug
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31218 bug
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Dynamic Knowles microphone
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31218 bug compared to the size of a hand
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31218-1 with original envelope

The diagram below shows the basic setup of an 31218-1 unit. At the left is the transmitter, which consists of an 31218-1 driver stage (which is actually a 31217-1 transmitter) and an 31218-ES power amplifier (PA). At the right is the matching 31225 surveillance receiver for 940-980 MHz.

For specific bugging operations, especially in the vicinity of the West-German border, the Stasi often chose to use audio-masking by means of subcarrier-modulation, to avoid accidental or deliberate interception of a conversation. The 31218-14 variant was made especially for this application. It consists of a 31218-1 from which the audio amplifier has been removed.

In this variant the RF oscillator is driven directly by an external SVM-44 audio masking unit, which modulates the audio on a 22 or 24 kHz subcarrier (double FM) and injects a 80 or 100 Hz hum into the baseband. In German this is known as Sprachverschleierung mit Maskerator (SVM).

The above solution was also available in a single package as the 31218-145, which basically consists of a 31218-14 unit with an integrated SVM-44 speech masking unit. Other variants were available for connection of electret microphones instead of dynamic ones.

Construction of the project number (31218-145)

The diagram above shows how the project number is constructed. The first digit tells us which department was responsible for it. In this case it is department 33, which was Außenstelle Beucha (Outstation Beucha). Before 1977, the prefix '3' was omitted, or the prefix 'AB' was used. The next two digits define the theme and the group within the theme. The next two digits define the actual project (within the group). If a device is part of a kit, the number behind the dash specifies the item number (within the kit). The last two digits are optional, and specify the version or variant.

Known versions and options
  • 31218-1
    Basic version for dynamic microphone, with RF PA
  • 31218-11
    Basic version for dynamic microphone, with RF PA
  • 31218-111
    Suitable for electret microphone
  • 31218-14
    For use with audio masking module SVM 144 or SVM 145
  • 31218-145
    Combined with SVM 144 audio masking module
  • 31218-10
    200 mW version
  • 31218-20
    Mains power upply unit (PSU)
  • 31218-SS
    Steuer-Stufe (driver stage only)
  • 31218-ES
    End-Stufe (power amplifier only)

Interior of the 31218-1 bug. Click for a closer view.

The 31218-1 bug is housed inside a protective case or sleeve, which consists of two thin PVC case shells, with cut-outs for the wires at both of the short sides. The case shells, and in some cases part of the wiring, are held together by means of cellotape. Inside is a silver-plated can.

After cutting through the cellotape, the two case shells can be removed and the silver-plated copper can is exposed. The can is close with a removable cap that is usually soldered in place.

The image on the right shows the enclosure of the 31218-1 after the cap has been removed from the can. Inside the can are two ceramic PCBs with miniature components, each of which takes about half the space. The PCB at the left is identical to the one inside the 31217-1 bug. It contains a free-running RF oscillator (from the 31216 bug) plus a two-stage audio amplifier.
31218 interior, compared to the size of a hand

The PCB at the right holds a single-stage power amplifier (PA) that takes the signal from the RF oscillator and raises it to approx. 100 mW. The S-shaped copper track on the rightmost PCB is a stripline filter. Due to the fact that a PA-stage is present between the RF oscillator and the antenna, the device suffers less from the so-called hand-effect than the 31216 and the 31217.

31218 bug with removed cap and PVC shells 31218 bug with removed cap 31218 bug - interior Interior - top view AF amplifier and RF oscillator stages Two PCBs jointed together RF amplifier stage 31218 interior, compared to the size of a hand
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31218 bug with removed cap and PVC shells
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31218 bug with removed cap
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31218 bug - interior
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Interior - top view
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AF amplifier and RF oscillator stages
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Two PCBs jointed together
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RF amplifier stage
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31218 interior, compared to the size of a hand

Circuit diagram
The basic 31218-1 transmitter consists of two parts: a driver stage with a two-stage audio amplifier and an RF oscillator, and a power amplifier (PA), mounted together in a single metal enclosure. The driver stage is in fact an exact copy of the complete 31217-1 transmitter:

Circuit diagram of the 31218-1 bug (part 1)

This circuit is complemented by an RF power amplifier (PA) which takes the output from the oscillator at point (X) and raises its level to ~ 100 mW. The PA also isolates the RF oscillator (BFS17) from the antenna, so that the unit suffers less from the hand-effect. The filter in the output circuit (L3 and 1-4pF) reduces the transmission of harmonic frequencies somewhat. Transistor (T4) is powered via L5. Note that the unit has the (+) terminal connected to ground.

Circuit diagram of the 31218-ES power amplifier (Endstufe)

If the 31218 was used in combination with an SVM-44 or SVM-45 audio-masking or speech concealment unit, the two-stage audio amplifier (T1, T2) was omitted and the bare RF oscillator was used to drive the PA. This oscillator is known as the 31216-SS driver stage (Steuerstufe) and is nearly identical to the oscillator part of the first diagram (31218-1) and to the 31216-1 bug.

Circuit diagram of the 31218-SS driver stage (Steurstufe)

In this case, the output of the 31218-SS oscillator (X) is connected to the input of the 31218-ES power amplifier (X), whilst the signal from the SVM-44 audio masking unit is provided to the LF input terminal (3). The nominal frequency of the oscillator is determined by the position of the 4.7pF capacitor on the longest arm of the stripline transformer (TR).

  1. GND
    +9V ±1V
  2. 0V
  3. Input
  4. ANT
  1. GND
    +9V ±1V
  2. 0V
  3. Input
  4. ANT
  • Power supply
    9V DC
  • Current
    40 mA
  • Frequency
    940 - 980 MHz (fixed spot frequency in band V)
  • HF power
    100 mW @ 9V DC supply
  • Antenna
    Wire, 11 cm (¼λ)
  • Audio
    200 Hz - 8 kHz
  • Deviation
    ± 75 kHz
  • Modulation
    FM (F3)
  • Subcarrier
    22 or 24 kHz, with 80 or 100 Hz hum 1
  • Dimensions
    53 x 17 x 8 mm
  • Weight
    24 grams
  1. Only for units with audio masking option (SVM).

  1. Kennblatt Technik 31218-14
    Datasheet (German). 2 July 1984 — 3 December 1984.

  2. 31218-14 SS (Steuer-Stufe)
    Technical documentation driver stage (German).
    12 June 1984 — 3 December 1984.

  3. 31218-14 ES (End-Stufe)
    Technical documentation final stage, power amplifier (German).
    11 June 1984 — 3 December 1984.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, 31218-1, technical description and operating instructions
    Personal correspondence, May - August 2018.

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

  3. Louis Meulstee, 31218-1 (GDR bugs V)
    Wireless for the Warrier, Volume 4 Supplement, Chapter 129 v1.01.
    Retrieved August 2018.

  4. Hinweise fur die Erprobung der Technik 31216, 31217, 31218 1
    Recommendations for application of 31216, 31217 and 31218 (German).
    MfS, BV Gera OTS 0102. 8 September 1976.
  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.

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© Crypto Museum. Created: Saturday 04 August 2018. Last changed: Monday, 13 August 2018 - 12:39 CET.
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