Click for homepage
KOS-11   КОС-11
972 MHz radio bug with subcarrier audio-masking

KOS-11 (Bulgarian: КОС-11) was a Cold War radio frequency (RF) covert listening device (bug), developed in the early 1980s in Bulgaria, at that time a member of the Warsaw Pact. The device is crystal-operated and works in the 950 MHz band. It was used from 1984 onwards by the East-German (DDR) security service — MfS or Stasi — for covert overhearing of conversations in a car. It is also known by its Stasi project number 33212 and belongs to the 3rd generation of bugs.

The device measures 130 x 28 x 19 mm and weights 64 grams. It can be powered by any DC power supply between 8 and 15V, such as the 12V battery of a vehicle. When it was used for overhearing a conversation inside a (moving) car, the device was commonly hidden between the lining and the metal roof of the car, in such a way that the wire antenna was least obstructed.

It has a fixed thin 3 metre shielded cable, with a Knowles BT1751 electret micro­phone at the end. This allows the microphone to be installed at the optimal position for picking up the conversation.
KOS-11 bug

Apart from the electret microphone, the device features a two-stage pre-amplifier, as a result of which it belongs to Sensitivity Class III. To avoid accidental or intentional eavesdropping on the signal, it features subcarrier audio-masking. This means that on a regular receiver, only a silent carrier will be heard. The device has an output power of 10 mW which was sufficient for a 150 to 500 metre range. Because it has a crystal-driven oscillator, the transmitter is extremely stable.

It is currently unknown when exactly the Bulgarian KOS-11 was developed and in which countries it was used. Judging from the technology and the components used inside it, it is estimated that it was developed in the early 1980s. From a surviving Stasi document it is known that it was tested by the Stasi in late 1984, before it was approved for use in mobile (covert) operations [A].

KOS-11 bug Alignment access holes Screw-operated switch KOS-11 with wiring and microphone Knowles BT1752 microphone (with sound port at the side) Opened KOS-11 with microphone on a long cable KOS-11 compared to the size of a hand KOS-11 bug on top of its original envelope
1 / 8
KOS-11 bug
2 / 8
Alignment access holes
3 / 8
Screw-operated switch
4 / 8
KOS-11 with wiring and microphone
5 / 8
Knowles BT1752 microphone (with sound port at the side)
6 / 8
Opened KOS-11 with microphone on a long cable
7 / 8
KOS-11 compared to the size of a hand
8 / 8
KOS-11 bug on top of its original envelope

The diagram below shows how the KOS-11 was typically used. At the left is the actual bug, which consists of a sub-carrier modulator (FM-FM) and a transmitter, or RF unit. At the left is a sensitive external Knowles BT1751 or BT1752 1 electret microphone. It is powered by the car battery (12V).

At the right is a matching receiver. In the case of the Stasi this was a 31215 or 31225 receiver with a suitable (external or internal) demodulator to recover the double-modulated FM signal.

  1. The Knowles BT1751 and BT1752 microphones are electrically identical. The 1751 has its soundport at the end (at one of the short sides), whereas the 1752 has the soundport at the side (i.e. in the middle of one of the long sides). The KOS-11 featured on this page, is fitted with a BT1752.

Block diagram
Below is the block diagram of the KOS-11 bug. At the left is an externally connected miniature electret microphone, such as the Knowles BT1751. It modulates a 24kHz sub-carrier generator, which in turn modulates the 81 MHz crystal-controlled master oscillator. This technique is known as subcarrier modulation, or double FM, and is used to hide the audio from an eavesdropper.

The signal from the 81 MHz oscillator is multiplied by 12 and then amplified, before it is applied to the ¼λ wire antenna at the right. Note that apart from the desired signal at 973 MHz, it also eminates some unwanted side products, caused by harmonics of the crystal frequency.

Due to lack of selectivity in the multiplier stage of the KOS-11, the transmitter produces signals at the following frequencies: 242, 322, 406, 486, 566, 649, 741, 811, 891 and 973 MHz. These signals are all multiples of the 81 MHz oscillator frequency, and apart from the final one (973 MHz), they are unwanted. The lowest three frequencies (242, 322 and 406 MHz) could safely be ignored as they were not detectable more than 1 metre away from the KOS-11 transmitter.

The other frequencies (show in red) were more serious as they could potentially interfere with regular radio and TV transmissions, which might lead to discovery of the device. In particular the 566 MHz signal was close to the carrier of the 2nd West-German TV channel at 565.5 MHz. In close vicinity of a TV set, the KOS-11 could cause the image to become darker [A]. When used from inside a car however, no interference was noticed and the device was therefore approved.

The KOS-11 produces an output power of 10 mW. Depending on the position of its antenna, this should be sufficient for a maximum range between 150 and 500 metres. In practice, the device was often installed between the lining and the metal roof of a car, in which case the antenna was somewhat obstructed by the roof. Experiments by the Stasi showed the following results [A]:

  • 150 m
    Good quality reception
  • 250 m
    Good quality reception when using a Yagi antenna with the receiver
  • 300 m
    Steady car: good reception, moving car: no reception
  • 500 m
    Reception only when motor and lights are off (using Yagi antenna)
Audio masking
Although the KOS-11 operates at a very high frequency (972 MHz), and very few people in the DDR were able to receive radio signals at that frequency, there was always the danger that some­one accidently (or intentionally) picks up the signal from the bug and overhears the conversation. This was particularly the case in the vicinity of the West-German border, which was constantly under surveillance of West-German intelligence service like the BND (civil) and MAD (military).

In order to protect against eavesdropping, the Stasi sometimes used audio-masking, by which the audio is modulated onto a carrier above the audible frequency range. This technique is also known as subcarrier modulation or double FM (FM-FM). On a regular receiver, an eavesdropper will only hear a silent carrier. In order to hear the conversation, the seemingly silent signal has to be demodulated once more. Although this defeats the average surveillance receiver, the method was well-known by intelligence services at the other side, and counter-measures were in place.

 More about sub-carrier audio-masking

Receiver   31225
For reception of the double-FM-modulated KOS-11, the Stasi used a 31215 or 31225 surveillance receiver, with either an external demodulator (LWE6-1) or with an internally fitted module.

More information to follow
Stasi surveillance receiver 31225

Despite the fact that SC-modulated bugs are often used by intelligence serices, even today, the system is easily defeated with a professional surveillance receiver, or bug tracer. One of the first bug tracers that was able to demodulate an SC signal, was the Scanlock Mark 3 in 1976.

Its successor, the ScanLock Mark VB shown in the image on the right, can even discover the SC frequency automatically and will find the subcarrier-modulated KOS-11 within seconds.

 More about the Scanlock range

Scanlock Mark VB in operation

The KOS-11 is housed in a die-cast metalised plastic enclosure with two compartments. It is closed with a metal panel at the bottom. The metal panel is held in place by four screws in the corners. Removing these screws and taking off the metal panel, reveals the interior of the device.

Inside the device are two printed circuit boards (PCBs): one holding the subcarrier modulator and a larger one that holds the transmitter. Each PCB occupies one compartment of the enclosure, and each one is held in place by four screws.

The PCBs have conventional components on one side (top) and SMD parts at the other (bottom). When opening the case, the bottom side of the PCBs is visible. The image on the right shows the top side of the PCBs after removing them from the enclosure. The PCBs are interconnected by means of two shielded wires (audio and power).
KOS-11 PCBs outside the enclosure

At the left are two wires for connection of the supply voltage. The white wire is for connection to the +12V rail of the car battery. The blue wire is for connection to the (-) terminal of the battery (0V). The electret microphone is connected via a long two-wire shielded cable. The 1.5V power for the electret microphone is supplied by the device. At the left is a screw-operated micro-switch that allows selection between the raw 8-15V power, or (better) a stabilized 10.4V supply.

The device is well-made, but is a strange mixture of conventional (through-hole) components and surface-mount parts (SMDs). Although the use of SMDs generally leads to smaller devices, this is not the case here, as the conventional parts (in particular the crystal) need a lot of space. Nevertheless, it gives a good impression of the state-of-the-art in Bulgaria in the early 1980s, and clearly shows that they were capable of making ever smaller listening devices (bugs).

KOS-11 interior KOS-11 interior KOS-11 PCBs outside the enclosure Top side of the PCBs Bottom side of the PCBs Subcarrier modulator - top side Oscillator with 27 MHz crystal RF output stage
1 / 8
KOS-11 interior
2 / 8
KOS-11 interior
3 / 8
KOS-11 PCBs outside the enclosure
4 / 8
Top side of the PCBs
5 / 8
Bottom side of the PCBs
6 / 8
Subcarrier modulator - top side
7 / 8
Oscillator with 27 MHz crystal
8 / 8
RF output stage

Circuit diagram
Below are the circuit diagrams 1 of the two PCBs of the KOS-11. The first PCB holds the subcarrier modulator and consists of four sub-circuits: A power stabilizer, built around T1 and T2, a micro­phone amplifier (T3 and T4), a modulator (T5) and a 24 kHz subcarrier oscillator (T6 and T7). The amplifier is suitable for connection of an electret microphone, for which 1.5V is made. All circuits are powered by 6.2V. The modulated 24 kHz sub-carrier signal is available at the right (SC out).

PCB 1 - Amplifier and subcarrier modulator

At the top left is the raw battery input. A diode in series with this rail protects the transmitter against revered polarity. Also at the top left is a microswitch that can be used to select between the raw +12V input voltage and the stabilized 10.4 V. This switch is operated with a screw at one of the short sides of the enclosure. When the screw is not fully inserted, the stabilized 10.4V rail is selected. This is the default setting. By turning the screw all the way in, the raw battery voltage is supplied to the transmitter. Be careful when doing this, as it might damage the transmitter.

The other PCB contains the actual transmitter, which consists of four sub-circuits: a modulator, a crystal oscillator, a multiplier and an RF amplifier. The circuit is powered by a stabilized 10.V DC voltage, that is supplied by the other PCB. Central to the circuit is a crystal oscillator that uses a 27.000 MHz in its 3rd overtone to get a stable 81 MHz signal. The crystal is FM-modulated by means of two varicap diodes (V1,2) to which the subcarrier output of the other PCB is supplied.

PCB 2 - Transmitter

In the 2nd stage (T2) the 81 MHz signal is multiplied by 12 to get the required 972 MHz signal. The last stage (T3) finally amplifies the signal to antenna level (approx. 10 mW). The antenna is connected to a tap on the stripline coil in the tuned circuit of T3. The circuit has three adjustable capacitors (C4, C7 and C12) that are accessible via holes in the top of the enclosure. The three SMD transistors are all marked -P1- which refers most likely to the RF transistor BFR-92 [B].

  1. As the original technical documentation has not been recovered, the circuit diagrams above have been created by us, by carefully studying the PCBs and making some educated guesses.

  • Power supply
    8—15V DC (typically 12V from a car battery)
  • Current
    11 - 40 mA
  • Frequency
    973 MHz (from 81 MHz crystal)
  • HF power
    10 mW
  • Modulation
    FM (F3)
  • Masking
    FM-FM subcarrier modulation
  • Subcarrier
    24 kHz
  • Temperature
    -10°C — +55°C
  • Microphone
    Knowles BT1751 or similar
  • Dimensions
    130 x 28 x 19 mm
  • Weight
    64 grams
Known frequencies
  • 972.2 MHz
    Crystal: 27.000 MHz
  • 973.81 MHz
    Crystal: 27.050 MHz
  1. Kennblatt KOS 11 (KOS-11 datasheet) 1
    MfS, Berlin, 22 October 1984.

  2. BFR92 datasheet
    Retrieved August 2016.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, KOS-11, technical backgrounds
    Personal correspondence, May - August 2018.

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

  3. Louis Meulstee, KOS-11 (GDR bugs V)
    Wireless for the Warrier, Volume 4 Supplement, Chapter 132 v1.01.
    Retrieved August 2018.
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: Wednesday 08 August 2018. Last changed: Wednesday, 15 August 2018 - 05:54 CET.
Click for homepage