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NATO
MI6
  
NERA VR-2
Microwave receiver for passive microphone

VR-2 or VR-2/B is a microwave receiver for a passive covert listening device (bug), developed around 1963 by NERA in Bergen (Norway) for use by NATO. 1 It is part of a complete listening system that comprises an activation transmitter, the VR-2 receiver and a passive microphone — the actual bug — which is different from the device found in 1952 at the residence of the US Ambassador in Moscow, hidden inside a wooden carving of the Great Seal of the United States.

The device works at a frequency around 3.4 GHz and consists of a pyramidal feed horn and a wave guide with a detector diode. The signal from the detector is amplified and fed to a pair of headphones and (optionally) to a recorder.

The VR-2 is powered by a regular 4.5V battery, that is installed inside the grip at the bottom. Inside the wave guide is a dielectic taper that focusses the microwave beam in the same way as a lens focusses light. The axial position of the dielectric taper can be adjusted with a plunjer at the rear. This also alters the frequency slightly.
  

At the side of the battery compartment is a 9-digit code (-25-101-1890), which is basically a NATO Stock Number (NSN) of which the first four digits are missing. 2 The complete number is NSN 5865-25-101-1890, which is described in the NSN database as DETECTING SET, RADAR. It's group code (5865) reveals that it belongs to the category of Electronic Countermeasures (ECM), Counter-Countermeasures (ECCM) and Quick Reaction Capability equipment (QRC). This group code was also used as a camouflage for dual-use equipment of which even the people on the production line were not supposed to know the true purpose. It is known that the device shown here was used in Germany between 1984 and 1989 by British intelligence service MI6 [1].

It is currently unknown when the VR-2/B was developed, but date codes on the internal parts indicate that it was built in late 1962 or early 1963. It is also likely that it was part of a series of devices that were developed between 1952 and 1980 — probably with help from the Norwegian Defence Research Establishment (FFI) — in response to the discovery in 1952 of a similar device at the residence of the American Ambassador in Moscow, which became known as the Thing [4].

  1. According to the NSN database, the device was made for the German market. The device shown here was used in Germany by the British intelligence service MI6 and has German text on its control panel. This was probably done to hide its real use by the British [1].
  2. Without the 4-digit prefix, an NSN is known as a National Item Identification Number (NIIN) [7]. It uniquely identifies the item. The 4-digit prefix only specifies the Supply Classification Group.

PLEASE HELP — At present, Crypto Museum only has a VR-2/B receiver in its collection. We are still looking for the activation transmitter and for the actual bug — the passive microphone. If you have additional information, documentation or, better, the actual hardware, please contact us, so that we can expand this page.
Handheld SHF receiver
Handheld SHF receiver
Identification and acceptance stamp
Dielictric lens
Beam forming adjustment plunjer
Controls and headset connector
Headphones
Model and serial number
A
×
A
1 / 8
Handheld SHF receiver
A
2 / 8
Handheld SHF receiver
A
3 / 8
Identification and acceptance stamp
A
4 / 8
Dielictric lens
A
5 / 8
Beam forming adjustment plunjer
A
6 / 8
Controls and headset connector
A
7 / 8
Headphones
A
8 / 8
Model and serial number

Features
The VR-2/B is an extremely simple device that measures 490 x 200 x 156 mm and weights 2235 grams, battery included. The actual receiver is housed inside the cylindrical part of the device and consists of a wave guide, a detector diode and a 6-transistor electronic circuit. It is powered by a regular 4.5V 'flat' battery, that is installed inside the grip. At the rear of the device is a BNC socket for connection of a pair of Headphones and – with a T-adapter – an (optional) recording device.


Inside the wave guide is a rectangular piece of plexiglass that can be moved in and out of the wave guide by means of a plunger at the rear. It acts as a dielectric taper and has a pattern of drilled holes to provide a focussing capability. The dielectric taper is covered by a feed horn that is attached to the entry port of the wave guide by means of a threaded knurled ring.

Controls and headset connector
Beam forming adjustment plunjer
Plunjer all the way out
Frequency fine tuning
Battery removed from the battery compartment (inside grip)
Beam forming dielectric lens
Handheld receiver with headset
Removing the horn
B
×
B
1 / 8
Controls and headset connector
B
2 / 8
Beam forming adjustment plunjer
B
3 / 8
Plunjer all the way out
B
4 / 8
Frequency fine tuning
B
5 / 8
Battery removed from the battery compartment (inside grip)
B
6 / 8
Beam forming dielectric lens
B
7 / 8
Handheld receiver with headset
B
8 / 8
Removing the horn

Setup
The diagram below shows how the devices were used. At the bottom left is the activation trans­mitter (1) which comprises a klystron oscillator and a feed horn. It illuminates the target from a distance of 500-600 metres with a strong continuous wave (CW) microwave signal of a specific frequency. A portion of this signal hits the concealed bug — a passive microphone (2) — that is hidden in the target area. If the incident illumination frequency is the same as the microphone's resonant frequency, it will cause the microphone to resonate and (re)emit some of the energy.


The signal emitted by the bug has the same frequency as the illumination signal, but is Amplitude Modulated (AM) with the sound picked up in the room where the bug is hidden. It is intercepted by the VR-2 receiver (3), which can be up to 1 km away. Note that transmitter and receiver are at different locations to avoid spillover from the transmitter into the receiver. They are typically placed at 90° angles. Note that such distances were possible as there were virtually no other signals or sources of interference in the 3-4 GHz band and the time.

Transmitter
The activation transmitter – used to illuminate the target – is built around a klystron oscillator. It is housed in a rectangular enclosure inside which the feed horn, the klystron, the HT power circuits, a timer and the batteries are housed.

The timer was used for unmanned operation and allowed the illumination signal to be transmitted only during specific times of the day. At the centre is the klystron, which is used as a free-running oscillator. It has a knob by means of which the frequency can be tuned somewhat.

The device is powered either by the internal batteries or by means of the external 12V DC power supply unit, which was stowed inside the top lid. The transmitter is currently missing from our collection. The image on the right shows an educated guess of what it looked like, based on the recollections of an eye witness [1].

No photograph available

  
Educated guess of the construction of the illumination transmitter

Passive element
The exact operation and construction of the actual bug — the passive element (PE) — is currently unknown, and no surviving specimen has turned up so far. From the description of a former MI6 operative however, we were able to create the educated guess shown in the diagram below [1]. The device is concealed inside a plastic cigar tube. As soon as the screw cap is removed from the cigar tube, a spring ensures that the microphone end of the device is raised above the tube.

Educated guess of a possible construction of the passive element. The exact operation of this device is currently unknown.
Educated guess of the construction of the passive element

The microphone consists of a thin plastic cone that is suspended from the top of a cylindrical frame. At the bottom of the cone is a metal needle that enters a resonant unit. Any sound in the target area causes the cone to vibrate and the needle to move in and out of the resonant unit. Note that this construction makes the microphone extremely sensitive to other types of vibration, such as walking through the room, slamming a door, or placing coffee cups on the table top.

It is currently unclear whether the actual device is a resonant cavity – like the original Russian device – or an open dipole with a capacitive load. In the former case, the moving needle alters the volume of the cavity. In the latter case, the needle extends through the upper arm of the dipole, half way into the lower arm. Its movement alters the capacity between the two arms of the dipole.

Possible construction of the passive element. Please note that the exact operation is still unknown and that the actual construction might be totally different.
Possible construction of the passive element (cross section of the dipole)

The microphone is constructed in such a way that it can be used in radial as well as axial mode. In most cases the PE was used in vertical position — it was typically hidden inside a table leg — in which case the microphone is used in radial mode and sound enters it from the side(s). In this mode, the microphone is omni-directional. The diagram below shows how it was usually hidden:

Passive element hidden inside a 1" hole in a table leg

Note that the plastic cigar tube was not removed (only the cap). It was also possible to use the PE as a contact microphone, in which case the open end of the cone was attached to, say, a wall or a floor board. In that case, the microphone is used in axial mode and is uni-directional, but the sound quality is not as good as with a real contact microphone, such as the CIA's SWM-25.

Passive microphone used in axial mode (as contact microphone)



History
In September 1952, a hitherto unknown type of covert listening device (bug) was discovered at the residence of the American Ambassador in Moscow. It was hidden inside a wooden carving of the Great Seal of the United States that hang on the wall behind the ambassador's desk.

The carving had been given by the Russians in August 1945, to commemorate the close friend­ship between the United States and the Soviet Union (USSR) during World War II (WWII) and the subsequent defeat of Nazi Germany. For more than seven years it had provided the Russians with the most sensitive first-class intelligence.

The mysterious bug, which became known as The Great Seal Bug, didn't contain any electronic parts and did not have its own power source. As it was initially unclear how it worked, it soon became known by its nickname — the Thing [4].
  

Immediately after the discovery of the Thing, the device was brought to the US, where it was investigated by the Federal Bureau of Investigation (FBI) and the Naval Research Laboratory (NRL). From several publications, such as Peter Wright's book Spy Catcher [2] and David Wise's book Molehunt [3], it is known that part of the initial investigation was carried out abroad, including in the UK and the Netherlands. It is also known that the CIA conducted its own investigation.

The final FBI/NRL report, that was released on 1 December 1952 (declassified in 2019), contains a detailed drawing of the bug in which the dimension are given in centimetres rather than inches, which reveals the involvement of a metric country [5]. The most likely candidates are the Norway and the Netherlands, both of which had strong ties with the US at the time, and had the required technological expertise. The Netherlands can be ruled out, as they were already involved in the secret EASYCHAIR research program of the CIA, leaving Norway as the most likely FBI candidate.

It is possible that the FBI research was partially carried out at the Norwegian Defence Research Establishment (FFI) in Bergen (Norway), which had been established shortly after WWII in 1946. Many of its scientists and engineers had escaped to the UK during the war, where they worked on technological developments in the field of radio and radar. The first post-war Norwegian radar systems were developed at the FFI and were manufactured by NERA, which was co-located.

Targets
The device featured here, was used in Germany by the British intelligence service MI6 against various types of targets, both foreign and domestic. This involved breaking into offices and homes when the target area was abandoned, installing the bug (the PE), removing any traces of the operation, leaving the target area and testing the bug. On the way to the target area, the MI6 operative carried the PE in a cigar box filled with real cigars, just in case he was ever checked.

He would then break into the building by means of surreptitious entry, inspect the target area and decide where to hide the PE. In most cases it was hidden inside a table leg or inside a heavy piece of furniture. The operative would partly disassemble a table or desk, drill a hole of the desired diameter and depth, remove the cap from the PE and install it in the freshly drilled hole.

The table was then reassembled and all traces of the operation (such as wood chips and dust from the drilling) were removed. Note that it was necessary to either drill an additional hole for the PE's microphone, or leave enough of an air gap between the top of the table leg and the table top, to allow sound to reach the microphone.


An example of a domestic target of the mid-1980s is KUKA Wehrtechnik GmbH in Augsburg (Germany) [8]. From 1981 onwards, KUKA was one of the suppliers of anti-aircraft equipment to the Germany Army and NATO. To ensure that KUKA stayed within NATO guidelines and did not sell its technology to proscribed countries, a bug was planted in its office [1]. KUKA Wehrtechnik was sold in 1999 to Rheinmetall [8]. The remainder of KUKA — in particular the industrial robot business — was bought in 2016 by the Chinese company Midea Group [8].


Hitler's desk
Rumour has it that a Passive Element (PE) even ended up in one of Adolf Hitler's desks [1]. US forces had secured the desk from the Reichs Chancellery 1 in Berlin at the end of WWII, and had brought it to the Allied Headquarters in Heidelberg [9] as a trophy — one of the spoils of war.

In Heidelberg, the desk was used by subsequent American commanders, starting directly after the war with General Patton. Knowing that the British Forces were being spied on by the US – despite being allies – the British decided to do the same, and place the US commander under surveillance.

One night, when the desk was at the workshop of the 21st Support Command in Mannheim-Seckenheim to be repolished, an MI6 operative was able to get access to the workshop. There he drilled a hole in the rightmost vertical timber, just below the top, and covertly installed a PE.
  

It is currently unknown what happened to the desk and whether the bug is still inside it today. In 2006, a desk that alledgedly came from Hitler's appartment in München was auctioned in the US. It was said to have been purchased by a private collector from a government auction in the early 1970s [10]. As it was sold before the PE was installed, this desk in an unlikely candidate.

Another Hitler desk was rediscovered in Germany in 2014 [11]. It reportedly came from Hitler's bunker in Berlin 1 and was held for many years by the US military. In 1996 it was handed over to the German authorities and placed in a government storeroom in 2000, where it was rediscovered in 2014. It is quite possible, if not likely, that the latter is the actual desk in which the bug was hidden by MI6 in the mid-1980s. It is unclear however, whether the bug is still present...

  1. The accounts differ. Some sources say that the desk was taken from Htiler's Bunker in Berlin, whilst others claim that it came from the 'Eagle's Nest' in Bechtesgaden [12]. Our eye witness recalled that it was probably recovered from the Reichs Chancellery in Berlin [1].

Circuit diagram
Below is the circuit diagram of the VR-2/B receiver taken down from the device in the Crypto Museum collection. It is built around one OC44 and five 2N346 transistors, all of which are Germanium PNP types. At the far left is the detector diode (D1), which is mounted inside a wave guide with an optimum frequency around 3.4 GHz.


Q1 to Q5 are all 2N346 Ge PNP transistors with a hFE of 20. The last transistor (Q6) is an OC44 Ge PNP transistor with a hFE of 100. It is used here as an Automatic Volume Control (AVC) by feeding back a portion of the counterphase of the output signal to the base of Q5.


Interior
The interior of the VR-2 can be accessed by removing the feed horn from the front, and the knobs from the rear panel, after which the wave guide can be separated from the brass enclosure by means of a screw driver. This may take some effort as the interior is firmly held in place by the friction between the silicone gasket around the entry port of the wave guide, and the enclosure.


The image above shows the interior of the device once it's extracted from the enclosure. A 6-transistor electronic circuit — the actual receiver — is mounted to one side of the wave guide. The input from the detector diode is at the right, whilst the output transformer is visible at the left. The detector diode is held in place by a knurled cap at the right. At the other side, the wave guide has a longitudinal slot into which a thin carbon card can be inserted by means of a knob at the rear. This is a so-called card attenuator that allows strong signals to be suppressed.

The unit is constructed in such a way that the interior can be removed without disconnecting any wires. The battery compartment is connected to the circuit by means of a slide contact that mates with the (+) terminal from the battery. The (-) battery terminal is connected via the enclosure. When reassembling the unit, an index stub near the entry port of the wave guide must mate with the index notch in the threaded ring of the enclosure, to ensure that it is correctly orientated.

Handheld SHF receiver
Removing the horn
Horn removed from the receiver
Receiver with dielectric lens
Receiver with horn removed
Dielictric lens
Dielectric lens inside the wave guide
Interior removed from the enclosure
Interior
Interior - frequency adjustment side
Interior - PCB side
Printed Circuit Board
PCB detail (date code 6240 on transistor)
Detector diode
Frequency adjustment
(+) terminal slide contact
C
×
C
1 / 16
Handheld SHF receiver
C
2 / 16
Removing the horn
C
3 / 16
Horn removed from the receiver
C
4 / 16
Receiver with dielectric lens
C
5 / 16
Receiver with horn removed
C
6 / 16
Dielictric lens
C
7 / 16
Dielectric lens inside the wave guide
C
8 / 16
Interior removed from the enclosure
C
9 / 16
Interior
C
10 / 16
Interior - frequency adjustment side
C
11 / 16
Interior - PCB side
C
12 / 16
Printed Circuit Board
C
13 / 16
PCB detail (date code 6240 on transistor)
C
14 / 16
Detector diode
C
15 / 16
Frequency adjustment
C
16 / 16
(+) terminal slide contact

Restoration
When we received the microwave receiver featured on this page in May 2023, it was in unknown condition. A first test with an improvised battery produced noise in the connected headphones, which was hopeful. After opening the unit, the next test consisted of measuring the voltage over the detector diode, which was as expected, and taking down the circuit diagram from the PCB.

Once that was done, the voltages at all terminals of the transistors were checked, to see if they were within expected ranges. As this appeared to be the case, the unit was assumed to be fully functional. In order to test the detector however, it was necessary to determine the average operating frequency. This was done by first finding the cut-off frequency of the SHF part.


Cut-off frequency
The SHF part of the receiver consists of a feed horn, a wave guide with a dielectric taper and a detector diode. The cut-off frequency of this device is largely determined by the longest inside length of the waveguide, denoted as 'a' in the diagram above. For the fundamental mode, the cut-off frequency is calculated with the formula below. In this case, the value of 'a' is 38.75 mm, whilst 'c' is the velocity of light (~ 3 x 108) and εr — the permittivity of air — is approx. 1.


In reality, the cut-off frequency is lower due to the effects of the dielectric taper, but the precise effect is currently unknown. To determine the optimum frequency, an antenna was con­nec­ted to a signal generator and placed in front of the generator's feed horn. The frequency was then gradu­ally lowered until a maximum strength signal was detected in the receiver. This appe­ared to be around 3.4 GHz, depending on the axial position of the dielectric taper (i.e. the plunger).




Similar devices
The Thing   1945
In 1952, this bug was found at the residence of the American Ambassador in Moscow (Russia), hidden inside a wooden carving of the Great Seal of the United States. The wooden carving had been presented by the Russians as a gift in 1945, as a token of friendship after WWII.

It was the first application of a resonant cavity microphone as a covert listening device, and is also known as 'the Thing'.

 More information

  

SATYR   1953
One of the first people to investigate the Russian resonant cavity — The Thing, or a copy of it — on behalf of the British intelligence service MI5, was British engineer Peter Wright. It took him 10 weeks to discover the operating principle.

In the following year (1953), he developed a similar system for MI5 under the name SATYR. It used modified British umbrellas as the transmit and receive antennas.

 More information

  

EASYCHAIR   1955
After The Thing was found, the CIA started its own research under the codename EASYCHAIR, with the aim to develop similar devices that could be used by the CIA. The research was carried out in The Netherlands, by the Dutch Radar Laboratory (NRP) in Noordwijk.

The EASYCHAIR program led to the development of a series of Passive Elements (PEs), the first of which was Easy Chair I, developed in 1955.

 More information

  

EC Pulsed Cavity   1965
As part of the EASYCHAIR research, the Dutch Radar Laboratory (NRP) also tried to develop a fully passive resonant cavity microphone. Initially they didn't succeed, but between 1964 and 1965 they managed to produce a reliably working de­vice, that was illuminated by a pulse transmitter.

The image on the right shows two variants of the device — for 1100 and 360 MHz respectively — that are presented in the original CIA report.

 More information

  



Specifications
  • Device
    Microwave AM receiver
  • Purpose
    Reception of passive microphone
  • Manufacturer
    NERA ASA, Bergen
  • Country
    Norway
  • Users
    NATO, MI6
  • Year
    1963 (est.)
  • Operational
    1963-1990 (est.)
  • NSN
    5865-25-101-1890
  • Frequency
    3.4 GHz
  • Transistors
    6 (5 × 2N346, 1 × OC44)
  • Supply
    4.5V DC (flat pack battery)
  • Dimensions
    490 × 200 × 156 mm
  • Weight
    2235 g
Nomenclature
The device is known under the following names:

  • VR2 B
  • Detecting set, radar
  • NSN 5865-25-101-1890
  • NIIN 251011890
  • NE3BVR2
Contributors
The following people have contributed to this page:

  • Simon Hemour
  • Paul Reuvers
  • Marc Simons
  • Peter Wright [1]
References
  1. Peter Wright (ex-MI6), 1 NERA VR-2/B microwave receiver — THANKS !
    Personal correspondence. Crypto Museum, May 2023.

  2. Peter Wright (ex-MI5), Spycatcher
    1987-1988. ISBN 0-440-29504-1. pp. 24-xx.

  3. David Wise, Molehunt
    10 March 1992. ISBN 978-0394585147.

  4. Paul Reuvers & Marc Simons, The Thing (The Great Seal Bug)
    Crypto Museum, 18 September 2015 (updated 14 May 2023).

  5. Paul Reuvers & Marc Simons, EASYCHAIR (covert listening devices)
    Crypto Museum, 12 January 2016.

  6. J. Edgar Hoover to John W. Ford
    Drawing and Photographs, Russian Resonant Cavity Microphone
    FBI. 1 December 1952. Released to a selected group on 4 December 1952.
    Declassified and approved for release by the FBI on 24 April 2019 persuant to E.O. 13526.

  7. Wikipedia, NATO Stock Number
    Visited 12 May 2023.

  8. Wikipedia, KUKA
    Visited 14 May 2023.  German version

  9. Wikipedia, Headquarters Allied Force Command Heidelberg
    Visited 15 May 2023.

  10. Hitler desk could sell for half a million
    Evening Standard, 4 September 2006.

  11. Found, Hitler's old office desk:
    But piece of 1937 furniture will never be sold, say German officials

    Mail Online, 1 June 2014.

  12. Hitler haul is give back
    Sunday Mail, 4 January 1998.
  1. Not to be confused with the ex-MI5 officer Peter Wright, who was the author of the book Spy Catcher [2] and the developer of the SATYR covert listening device. The two are not related.

Further information
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Crypto Museum. Created: Tuesday 28 March 2023. Last changed: Tuesday, 02 January 2024 - 14:14 CET.
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