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MRK-1
Radio distress beacon finder

MRK-1 was a radio direction finder (RDF), developed in 1980 by Mechanikai Laboratórium (ML) in Budapest (Hungary). The device was intended for locating the exact position of an activated civil emergency distress beacon — VHF Guard, or International Air Distress (IAD) — operating at 121.5 MHz. During the Cold War, it was used for locating downed enemy pilots, by the Soviet Union's and Warsaw Pact's intelligence services, including the KGB, the GRU and the East German Stasi.

The device consists of a plastic enclosure that houses a highly selective receiver for 121.5 MHz, plus a rather special magnetic loop antenna that has a very good directivity, as it is only sensitive to the magnetic (H) field of an RF signal. This makes direction finding in the near-field of the transmitter easier than with electric (E) fields.

The receiver has a fixed Y-cable that connects the receiver to an external battery and a regular pair of headphones through which a 1 kHz tone is produced, with an amplitude that is related to the signal strength of the intercepted RF signal.
  
MRK-1 receiver with leather strap and cable

The MRK-1 was first introduced in 1980, and was clearly intented for use throughout the Soviet Union (USSR), as the manual and the passport that came with the device in our collection, are in Russian. It was in production until at least 1986, and probably until the end of the Cold War [A].

Black leather briefcase (MRK-1) Leather briefcase with MRK-1 Leather briefcase with MRK-1 MRK-1 receiver with leather strap and cable MRK-1 with accessories Front panel Y-cable with hook Battery (Priboj-2)
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Black leather briefcase (MRK-1)
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Leather briefcase with MRK-1
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Leather briefcase with MRK-1
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MRK-1 receiver with leather strap and cable
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MRK-1 with accessories
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Front panel
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Y-cable with hook
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Battery (Priboj-2)

Features
The diagram below provides an overview of a complete MRK-1 set, in which all parts are inter­connected and ready for use. At the top right is the actual receiver. It has a fixed cable with a Y-junction towards the end, that connects to a pair of headphones and to a yellow 9.4V battery.


The receiver has a fixed leather carrying strap, that allows it to be carried around the neck, with the antenna pointing upwards. In that case, the control panel will be at the bottom. The device is operated with a single 5-position rotary dial on the control panel. This is used to turn the device ON and OFF, and to selected the desired RF attenuation in four steps: 0dB, 20dB, 40dB and 60dB.

The operator should hold the device in front of his chest, with the antenna (i.e. the rounded end) up, and the side with the 3 LEDs towards his chest — for finding the minimum signal strength — or the (short) side with the 2 LEDs towards hist chest for finding the maximum signal strength.

Antenna
The MRK-1 uses a resonant magnetic loop antenna that is mounted inside the rounded part of the grey plastic enclosure. It consists of a semi-rigid coaxial line that is mounted inside a metal enclosure with a narrow slit at the top. This double shielding ensures that the electric field of the RF wave is fully ignored, resulting in an antenna that is sensitive to the magnetic field only [3].

Schematic view of the magnetic loop antenna

The diagram above shows the construction of the antenna. The aluminium enclosure forms an extra shield for electric fields — in addition to the shielding of the semi-rigid coax — whilst the narrow slit at the top allows the magnetic field to enter the loop perpen­dicular. At the bottom is the symmetric feedpoint to the inner conductor of the semi-rigid coax line. As it is a resonant antenna, an adjustable capacitor is connected in parallel to the feedline, as shown above.

Antenna directivity

The diagram above shows the radiation pattern of the antenna when the MRK-1 is held in front of the chest as indicated. The radiation pattern is symmetric, but one of the lobes is affected by the body of the operator, which resolves the front-back ambiguity. The device can be used to find the direction to a transmitter, by searching for the maximum or the minimum signal strength, which­ever the operator prefers. The technical manual gives clear instructions on the intended use [A].


Parts
Leather briefcase MRK-1 direction finder 9.4V battery (from emergency beacon) Headphones Leather carrying strap and spring hook Technical description, operating instructions and passport
Briefcase
The MRK-1 was supplied in an unobtrusive black leather briefcase, that measures 42 x 33 x 8 cm and weights ~4.5 kg, when all items are present.

The case is marked with a metal tag that carries the model and the serial number. It also has a metal stub with a through-hole, allowing it to be sealed before distribution. The image on the right shows the opened briefcase. The papers are stowed in the lid of the briefcase, whilst the receiver is at the bottom right.

  
Leather briefcase with MRK-1

Receiver   MRK-1
The actual receiver is housed in a grey plastic (PVC) enclosure, that measures 300 x 145 x 40 mm and weights 1450 gram, excluding the (external) battery. The device has one rounded side – in which the antenna is located – and a leather strap for carrying it around the neck.

The receiver is suitable for signals on the 121.5 MHz emergency frequency only. It has three indicator LEDs on its largest surface, 2 LEDs at the side and a 4-step attenuator on the control panel. Radio Direction Finding (RDF) is done by means of a 1 kHz tone in combination with the indicator LEDs.

  
MRK-1 RDF receiver

Battery
The receiver is powered by a 9.4V DC source, typically the yellow watertight PRIBOJ-2 (Russian: ПРИБОЙ-2) battery shown on the right. It has a watertight connector, which should be fitted to one branch of the Y-cable of the receiver by means of a (supplied) conversion cable.

This is the same battery as used with the actual emergency transmitters, such as the Russian R-855UM (Р-855УМ). A handwritten date code on the label indicates when the battery was made.

  
Battery (Priboj-2)

Headphones
A common pair of (Russian) headphones was supplied with the MRK-1, such as the one shown in the image on the right. The only thing special about it, is the rather uncommon connector, that allows it to be fitted to one branch of the fixed Y-cable of the receiver.

The headphones are used for listening to the 1 kHz detector done, the amplitude of which is a measure for the signal strength of the received 121.5 MHz signal from the distress transmitter.

  
Headphones

Carrying strap and spring hook
The receiver came with a leather carrying strap that was affixed to both of the long sides. The strap allows the device to be carried around the neck, hanging on the chest.

Also in the kit, was a 4 cm spring hook (Russian: карабин) that should be fitted to the Y-junction in the fixed cable. It allows the cable to be attached to the clothing.
  
Y-cable with hook

Manuals
Each MRK-1 came with a maintenance booklet, or passport, in which the manufacturing date and acceptance stamps were recorded, plus all repairs and modifications that were carried out during its lifetime [B].

In addition, each receiver was supplied with full technical manual that contains clear operating instructions, circuit descriptions and circuit diagrams. The documents supplied with the MRK-1 featured here, are in Russian.

 Download the technical manual
 Download the maintenance booklet

  
MRK-1 documentation

Black leather briefcase (MRK-1) Model and serial number on the lid of the briefcase MRK-1 documentation MRK-1 passport Battery adapter cable Battery connector Y-cable with hook
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Black leather briefcase (MRK-1)
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Model and serial number on the lid of the briefcase
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MRK-1 documentation
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MRK-1 passport
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Battery adapter cable
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Battery connector
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Y-cable with hook

Distress beacon
In aviation, two aircraft emergency frequencies, or GUARDs, have been assigned internationally for emergency communications for aircraft in distress. One is in the VHF band at 121.5 MHz, and is intended for civilian use. It is known as International Air Distress (IAD), or VHF Guard [2].

The other one is in the UHF band at 243.0 MHz and is intended for military use. It is known as Military Air Distress (MAD) or UHF Guard [2].

In the past, these frequencies were also used for Emergency Locator Transmitters (ELT) — also known as Distress Beacon [3]. In the US and UK they are also known as Special Forces Tactical Beacon or TACBE. The ELT is usually activated by a downed pilot immediately after a crash, and broadcasts a signal that can be picked up by monitoring stations, such as air traffic control centers, military air defense and civil aircraft.
  
R-855UM with external battery

The image above shows a typical Russian R-855UM (Р-855УМ) distress beacon that operates on 121.5 MHz. Apart from sending a tone, it can also be used for short range voice communication, for example to speak with a passing airplane. It uses the same external battery as the MRK-1.

Modern ELTs transmit their emergency signal on 406 MHz, with a low power beacon on 121.5 MHz for local homing. The 406 MHz beacon is encoded, so that its ID can be determined and false alarms can be verified. This frequency is monitored by a worldwide network of satellites.

  • 121.5 MHz 1
    VHF Guard
    International Air Distress (IAD)
  • 243.0 MHz
    UHF Guard
    Military Air Distress (MAD)
  • 406.0 MHz
    -
    Emergency Locator Transmitter (ELT)
WARNING — Please note that the above frequencies are still in use internationally for distress transmitters today (2019). Although decommissioned devices are frequently offered on auction sites, such as eBay, the use of such transmitters by the public is prohibited and undesired, and in some countries even a criminal offense.
  1. This frequency is currently in the process of being phased out [5]. Although the use of the equipment in distress situations is still permitted, the certification of newly built devices has been withdrown by the FCC.



Schematic drawing of the contents of the MRK-1 as seen from the LED side

Interior
The receiver is housed in a grey plastic (PVC) enclosure that is rounded at one side — the antenna — and flat at the other side — the control panel. The control panel is made of aluminium and is fitted to the plastic enclosure by means of 6 recessed screws, and sealed with a lot of silicone kit.

Although the kit protects the unit against water, it also acts as an adhesive between the control panel and the plastic enclosure. As a result, it appears to be virtually impossible to open the case without damaging the control panel.

For this reason we have decided to leave it in piece for now, and make a couple of high-quality x-ray images 1 so that we can see what is inside without opening the enclosure. The image on the right shows the feedpoint of the antenna plus part of the RF pre-amplifier, that is built around two 2N3823 FETs and a BFY90 transistor.
  
Interior - antenna feed point

The output of the pre-amplifier is passed through a 4-step attenuator which is mounted on the same board. The attenuator is operated by the 5-position rotary knob on the control panel, which also acts as the ON/OFF switch. A long shaft connects the knob to the attenuator/power switch.

The output of the attenuator is connected to the input of the 1st IF section, that is built around an SO42P IC. It is a typical circuit that was also used in the domestic radios of the era, and delivers an output signal – via a crystal filter – at 10.7 MHz. The first IF is shown in the image on the right.

The output from the IF1 board is fed to the input of the third PCB, which holds the 2nd IF stage – built around a TCA440, and crystal filtered at 455 kHz – an OA1180 detector diode, and an audio amplifier/oscillator built around a TBA222 in Wien-bridge configuration, running at 1 kHz.
  
PCB 2 - IF1 stage

The output of the TBA222 is fed to the headphones, so that the user can use the amplitude of the 1 kHz tone as a measure for the signal strength of the 121.5 MHz signal from the intercepted distress transmitter. The circuit does not allow any spoken messages (AM) to be demodulated.

  1. X-ray images made with Creative Electron TruView Prime.  More

Interior - antenna feed point Shielded antenna with air-gap Mechanics - power switch and attenuator PCB 1 - pre-amplifier (right) and attenuator (left) PCB 2 - IF1 stage PCB 3 - IF2 stage and audio amplifier Construction detail
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Interior - antenna feed point
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Shielded antenna with air-gap
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Mechanics - power switch and attenuator
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PCB 1 - pre-amplifier (right) and attenuator (left)
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PCB 2 - IF1 stage
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PCB 3 - IF2 stage and audio amplifier
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Construction detail

Block diagram
Below is the block diagram of the MRK-1. At the top left is the magnetic loop antenna which is coupled to an HF pre-amplifier, followed by a 4-step attenuator. The signal is then filtered and amplified, and fed to the 1st IF mixer — built around an SO42P — that produces an output signal at 10.7 MHz. This signal is then passed through a 10.7 MHz crystal filter, onto the 2nd IF mixer.


The 2nd IF mixer is built around a TCA440. It mixes the 10.7 MHz IF1 signal with the 11.155 MHz from a crystal oscillator, resulting in a 455 MHz IF2 signal, which is then fed to a diode detector. The detector output drives a 1kHz Wien bridge oscillator – built around a TBA220 – that produces a tone for the headphones. The amplitude of the tone is a measure for the signal strength.

The same output signal drives the 'modulation' LED at the bottom right. The output of the diode detector is also fed to an overload detector (fitted on the IF1 PCB) that causes an LED to light up when the antenna signal becomes too strong. The user should then increase the RF attenuation.


Specifications
  • Frequency
    121.5 MHz
  • Stability
    ≤ 50Hz/°C
  • Power
    9.4V DC (from external battery)
  • Antenna
    resonant magnetic loop
  • IF1
    10.7 MHz
  • IF2
    455 kHz
  • Audio
    1 kHz
  • Dimensions
    300 x 145 x 40 mm
  • Weight
    < 1.5 kg
Documentation
  1. Technical description and operating instructions MRK-1
    304-0000-00/01, Volume 1 (Russian). ML, 1980, updated 25 September 1986.

  2. MRK-1, Pasport No. 0107
    304-0000-00/24. ML, 25 September 1986.
References
  1. Louis Meulstee, MRK-1
    Retrieved July 2019.

  2. Wikipedia, Aircraft emergency frequency
    Retrieved July 2019.

  3. Wikipedia, Emergency position-indicating radiobeacon station
    Retrieved July 2019.

  4. Wikipedia, Loop antennas
    Retrieved July 2019.

  5. Dan Namowitz, New 121.5 MHz ELTs to be prohibited
    AOPA website, 18 December 2018.
Further information
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© Crypto Museum. Created: Sunday 14 July 2019. Last changed: Tuesday, 23 July 2019 - 06:00 CET.
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