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.
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
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
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.
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
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.
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].
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.
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.
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)
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.
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.
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.
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
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
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
In the past, these frequencies were also used for
Emergency Locator Transmitters (ELT) —
also known as Distress Beacon .
In the US and UK they are also known as Special Forces
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.
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 GuardInternational Air Distress (IAD)
UHF GuardMilitary Air Distress (MAD)
-Emergency Locator Transmitter (ELT)
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.
This frequency is currently in the process of being phased out .
Although the use of the equipment in distress situations is still
permitted, the certification of newly built devices has been
withdrown by the FCC.
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
plus part of the RF pre-amplifier,
that is built around two 2N3823 FETs and a BFY90 transistor.
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.
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.
X-ray images made with Creative Electron TruView Prime.
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
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.
Power9.4V DC (from external battery)
Antennaresonant magnetic loop
Dimensions300 x 145 x 40 mm
Weight< 1.5 kg