Spy radio
Burst encoders
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Strizh   Стриж
USSR digital spy radio set - Swift Mark IV

Strizh (Russian: Стриж) is a high-end digital HF spy radio station, developed and build in Russia in the early 1980s and used by all countries of the Warsaw Pact until the fall of the Iron Curtain in 1989. Strizh replaced earlier spy radio sets like the R-353 and the R-394K. It was used by Special Forces (SF) and for clandestine activities by agencies like the KGB and the GRU. The one shown here was developed especially for espionage and can be seen as the civil version of the R-394KM.
Strizh covers 1.5 to 13.5 MHz and consists of three modules: a receiver (left), a transmitter (right) and a digital storage unit (DSU) at the centre. Each module is housed in a grey metal enclosure and is connected to its neighbour by means of a large connector towards the front.

The DSU has a built-in burst encoder, which allows numerical (pre-coded) messages to be stored in its internal memory and to play them back at very high speed as soon as a link with the spy centre is established. This minimises the risk of interception and detection by the enemy.
Strizh spy radio set (R-394KM)

Spy radio stations of this kind generally have no model or serial number plate. Instead they are given code names. In this case, the radio is called Strizh (English: Swift) and the serial number is written on a label inside the DSU: 600402. Strizh was typically used by the secret services of the former Warsaw Pact, such as the KGB (Russia) and the Stasi (DDR), whereas the R-394KM was used by military special forces, reconnaissance units, behind-enemy-lines organisations, etc.

The R-394KM (Strizh) was developed in the early 1980s and remained in production until at least 1989. It was used by all countries of the Warsaw Pact. The Russian Army started using it around 1984, but the Army of the DDR (the NVA) was relatively late and introduced the set around 1988. The version shown here was build in April 1986 and was actually used by spies and agents operating in a target country, whereas the R-394KM would typically be stored in underground caches. It was the last spy radio set of this kind before the collapse of the Soviet Union (USSR).
Strizh spy radio set (R-394KM) The three functional modules of the Strizh The Digital Storage Unit (DSU) Control panel(s) of the Strizh The case in which Strizh was found The three functional modules of the Strizh Strizh in operation The case in which Strizh was found

The controls of the Strizh radio station are nearly identical to those of the R-394KM with some small differences. The set can only be used if all three units are mounted together and a 12V DC source is connected to the power socket at the top of the DSU. Turn the unit on with the toggle switch at the top centre of the DSU. Set the MODE switch at the bottom right to REC (Receive).

Operation of the set is identical to the R-394KM, but the connections are somewhat different. Rather than the typical Russian military connectors, this version has commercial sockets, similar to the ones found on western domestic equipment. Furthermore the set has English text on the front panel, whereas the military version was in Russian. All differences with the R-394KM are discussed below. For a description of the operation, please refer to the operating instructions.
Modular design
Like many other true spy radio sets, Strizh has a modular construction. This allows the individual modules to be hidden separately. It also allows simple replacement of a module in case of a failure. Like its brother, the R-394KM, Strizh consists of three functional modules: a receiver (RX) on the left, a transmitter (TX) on the right and a control unit at the centre. The Control Unit, or Digital Storage Unit (DSU), contains the burst transmitter and the burst receiver, but also the individual RX and TX synthesizers. Without the DSU, the other two modules can not be used.

The three functional modules of the Strizh

The three modules are connected via the large connectors towards the front of each unit. An ingenious spring-loaded clamp mechanism holds the units together. Each module has two such clamps just below the control panel: one towards the front and one towards the rear. The units can be separated again by pushing the spring-loaded clamps inwards with a screwdriver.
The three functional modules of the Strizh Close-up of one of the spring-loaded clamps Another clamp just above the connector Mounting two units together Two units mounted together Close-up of the clamp Extra bracket to prevent the modules from separating Bottom view of the assembled radio set

Digital Storage Unit   DSU
The Digital Storage Unit (DSU) is at the heart of the Strizh radio station. It contains individial digital synthesizers for the transmitter and the receiver, allowing split-frequency operation between 1.5 and 13.5 MHz. The DSU also contains an automatic digital message centre. Each feature has its own 5-digit red display, but the black numerical keypad is used for for all three.
The DSU allows 203 groups of five numbers each (typically called datagrams) to be stored in its internal memory. Once the connection with the spy centre has been established, the datagrams are sent at very high speed (burst) in order to reduce the time on the air and hence minimize the risk of interception and detection.

The DSU is also capable of receiving burst transmissions, typically from the spy centre, fully automatically, and can store messages in its internal message, so that they can be read later through the leftmost 5-digit red display.
The Digital Storage Unit (DSU)

In order to retain the data in the DSU's memory, a 9V backup battery has to be installed through a rectangular hole at the front of the DSU. Please note that the position of the battery is different from the R-394KM (see below). The DSU clearly shows the current state of technology in the USSR in the early 1980s. It shows that the Russians had abandonned valve-based technology in favour of transistors and integrated circuits (ICs) and that they had embraced digital solutions. Despite its compact dimensions, the DSU contains no less than 120 ICs! More about messages below.
Receiver   RX
The leftmost module is the receiver. It is connected to the left of the DSU and is a double-super­heterodyne receiver with the same frequency range as the transmitter (2-15 MHz). The frequency is adjustable in steps of 1 kHz and the intermediate frequencies are at 140.5 MHz and 500 kHz.
The type of modulation is set with a 3-position selector at the top. The receiver is suitable for AM (A3), CW (A1) and A2. The latter (A2) is for the reception of digital (telegraphy) signals that are modulated onto a sub-carrier. For the reception of normal morse code signals, the middle setting (CW or A1) should be used [2].

Below the modulation selector is the adjustment for the Beat Frequency Ocillator (BFO) that is used for making CW signals audible. Below the BFO are the LF audio adjustment (volume), the antenna adjustment and the HF band selector.
Receiver (RX)

The middle display at the centre of the DSU is used for the receiving frequency in kHz. This display is marked 'REC'. After switching on the radio, with the power switch at the top center of the DSU, set the MODE selector (bottom right of DSU) to REC. Next set the band selector to the desired frequency range. The following ranges are available:

2 → 3 MHz, 3 → 5 MHz, 5 → 8 MHz, 8 → 13 MHz

The RX frequency can be set by holding down the K-button below the display briefly, pressing the red C-button to clear the synthesizer and then (whilst holding down the K-button), typing 5 digits on the key pad. When finished, release the K-button. The display will now be blanked again. Please note that 4-digit frequencies, e.g. 7035 kHz need a leading '0' as shown here:


Now press and hold the unmarked TUNE button (just above the earphone socket) and tune the antenna adjustment of the receiver (ADJ ANT) for a maximum reading on the meter. And finally the volume knob should be used to ajust the audio level of the earphone to an appropriate level.
Transmitter   TX
The rightmost module is the transmitter. It is connected to the right of the DSU and delivers approx. 15W PEP. Manual telegraphy, using the built-in key or the numbers 0-9 on the key pad is possible in A1A modulation, (CW) but for a burst transmission Phase Modulation (PM) is used. This allows a transmission speed of 835 numbers per minute (equivalent to 167 datagrams).
Before using the transmitter, a suitable antenna wire length should be selected from the table on the DSU. For example: when transmitting on a frequency of 5125 kHz, the antenna wire (ANT) has to be 8 metres long, whilst the matching counterpoise (GND) should be 12 metres long.

Next, clear the existing transmission frequency by pressing the C-button under the rightmost display and enter the required frequency on the key pad, e.g. 05125. Use the K-button to check the frequency on the display. You may also hold down the K-button while entering the frequency.
Transmitter (TX)

Set the Antenna Matcher to the required position as indicated in the table on the DSU. For example: for a frequency of 5125 kHz, the Antenna Matcher (STEP) should be set to '8'. Now select the required modulation type with the MODE selector at the bottom right of the DSU. For burst transmissions, it should be set to FM, which is actually Phase Modulation (PM). Press and hold down the morse key and adjust the FINE knob for a maximum reading on the meter. Then release the morse key. → On this version of the radio, an external morse key should be used.

When using Strizh for manual telegraphy (morse code), connect an external key to the CINCH socket on the transmitter (KEY) or the EXT socket on the DSU, and set the MODE selector on the DSU to CW (A1). On the military version of the radio, the R-394KM, the internal morse key on the transmitter can be used for this. Mode A2 is used for high-security burst transmissions, in which the signal is modulated onto a sub-carrier. The push button (CONTROL POWER) can be pressed to check the battery voltage. It should put the needle of the meter somewhere in the green area.
Burst speed
The speed at which a message is sent, depends on the selected transmission mode:
  • CW - 10 GPM
    This is the so-called morse mode (CW). It is used for normal speed automatic and manual morse code and allows 10 groups (of 5 numbers each) to be transmitted per minute.

  • A2 - 167 GPM
    This is the so-called tone telegraphy mode (TT) which allows 167 groups per minute to be sent. This means that the longest possible message of 203 groups is sent in approx. 1.2 sec.

  • FM - 415 GPM
    This is so-called phase telegraphy mode (FT) which offers the highest security by sending the data at a speed of 415 groups per minute. In this mode, the longest possible message of 203 groups is sent in less than 0.5 sec.

Like most HF radio sets, Strizh is best used with a wire antenna. Depending on the selected operating frequency, a wirelength of 6 or 8 metres was used and a counterpoise of 4, 6, 8 or 12 metres. A table, printed on the control panel of the DSU, shows which wire lengths to use.
The two wires were connected to the two screw-terminals at the top right of the transmitter, to the right of the meter. The upper connector is for the counterpoise wire (GND) whilst the lower one is for the antenna (ANT). When transmitting, a small red light, just below the meter, gives an indication of the antenna current.

The length of the wires has been choosen carefully, so that the standing wave ratio of the transmiter can be matched easily by the built-in antenna tuner. Again, the table on the DSU is used to select the correct tuner preset.
Wire antenna and counterpoise

The table dictates the setting of the STEP selector on the transmitter, for any given frequency. For example: when transmitting on a frequency of 7035 kHz, the STEP selector should be set to 9. The FINE knob is then used to adjust the transmitter for maximum power output. Select the desired transmission mode (e.g. CW), activate the transmitter by holding down the external morse key and ajust the FINE knob for a maximum reading on the meter. Then release the key.
Unlike the military R-394KM, which uses a Russian military pair of headphones, Strizh has been converted for use with standard domestic earphones with a 3 mm jack at the end. The one shown here was made in Russia in the 1980s and came with a standard Western 3 mm jack.

When the operator lost his earphone or when it got broken, he would be able to buy a standard earphone from a local store whithout attracting any attention. The earphone is suitable for both the left and the right ear, as the plastic clip can be reversed.


9V backup battery at the front of the DSU Standard 3.5 mm earphone socket Wire antenna and counterpoise Antenna and counterpoise wires connected to Strizh Earphone Earphone Using a Russian brand earphone

Preparing a burst transmission
The leftmost display (DATA) on the DSU is used for the burst encoder, together with the leftmost column of keys (С, П, ПУСК, Р) and the black numerical key pad. After switching on the radio station, check the battery voltage (CONTROL POWER) and set the MODE selector to Receive (REC). A message consists of a 5-digit header followed by a series of 5-digit groups (datagrams) that are each separated by pressing ENTER (Р). The DSU can hold 203 of such 5-digit datagrams.
Next, set the Memory Control selector, at the bottom left of the DSU, to Program (PRGM) and enter the 5-digit header of the message using the numerical keys. The numbers should be visible in the display immediately. Terminate the header by pressing the ENTER-button (Р).

Now enter the message as a series of 5-digit groups and terminate each group by pressing the Р-button. If you made a mistake, the current group can be cleared by pressing the С-button. Multiple messages can be entered by pressing the Р-key twice at the end of a message.
The displays and the key pad of the DSU

When finished, set Memory Control to STORE. A message can be checked by setting the Memory selector to PRGM again, after which the first radiogram (i.e. the header) should be visible. Now use the Р-button to step through the groups. Errors can be corrected by pressing С, entering a new 5-number group and pressing Р. When finished, set the Memory selector to STORE again.

The message is now ready for transmission. Prepare the transmitter as described above and select the required transmission mode (FM or A2). Set the Memory Control selector to PRGM and keep the START-button (ПУСК) depressed until the DATA display lights up and the antenna current light starts flashing. Then release the ПУСК-button. The message will now be sent.

During the transmission, the display will show the datagrams and the morse signals can be heard through the earphone. When the message is completed, the display and the Antenna Current Indicator will go off. If you have another message ready to go, press the Р-key until the message header shows on the DATA display and press ПУСК to start. When finished set the Memory selector to STORE again. The message stays in the memory of the DSU as long as the main power switch is ON, or the Memory selector is set to STORE. In order to clear the message, turn the radio set off and set the Memory selector to 'OFF'. Wait a few seconds before switching on again.
Strizh in operation The displays and the key pad of the DSU Close-up of the data display Setting the RX frequency Checking the RX frequency Checking the TX frequency Using a Russian brand earphone

Power source
Strizh should be powered by a 12V DC source that can deliver 5A. Although it is possible to use an external power supply unit, the radio was commonly driven from the battery of a car. When the spy had to send a message to the spy centre in the homeland, he would drive to a quiet spot, e.g. in a forest, put up the antenna, connect the radio to the car battery and deliver the message.
In order to save time, he might have entered the (pre)coded message in the memory of the DSU at home. This is why the DSU has a backup battery.

As the battery cable might break or get lost, Strizh was equipped with an 8-pin DIN power socket, rather than a Russian military one. It allowed the spy to buy a new connector from a local electronics store. The cable shown here was probably made at a later date. As the former owner probably couldn't find a suitable 8-pin DIN connector, he used a 5-pin one and broke away the remaining three pins from the socket.
Power cable

The original cable would probably have had a cigarette-lighter plug in place of the two banana-type plugs shown above, or ultimately two battery clamps, allowing it to be connected directly to the car battery. As 12V is the most common voltage in domestic cars world-wide, Strizh can be used directly. For the same reason, older spy radio sets, such as the R-350 and the R-354, were powered by 6V DC, which was a common voltage in the cars of the 1950s and 60s.

 Pinout of the DIN socket
Differences with R-394KM
Strizh is fully based on the design of the (military) R-394KM. In fact, the radio station has been designed in such a way that it could be made suitable for a variety of purposes, including Special Forces, reconnaissance, Stay-Behind, underground caches and clandestine activities (espionage). Depending on the application, the radio station was adapted to the specific user requirements.

For espionage, the set was not housed in the typical military case of the R-394KM, but came as three separate units that could be adapted for any type of concealment (e.g. inside a common briefcase). Furthermore, the units had english lettering on their control panels. This was done for two reasons: It would not immediately expose the set as being Russian when it was accidently discovered by, say, the police. The most important reason however, was the fact that many spies and agents working abroad did not speak Russian and were not able to read Russian instructions.
Depending on the area in which the radio was to be used (i.e. the target country), the radio was adapted to the local availability of spare parts.

On this example, the typical Russian fuse holder has been replaced by a western alternative. In case the fuse was blown, the agent could obtain a new commonly available fuse from any local store, rather than ask for an original (smaller) Russian one, which was not available in the west and would certainly have raised eyebrows. In the same vein, the military 4-pin power socket was replaced by a more common 8-pin DIN socket.
A western-style fuse holder replacing the Russian one

On a standard R-394KM, a built-in morse key is present towards the front of the transmitter. It allows messages to be sent directly in morse code in case of an emergency. As this was not very convenient - most operators were not capable of giving morse code and if they were, they would like to use an external key - the internal key was replaced by a CINCH (RCA) socket.

The following differences with the R-394KM have been found:
Although Strizh was built on the chassis of the R-394KM, the holes in the grey front panels are positioned in such a way that they accomodate standard civil connectors rather than the usual military ones. It is entirely possible that some of the modifications mentioned above were carried out after the Cold War (i.e. by a former owner), but this seems unlikely. They are carried out in a professional manner and fit in with the idea of being able to source spare parts locally.
The position of the 9V backup battery needs some clarification. On the military version of the R-394KM, the backup battery is located under a removable oval lid on the control panel of the transmitter. Below this panel is space for a cylindrical Acacia battery (Russian: Акация), which is actually a stack of 6 circular 1.5V cells.

Although the holder for the Acacia battery is still present inside the transmitter, there is no lid in the control panel. As this type of battery was not available in the west, a western-style 9V battery holder was added to the front of the DSU.
Holder for the 9V backup battery

This allowed the agent to obtain a replacement battery from a local store and install it through a rectanglular hole in the front of the DSU. Another modification to this effect was the replacement of the 2-pin Russian headpones socket by a standard 3 mm jack socket, allowing any ordinary earphone to be used, such as the ones that were commonly supplied with portable radios.
The three functional modules of the Strizh DIN power socket A western-style fuse holder replacing the Russian one A standard CINCH socket replacing the built-in key Holder for the 9V backup battery Standard 3.5 mm earphone socket Earphone Power cable

Coding and decoding messages
It is often claimed that digital spy radio station, such as Strizh, have a built-in encryption system for the protection of the messages. This is not true however. Strizh can only send and receive pre-coded numerical messages at very high speed. Encryption should be done externally.

The spy radio set (i.e. Strizh) was generally not used for receiving messages as it was considered too risky. In most cases, a spy would use a domestic short-wave receiver (e.g. a Sony ICF-2000ID) and tune in to the broadcast of one of the so-called number stations. Short-Wave listeners will certainly remember the endless ranges of numbers read in German or English by a female voice.


These numerical messages contained instructions for spies and agents world wide and were generally encrypted with the unbreakable One-Time Pad (OTP). Occasionally, the lines of a poem or the pages from a popular book were used as the encryption key, but that was less secure.
In most cases, the spy radio station was only used for transmitting messages. The text-based messages were first converted to numbers using some kind of encoding scheme. The result was then encrypted by means of a One-Time Pad (OTP) and stored in the memory of the DSU.

The image on the right shows an original One-Time Pad as it was used by Eastern Block spies from the 1960s onwards. It is a small booklet that contains very thin pages, each with a series of random numbers in groups of 5 digits. These numbers were added to the numerical message.
Original Russian One-Time Pad booklet. Click for further information.

Only two copies of the number ranges existed: one with the spy and one at the spy centre in the homeland. Each 5-digit group was added to a 5-digit group of the message. A page from this booklet was only used once (hence the name one-time pad), and was destroyed immediately after use. When a one-time pad consists of truely random numbers, this code remains unbreakable.

 More about one-time pads
 More about number stations
Storage case
Unlike the military variant, the R-394KM, Strizh is not housed in a transit case. Instead, its casing is constructed in such a way that it could be built inside virtually any type of concealment, for example a common briefcase that would not attract unnecessary attention when carried around.
When this unit was (re)discovered in 2014, it was stored inside a green aluminium transport case that was originally used for a military 9S13 (Russian: 9С13) homing device. The radio set fits tightly inside this case, but the lid has been bulged somewhat in order to accomodate the knobs. The top lid of the case is rubber-sealed.

It is unlikely that Strizh was originally stored inside this case. Carefully hiding the fact that it is a Russian device (by putting English text on the front panel) and then storing it inside a case with Russian markings, doesn't make any sense.
The case in which Strizh was found

Furthermore, the case has no space for the accessories, such as the headphones, the power cable and the antenna wires. On the other hand, the case might have been used to protect the radio when it was stored for longer periods of time, e.g. as part of a cache. In that case, the accessories might have been stored in a separate container. If you know more about this, please let us know.
Circuit description
The transceiver consists of the following building blocks:
  1. Transmitter
  2. Synthesizer
  3. Receiver
Block 2 - Synthesizer
This is the largest of the three units that is physically placed between the transmitter (block 1) and the receiver (block 2). Although it is called the synthesizer, it is much more than that. It contains the internal power supply unit (PSU), a keypad and three displays, and provides the tuning signals for the receiver and the transmitter. Furthermore it contains the Storage-Keying Device (NMU) that allows messages to be stored in its internal RAM and sent at very high speed.
  • 2-01 - Reference oscillator (10 MHz)
  • 2-02 - Analogue board (synthesizer)
  • 2-03 - Display and keypad board
  • 2-04 - Logic board
  • 2-05 - PSU
  • 2-06 - PLL (phase detector)
  • 2-07 - NMU (see below)
Block NMU   block 2-07
Inside the synthesizer section (block 2) is a digital circuit that is provided as a closed metal block with a large connector at one end. This block is marked SECRET and is known as NMU (Russian: НМУ), which stands for: Накопительно-манипулируюшего устройства (Storage-keying device). Inside the NMU are five circuit boards, marked A1 thru A5, which contain the following circuits:
  • A1 - Tone Telegraphy keyer (TT)
  • A2 - Morse keyer (CW)
  • A3 - Address decoder and RAM
  • A4 - Pulse distributor
  • A5 - Phase Telegraphy keyer (FT)
Serial numbers
It is currently unknown how many R-394KM and Strizh transceivers were built. Judging from the serial numbers on recovered radio sets, it seems likely that two batches of the R-394KM were built and one batch of Strizh. So far, we've seen the following serial numbers:
  • 360133, 380995, 380996
    ← R-394KM
  • 547659, 547992
    ← R-394KM
  • 600402
    ← Strizh
There seem to be three number ranges: starting with '3' and '5' for the military R-394KM variant and '6' for the agent/spy radio sets. If we assume that the '6' series was indeed the spy variant, and that they were numbered from 1 onwards, it seems safe to estimate that several hundred units of this type were built. Serveral thousands of the military variant will have been built.
Power supply
The Strizh spy radio set featured on this page features an 8-pin male DIN socket for connection of the external 12V DC power source. This was a replacement of the original Russian 4-pin male socket that is found on the military R-394KM. The diagram below shows the pinout of the DIN power socket when looking into the socket. Note that a 5-pin socket would have been suitable. Pins 6, 7 and 8 are removed from the socket, so that a common 5-pin female plug can be fitted.
  1. 0V
  2. n.c.
  3. +12V
  4. 0V
  5. +12V
  6. n.c.
  7. n.c.
  8. n.c.
External connector
A 10-pin expansion connector is present at the center of the DSU, between the MEMORY and MODE selectors. This socket is sometimes protected by a black plastic cap and is intended for the connection of additional equipment such as an external morse keyer. It allows the transceiver to be partly remote-controlled by the external device. The connector has the following pin-out:

pinout of the 10-pin expansion socket, when looking into the socket.

An external key can be connected between KEY and GND. Please note that the radio has two KEY inputs: one used for AM (amplitude modulation) and one for PM (phase modulation, here called 'FM'). Also note that the pin-out of this socket is different from the same socket on the earlier R-394K radio. Connectors for this socket are difficult to obtain. Warning: connecting the wrong type of accessory may cause permanent damage. On the version of Strizh featured on this page, this socket was probably unused as a separate key input is available on the transmitter.
  1. This contact is NOT wired on most Strizh/R-394KM units. When wired, it provides a clock signal for an external keyer. In A2 mode, the clock signal is 100 Hz. In FM mode it is 250 Hz.

Technical specifications
  • Power
    12 - 13.8 V DC
  • Current
    0.7 A (RX) or 4.5 A (TX)
  • Output power
    15 Watt
  • Frequency
    2 - 15 MHz (or actually 1.5 - 14.999 MHz)
  • Weight
    8 KG
  1. Radio Station R-394KM Technical Description and Operating Instructions
    Full circuit description, block diagrams and wiring diagram (Russian).
    IV1.106 007 TO. 1988. SECRET. Serial number 10.
     Drawings and diagrams for this manual

  2. Radio Station R-394KM Technical Description and Operating Instructions. Appendix.
    Components list and circuit diagrams (Russian).
    IV1.106 007 TO1. 1988.

  3. Block NMU (Russian: Блок НМУ) Technical Description
    Circuit description and diagrams of the burst encoder (Russian).
    2.082.046 TO. 1988. SECRET. Serial number: 31.

  1. Gunter Fietsch, Nachrichtentechnik der Nationalen Volksarmee
    Part 2, 1996, p. 303. ISBN 3-88180-340-8.

  2. Wikipedia, Types of radio emissions
    Overview of modulation types. Retrieved September 2014.

Further information

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Crypto Museum. Created: Sunday 27 September 2015. Last changed: Sunday, 02 October 2016 - 09:09 CET.
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