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RU-2   RU-2/2K
Manpack VHF-L transceiver

The RU-2 radio was a family of low-band VHF transceivers (30-70 MHz), developed in the former Yugoslavia in the late 1970s [1] and built by the Rudi Čajavec factory in Banja Luka (Bosnia and Herzegovina). It was the successor to the RUP-12 manpack radio that was housed in the same enclosure, and was intended for communication between infantry companies and battalions. In practice however, the RU-2 became the 'work horse' for the entire Yugoslav National Army (JNA).
The RU-2 is available in three variants. The basic model was intended for mobile applications, soon followed by the RU-2/1 that was available as a manpack by attaching a battery at the bottom and mounting it in a backpack frame.

When voice scramblers and (later) encryption devices became available, a further model was added to the range. It had an extra socket at the front panel for the connection of the KzU-61 scrambler or the KzU-63 encryption unit. The image on the right shows the RU-2/2K, which has a crypto socket, marked 'K', on the right.
RU-2/2K Front panel

In the front-end of the radio, a MOSFET-based low-noise pre-amplifier is used. Furthermore, the radio has a sophisticated Automatic Gain Control (AGC) and features varicap tuning. The circuits are built with transistors and Integrated Circuits (ICs). The intermediate frequency sections (IF) are built with ICs and use high-quality crystal filters manufactured by the Mihailo Pupin company.

The radio offers wide-band Frequency Modulation (FM) and covers the lower VHF frequencies from 30 to 69.95 MHz, divided over 800 channels with a channel spacing of 50 kHz. The desired communication channel is selected by 3 rotary switches at the front panel (000 - 799). In the early days, the RU-2 was sometimes used with the American KY-189 voice scrambler handset.
RU-2/2K RU-2/2K with battery case present RU-2/2K Front panel Front panel with the protective caps in place Front panel with the protective caps removed RU-2/2K front panel with handset connected RU-2/2K with KzU-63 voice encryptor and handset Long rod antenna installed

All controls and connections are at the front panel of the radio (or the top panel if the radio is placed vertically). When used as a portable device, a battery is connected at the bottom and the radio is usually placed in a metal frame or in a backpack, so that it can easily be carried around.

The image above show the control panel of the RU-2/2K which is slightly different from the other models. The antenna is installed or connected at the top left, with a tuning knob to its right. The knob immediately to the right of that is the MODE selector. It is used for turning the set ON/OFF.

The desired frequency is selected by means of three rotary dials that select one of 800 channels (000-799). A simple formula allows conversion between channel numbers and frequencies. At the bottom right is an indicator that is used for tuning the antenna as wel as measuring the battery voltage. It has three areas: empty (prazan), half-full (polupun) and full (pun). The battery voltage can be checked by putting the MODE selector to the AK position. A 2-wire field telephone line can be connected at the right. At night, the meter and the channel selectors can be illuminated.

RU-2/2K with KzU-63 voice encryption unit

A handset, microphone or headset is connected to the socket marked 'MK.' at the top right. In case of the RU-2/2K, the handset might also be connected to an external encryption device, such as the KzU-63, in which case the crypto device is connected to the socket marked 'K'. On other models of the RU-2, this socket is not available and its place is taken by the headphones socket, which on the RU-2/2K is at the center of the front panel, left of the Volume knob (Jačina).
Removing the dummy plug from the crypto socket Crypto shorting plug removed Headphones socket (SL) Switching the radio off Channel selectors (marked A, B and C) Antenna tuning and battery check indicator Close-up of one of the scale lights Illuminated scale and meter

Externally, the radio is identical to its predecessor, the RUP-12. It is built on the same chassis and the controls are identical. The internals are completely redesigned however, with more modern components, many of which were imported from the West. The radios were built by Rudi Čajavec in Banja Luka and came in 3 variants, of which only one is suitable for use with the KzU-63:
  • RU-2
    This is the basic radio that was suitable for use in base stations as well as inside a vehicle. It is powered by a 12V DC source and produces an RF output power of 1.4W which is rather low, considering the fact that the chassis has sufficient space for a 5-6W PA [1]. Output power can be increased to 20W by using the external P-25 power amplifier.

  • RU-2/1
    This is the manpack version of the radio. It is intended for portable use and has a battery pack mounted to the bottom of the radio and a fixed antenna rod at the top left. Using a handset, the radio can be used directly for voice communication. It is normally powered by a 12V/7A DC battery that is attached at the bottom.

  • RU-2/2K
    This version is nearly identical to the R-2/1 (above) but has a special socket for the connection of an external voice protection unit, such as the KzU-61 voice scrambler or KzU-63 voice encryptor. If the KzU-63 is not connected to the radio, a dummy plug has to be inserted into the socket marked 'K' (Kripto).

The image below shows the RU-2/2K, which is the only version of the RU-2 that is suitable for use with the KzU-63. The extra socket, needed for connection to the KzU-63, is to the left of the handset plug. It is shown here with a black dummy or shorting plug installed. If the KzU-63 is not connected to the radio, this shorting plug needs to be in place as otherwise the unfiltered audio lines of the transmitter and the receiver are interrupted.

RU-2/2K with shorting plug installed in the Crypto socket

MODE selector
The MODE selector acts as the main power switch. It has the following settings:
  • AK.
    Battery check (momentary)
  • ISKLJ.
  • UKLJ.
    ON (squelch open)
  • 1
    ON (squelch closed)
  • 2
    ON (squelch closed further)
Channel selection
The RU-2 is suitable for all low-band VHF frequencies between 30 and 70 MHz (actually 69.95), with a channel spacing of 50 kHz, resulting in 800 distinct channels. The desired frequency is selected by means of 3 rotary switches at the front panel, between 000 and 799. For conversion between the channel number and the actual frequency, the following formulas can be used:

As an example, a scale setting of 367, results in (367+600)/20 = 48.35 MHz. The lowest setting (000), results in 30 MHz, whilst the highest setting (799) gives 69.95 MHz. Use formula (2) to calculate backwards. A frequency of 52 MHz, results in a scale setting of 20 · (52-30) = 440.

When setting up a channel, the number on the scale of the rotary selector has to line up with a mark (a small white dash) located at the top right of each scale. A small indicator lamp is hidden inside each mark, so that the radio can also be used in the dark. Press the black rubber button marked 'SVETLO' at the bottom right to turn on scale illumination. The meter is also illuminated.
The three channel selectors (A, B and C) along the lower edge of the front panel Channel selectors (marked A, B and C) Channel selector and scale lamp hidden inside the mark Close-up of one of the scale lights Illuminated scale and meter Illuminated scale and meter Even in the dark the transceiver can be operated Antenna tuning and battery check indicator

A wide range of accessories was available for the RU-2, some of which are described below. Please note that some of these accessories were optional.
KzU-63 KY-189 scrambler handset Battery box Foldable long rod antenna Handset

Voice encryption unit
Like most military VHF FM transceivers, the RU-2 is an easy target for radio direction finding and interception. In order to make eavesdropping impossible, or at least extremely difficult, the RU-2 was sometimes equipped with the external KzU-63 voice encryption unit, as shown in the image.
When the KzU-63 was used, it was connected to the special Crypto socket of the RU-2/2K (instead of the shorting plug) by means of a short cable that is also known as the 'dog bone'.

The handset should now be connected to the MTK-socket on the KzU-63 instead of the MK-socket on the RU-2/2K, and the empty socket on the RU-2 should be covered by a rubber cap in order to protect it agains moist and dirt. The image on the right shows a KzU-63 with its RUP socket connected to the socket marked 'K' on the RU-2/2K radio via a U-shaped cable.
RU-2/2K with KzU-63 voice encryptor connected

The KzU-63 is powered directly by the RU-2/2K. Once the encryption unit is connected to the radio, it has to be loaded with suitable encryption keys, by means of a dedicated device. In the image above, the KzU-63 is shown to the right of the RU-2/2K. In practice it was installed in a side-pocket of the canvas manpack, or strapped to the main body of the radio by means of canvas straps. When the KzU-63 is used, the encrypted speech sounds just like (radio) noise.

 More about the KzU-63 speech encryptor
RU-2/2K with KzU-63 voice encryptor and handset RU-2/2K with KzU-63 voice encryptor connected KzU-63 U-shape cable for connection to the radio Crypto connector (K) and handset socket (MK) Removing the dummy plug from the crypto socket Crypto shorting plug removed Handset connected to the MTK socket on the encryptor

Voice scrambler
In the days before the KzU-63 voice encryption unit became available, the RU-2 was sometimes used in combination with a voice scrambler, such as the Yugoslav KzU-61, or the American KY-189 that is shown in the image on the right.

This was also the case with the non-crypto-capable version of the RU-2 (i.e. any version other than the RU-2/2K). The KY-189 shown here was adapted especially for the Yugoslav RUP-12 and the RU-2.

 More information
KY-189 scrambler handset

The RU-2 should be powered by a 12V DC source, such as the battery of a car or a dedicated mains power supply unit (PSU). When used as a manpack radio, a standard 12V Yugoslav Army battery was usually installed in a metal box that was attached at the bottom of the radio.
The image on the right shows the standard battery box that was supplied with the RU-2. The one shown here is empty, but it normally holds 6 wet rechargeable battery cells that supply 2V each. Together they form the BAJ-13,5 battery pack that should be connected to the wire terminals at the bottom of the radio.

The battery is held in place by two snap-locks at the sides and is protected against moist and dirt by a rubber gasket at the bottom of the radio. The battery supplies 12V at 6 Ah and should be charged by an external 13.5V DC source.
Battery box

As an alterative to using batteries, it was also possible to use the PT-6 DC power supply unit (PSU). It had the same form factor as the standard battery and was fitted in its place. The PT-6 allows the RU-2 to be powered from any DC voltage between 11 and 30V, making it ideal for installations in vehicles with 12V or 24V batteries. The PT-6 was also used with the RUP-12.
Long rod antenna installed Battery box Rear view Bottom view of the radio RU-2/2K with battery case present

Three different types of antenna were available for the RU-2. These are the same types as for the earlier RUP-12 radio. When delivered, the radio came with a standard AT-17 long rod antenna. It consists of 8 short pieces that are held together by short straps. When assembled, the antenna is approx 311 cm long and should be installed in the black socket at the top left of the front panel.
As an alternative to the long rod, a short flexible variant (AT-19) was available. It is 112 cm long and is also mounted on the black antenna base at the top left. Although its performance is much worse than that of the long rod, it is often more practicle in combat situations. The lower 20 cm of the antenna are flexible, so that it can be erected whilst the operator lies on the ground.

If the operational conditions are suitable and performance is mandatory, A long wire antenna of up to 50 metres can be connected to the BNC socket marked ŽIČ at the left edge of the front.
Foldable long rod antenna

When using a wire antenna, a suitable BALUN should be used in order to match the antenna to the 50 ohm impedance of the transceiver. The standard KS-28 wire antenna included a suitable BALUN. The ŽIČ-socket can also be used for any other suitable antenna with a 50 ohm load. In any case, the antenna tuning knob (PODEŠ. ANT) had to be adjusted during transmission for a maximum reading of the antenna indicator (meter) at the bottom right.
Foldable long rod antenna Close-up of an antenna joint Assembling the antenna Two pieces assembled together Long rod antenna installed Close-up of the antenna sockets Antenna tuning and battery check indicator

The socket marked 'MK.' at the top right of the control panel is for the connection of the audio ancillaries, such as the standard handset shown here. It consists of a metal assembly with a microphone and a speaker, much like the handset of an ordinary telephone set, with a Push-To-Talk (PTT) switch in the middle.

The MK-socket can also be used connecting a headset or a single microphone. In the latter case, a standard pair of headphones should be connected to the SL-socket at the center of the front panel. 1

  1. On the RU-2/2K featured here, the SL-socket is at the center of the front panel. On all other models, it is at the top right, to the left of the MK-socket, in place of the K-socket.

Handset Handset Operating the handset Sockets K (crypto) and MK (handset) Headphones socket (SL) RU-2/2K front panel with handset connected Handset connected to the KzU-63 encryption device RU-2/2K with handset

Block diagram
The block diagram of the RU-2 is nearly identical to that of the earlier RUP-12 transceiver. The image below shows a simplified block diagram of the RUP-12, and is based on several more detailed block diagrams from the operator's manual [3]. The transceiver parts (TX) are show in red, whilst the various stages of the receiver are yellow. The audio frequency stages (AF) are shown in blue. The receiver is a straightforward heterodyne with a 1st IF frequency of 10.7 MHz. Some parts of the 10.7 MHz section of the receiver are shared with the transmitter (AFC).

The transmitter section is shown at the top (in red). The modulator at the right mixes the audio signal from the microphone with a 10.7 MHz signal from a crystal oscillator. The advantage of using 10.7 MHz is that the crystal reference of the receiver can be used for selecting the desired transmission frequency. The most complex part of the transceiver is the crystal reference itself:

The block diagram above shows how the crystal reference is synthesized. The analogue parts are shown in blue, whilst the (digital) logic parts are red. The three parts that determine the actual frequency are shown in blue. The leftmost one is based on a 5 MHz crystal oscillator with a configurable multiplier behind it. It allows the 5 MHz signal to be multiplied by 1, 2, 3 or 4. This signal is fed to the first mixer, where it produces a sum and a difference signal. If we also take 0 MHz into account (i.e. multiply by 0, i.e. no signal), the following 8 steps are possible:

-20, -15, -10, -5, 0, 5, 10, 15 MHz

These steps are controlled by the leftmost frequency selector on the front panel of the transceiver (0-7). The other two digits are determined by two crystal banks with 10 crystals each (0-9). The first one is added in the second mixer, whilst the third one is fed into a phase discriminator and a differential amplifier followed by a complex logic circuit that is finally mixed at the heterodyne.
The drawing below shows the wiring of the 7-pin socket marked 'MK.' at the top right. It is used for the connection of a handset, headset or another microphone/speaker combination. The colours are of the internal wiring when looking into the socket from the front panel of the device.

Connector MK - Handset

The drawing below shows the identification of the pins of the socket marked 'K' (Kripto). It is used for connection to the RU-2/2K radio set and provides access to the unfiltered audio signals of the transmitter and the receiver. It also supplied 12V DC to the KzU-63. Wiring currently unknown.

Connector K - Crypto Unit

  1. Unknown author, RU-2/1 schematic diagram
    Via Radista (Website) [1]. Date unknown, but probably around 1995. Retrieved April 2015.

  1. Radista, VHF Radio RU-2 / 2K
    Website. Retrieved April 2015.

  2. Unknown author, RU-2/1 schematic diagram
    Via Radista (Website) [1]. Date unknown, but probably around 1995. Retrieved April 2015.

  3. Unknown author, RUP-12 manual
    Via Radista (Website) [1]. 30 July 1970. Retrieved April 2015.

Further information

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Crypto Museum. Last changed: Thursday, 27 August 2015 - 06:25 CET.
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