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Special Forces radio station

The Ra-190 was a valve-based spy radio station that was developed by Svenska Radio­aktie­bolaget (SRA) in Sweden in the late 1950s. The green radio set was intended for covert activities, mainly by the Swedish Special Forces (SF), and consists of a transmitter (M-3950-190), a receiver (M-3951-190), a battery pack and a green wallet with various accessories including crystals.
The Ra-190 had two frequency bands (1.6 - 8 MHz) and (8 - 16 MHz) and produced an output power of 0.8 and 0.4 Watt respectively, making it suitable for short to mid-range CW (morse). The transmitter is crystal operated whilst the receiver features a Variable Frequency Oscillator (VFO).

A complete Ra-190 station, with all accessories, weights around 6.4 kg. It was usually carried by a single person, but could also be shared by several individuals. It was intended for stationary use (i.e. not portable), as it requires a halve (½λ) wave-length or full wavelength (λ) wire antenna.
Ra-190 transmitter (rear) and receiver (front)

The transmitter is suitable for CW (morse) only and can be operated by a built-in morse key, that is located at the right side of the control panel, just below the meter. This key was for emergency purposes only however, as a suitable external morse key was usually supplied with the set.

The receiver covers 1.1 - 16 MHz, divided over five ranges, with a gap between 1.7 and 2.5 MHz. The Ra-190 was powered by two sets of dry batteries: 67.5 V (HT) and 3 V for the filaments of the valves (LT). The batteries were housed in a wallet that could be carried under the clothing or uniform of the operator, in order to keep them warm and dry when used under harsh conditions.
Complete Ra-190 kit Ra-190 transmitter (rear) and receiver (front) M-3950-190 transmitter M-3951-190 Receiver Accessory wallet Battery pack Complete Ra-190 radio station Operating the internal key of the Ra-190

The crystal-operated transmitter is marked M-3950-190 and covers 1.6 - 16 MHz, divided over four frequency ranges (two bands): The lower frequency band (1.6 - 8 MHz) uses the crystal in the fundamental mode and produces an output power of 0.8 Watt. At the higher frequency band (8 - 16 MHz) the crystal frequency is doubled and the output power is 0.4 Watt.
The transmitter is housed in a sturdy green metal case with a well-structured recessed front panel. The antenna socket (A) is at the top, with a switched loop output 'Mt(A)' for the receiver.

The valve-based transmitter is driven by a crystal that should be inserted into the socket marked KRISTALL along the lower edge of the control panel. The external battery pack should be connected to the 6-pin socket in the lower right corner. It caries the 3V LT and 2 x 67.5V HT voltages. A fixed 'flying wire' with a 6-pin plug at the end is supplies power to the receiver.
M-3950-190 transmitter

At the top right is a meter that allows the antenna to be tuned for maximum antenna current. The transmitter has a built-in morse key that is located at the lower right corner of the meter. For this reason one of the corners of the protective plexiglass over the meter has been cut off.

The transmitter is suitable for A1 modulation only (morse, CW). The output power depends on the selected frequency range and is either 0.4 or 0.8 Watt. When the conditions are sufficiently good, the output can further be reduced to about ¼ of the nominal power. This minimises the risk of detection and is extremely useful for Special Forces operating behind enemy lines.

A 4-position rotary switch at the right half is used for selecting the required mode of operation (MODE). In the leftmost position (FRÅN), the radio station is switched off. In the rightmost position (1/1 EFF) the transmitter is operated at maximum power. In the (1/4 EFF) setting, the output power is reduced to about 25% and in the (Mt) setting, the receiver is enabled (Mottagare).
M-3950-190 transmitter M-3950-190 transmitter Transmitter control panel The meter in the top right corner and the built-in morse key Power socket and socket for external morse key Loop wire to receiver antenna input Operating the MODE selector Operating the built-in morse key

The receiver is slightly larger than the transmitter and is housed in a similar case, with a recessed control panel. It has a built-in Variable Frequency Oscillator (VFO) and can be tuned freely from 1.1 to 16 MHz, divided over five frequency ranges, with a gap between 1.7 and 2.5 MHz.
The rather sophisticated receiver is built around 8 valves (tubes) and consists of an RF stage, mixer, oscillator, 2nd IF, detector, BFO and AF output stage. The latter is suitable for the connection of a pair of 600 Ohm headphones.

The receiver has a sensitivity of 15 µV at a S/N ratio of 7 dB. The 2nd IF frequency is at 470 kHz. A toggle switch at the right hand side is used to select the desired modulation type A1 (CW) or A3 (AM). In A1 mode, the Beat Frequency Oscillator (BFO) at the top right is enabled. By default, it should be set to the 0-position.
M-3951-190 Receiver

The receiver is powered by the transmitter, whch in turn is powered by the battery pack. When in operation, the 3V filament power is always supplied, so that the receiver is ready for use. The HT voltage is controlled by the MODE selector on the transmitter. Set it to (Mt) for reception. If no transmitter is required, the battery pack can also be used to power the receiver directly. In that case, the two 67.5 V HT batteries are connected in parallel, so that they will last longer.

The antenna signal is also supplied by the transmitter by connecting a short loop cable between Mt(A) on the transmitter and S(A) on the receiver. The antenna signal is controlled by the MODE selector on the transmitter. When in operation, the ground terminal (J) should be connected to ground or to a suitable counterpoise. The knob at the centre is used for selecting the required frequency range, each of which corresponds to a horizontal line on the tuning scale (top left).

The 600 Ohm headphones, that comes with the Ra-190, should be connected to the socket marked 'HÖRTEL' at the bottom edge of the control panel. The volume level can be adjust with the potentiometer marked 'STRYKA' to the right of the socket.
M-3951-190 Receiver M-3951-190 Receiver front panel Frequency scale and tuning knob Frequency range selector Headphones socket Antenna input (from transmitter) Adjusting the BFO for optimum CW reception Selecting the desired frequency

Battery pack
As the Ra-190 was intended for field use, it was supplied with it own battery pack that supplies the two voltages (LT and HT) that are needed to operate the radio station. To allow the station to be used under harsh conditions, such as extreme cold, the battery pack came in the shape of a belt-pack that could be worn by the operator under his clothing, close to his body.
This was done to keep the battery dry and warm, so that it would last longer. Dry 1.5 V batteries were used to power the filaments of the valves. When bringing an old Ra-190 back to life, these batteries can easily be replaced by four standard 1.5 V D-cell batteries (see the diagram below).

For the HT voltage, the radio requires 67.5 V DC with a relatively low current. A suitable HT battery can easily be created by connecting 7 standard 9V block batteries in series (63 V). In order to be able to supply the required voltage, two such packs are used in series/parallel.
Battery pack

The belt-pack supplied with the R-190 is made of green water-resistant cloth and has two large pockets; one for the LT battery and one for the HT battery. Inside each pocket are two smaller pockets in which the individual batteries are stored. The belt-pack can be carried around the waist by means of a green canvas strap. A short 6-wire cable, with a rather strange 6-pin plug at the end, connects the battery to the transmitter. The receiver is powered from the transmitter.

Battery belt Battery pack Power connector Close-up of power plug Single pocket of the battery belt Four D-cells inside a pocket of the battery belt Rubber straps to keep the pockets of the battery belt closed Improvised 67.5V batteries

The accessories of the Ra-190 were stored in a large green wallet that was supplied with the set. The wallet has two large compartments in which the headphones, an external key, the antenna wires and some spare valves were stored. The lid of the wallet has 10 small pockets in which the crystals are stored. The batteries are housed in a separate wallet that can be carried on the body.
Wallet in which the accessories are stored A pair of headphones with a rubber cable Morse key with leg mount
Wire antenna Ground wire (counterpoise) Rope on spool for fixating the wire antenna Lead weight for throwing the antenna over the branch of a tree Body wearable wallet with batteries
Set of 10 crystals Spare valves Standard small flat screwdriver with green plastic grip Antenna loop wire

The accessories and spare parts were usually stored in a large wallet that was made of green water-resistant cloth. Inside the wallet are two large pockets of which the smaller one is used for storing the spare valves and some tools.

The larger pocket is use for storing the accessories, such as the antenna wires, a throwing weight, the headphone and an external morse key. These accessories are further listed below. Inside the flap of the wallet are 10 small pockets that are used to store the crystals.

The audio output level of the receiver is just strong enough to drive a pair of headphones, such as the ones shown in the image on the right. These headphones were issued with the Ra-190 and should be connected to the HÖRTEL socket at the bottom center of the control panel.

The headset has a rubber cable with a moulded 2-pin plug at the end.
Headphone with rubber wire and plug

Morse key
The station was supplied with an external morse key that could be mounted to the operator's leg. It was connected to the NYCKEL socket at the bottom right of the transmitter's control panel.

If the external key is missing, or when the operator has insufficient space, the built-in morse key can be used as an alternative. It is located just above the NYCKEL socket and has a large black knob. In order to accomodate this key, one corner of the protective plexiglass shield over the meter has been cut-off.
External morse key

Wire antenna
The Ra-190 is supplied with two wires that can both be used as antennas. The longer one is green and is intended as a half-wave antenna for the lower frequencies or a full-wave antenna for the higher frequencies.

When not in use, this wire can also be used for counterpoinse (ground).
Wire antenna

The black wire is the shorter of the two. It can be used as half-wave antenna for frequencies around 8 MHz and full-wave antenna for frequencies between 14 and 16 MHz. When not in use, this wire can also be used as counterpoise (ground).

The table on page 11 of the manual [3] shows which cable is best used for each of the available frequency ranges.
Counterpoise wire

When fixing an antenna, e.g. between two trees, it might be useful to attach one end to an (isolated) rope, so that it can be hanged freely in the air, away from obstacles.

A red rope on a spool is supplied with the Ra-190. The spool has a grip for easier unwinding.
Antenna fixating rope

Throwing weight
When using the Ra-190 in the field or in a forest, which was usually the case, the antenna often had to be improvised. In most cases, a nearby tree would be used as a supporing mast for the ½λ or λ wire antenna.

As the antenna wire should be mounted as high as possible, the throwing weight, shown in the image on the right, was supplied. It is attached to one end of the wire and and then thrown over a branch of the supporting tree.
Throwing weight

Spare valves
In total, 10 valves are used in the Ra-190: 2 in the transmitter and 8 in the receiver. As only five different types of valves are used, one of each type is supplied as a spare. They are stored inside the smaller pocket of the accessory wallet.

For an overview of the used valves, please refer to the technical specifications below.
Spare valves

Antenna loop wire
A short loop wire, with a banana-type plug at either end, is needed to distribute the antenna signal from the transmitter to the receiver.

It should be connected between the transmitter loop output marked 'Mt(A)' and the antenna input marked 'A' of the receiver.
Antenna loop wire

A small flat screwdriver was supplied with the Ra-190. It was kept in a small pocket inside the flap of the accessory wallet and was suitable for opening the cases of the transmitter and the receiver, when servicing the units or replacing the valves.   

As the transmitter of the Ra-190 is crystal operated, as set of 10 different crystals was supplied with the radio station. Each crystal was stored in a small padded pocket inside the flap of the accessory wallet. A full overview of the available crystals is given below [5].

A crystal can be removed from its pocket by pulling a green ribbon that is folded around the crystal. After use, the crystal should be restored in its pocket by pushing the crystal (and the ribbon) back in place, with the contact pins first.
Pulling a crystal from its pocket

Order # Code f0 (kHz) TX < 8 MHz TX > 8 MHz
F1080-033057 Tc 33057 2555 2.555 -
F1080-033058 Tc 33058 2840 2.840 -
F1080-033059 Tc 33059 3395 3.395 -
F1080-033060 Tc 33060 3755 3.755 -
F1080-033061 Tc 33061 4385 4.385 8.770
F1080-033062 Tc 33062 4880 4.880 9.760
F1080-033063 Tc 33063 5060 5.060 10.120
F1080-033064 Tc 33064 6005 6.005 12.010
F1080-033065 Tc 33065 7325 7.325 14.650
F1080-033066 Tc 33066 7955 7.955 15.910

Accessory wallet Accessory wallet (rear view) Accessory wallet with flap open Looking into the accessory wallet Screwdriver Spare valves Spare valves
Close-up of the flap with the crystal pockets Opening the crystal pockets View of 5 crystals in their pockets The flap of the wallet with the 10 crystal pockets (open) Pulling a crystal from its pocket Product code engraved in the top of the crystal Storing the crystal Pushing the crystal back into its pocket
Headphone with rubber wire and plug Close-up of the headphones One speaker of the headphones Headphones connected to the HÖRTEL socket of the receiver Headphones connected to the Ra-190 station Wire antenna Looking inside the wallet Counterpoise wire
Antenna wire detail Antenna fixating rope Unwinding the spool Throwing weight Antenna loop wire Battery belt Battery pack Power connector
External morse key External morse key Morse key External morse key Operating the morse key Operating the knee-mounted morse key Cable mount Close-up of the cable mount

The interior of both the transmitter and the receiver can easily be accessed by loosening the four bolts in the corners of the control panel of each unit. This allows the front panel, to which all electronic components are mounted, to be extracted from the case, which is very service-friendly.
The design of the transmitter is pretty simple and consists of no more than two identical 3A4 valves. As it is only suitable for CW (morse), there is no modulator. The first 3A4 valve acts as the oscillator which runs up from 0 to 8 MHz.

The second stage has a double function. When the transmitter is used at frequencies up to 8 MHz, it acts as a straight power amplifier (PA), and produces approx. 0.8 W. At frequencies above 8 MHz however, it acts as a frequency doubler as well. As a result, the output power is reduced by half at this range, to about 0.4 W.
Ra-190 transmitter interior

The output can further be reduced to about 1/4 of the nominal power, by setting the the MODE-selector to 1/4 EFF. The image above shows the interior of the transmitter, which is rather clean. The valves are protected by aluminium cans and can easily be swapped in case of a failure.
As usual, the design of the receiver is much more complicated, which is why the case of the receiver is slightly larger than that of the transmitter. It is a single superheterodyne design with an Intermediate Frequency (IF) of 470 kHz, and contains no less than 8 valves

The design of the interior is very compact, with all passive components located in a couple of metal enclosures at the centre. All valves and the adjustable coils are mounted to the side of these enclosures, so that they can easily be accessed when the device needs calibration or repair.
Ra-190 receiver interior

The block diagram below show how the receiver is constructed. At the left is the HF pre-amplifier built around an 1L4 valve. It is followed by a mixer (1R5) that combines the HF input with the signal from the oscillator (3V4), resulting in an 470 MHz IF signal. The IF signal is amplified twice (2 x 1L4) before it is fed to the detector (1U5) which is also used as the LF pre-amplifier.

The LF-signal if finally amplified to headphones level by the LF output amplifier (1L4). For the reception of CW signals, a Beat Frequency Oscillator (BFO) built around a 1L4 is present. It is activated by setting the Modulation switch to A1 and can be tuned with the A1-OSC knob.
The transmitter lifted from its case Ra-190 transmitter interior Close-up of the valves Transmitter detail The receiver taken from its case Ra-190 receiver interior Close-up of the valves and the adjustable coils Close-up of the tuning scale mechanism

Technical Specifications
  • Total weight: 6.4 kg (including battery belt)
  • Range 1: 1.6 - 3 MHz
  • Range 2: 3 - 5 MHz
  • Range 3: 5 - 9 MHz
  • Range 4: 9 - 16 MHz
  • Valves: 2 x 3A4
  • Power output: 0.8 W (1.6 - 8 MHz), or 0.4 W (8 - 16 MHz)
  • Dimensions: 11.5 x 18.5 x 7.5 cm
  • Weight: 1.2 kg
  • Range 1: 1.1 - 1.7 MHz
  • Range 2: 2.5 - 3.9 MHz
  • Range 3: 3.9 - 6.3 MHz
  • Range 4: 6.3 - 10 MHz
  • Range 5: 10 - 16 MHz
  • Valves: 5 x 1L4, 1 x 1R5, 1 x 1U5 and 1 x 3V4
  • Sensitivity:15 µV @ 7 dB S/N
  • IF: 470 kHz
  • Headphones: 600 Ohm
  • Dimensions: 11.5 x 21 x 10 cm
  • Weight: 1.8 kg
  • LT: 3 V (filament)
  • HT: 2 x 67.5 V
About Svenska Radioaktiebolaget (SRA)
The manufacturer of the Ra-190, Svenska Radioaktiebolaget (SRA), was founded in Stockholm (Sweden) in 1919 by five Swedish companies who had developed an interest in the (then) new radio technology. Amoung these companies were AGA, ASEA and LM Ericsson. In the first years of its existence, the company faced serious problems with the development and manufacturing of radio equipment, due to patent protection by large international corporations.

SRA company logo

All that changed when, in 1921, SRA was restructured after Marconi had acquired 43% of its shares, allowing the company to use proprietary technology from Marconi. During the early 1920s, SRA developed equipment for wireless telegraphy and broadcasting. The first domestic radio was released in 1922. Like most other brands in those days, it required headphones in order to listen to a broadcast. Later improved models were sold under the Radiola brand [1].

In autumn 1927, LM Ericsson became the major shareholder (57%) when it took over the shares from the four Swedish partners, whilst Marconi remained a minority shareholder with just 43%. The first Radiola radios with built-in speakers were introduced in 1928, eventually followed by the first portable radio (1939), the first gramophone (pickup) and the first transistor radio (1958).

In 1944, SRA started developing two-way radio systems, initially for the police and other law enforcement agencies. During the 1950s, SRA also developed two-way radio systems for the Department of Defense (DoD), such as the Ra-190 special forces radio. As these developments competed with former shareholder AGA, the principle owner - LM Ericsson - decided to sell its Radiola brand to AGA in 1964, allowing SRA to concentrate on the development of two-way radio.

During the 1970s and 1980s, SRA further developed itself as a specialist company in the field of two-way radio, mainly by acquiring several other companies. The first large company to be taken over by SRA in 1978 was Sonab Communications, which itself was formed in 1974, by merging Sonab and AGA's radio operations. These developments eventually led to Ericsson becoming a major player in mobile telecommunications. After acquiring the remaining shares from Marconi, SRA was fully integrated with LM Ericsson and went on as Ericsson Radio Systems AB.
  1. Wikipedia, Svenska Radioaktiebolaget
    Swedish. Retrieved June 2014.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

  3. Beskrivning av RADIOSTATION 190, Ra 190, Del I
    Description of Radio Station 190 (Ra 190) Part 1 (Swedish).
    Swedish Royal Army Admnistration. 1958. Retrieved June 2014. 1

  4. Kompendium över Radiostation Ra 190, M 3955-190, Del II
    Technical Description of Radio Station Ra 190, Part 2 (Swedish).
    Date unknown. Retrieved June 2014. 1

  5. Thomas Hörstedt (SM7DLF), Ra 190
    Website Grön Radio. Retrieved June 2014.

  1. Ra 190 manuals re-typeset and published by Jan Andersson (SM0OFV).
    More information on the Rig Pix Database website.

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© Crypto Museum. Created: Thursday 12 June 2014. Last changed: Monday, 04 July 2016 - 21:11 CET.
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