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Cold War
PRM-4150   TITHE
Automatic clandestine transceiver - wanted item

The PRM-4150 was a spy radio station developed by HMGCC at Hanslope Park (UK) and built by Racal Tacticom in Reading (UK) from 1979 onwards [7]. It was a complete self-contained radio station with a fully digital messaging system, supplied in a standard attaché case. It was intended for use by Stay-Behind Organisations (SBO), the Diplomatic Wireless Service (DWS) and Special Forces (SF). The PRM-4150 is also known as TITHE and (in The Netherlands) as DZO-81.

The image on the right shows a typical PRM-4150 radio station packed in a standard executive style briefcase. It consists of three main units:
  • RA-4151 Receiver (right)
  • TA-4152 Transmitter (left)
  • MA-4153 Message store (centre)
Each of the units is self-contained and is powered by its own built-in rechargeable batteries. A brief description of each unit is given below. The Operator's Instruction Manual is available for download at the bottom of this page.

The radio station could be extended with secure communication features by adding the optional external MA-4245 data encryption unit.

Message store (MA-4153)
Transmitter (TA-4152)
Receiver (RA-4151)
Message Store   MA-4153
This is a message compilation and storage unit capable of storing a coded message of up to 1020 numeric characters, for periods in excess of 7 days when switched OFF. The message is displayed 2 groups at a time, and each group consists of 5 digits.   

Transmitter   TA-4152
The transmitter accepts data messages from the Message Store (MA-4153) or from an external Data Unit, such as the Racal MA-4245 portable cipher unit. It transmits messages as a two-tone Frequency Shift Keying (FSK) signal on the selected frequency channel at a rate of 50 characters per second. This is equivalent to 600 groups per minute. Furthermore, facilities are provided for transmissions in morse code (CW).   

Receiver   RA-4151
When set to DATA, this self-contained unit is capable of receiving data messages on a selected frequency channel and storing this message until reset by the user. The stored message can be recalled at any time up to at least 7 days after reception. It is displayed on the receiver's display. Facilities are available to process the data message externally, e.g. on a Racal MA-4245 portable cipher unit, whilst it is being received. When in VOICE or CW mode, the receiver acts as a standard radio with frequency pre-selection, capable of receiving voice (LSB or AM LSB) or morse (CW).


Data encryption unit   MA-4245
The PRM-4150 could optionally be expanded with an MA-4245 Cipher Unit that is compatible with the data format of the MA-4153 Message Store and can be connected directly to the set.

It allows the entry and encryption of alpha-numeric messages directly from a full Latin keyboard.

 More information


Stay-behind organisation O&I

The PRM-4150 was also used by the Dutch Stay-behind Operation (SBO) O&I, where it was known as DZO-81, short for Digitale ZendOntvanger (Digital Transceiver), 1981. It was introduced in 1981 as a gap-fil solution between the ageing SP-15 and the pan-European FS-5000 Harpoon.

Not many PRM-4150 units of the Dutch Stay-Behind Organisation have survived. In fact, the only one known to have survived is now in the collection of the Dutch Signals Museum [5].   

State Security Service (SSS)

Following WWII, Yugoslavia was a communist state that was more or less trapped between two military powers: the Soviet Union (USSR), led by Russia, and the free West, led by the UK and the US. Initially, only the UK and the US were seen its most serious threat, but after the 1948 Informbiro Crisis, the Soviet union and its allies were added to the list of enemies.

An efficient weapon against these enemies, turned out to be a wll-organised network of agents, for which modern communication equipment was needed. Around 1980, the State Security Service (SSS) of the (then) communist republic of Slovenia purchased an unknown number of MA-4150 spy radio field stations, plus one base station [8][9]. 1

  1. It is known that Slovenia had plans for ordering 13 field stations, but it is unknown how many units were actually ordered [8].

Technical specifications
  • Device
    Spy radio set
  • Purpose
    Covert clandestine and diplomatic communication
  • Manufacturer
  • Model
    PRM-4150 (MA-4150)
  • Country
  • Year
  • Frequency
    2-29.999 MHz
  • Channels
    28,000 (1 kHz steps)
  • Speed
    200 baud (synchronous)
  • Data
    600 five-figure groups/minute
  • Message
    204 groups (1020 digits)
  • Duration
    21 sec. max.
  • Modulation
    2-tone FSK (1563 and 2083 Hz)
  • Shift
    520 Hz
  • Error det.
    1 parity bit per character pair
  • Model
  • Modulation
    LSB FSK (F2J), LSB CW (A2J)
  • Output
    5W minimum from ATU into 50 ohm load
  • Harmonics
    < -40 dB (ralative to PEP in 50 ohms load)
  • Spurious
    < -45 dB
  • Carier sup.
    Better than -40 dB
  • IMD3
    Better than -25 dB
  • Tuning
    Automatic ATU to tune long wire antenna (< 10 s)
  • Storage
    1 message with up to 1020 digits (204 groups)
  • Memory
    Volatile (erased when transmitter is switched OFF)
  • Model
  • Modulation
    LSB FSK (F2J), LSB CW (A2J), LSB Voice (A3J)
  • Display
    2 x 5 digit LED (2 groups)
  • Sensitivity
    > mW into 300 ohms for 2µV EMF RF input (into 50 ohms)
  • S/N ratio
    CW 22dB @ 2µV EMF, Voice 15dB @ at 2µV EMF
  • Selectivity
    CW 3dB bandwidth 100Hz ±30Hz, Voice 6dB/2kHz, 40dB/5kHz
  • Image rej.
    Better than 70dB
  • IF rej.
    Better than 70dB
  • AF output
    20mW into 300 ohms
  • Storage
    1 message of up to 120 digits (204 groups)
  • Memory
    Non volatile
Message Store
  • Model
  • Input
    Numeric keyboard
  • Display
    2 x 5 digit LED
  • Output
    Serial data multiplexed over power line to transmitter
  • Storage
    1020 digits (204 groups)
  • Memory
    Non volatile
Known users
  • Diplomatic Wireless Service (UK)
  • Stay-behind organisation O7I (Netherlands)
  • Ministry of Defense (Netherlands)
  • Bundeswehr (Germany)
  • State Security Service (Slovenia, Yugoslavia)
  1. Racal Tacticom Ltd., PRM-4150 Portable High Speed Data Station
    TH1344/T. Operator's Instruction sheet (8-pages fold-out). 2 September 1980.

  2. Racal Tacticom Ltd., PRM-4150 Technical Manual Vol. 1 (Parts 1 to 5)
    Technical Service Manual with updates. 1 May 1981.

  3. Racal Tacticom Ltd., PRM-4150 Technical Manual Vol. 2 (Parts 6 to 10)
    Technical Service Manual and updates. 1 May 1981.

  4. Racal Mobical Ltd. Racal PRM-4150 Technical Assembly Manual
    Original PCB assembly instructions, with hand-written notes (2 binders).
    Various dates - June 1978 to June 1981.

  5. Museum Verbindingsdienst, PRM-4150 in display case
    PRM-4150 (without suitcase) formerly used by Dutch Stay Behind organization.
    Photograph Copyright Crypto Museum, 25 February 2009.

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

  7. Electronics & Power, Five more comms. products announced by Racal
    September 1979. Page 599.

  8. Technology of the Yugoslav State Security Service's James Bond
    Republic of Slovenia (website: 1 June 2019.

  9. Lojze Golob (Chief Department VI of USDV) to Danilo Kovačevič (Security Institute)
    VI-Z-18/92-83. 30 June 1983.
Further information
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Crypto Museum. Created: Wednesday 03 February 2010. Last changed: Tuesday, 19 December 2023 - 12:20 CET.
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