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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.
  

MA-4153 Message Store
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.

TA-4152 Transmitter
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).

RA-4151 Receiver
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).

The Dutch Stay-Behind Organisation
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].   
The PRM-4150 at the Dutch Signals Museum

Technical specifications
  • Frequency range: 2-29.999 MHz
  • Channels: 28,000 (1 kHz steps)
  • Telegraphy speed: 200 baud (synchronous)
  • Data rate: 600 five-figure groups/minute
  • Message length: 204 groups (1020 digits)
  • Message duration: 21 sec. max.
  • Modulation: 2-tone FSK (1563 and 2083 Hz)
  • Frequency shift: 520 Hz
  • Error detection: 1 parity bit per character pair
Transmitter
  • Modulation: LSB FSK (F2J), LSB CW (A2J)
  • Output power: 5W minimum from ATU into 50 ohm load
  • Harmonic emissions: < -40 dB (ralative to PEP in 50 ohms load)
  • Spurious emissions: < -45 dB
  • Carier suppression: Better than -40 dB
  • 3rd order intermodulation distortion: Better than -25 dB
  • Antenna Tuning: Automatic ATU to tune long wire antenna
  • Tuning time: < 10 sec.
  • Storage capacity: 1 message with up to 1020 digits (204 groups)
  • memory type: Volatile (erased when transmitter is switched OFF)
Receiver
  • 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 rejection: Better than 70dB
  • IF rejection: Better than 70dB
  • Maximum AF output: 20mW into 300 ohms
  • Storage capacity: 1 message of up to 120 digits (204 groups)
  • Memory type: Non volatile
Message Store
  • Data input: Numeric keyboard
  • Display: 2 x 5 digit LED
  • Data output: Serial data multiplexed over power line to transmitter
  • Storage capacity: 1020 digits (204 groups)
  • Memory type: Non volatile
References
  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.
Further information
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