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UK PRM-4150 →
Mobile Data Radio Station
- under construction
The FRM-4750 was a automatic transceiver developed by
HMGCC at Hanslope Park (UK)
and built by Racal in the late 1970s.
It was a self-contained
radio station with a digital messaging system, that was intended
as a mobile emergency system for the UK's
Diplomatic Wireless Service (DWS).
As such, it replaced earlier
Piccolo systems and,
to a lesser extent, manually operated morse-code-based systems
such as the Mk.123.
The system is also known by its codename KAYNARD.
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Kaynard consisted of three units: a transceiver, a power amplifier
and a coder/decoder.
The radio station was available in three versions:
(1) as a single device, with all three units mounted in a 19" rack, or
(2) as three separate units that were each packed inside a large Samsonite
briefcase. The latter version was intended for British embassies abroad,
as it was easier to bring three briefcase into a country via diplomatic mail.
The image on the right shows the all-in-one C-version of the radio station.
It was is use at a site of the British Foreign Office for many years.
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The FRM-4750 (Kaynard) was suitable for several purposes.
It could be used as an alternative to the portable
PRM-4150 suitcase radios,
for example when used in an embassy abroad. In that case the
radio station would act as a field station, typically with an
FRM-4780 Base Station at the other end.
It could also be used as an extended-range field station, in which
case it was equipped with a 400W power amplifier.
When used with a 100W power amplifier, the FRM-4750
could be used as a medium-power base station, suitable for communication
with other FRM-4750 units.
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Messages are passed to the FRM-4750 in digital form on 5-level
punched paper tape via a serial communications interface.
Transmission of CCITT characters (ITA2) at medium speed (20 cps)
is achieved using narrow-band 6-tone Multiple Frequency-Shift Keying
(MFSK) modulation [4].
This system is a variant of Piccolo II,
which has been in use by the DWS since 1962.
In most cases, the FRM-4780
is used as the base station. Although Piccolo has been largely phased
out now, the latest Piccolo Mark IV standard is still occasionally
being used for military point-to-point links.
In addition, direct communication between two stations is possible
by using the numerical keypad for input and the 7-digit LED display
for showing the output.
The FRM-4750 has a 3000-character memory for the transmission or
reception of messages consisting of several A4 pages.
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- MA-4751 (RT) receiver/transmitter
- MA-4752 (PA) power amplifier
- MA-4753 (CD) coder/decoder
- MA-4754 Suitcase with AAMTU, BCC 540B and system cables
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- FRM-4750A - All three units housed in an anti-vibration cabinet
- FRM-4750B - As three separate units, each stored in a suitcase
- FRM-4750C - All three units housed in a desktop cabinet
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The standard FRM-4750 radio stations delivers an RF output power
of 20W, which is sufficient for world-wide communication under good
(antenna) conditions. Two enhanced power options were available,
enabling the radio station to be used in a medium power base station role,
or as an extended range field station.
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- 400W - For use as an extended range field station
- 100W - For use as medium power base station
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In this version, the RT unit, the PA unit and the CD unit are housed in an
anti-vibration cabinet, making it suitable for transportation and operation
in mobile environments. The AAMTU, the cables and the other accessories are
caried in a separate MA-4754 executive-style suitcase.
The image on the right shows an example of the A-version, taken from the
Technical Manual [1].
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In the B-version, the three units (RT, PA and CD) are housed in individual
desktop cabinets and are carried around separately in executive-style
suitcases. They are stacked for operation after unpacking.
The AAMTU, the cables and the other accessories are
caried in a (fourth suitcase).
The image on the right shows an example of the B-version, as it is
present in the collection of the IWM in London [3]. The transceiver
(RT-unit) is missing from this photograph, but is shown in the image
below, together with the 4th suitcase that carries the accessories.
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The image on the right shows the MA-4754 executive-style suitcase with
the Automatic Antenna Matching and Tuning Unit (AAMTU), the various cable
for connecting the three units together, and various other accessories.
The suitcase on the right contains the RT-unit.
The entire radio station could be carried around in four high-profile
Samsonite briefcases. After unpacking, the three units were stacked and
connected. If the antenna was already present, the station could
be operational within minutes.
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In the C-version, the three units (RT, PA and CD) are housed in a single
desktop cabinet, ready for operation.
The AAMTU, the cables and the other accessories are
caried in a separate MA-4754 executive-style suitcase.
The image on the right shows an example of the C-version, which is part
of the Crypto Museum collection. It was used for many years by the
British Foreign Office on Diplomatic Wireless Service links
with their embassies abroad. It was donated to Crypto Museum in 2011.
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The MA-4754 suitcase was common to all three versions.
It contained the cable for connecting the three units together,
the antenna wires, the BCC-540B Automatic Antenna Matching and
Tuning Unit (AAMTU), ground spike, etc.
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- Frequency coverage: 3 - 30 MHz
- Channels: 27000 (in 1kHZ steps)
- Operating mode: SSB, Upper Sideband (USB)
- Modulation: 6-tone multi-frequency shift keying (MFSK)
- MFSK: 6 tones with 40Hz spacing centered around 2kHz
- Data rate: 200 words per minute (WPM) at 6 characters per word
- Frequency stability: Better than 2Hz at 30 MHz
- Power output: 20 Watts (nominal)
- Receiver S/N ratio: 15dB minimum in a 400Hz bandwidth for 1µV emf.
- Receiver bandwidth: 400Hz or 2kHz minimum (selectable)
- Power supply: 110V or 230V AC (50-60Hz), or 18-32V DC
- Power consumption: 45W (RX) and 125W (TX) at 24V DC supply
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© Crypto Museum. Created: Monday 10 December 2012. Last changed: Saturday, 17 December 2022 - 00:02 CET.
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![FRM-4750B in suitcases. Copyright IWM London. Reproduced here by kind permission [3].](img/iwm_frm4750b.jpg)
![FRM-4750B in suitcases. Copyright IWM London. Reproduced here by kind permission [3].](img/iwm_frm4750b_2.jpg)