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NESTOR VINSON
Portable military VHF/FM transceiver
AN/PRC-77, also known under different names, is a portable military
VHF/FM transceiver for the 30 to 86 MHz band,
introduced in 1968 by the US Army as the successor to the
AN/PRC-25.
The radio was used heavily during the Vietnam War (1955-1975),
and differs from its predecessor in that its RF power amplifier stage
(PA) is fully solid state, whereas the PA of the PRC-25
was made with thermionic valves (vacuum tubes).
It also adds support for digital voice encryption devices.
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The PRC-77 consists of an RT-841 transceiver and some additional components, made by various companies.
It is suitable for secure voice operation, by selecting the so-called
X-MODE and adding an external voice encryption unit,
such as KY-38 (NESTOR) and
KY-57 (VINSON).
For the European market, it was made by
Telemit Electronic GmbH
in München (Germany). Telemit improved the radio in
several ways, including changing the channel spacing from 50 kHz to 25 kHz.
As a result, the radio is still interoperable 1 with most modern military
VHF/FM radios today.
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Both receiver and transmitter cover the 30-75.95 MHz frequency range,
divided over two bands, in steps of 50 kHz (or 25 kHz for the
European version). 2
The receiver has a sensitivity of 0.5 µV, whilst the transmitter delivers
between 1.3 and 4 Watts to the 50Ω antenna. The device measures
273 x 273 x 100 mm and weights 6.2 kg. It should powered by a 12.5 to 15V DC
voltage that is usually supplied by military batteries, such as the
(BA-4386/U, BA-398/U or BA-55984) [1].
From the late 1980s onwards, the PRC-77 was gradually replaced by the newly
introduced Single Channel Ground and Airborne Radio System
(SINCGARS), starting in 1988 with the RT-1439.
The SINCGARS radios are backward compatible with the PRC-77 and offer
Frequency Hopping (FH) as an Electronic Counter Counter Measure (ECCM).
It also supports the same encryption devices, and
offers digital communication in addition to analogue voice (FM).
Nevertheless, many PRC-77 units remained in use well into the 1990s,
and in some countries even as late as 2018 [2].
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Not compatible with the SINCGARS frequency hopping mode (FH).
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Commonly designated AN/PRC-1177 or AN/PRC/77A.
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The diagram below provides a quick overview of the controls and connections
of the PRC-77, all of which are located at the front panel (which is at the
top when carried on the back of a soldier). The device is powered by
internal batteries,
or by an external power source – such as the battery of a vehicle –
connected via the POWER socket.
It has provision for a local antenna (fitted
to the antenna base mount) or an external one, that can be connected to the
BNC socket at the top left.
The radio has two bands: (1) 30 to 52.950 MHz and (2) 53 to 75.950 MHz,
selectable with the BAND selector at the bottom left. The desired frequency
should be set with the two rotary dials at the centre. The leftmost one is
used for setting the MHz, whilst the rightmost one is for the kHz.
The standard version of the radio has a channel spacing of 50 kHz, but some
of the later units have a 25 kHz spacing.
On such devices a +25kHz switch has been added to the front panel.
For normal use, a standard handset with
U-229 connector
should be connected to one of the audio sockets at the top right.
When using voice encryption, the encryptor should be connected to the power
socket, and the handset should be connected directly to the
encryption device.
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Differences with the PRC-25
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- Transistorised Power Amplifier (PA) instead of valve (tube)
- No DC-DC converter (needed for the anode voltage of the valve)
- Transmitter spurious improved
- Large signal behaviour of receiver improved
- Single supply voltage (no separate filament voltage needed)
- Wider audio bandwidth
- Support for external (digital) encryption device (X-Mode)
- Longer operation with the same battery
- Less weight
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Voice encryption devices used with the PRC-77
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KY-38 was one of the first secure voice units that provided real
high-end encryption. It is part of the
NESTOR family of devices
– developed by the US National Security Agency (NSA) –
and was introduced during the Vietnam War in 1964.
The KY-38 was a large and heavy device, that has nearly the same size as
the radio itself. The image on the right shows an American soldier during
the Vietname War, carrying a PRC-77 in manpack configuration
with a KY-38 mounted at the bottom. Note that the handset is connected
to the KY-38 rather than to the radio itself.
➤ More information
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In the mid-1970s, Telemit
added the
CVX-396 voice encryption device
to the European version of the PRC-77.
The CVX-396 was in fact a rebranded version of the SVZ-B made by
Crypto AG (Hagelin) in Switzerland.
It has the same width as the PRC-77 and can be mounted on top of it.
Note that it is less secure than the Swiss national version SVZ-B,
which looks nearly identical.
➤ More information
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In 1978, Siemens in München (Germany) introduced the MSC-2001,
a compact tactical voice encryption unit that could be mounted to the
side of the PRC-77. It was initially developed especially for the PRC-77,
but was later adapted for use with other radios as well.
It can hold up to 8 different 50-bit cryptographic keys, selectable with a
rotary switch at the front.
➤ More information
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in the mid-1980s, Telescurity Timmann (TST) developed the TST-7698 voice
encryptor that uses an LPC-10 or a RELP vocoder,
allowing it to be used on HF, VHF and UHF radio channels,
as well as via analogue telephone lines.
The device is constructed in such as way that it can be fitted to one of
the short sides of the PRC-77, using a special
mounting bracket.
➤ More information
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KY-189 was a voice scrambler
for tactical use, developed around 1990 by Napco in the US.
The entire device is housed in a somewhat bulged radio handset,
and has controls on the inside.
The device was suitable for virtually any HF, VHF and UHF radio set,
including the PRC-77, and was used in the Yugoslav Wars (1991-2001).
➤ More information
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The RT-841 radio is the core part of the set. It measures 273 × 273 × 100 mm and
weights 6.2 kg. It can be used in a vehicle by using a suitable mounting
kit or, more commonly, as a manpack unit mounted in a metal
carrying frame.
For portable use, it is powered by an
internal battery pack and uses
a local antenna,
mounted directly to the control panel. For mobile use,
power can be supplied directly to the POWER socket,
whilst an external antenna can be connected to the BNC socket marked 'ANT'.
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The radio was usually powered by an internal battery pack, of which various
types were available: chargeable and non-chargeable. The image on the right
shows the BA-386-A/PRC non-chargeable type, that accepts 10 standard 1.5V
D-size cells.
The battery pack is installed at the bottom of the radio and has a
socket
that should mate with a
connector inside the radio.
The battery pack is protected by a metal cover.
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The radio should be used with a standard dynamic handset with an
U-229 connector
at the end of its coiled cable, such as the one shown in the image
on the right made by TMC.
Note that, when using an external voice encryptor, another type
of handset might be required.
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By default, the PRC-77 is used
with its own antenna that should be mounted on the screw terminal marked ANT
at the bottom left of the control panel. It consists of a gooseneck with a
foldable steel-tape part mounted on top and is also known as bush-whip.
When folded, this antenna requires very little space.
When using the PRC-77 with an external antenna, it should be connected to the
BNC terminal marked ANT. In that case, the foldable antenna should be removed
and a cover plate should be installed on the screw terminal.
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Several mounting brackets were available for mounting an encryption device
to the body of the PRC-77, such as the one shown in the image on the right,
which was made especially for the
TST-7698
by Timmann Telesecurity (TST).
Other types of brackets were available for the other encryption devices
listed above.
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The AN/PRC-77 is a service-friendly device that was designed in such a way
that it could be repaired in the field and that it could be built under licence
by different companies in different parts of the world, like
Associated Industries (USA), Telefunken (Brasil)
and Telemit (Germany).
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The device is housed in a watertight enclosure that consists of a die cast
aluminium control planel and an extruded case shell.
The interior can be accessed by releasing
just four bolts,
located at the rear side of the front panel grips.
After releasing the bolts, the case shell can be removed and the interior —
which is completely mounted to the rear of the control panel — will be exposed.
The image on the right shows the various modules that are visible at the bottom
of the radio. This side also exposes the extremely complex
MHz-stepping mechanism of the tuner.
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Additional (smaller) modules are located at the
upper side of the radio,
mounted onto a hinged panel that can be
swung to the side in order to get
access to the tuning assembly, the PA stage and the audio circuitry
mounted below it. Further details can be observed in the images below.
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Device VHF/FM transceiver Purpose Military communications Year 1968 Model AN/PRC-77 Nickname Prick-77 Manufacturer Associated Industries, USA
Telefunken, Brasil
Telemit, Germany
Thomson-CSF, France Predecessor AN/PRC-25 Successor SINCGARS Frequency 30 - 86 MHz Modulation FM Bands 2 (30-52.95 MHz and 53-75.950 MHz) Channels 920 Spacing 50 kHz 1 Bandwidth 12 kHz Output 1.3 to 4 Watts Range 8 km (est.) Power 12 to 15 V Batteries BA-4386, BA-398, BA-5598, BB-386, BB-2598 Antenna 1 AT-271A/PRC — 3 m multi-section whip Antenna 2 AT-892/PRC-25 — steel tape bush-whip Dimensions 273 × 273 × 100 mm Weight 6.2 kg
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Some units were modified for a 25 kHz channel spacing and a reduced voice
bandwidth of 6 kHz. These units were generally designated AN/PRC-1177 or
PRC-77A, and can be recognised by an extra switch (25 kHz) on the front panel.
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Model AN/PRC-1177 Manufacturer Telemit (Germany) Channels 1840 Spacing 25 KHz Bandwidth 6 kHz
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- Austria
- Bangladesh
- Brazil
- Chile
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- El Savador
- Finland
- France
- Isreal
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- New Zealand
- Norway
- Pakistan
- Philippines
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- Spain
- Sweden
- Switzerland
- Taiwan
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The PRC-77 is known under the following names:
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- PRC-77
- AN/PRC-77
- AN/PRC-77A
- AN/PRC-1177
TR-PP-13A France ER-95B France EB-11/ERC-110 Brazil LV 217 Finland Ra 145/146 Sweden SE-227 Switzerland
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© Crypto Museum. Created: Sunday 11 March 2018. Last changed: Wednesday, 01 May 2024 - 08:19 CET.
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