Portable military VHF/FM transceiver
AN/PRC-77 was a military portable VHF transceiver,
also known as a backpack radio,
introduced by the US Army in 1968 as the successor to the
The radio was used heavily during the Vietnam War, 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)
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, like the
or the 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.
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) .
Not compatible with the SINCGARS frequency hopping mode (FH).
Commonly designated AN/PRC-1177.
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
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
units – made by Telemit
in Germany – have a 25 kHz spacing. On such device
an extra +25kHz switch is present.
For regular use, a standard handset with an
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
Voice encryption devices used with the PRC-77
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 unit, 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
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
in the mid-1980s, Telescurity Timmann (TST) developed the TST-7698 voice
encryptor that used 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
➤ More information
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
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
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'.
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
The battery pack is installed at the bottom of the radio and has a
that should mate with a
connector inside the radio.
The battery pack is protected by a metal cover.
The radio should be used with a standard dynamic handset with an
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.
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.
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
by Timmann Telesecurity (TST).
Other types of brackets were available for the other encryption devices
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, such as
Assiciated Industries (USA), Telefunken (Brasil) and Telemit (Germany).
The device is housed in a watertight enclosure that consists of a die cast
aluminium control planel and an extruded cases 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.
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.
DeviceMilitary VHF/FM transceiver
Frequency30 - 86 MHz
Bands2 (30-52.95 MHz and 53-75.950 MHz)
Spacing50 kHz (25 kHz on some units) 1
Output1.3 to 4 Watts
Power12.5V and 2.5V supplied by battery pack
Antenna 1AT-271A/PRC — 3 m multi-section whip
Antenna 2AT-892/PRC-25 — steel tape bush-whip
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 and
can be recognised by an extra switch (25 kHz) at the front panel.
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© Crypto Museum. Created: Sunday 11 March 2018. Last changed: Friday, 07 July 2023 - 11:12 CET.