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← Easy Chair CIA NRP
The SRT-52 consisted of two or three cylindrical modules that contained
the actual transmitter,
the audio masking unit
(also known as the video encoder)
and an optional power supply unit (PSU).
The latter could also be replaced by a set of Mercury batteries
for short-term operation.
The transmitter is adjusted at the factory to a spot frequency in
the 265-315 MHz 1 range.
The image on the right shows a typical SRT-52 set, as used by the CIA.
The units can be identified by their typical blue hamerite exterior,
although the varnish on some may have
faded into green.
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The SRT-52 is very similar to the SRT-56,
which was developed around the
same time, but uses a different frequency range and a
different audio masking scheme.
Development of the SRT-52 was started in 1964,
with the first prototypes of the transmitter available in May 1967 [A].
The device was in production from 1969 to 1971, after which
its TP audio masking scheme was abandoned.
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At the request of the CIA, some SRT-52 devices could be tuned somewhat
outside this range. In addition, it was also possible to use the RF unit
of the SRT-56, which provided the 315 - 385 MHz range.
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The individual modules are housed in a cylindrical brass enclosure
with a diameter of 26 mm and a length of approx. 65 mm.
The modules could be hidden inside a hole of 1 1/8",
that was drilled inside, say, a wooden piece of furniture or a window pane.
All parts are connected by means of high-quality 6-pin
Socapex plugs to a central
connector that is part of the video coder's wiring.
One side of this central connector is
marked SRT,
whilst the other side
carries the marking UWP.
The image above shows all parts of an SRT-52 target element, except
for the microphone, which had to be connected to one of the
BNC plugs. If space was limited, the microphone could also be soldered
directly to the white teflon coax cable. The small red connectors at
the bottom are used for selecting the operating voltage of the
transmitter, resulting in a peak output power of either 100 mW or 400 mW,
subject to the distance between the bug and the listening post (LP).
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- Standard version (290 MHz)
The standard version of the SRT-52 operates in the 300 MHz band.
In a minimum configuration, it consists of an SRK-29 transmitter
and an SWE-52 video coder. When necessary, the SRT-52 could be
made to work somewhat outside its frequency range.
- Alternative version (350 MHz)
In situations where a higher frequency was required, for example
when the SRT-52 caused interference in the reception of TV signals,
the electrically compatible SRK-35
RF unit of the SRT-56 was used.
It supported a frequency range of 315 to 285 MHz.
- High-band version (1500 MHz)
In 1971, the CIA decided to move the operating frequency of
their bugs from 300 MHz to the 1500 MHz band. In the SRT-52
this was done by swapping the SRK-29 RF unit for an
SRK-145,
and replacing the Sleevex antenna
by an SRN-58 plexiglass antenna.
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To hide the RF carrier and its modulation from regular
surveillance receivers,
professional bugs often use a special technique
that is known as
audio masking.
The SRT-52 uses a sophisticated masking scheme based on Pulse Position
Modulation (PPM), known as Triple Pulse (TP) masking.
This masking scheme is characterised by an AM carrier with a rather large
bandwidth (~ 7 MHz) and a multitude of sidebands at either side,
caused by the short square-wave pulses, as shown in the diagram
above. There are currently no known commercially available surveillance
receivers that can readily demodulate an RP-masked signal.
Most receivers won't even lock onto the signal.
➤ More about TP audio masking
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The SRR-52
was released in January 1969, and covers 240 to 330 MHz in one
contiguous range. It has a built-in decoder for recovering the
TP-masked signals. As far as we currently know, it is the only
receiver that supports TP signals.
In 1972, most of the existing SRR-52 receivers were retrofitted with
a modification that made it compatible with the
RP audio masking scheme
used by the SRT-56
and the DP scheme
used by the SRT-91.
Furthermore, its frequency range was extended so that it included
the SRT-56 frequency band and part of the
SRT-91 band.
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The image above shows the SRR-52-M, which is the modified version of
the SRR-52.
The receiver was built in 1968 and was retrofitted with the
modification in 1972, which resulted in an upgrade of the front panel
as well. The bright part is the upgraded section, whilst the rest
has colourized.
➤ More about the SRR-52 receiver
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Detection and discovery of the bug is possible, but is not evident.
As far as we know, there are no commercially available
surveillance receivers that can readily
demodulate a TP-masked signal. Furthermore, existing bug tracers like the
Scanlock
do not lock onto its signal at all.
Finding and locating the bug is possible with a portable spectrum analyzer,
such as the
Rohde & Schwarz FSH-3,
and with a modern monitoring receiver like
the R&S PR-100 shown on the right.
➤ Read the full story
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A complete SRT-52 transmitter consists of one or more of the following items:
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This is the SRK-29 transmitter 1 that operates on a fixed frequency
between 265 and 315 MHz, driven by a pulse shaper, that in
turn is driven by extremely short pulses from the video encoder.
The pulse shaper ensures that all pulses are of the same amplitude
and length (approx. 0.5 µs).
The RF unit is powered by a DC source between +5V and +12V, and consumes
between 1.5 and 5.5 mA. It produces a peak output power of 100 to 400 mW.
It can be driven by an SWE-52,
SWE-56 or SWE-56-C video coder.
In 1971, it was replaced by the SRK-145 for use at 1500 MHz.
➤ Look inside the SRK-29
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In SRK-29, the suffix 29 indicates the center of its frequency
range, i.e. 290 MHz.
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In situations were the SRT-52 could be powered from the mains, the UWP-52 1
power supply unit (PSU) could be used. Like the RF unit, it is housed in
a cylindrical brass enclosure, and is cast in epoxy. Inside the cylinder is
a miniature toroid transformer that is suitable for 110V and 220V AC mains
networks.
Powering a bug from the mains, virtually gives it an endless life, but
increases the chance of discovery. For this reason a
QRR-25 switch receiver
was sometimes added to the setup.
➤ More about the PSU
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In situations where it was not possible to power the SRT-52 from
the mains, a series of stacked long-life Mercury cells was sometimes
used. Although this reduces the operational life of the bug, it make
it's installation a lot easier.
Mercury cells use a reaction between mercuric oxide and zinc electrodes
in alkaline electrolite, and deliver 1.35V per cell [2]. When using four
stacked cells, the battery provides 5.4V, which remains practically constant
during discharge. Due to the presence of toxic elements,
mercury batteries are now banned in most countries [2].
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Although the SRT-52 can be used with virtually any type of
sensitive dynamic microphone, it was commonly used in combination
with a Knowles BA-1501
or BA-1502 element.
Measuring just 10 x 10 x 5 mm, it was one of the smallest
dynamic microphones available. It has an excellent
dynamic behaviour and a good frequency response
curve, and was commonly used in military equipment for many years.
➤ More information
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The SRT-52 was commonly used in combination with a so-called
Sleevex antenna,
which was also developed by the NRP.
Made from a piece of rigid coax cable, Sleevex antennas
were available for a variety of frequency ranges.
Furthermore, different types of Sleevex antennas were available
for embedding in a variety of environments, such as wood and concrete.
➤ More information
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High-band version
SRK-145
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In 1971, following a series of studies into the use of the higher frequency
bands for covert listening devices, the SRK-145 RF unit was introduced
as an alternative for the existing SRK-29 units.
It is somewhat shorter than the existing ones,
and is normally painted in the same colour as
the SRT-52. The one shown here is a laboratory model, which is why it is beige.
The SRK-145 works on a spot frequency in the 1300 - 1600 MHz band
and requires the use of the SRN-58 antenna instead of the
Sleevex.
➤ Look inside the SRK-145
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When the SRK-145 high-band RF unit was used, the
Sleevex antenna
had to be swapped for the small end-fed vertical dipole shown
in the image on the right. The antenna is fore-shorted and is embedded
in a plexiglass (perspex) stick that has the same diameter as the
SRT-52 modules.
The plexiglass SRN-58 antenna was also used
with the high-band version of the SRT-56
and with the later integrated SRT-107
transmitter.
➤ More information
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Showing the interior of the SRT-52 is difficult, as all of its modules
were soldered hermetically. Furthermore, the video encoder and the
PSU are fully cast in black epoxy, making it impossible to show
the interior. Nevertheless we are able to show the interior of some
laboratory models below.
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The standard RF unit SRK-29 is the only module of the set, that is not cast in
expoxy, because of the dielectric effects of the material on the RF components.
Furthermore, it has to be possible to adjust the transmission frequency
from the outside of the case. Instead, the components are protected against moist
by a conformal coating.
The image on the right shows the interior of the RF unit of an SRT-52.
At the centre is the main oscillator, built around a
TR1062 transistor. The transistor is directly
coupled to the tuned circuit that determines the bug's operating frequency.
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The oscillator is keyed by a pulse-shaper that is located at the top of the
unit (assuming that the wiring is at the bottom), and is built around a
2N3866 transistor. It is driven by the modulated pulses from the video encoder,
and shapes them so that they are of equal length and amplitude.
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The most complex part of the SRT-52 is without doubt the video coder.
It converts the sound that is picked up by the
microphone, into a series
of seemingly random pulses that are used to key the RF unit.
This is known as audio masking.
The video encoder is housed in a brass cylinder, roughly the same size
as the transmitter, and contains five electronic circuit blocks,
known as cordwood structures.
The entire unit is cast in an elastic substance and is fixated inside
the brass cylinder at both ends with a strong black epoxy,
due to which the SWE-52 cannnot be serviced.
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For this photograph, we've removed the enclosure from a broken SWE-52
video encoder, so that we can see how it is constructed internally.
The five cordwood circuit blocks are clearly visible and are separated
by isolated discs, that have cut-outs for the wirings. A long blank wire,
running over the full length of the assembly, provides ground (0V)
to each of the cordwood modules.
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The power supply unit (PSU) was initially much longer than the
RF unit and the video encoder, but was later redesigned to fit
inside the same size enclosure.
As the entire PSU is cast in black epoxy, we are
unable to show its contents, but from the documentation,
its circuits are known.
At the heart of the PSU is a miniature toroid transformer that is
connected directly to the mains network. It has separate windings
for 110V and 220V AC.
It was probably made by the Radio Corporation of America, especially
for the UWP-52. It came with an individual test sheet.
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The transformer had to be this small in order to be fitted inside
a standard brass cylinder that has an inner diameter of 25 mm.
The rest of the PSU's circuit is placed on
a single cordwood module.
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Like the PSU, the battery provides two voltages, in order to allow adjustment
of the transmitter's output power between 100 mW and 400 mW.
In 1971, new frequencies in the 1500 MHz band were allocated for covert
listening devices, and the SRK-145 module was developed to replace the standard
SRK-29 transmitter.
The image on the right shows a lab model of the new SRK-145.
Due to the higher frequency, the tuned circuit
is smaller, as a result of which the entire unit is shorter.
The cordwood structure at the right
provides the -20V oscillator keying voltage.
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The 1500 MHz RF unit was also used as a replacement for the 350 MHz
RF-unit of the SRT-56,
where it was combined with the SWE-56 video coder.
It was later used as the basis for the design of the
SRT-107 transmitter,
in which the RF unit was integrated with the
SWE-56 video coder.
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- Manual for SRT-52 and UWP-52 prototype equipment
CM302489/A, May 1967.
- Operating Manual for SRS-52 Equipment
CM302489/B, October 1968.
- Technical Manual for SRS-52 Equipment
CM302489/C, October 1968.
- Operating Manual for SRS-52 Equipment
CM302489/D, December 1968.
- Technical Manual for SRS-52 Equipment
CM302489/E, December 1968.
- Operating Manual for SRS-52 Equipment
CM302489/F, January 1969.
- Technical Manual for SRS-52 Equipment
CM302489/G, January 1969.
- Operating Manual for SRS-52 Equipment
CM302489/H, October 1969.
- Technical Manual for SRS-52 Equipment
CM302489/I, October 1969.
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- NRP/CIA, Collection of documents related to SRS-52
Crypto Museum Archive, CM302489 (see above).
- Wikipedia, Mercury battery
Retrieved, April 2017.
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© Crypto Museum. Created: Monday 10 April 2017. Last changed: Friday, 05 January 2018 - 22:26 CET.
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