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CIA NRP EC
The SRT-58 is a crystal controlled 450 mW FM
transmitter that operates in the 280-360 MHz frequency range.
It features with high-quality sound and has a built-in audio compressor.
The device is housed in a 300 × 35 mm Ø watertight cylindrical brass enclosure,
and weights 690 g. It consists of a threaded base part that holds
the electronics and accomodates all connections, and a grey painted case shell,
as shown in the image on the right.
All it needs is a 12V power source (car battery), a sensitive
microphone and an improvised wire antenna.
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The SRT-58 produces an extremely clean well-filtered RF signal, that
was strong enough to be picked up by a following monitoring
vehicle up to a distance of several kilometres (depending on the position
of the antenna). The device does not feature audio masking, as it was
assumed at the time that criminals had no easy way of monitoring the
280 to 360 MHz frequency range. 1
It is unknown how many SRT-58 units were made, but it is likely that
after the final prototype had been delivered to the CIA in 1974,
further units were manufactured by a CIA contractor in the US.
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A few years later the first computer scanners became available from
brands like Handic and Bearcat, that could receive up to 512 MHz.
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The device is polarity-insensitive, which means that the polarity of the 12V
DC power source (from the car battery) may be reversed.
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The SRT-58 is designed for operation with a so-called reluctance microphone,
like the 4000Ω Knowles 1501.
A reluctance microphone is a variation of the dynamic microphone, which
performs better under extreme temperature and humidity conditions.
The microphone should be in acoustical contact with the car interior, and must
be connected to the SRT-58 by means of a (twin) shielded cable.
➤ More information
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Designing an inconspicuous antenna for the SRT-58 was probably the biggest
challenge when covertly installing the device in a car. It was therefore
suggested to modify one of the external mirrors (by isolating it from
the car body) so that it could be used as antenna.
The image on the right shows the door mirror of a 1967 Chevrolet C10, which already has a rubber gasket between its base and the car door. All one has to
do is isolage the three mounting screws from the door chassis.
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The following receivers were used by the CIA for repection of the SRT-58:
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SRR-5 is a general communications receiver.
It was the solid state successor to the SRR-4
and covered the entire frequency range from 50 to 400 MHz in two bands.
Suitable for AM, FM and CW, it was powered from mains or
battery.
➤ More information
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The SRR-8 was the successor to the SRR-5 (above). It was developed around
1960 by the Technical Services Division (TSD) of the CIA and covers the
entire 30 to 1000 MHz range.
Although intended as a countermeasures receiver, it was also used as a general
surveillance reciever. It is powered from the AC mains, or from an internal
battery pack that replaced the mains PSU.
➤ More information
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The SR-209 is modular HF/VHF/UHF/SHF surveillance receiver,
developed around 1965 by ACL. The wide frequency
coverage is possible by using plug-in modules for each sub-band.
For reception of the SRT-58, the SH-203P-1 tuning head is required.
It covers 250 to 500 MHz and is present in the device shown in the image
on the right.
➤ More information
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Below is the block diagram of the SRT-58 transmitter. At the left is the
external microphone of which the signal is amplified and fed to the crystal
oscillator where it is frequency modulated (FM). The fundamental crystal
frequency is then multiplied 18 times, so that it ends up in the 280-360
MHz range. Each multiplier stage can be tuned individually. The signal is
then amplified in the Power Amplifier (PA), low-pass filtered (LPF)
(to remove any residual harmonic frequencies).
From the LPF, the signal is fed to the external antenna via a built-in
directional coupler. The latter is present to measure how much energy is
delivered to the antenna and how much is reflected. Inside the device is
a connector to which an external test box can be connected. It allows the
antenna to be optimised for the chosen concealment, which has a direct
effect on the range.
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The first prototype was ready in February 1967.
It was called CO-EV (car overhearing
evaluation) and weighted 950 grams. It was housed in a 440 mm long
(connectors excluded) cylindrical enclosure with adiameter of 38 mm.
Antenna, microhpone and power were connected to
dedicated receptacles at the bottom of the unit. It covered 320 - 360 MHz
and was powered by the 12V battery of the car under surveillance.
As some cars still had (+) on ground, the polarity of the device could be
adjusted by re-soldering two internal jump wires.
➤ Read the CO-EV manual
Apparently the device was found too large, as in April 1968 a new prototype
of the device — meanwhile named SRT-58 — was released to the CIA for
evalution. This time it consisted of two cylindrical units with a diameter
of 35 mm: a 230 mm long RF unit and a 15 mm long DC and AF unit,
each with their own receptacles at the bottom. The device had an operational
temperature range of 0 to +50°C and weighted 940 grams.
➤ Read the prototype manual
As the two-piece soltion was not acceptable for the CIA, the engineers
went back to the drawing board and reconsidered all design decisions.
Eventually, the final product was released in 1974, as a single 300 mm long
cylindrical unit with a diameter of 35 mm, that contained all electronic
circuits. It was self-adapting for reverse polarity of the battery,
and had solder terminals at the bottom, rather than dedicated receptacles.
In addition, the operational temperature range had been improved: from
-18°C to +70°C.
➤ Read the final manual
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The cylindrical SRT-58 consists of two parts: a threaded base unit that hold
all circuits and connections, and a tubular case shell that is screwed onto
the threaded base unit. A rubber O-ring between the two parts makes it
watertight.
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All connections of the SRT-58 are at the bottom of the device, as
shown in the image below. There are six glass-to-metal feedthroughs in the
bottom part and one flat braided conductor at the circumference. The braided
conductor should be connected to the chassis of the car. In most cases this is
connected the (-) terminal of the car battery. The other battery terminal —
usually the (+) terminal — must be connected to the rightmost contact (1).
The device is also suitable for vehicles in which the (+) terminal of the
battery is connected to the chassis (reversed polarity).
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- Battery
- GND
- Antenna
- GND
- Microphone
- Mic GND
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Device Covert FM voice transmitter Developer NRP Customer CIA Frequency 280 - 360 MHz Modulation FM Deviation 25 kHz Crystal 1/18 of output frequency Impedance 50Ω VSWR 3 or better Harmonics 60dB down or better Spurious 50dB down or better Output ≥ 450 mW (at 12V, +20°C) ≥ 350 mW (at 12V, -18°C or +70°C) Input 500 µV RMS Response 450 Hz - 5000 Hz (-6dB) Masking none Power 11.0 - 16.5 V DC Current < 325 mA Polarity Self-adapting (reversible) Dimensions 300 × 35 mm Ø Weight 690 grams
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- SRT-58 transmitter
- Test and adjustment unit (missing here)
- Operation and test manual
- Laboratory test data sheet
- Final acceptance test data sheet
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© Crypto Museum. Created: Thursday 09 March 2017. Last changed: Monday, 28 November 2022 - 22:43 CET.
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