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SRT-58
Car overhearing system - this page is a stub

SRT-58 is a covert listening device (bug), developed in 1974 by the Dutch Radar laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract code­named Easy Chair. The device was intended for overhearing conversations in a car whilst driving around, followed at some distance by a mobile listening post with a VHF/UHF FM surveillance receiver.

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.
  
SRT-58 car overhearing transmitter

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.

  1. A few years later the first computer scanners became available from brands like Handic and Bearcat, that could receive up to 512 MHz.

SRT-58 car overhearing transmitter
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SRT-58 car overhearing transmitter

Compatible receivers
CIA surveillance receiver SRR-5
CIA surveillance and countermeasures receiver SRR-8
ACL SR-209 HF/VHF/UHF/SHF surveillance receiver
Features
...

The device is polarity-insensitive, which means that the polarity of the 12V DC power source (from the car battery) may be reversed.

Parts
Transmitter   SRT-58
...   
SRT-58 car overhearing transmitter

Microphone
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

  
Knowles BA-1501

Power supply
...   

Antenna
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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Receivers
The following receivers were used by the CIA for repection of the SRT-58:

SRR-5
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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SRR-8
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

  
XRR-8 (SRR-8) CIA surveillance receiver

ACL SR-209
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

  
SR-209 surveillance receiver

Block diagram
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.


History
...

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


Interior
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.

...

Connections
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).

  1. Battery
  2. GND
  3. Antenna
  4. GND
  5. Microphone
  6. Mic GND
Specifications
  • 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
Deliverables
  • SRT-58 transmitter
  • Test and adjustment unit (missing here)
  • Operation and test manual
  • Laboratory test data sheet
  • Final acceptance test data sheet
External requirements
Documentation
  1. Manual for CO-EV. Equipment
    NRP/CIA, February 1967.

  2. Manual for SRT-58 Protype Equipment
    NRP/CIA, April 1968.

  3. Operation and Test Manual for SRT-58 Transmitter
    NRP/CIA, August 1974.
Further information
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© Crypto Museum. Created: Thursday 09 March 2017. Last changed: Monday, 28 November 2022 - 22:43 CET.
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