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RT-3R   ST-3, SRT-3
Solid state surveillance transmitter · 1959 - this page is a stub

RT-3R was a solid-state 1 covert listening device, also known as a surveillance transmitter or a bug, developed around 1959 by the Technical Services Division (TSD, later OTS) of the US Central Intelligence Agency (CIA). It was an improved version of the RT-3 transmitter [A], which in turn was the successor to the ST-2A. The device operates in the VHF-L band (56 to 84 MHz) and uses Frequency Modulation (WBFM). Confusingly, the device is also known as ST-3 and as SRT-3.

The bare transmitter measures just 97 x 45 x 19 mm and weight no more than 170 grams. It is housed in an aluminium enclosure with black wrinkle paint finish. Power is provided by an external battery pack or by the purpose-built ST-3PS mains PSU. The transmitter was supplied with a Shure MC-30 dynamic microphone, attached to the end of a long shielded cable.

The circuit is nearly identical to that of the ST-2(A), except that the 1AD4 valves in the RF-stages have been replaced by 2N500 transistors. The image on the right shows a complete kit.
Complete RT-3 kit as shown in the manual [A]

The ST-3PS power supply unit is now shown in this image, but is fully described in the manual [A]. A switch receiver, or Latching Radio System — LRS-1 — was later tested with the RT-3. It allowed the transmitter to be remote controlled, making it more difficult to discover.

There seems to be some confusion in the nomenclature of this device. Initially, it was assigned the designator ST-3, in line with earlier surveillance transmitters like the ST-1 and ST-2A. At that point, the unit still had a built-in AC mains PSU. The name BT-2 was used for a battery powered variant. Along the project, it was renamed RT-3, (Radio Transmitter). It is likely that the prefixes 'ST' and 'RT' were later merged into 'SRT', which stands for Surveillance Radio Transmitter.

  1. In this context, solid-state means that it was built with transistors rather than valves (tubes).

  • Device
    Covert listening device (bug)
  • Purpose
    Room overhearing
  • Developer
    Technical Services Division (TSD) of the CIA
  • Model
    RT-3R, ST-3, SRT-3
  • User
  • Frequency
    55 to 81 MHz (see below)
  • Modulation
  • Sensitivity
    1 to 3 kHz/100µV
  • Response
    500 - 5000 Hz ±1dB
  • Hum/noise
    500 Hz max.
  • Microphone
    Shure MC-30
  • Output
    ≥ 7 mW into 50Ω load
  • Power
    ≤ 65 mW
  • Voltage
    6.5V ±0.5V
  • Current
    ≤ 10 mA
  • Antenna
    ¼λ whip
  • Dimensions
    97 x 45 x 19 mm
  • Weight
    170 g
  • Finish
    Black wrinkle paint
  • Quantity
    305 (est.)
  • Green
    55 - 63 MHz
  • Blue
    64 - 72 MHz
  • White
    73 - 81 MHz
Battery pack
  • Batteries
    5 × Mallory RM-12R cells connection in series
  • Exterior
    Carboard case.
  • Lifetime
    ≥ 360 hours
  • Wiring
    Cable for connecting further battery packs in parallel
Mains PSU   RT-3PS
  • Input
    105/210V AC ±20%, 50-60 Hz
  • Output
    6.5V DC ±0.5V
  • Current
    0 - 15 mA
  • Ripple
    ≤ 0.5%
  • Dimensions
    132 x 22 x 24 mm
  • Weight
    285 g
  • Finish
    Black wrinkle paint
  • Green
    55 - 63 MHz
  • Blue
    64 - 72 MHz
  • White
    73 - 81 MHz
  1. Operating and Maintenance Manual for Transmitter RT-3R and PSU RT-3PS
    CIA, Date unknown. Copy 181 of 305. 1
  1. Declassified in part and approved for release by CIA on 29 May 2012.

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Crypto Museum. Created: Tuesday 22 September 2020. Last changed: Monday, 28 August 2023 - 14:36 CET.
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