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Professional radio bug

The S-35 was a small professional crystal-based eavesdropping device (bug), developed and made in the early 1990s by ATET in Turin (Italy). The device was intended for use by governments world-wide, but was also sold to authorized private customers. It was succeeded by the S-353.
The image on the right shows the bare S-35 transmitter. It is housed in the metal case of a large crystal. At the top are the antenna socket and a screw that hold the unit together. At the bottom is a 4-pin connector to which the 9V DC power and the microphone can be connected.

The S-35 is suitable for a variety of applications, ranging from body worn covert operations to eavesdropping (bugging) and even tracing of a person or object (homing). It can be powered by a battery, an (optional) transformer or (with a special adapter) the 12V battery of a car.
S-35 radio bug

The S-35 is driven by a crystal which makes the transmitter extremely stable. The audio signal from the sensitive electret microphone with built-in pre-amplifier is fed into the modulator. The modulated signal is mixed with the crystal oscillator and multiplied to the desired frequency.

The signal is then amplified and filtered before it leaves the transmitter through the antenna. The transmitter has a bandwidth of approx. 25 kHz and can be received with a professional intercept receiver, such as the Rohde & Schwartz EB-100. The S-35 was succeeded by the S-353, which is still available from ATET today (2013).
S-35 transmitter amidst a number of adapters S-35 radio bug ATET S-35 professional radio bug S-35 seen from the top, with wire antenna S-35 seen from the bottom, with connected wire antenna Interior of the ATET S-35 S-35 interior (top side) S-35 interior (bottom side)

Controls and connections
The image below shows the S-35 as seen from the front side. At the left is the antenna socket and the screw that keeps the case closed. At the right is the 4-pin male connector to which the various cables and adapters can be connected. Above this connector is the internal microphone.

The S-35 can be used in the following applications:
  • Battery operated bug
    A standard 9V block-type battery can be connected by using the S-35-B cable. This is the basic mode of operation of the S-35 in which the internal microphone, located aside the connector, is used. The internal microphone is looped back with a link in the connector.

  • Car battery operation
    The S-35 can be powered by the 12V battery of a car by using the S-35-AU adapter cable. The adapter converts the 12V battery voltage to 7.5V DC for the transmitter. In this mode, the unit draws approx. 100mA.

  • Homing
    Rather than eavesdrop on a conversation, the S-35 can also be used to follow or trace a vehicle, using the transmitter as a homing device. Being connected to the car battery, the S-35 can be powered for a virtually unlimited time. The car can be traced using a professional direction finder.

  • Body worn operation
    Instead of the sensitive internal microphone, the S-35 can also be used with the S-35-M external microphone. This small unit connects to the 4-pin connector of the S-35 and has a 55cm lead with a small electret microphone at the end, plus a short lead with a 9V battery clip. If Voice Operated Transmission (VOX) is needed, the small S-35-VX adapter can be connected in between the S-35 and the S-35-M.

  • Wire tapping
    The S-35 can be connected directly to an analogue telephone line for eavesdropping on phone conversations, using the S-35-AT adapter. In this mode, the S-35 uses the telephone line for three purposes: alongside the conversation, it 'steals' power from the line and uses it as antenna at the same time.

S-35 transmitter amidst a number of adapters S-35-B battery adapter S-35-M external microphone S-35 transmitter and S-35-M external microphone S-35-AT analogue telephone line adapter S-35 transmitter and S-35-AT telephone adapter Microphone with extra long wire S-35 seen from the top, with wire antenna

The ATET S35 is housed in the metal casing of a large crystal. The case shell is held in place by a single screw on the top surface (i.e. the same surface that holds the antenna socket). After this screw is removed, the case shell can be taken off, revealing a tiny printed circuit board (PCB).
The PCB has electronic components on both sides and is fitted to the black plastic bottom of the enclosure. The image on the right shows one side of the PCB, on which the quarz crystal and the Knowles microhone are clearly visible. Also visible in this image is an LM358 dual OpAmp.

The other side of the board holds the actual transmitter, which consists of various coils, capacitors and transistors. Wherever possible, surface-mount components (SMD) are used. Power and accessories should be connected to the 4-pin header at the plastic bottom plate.
S-35 interior (bottom side)

Considering its age, the ATET S-35 is very well built, with first-class professional components. Unlike many of its competitors however, it does not feature audio masking, which makes it less suitable for government-type covert listening operations. As it features continuous wave (CW) transmission only, it can easily be detected with professional bug tracers, such as the Scanlock.
Interior of the ATET S-35 S-35 interior (bottom side) S-35 interior (top side)

Technical specifications
  • Power
    5-10V DC
  • Current
    19-34 mA
  • Output
    10-60 mW
  • Impedance
    50 Ω
  • Spurious
    < 30 dB
  • Frequency
    150-174 MHz (105-250 MHz on special order)
  • Microphone
    internal and (optionally) external
  • Dimensions
    18 x 20 x 8 mm
  • S 35 B
  • S 35 AL
  • S 35 AU
    Adapter for car battery
  • S 35 M
    External microphone
  • S 35 AT
    Telephone adapter
  • S 35 VX
    Voice operated transmission (VOX)
About ATET
ATET is an electronics company that develops, manufactures and sells equipment, systems and solutions in the field of communications and security, for both private and government use. The company is based in Torino and Rome (Italy) and sells to customers in Italy and world-wide.

Since 1960, ATET has specialized in the development and production of miniature and subminiature transmitters for surveillance and covert listing, mainly intended for use by the Army and Goverment Agencies. Products like the S-35 were produced during the 1980s and were sold by the Security Division of ATET. ATET is still in business today and its Security Division is selling the lates generation of remote controlled programmable radio bugs. Information about such devices is available only to authorized customers.
  1. ATET S-35 Operational Manual
    Date unknown.

  1. A.T.E.T. website
    Retrieved April 2013.

  2. A.T.E.T., Operational Manual S-35
    7-page instruction manual with drawings. Date unknown.

Further information

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Crypto Museum. Created: Monday 29 April 2013. Last changed: Thursday, 30 March 2017 - 18:13 CET.
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