The SRT-107 consists of a fairly large cylindrical transmitter and a partly
transparent SRN-58 antenna.
The two parts are connected via a 25 cm fixed
coaxial cable. A power source and a suitable microphone should be connected
to the flying lead with a 5-pin connector at the end.
Contrary to early Easy Chair bugs,
the SRT-107 is a so-called active target element (ATE),
which means that it is powered locally from batteries,
the mains or via a telephone line.
The device operates in the 1500 MHz band
and features sophisticated masking of speech modulation.
The unit is powered by an external 5.2V DC source and consumes typically
6.5 mA when in full operation. Yet it delivers an output power of 200 mW, due
to the fact that it uses Pulse Position Modulation (PPM). Speech information
is masked by using a noise generator to randomly reject some of the pulses,
as a result of which the transmitter appears to be producing random noise.
The diagram below shows a typical SRT-107 unit, without an external microphone.
At the right is the actual transmitter which is housed in a hermetically
sealed brass cylinder, that is covered in a strong (green) two-component epoxy
paint. Inside the cylinder is a fairly large RF section plus six so-called cordwoord modules that contain the electronic circuits. At the bottom of the
cylinder is an endplate with three hermetic glass feedthroughs to which the
cables are connected. In order to protect the internal circuits against
corrosion, the cylinder has been filled with dry nitrogen.
At the top is the SRN-58 antenna
which is connected to the transmitter by means of a fixed 25 cm long
teflon coax cable. It consists of two brass pipes that form an end-fed
1/2λ dipole with vertical polarization. The antenna is cast in a
solid 26 mm thick transparent epoxy cylinder. The dimensions of the antenna
have been compensated for the dielectric effects (εr)
of the cylinder.
A flying lead is provided for connection of a power source and an external
microphone. At the end of the flying lead is a 5-pin quick-release connector
for testing and installation.
The unit is powered by a 5.2V DC source and consumes approx.
6.5 mA when in full operation. It provides a peak output power of 200 mW.
A recommended miniature microphone is the Knowles 1501.
A complete setup typically consists of the following parts:
This is further illustrated in the diagram below. At the left is the
SRT-107 transmitter with its fixed SRN-58 antenna. In the example it is
powered by batteries. The transmitter could be further extended by adding
an (optional) receiver module so that it can be ON/OFF controlled remotely.
At the Listening Post (LP), the signal from the SRN-55 antenna is first
converted from 1300 MHz to 300 MHz and then fed to an SRR-56 or
SRR-91A receiver, which is capable of recovering the masked audio. The output
of the receiver is typically fed to a pair of headphones or a recorder.
Although the SRT-107 is suitable for virtually any type of dynamic microphone,
it was commonly used in combination with a Knowles 1501 miniature element,
which measures 10 x 10 x 5 mm. This might seem rather
large by today's standards, but was really state-of-the-art at the time.
The microphone has a frequency range of 400 to 3500 Hz and was also used
in military applications, such as headsets, at the time.
The Knowles 1501 is also known as BA-1501 and by its National Stock Number
The SRT-107 transmitter comes with a fixed 1500 MHz SRN-58 antenna that is
embedded in a plexiglass stick. The stick has the same width as the
transmitter itself so that it can be fitted inside the same concealment
in a 1 1/8' hole.
The antenna is suitable for the entire 1300 to 1600 MHz range and its
dimension have been compensated for the dielectric effects of its
➤ More information
Despite the fact that the SRT-107 is difficult to detect, it seems likely
that eventually the Russians were able to intercept and locate them.
At a press conference in Washington on 10 April 1987, the Soviets presented
a range of bugging devices that had been found during the past week in the
walls of their Washington Embassy, claiming they had been planted there
by the Americans.
The photograph on the right  was taken at the press conference and shows
two Russian experts behind a table, explaining to the press what they had
On the wall behind them are several panels with photographs and real samples
of the equipment that was said to be planted by the US.
At the bottom of the second panel, highlighted here with a red circle, are
four cylindrical devices that are interconnected by a bunch of wires.
It is very likely that this is one of the predecessors of the SRT-107.
In any case it is an obsolete device that could not have been planted there
In seems likely that the Soviets wanted to amplify their claim by exposing
all types of bugs that had been found in their walls over the years.
The image quality is too low to determine the actual model of the bug,
but luckily a close-up appeared in a television newscast that evening .
The image on the right was taken from that newscast. At the far right we
clearly recognise the SRN-58 antenna,
which was typical for many SRT
transmitters used by the CIA. To the left of the antenna is a long cylinder
which probably contains four long-life Mercury battery cells.
The smaller black cylinder (second from the left) contains the RF transmitter,
whilst the leftmost one is probably the video coder that masks the audio
modulation. In the SRT-107 these two modules were combined. It is also possible, that the leftmost module contains a receiver that was used to turn
the transmitter ON and OFF at will.
Either way, it proves that the NRP-built CIA bugs were known to the Soviets
in 1987, and also that they had been used by the Americans against the Soviets
in Washington at some point. However, if that was the case, they were installed
there many years earlier, as by the time of the press conference this
particular model, and many of its successors, were no longer in production.
The basic operation of the SRT-107 is explained in the block diagram below.
At the far left is the microphone pre-amplifier which has a built-in Automatic
Gain Control (AGC) and dynamic range compressor. The output of the amplifier
is fed to the so-called video-coder. At the far right is the RF
oscillator/transmitter which is ON/OFF controlled by the output pulses
of the video-coder.
At the center is the video-coder which is responsible for masking the audio
signal. It starts with a Pulse Position Modulator (PPM) in which the signal
from the audio amplifier is used to modulate the phase of the trigger pulses
provided by the clock oscillator. A noise generator is used to
randomly reject pulses from the resulting pulse-train, which results in a
randomly varying output pulse rate, that resembles a noise pattern, similar
to the background noise of a radio channel.
A pulse shaper is present to ensure that the output pulses are of uniform
length and amplitude, before they are passed on to the keyer/booster, which
turns the RF oscillator ON and OFF at the pulse rate. The booster converts
the +5.2V DC supply into -20V pulses, allowing the transmitter to produce
a peak power of 200 mW. Due to the noise-resembling pulse pattern,
the transmitter produces a masked signal that will defeat
demodulation in a non-compatible intercept receiver.
Although the transmitter's RF oscillator is properly matched to the
in practice there will always be reflections of some kind.
This the case for example, when the antenna is positioned close to a metal
object and a significant amount of the energy is returned to the transmitter
where it must be dissipated. In reaction to this, the transmitter will
consume more power in order to overcome the returned energy.
This may potentially damage the transmitter.
In transmitters like the SRT-107,
this is solved by inserting an
between the output of the
transmitter and the antenna. An isolator is in fact a 3-port
circulator of which the return port is connected to ground, as illustrated
in the diagram above . In a circulator, the energy is always delivered to
the next port. The energy from the transmitter (1) is delivered at the antenna
(2), but the energy returned from the antenna (2) is delivered at port (3)
which is connected to ground.
The interior of the SRT-107 is not easily accessible, as the
entire unit is mounted inside a hermetically sealed brass cylinder that is
covered in a strong two-component expoxy paint. Furthermore, the circuits
inside the nitrogen-filled cylinder are covered in a conformal coating.
Inside the cylinder are six small circular cordwood 1
building blocks, plus a larger one that contains the RF section (the actual
Each of the cordwood structures
consist of two circular epoxy PCBs with
electronic components fitted inbetween. They are similar in design to the
so-called FLYBALL modules
that were used by the NSA during the 1950s
in cipher machines like the KW-7,
The image on the right shows the RF section (right) and one of the cordwood
modules (left). The remaining five cordwood structures are not shown
in the image.
The cordwood structures are connected together by means of flexible and rigid
wires between them. Together they form the complete circuit of the SRT-107.
The RF section
is built around an HP 35831B microwave transistor that was
introduced in 1971 as a military OEM component .
It forms a free-running
oscillator that is pulse-driven by the video coder.
At the far right is a rather large isolator
that is responsible for an optimum matching to the externanal
SRN-58 antenna .
It also makes the device insensitive to the so-called hand effect 2 ,
which makes it therefore more difficult to detect its presence.
The image on the right shows the rear side of the RF section, where two
capacitive trimmers are visible: a larger one
that is used for setting the transmission frequency, and a smaller one
for adjusting the optimum antenna loading.
The bottom of the entire structure consists of a
metal disc with 3 hermetic glass feedthroughs,
to which the external wires are soldered.
After placing the construction inside the outer cylinder, the edges
of the disc are soldered hermetically. Through a small hole in the metal disc,
the cylinder is later filled (under vacuum) with dry nitrogen, after which
the small hole is soldered as well.
In a cordwood construction, the electronic components are mounted vertically
between two parallel panels or printed circuit boards (PCBs).
In a free-running oscillator, the resonant frequency will vary when it is
aproached by, say, a hand. This so-called hand-effect was often used
to locate listening devices (bugs) by observing the frequency shift on a
spectrum analyser whilst moving the hand over the suspected area.
Supply+5.2V DC (max. 6V)
CurrentTypically 6.5 mA (max. 8 mA)
Frequency1300 - 1600 MHz (factory set)
Grid30 MHz (1305 - 1335 ... 1515 - 1545 MHz)
Accuracy± 7.5 MHz at 5.2V DC and 25°C
Drift< 10 MHz (0°C - 60 °C)
Output+24 dBm ±1.5dB peak (~200 mW)
Pulse width0.5 ± 0.1 µS
Duty cycle1.2 ± 0.35 %
Sensitivity500 µV RMS into 4000 Ω
Response300 - 5000 Hz (-3dB)
AntennaShortened asymmetrically fed ½λ dipole in dielectric material
VSWR< 2.5 (various environments, air - concrete)
PatternDoughnut pattern, vertical polarization
Gain0 dB omnidirectional
Active Target Element
Active bugging device that has its own power source.
Some ATEs can be controlled remotely and some have full masking
of its audio signal.
Pulse Position Modulation
Surveillance Radio Transmitter
Common abbreviation used by the CIA to identify surveillance transmitters
(bugs). Often used as part of the model number.
Surveillance Radio Receiver
Common abbreviation used by the CIA to identify surveillance receivers.
Often used as part of the model number.
- Operation and Test manual for SRT-107 Transmitter
July 1974. Confidential. #CM302454
Any links shown in red are currently unavailable.
If you like the information on this website, why not make a donation?|
© Crypto Museum. Created: Sunday 01 January 2017. Last changed: Wednesday, 11 January 2017 - 07:53 CET.