Professional mains RF bug
The Adapter Bug shown below, was a professional crystal-based
that was built completely inside the plastic enclosure
of a mains power adapter. It was powered directly from the AC mains and
contained a sensitive microphone. The unit was probably built by
The image on the right shows a typical example of such a bug.
It is built inside the case of a standard power adapter, similar to the
ones that are usually supplied with domestic equipment, such as portable
radios, calculators, mobile phones, cameras, portable flashlights, etc.
Inside the case however, is a sophisticated FM radio transmitter with
a sensitive microphone that can pick up any conversations in the room.
The unit is powered directly from the mains, using a built-in power supply
unit (PSU). A thin black 50 cm wire is used as the antenna.
The bug was professionally built and was extremely stable, due to the
fact that it was driven by a quartz crystal. As it uses narrow-band
frequency modulation (NBFM) it uses its RF power more efficiently than
similar wide-band transmitters, resulting in a range of several
The image on the right shows the interior of the Adapter Bug.
As the entire circuit is covered in hard brown epoxy, we can't really
see any details of the design. As there are no serial numbers, logos
or other manufacturer marks on the device, we can't determine the identity
of the manufacturer with absolute certainty either.
After closely examining the interior of the bug, and by knowing the route
by which this bug ended up in our collection, it seems safe to assume that
it was built by Mactron Electronics
in Oosterbeek (Netherlands) in the early 1990s.
Looking at the image above, the design can be split in two halfs along
the long axis. The front half contains the actual transmitter with the
crystal roughly in the middle (sticking out of the epoxy). To the left
of the crystal is the RF output section with some adjustments for tuning.
The microphone pre-amplifier must be to the right of the crystal,
as the grey wire to the microphone comes out of the epoxy at the far right.
The layout and size of this circuit is nearly identical to the
Mactron Mark II Bug. An example of such a bug
is shown in the image on the right.
The rear half of the unit contains the mains power supply unit,
with three blue electrolytic capacitors just visible through the epoxy.
As the unit is rather old now (more than twenty years)
the capacitors have degraded somewhat, causing a typical 50Hz
hum in the transmitted signal.
During the 1970s and 1980s, many FM transmitters built inside adapter
enclosures, similar to the one shown here, were sold in electronics shops
around Europe. Most of these however, contained a
simple free-running FM bug
that was powered directly from the mains by means of a series capacitor.
Such transmitters were generally very unstable and suffered badly from the
so-called hand-effect. They were often used as Baby Monitors or as
simple eavesdropping transmitters.
Although the Adapter Bug shown here looks just like the cheaper alternatives,
it is in fact a professional transmitter. Rather than operating in the
FM broadcast band (88-108 MHz) using wide-band modulation, it works in
the 2-meter band (135-174 MHz) in narrow-band FM. Bugs of this type were
often used as eavesdropping transmitters as they could easily be installed
in, say, a home or an office and did not stand-out between the other adapters
behind the stereo.
Adapter Bugs were also used as training devices for (bug) sweep teams
when learning how to use a Scanlock receiver
or to keep their skills up to date. One colleague would then hide the training
bug somewhere in the building, whilst the other one had to search for it.
The Adapter Bug should not be confused with a
Power Line Bug, such as the Audiotel MCX.
These bugs, also known as Mains Carrier Bugs, are not RF devices but
use the mains power lines as transmission medium.
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 19 May 2013. Last changed: Tuesday, 13 June 2017 - 06:47 CET.