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Adapter bug
Professional mains RF bug

The Adapter Bug shown below, was a professional crystal-based FM transmitter 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 Mactron.

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.
Mains plug FM radio bug

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 kilometres.

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.
Close-up of the interior

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.
Advanced version of the Mactron bug

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.

Mains plug FM transmitter Mains plug FM radio bug Opening for tuning and microphone Similar to a power adapter Interior Close-up of the interior
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Mains plug FM transmitter
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Mains plug FM radio bug
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Opening for tuning and microphone
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Similar to a power adapter
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Close-up of the interior

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Crypto Museum. Created: Sunday 19 May 2013. Last changed: Tuesday, 13 June 2017 - 06:47 CET.
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