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BND AID
Covert flat-pack transmitter
TX-873 is an embeddable covert listening device (bug) for
the VHF-M band (150-174 MHz), developed around 1986 by
Audio Intelligence Devices
(AID) in Fort Lauderdale (Florida, USA). It is a transmitter with
a flat construction — it is just 7 mm high — that was intended for concealment
in regular objects, such as behind the false lid of a briefcase.
The device shown here was used by the German intelligence service BND, in
the fight against corruption by government officials [1].
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The custom-made device consists of a large printed circuit board (PCB)
that acts as a carrier for a transmitter, a battery pack and an antenna.
The whole package is not thicker than 7 mm, which allows it to be hidden
easily in any type of concealment, such as behind the false bottom or
lid of a Samsonite executive style briefcase.
The device is powered by a Polaroid P100 or P500 flat-pack battery, which
was the only battery at the time that was thin enough for this type of
concealment. At the time, it was a commercial-of-the-shelf (COTS)
camera battery.
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The actual transmitter was a standard product of AID at the time,
which was frequently used in custom-made solutions and concealments.
Its design is very similar to that of the contemporary
TX-775 body-wire,
except that all parts are placed on one side of the board rather using
a double-sided construction. This was done to reduce the overall height
of the listening device.
The device shown here, was used by the
Bundesnachrichtendienst (BND)
— the German Federal Intelligence Service — in its
fight against corruption by government officials [1]. It is likely that it was
hidden inside a briefcase or a backpack, in order to overhear compromising
conversations.
The small internal microphone is extremely sensitive and produces an
exceptionally clear audio signal. The device has an output power of approx.
200 mW. In combination with the antenna on the carrier board, this should
give it a range of ~ 1 km under ideal conditions. In case a larger distance
was required, a mobile repeater like the LEA 6369 could be added
to the setup.
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The image below provides a overview of the features of the TX-873.
At the bottom is a large PCB that measures 250 × 96 mm. It acts
as a carrier for the transmitter (left), the battery (right) and
the antenna (front). The latter is implemented in the form of tracks
on the PCB. The transmitter is a standard AID design that was
also used in other contemporary bugs, such as the TX-775. It had been
redesigned to fit all components on one side of the board in order
to reduce its height.
To reduce the height even further, the transmitter board is fitted
in a cut-out space of the carrier board. Furthermore, it is covered
by a metal RF shield, so that its performance is not influenced
by nearby objects. At the bottom of the carrier board (in the far
corners) are thin push-buttons: one for turning the device ON and
one for turning it OFF. When the device was hidden behind the lining
of the lid of an executive-style briefcase, the buttons could be
operated through the lining.
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The diagram below shows how the TX-873 is constructed.
The large PCB is the carrier board, which was designed especially for
this application. It has a rectangular cut-out in which a universal
transmitter board is placed. This transmitter was also use in other
types of concealment. At the bottom, the transmitter is
soldered to the carrier board.
At the top, it is covered by a metal RF shield, with
six holes for accessing the various trimmers. A seventh hole is for the
microphone.
At the right is a cradle for the battery. It is constructed from three
metal rigs that hold the battery in place. Two curved contacts on the
carrier board mate with the (+) and (-) terminals of the battery.
At the bottom of the carrier board – in the far corners – are two
fairly large integrated tactile push-buttons .
They are used for turning the device ON and OFF respectively.
The complete construction is less than 7 mm high.
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The image below shows the TX-873 after the metal RF shield has been
removed from the transmitter. The large PCB is the carrier board,
which was made especially for this application.
The area within the yellow rectangle is the actual transmitter.
It is placed in a same-size cut-out in the carrier board.
The left half of the carrier board is taken by the battery cradle.
The area in front of the transmitter is for additional components,
which allows the device to be customised for specific applications.
With the device featured here, an improvised power circuit has been added
at the right. It sits between the (+) terminal of the battery and the transmitter
(via a thin red wire). It comprises a power regulator and some filtering.
It is not clear whether this is part of the original
design, or whether it was an aftermarket modification by a former user.
The small foam blob at the left side of the transmitter holds a
miniature Knowles electret microphone, which produces a remarkably
clear sound. The signal from the microphone is amplified in an
LM324 and used to modulate the crystal oscillator by means
of a varicap diode. At the right is the RF power amplifier (PA),
which is built around a Telefunken BFR96 transistor.
After filtering, the signal from the PA is supplied to the antenna,
which is integrated with the carrier board.
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When we received the TX-873 featured here in March 2026, it was in
unknown condition but probably working. It came with three Polaroid
P100 batteries, which were (of course) dead after 40 years in
storage. Luckily it is simple to test the device, as the power contacts
in the battery cradly are eaily accessible. After connecting them to
a 6 V/DC PSU, it came alive immediately. As expected, it
appears to transmits at 168.360 MHz, which was a
popular frequency with the BND.
Some corrosion is noticeable on the metal shield over the transmitter,
and also on the battery cradle.
This is caused by the foam inside the storage case, which has become
unstable due to hydrolysis. To avoid further degradation,
we've removed and discarded the foam. As it has not affected the
operation, we decided to leave it in this condition and deal with
the foam later.
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The battery was initially developed by Polaroid in 1982 as the P80 for
its range of instant photo cameras, for which instant film cartridges
were supplied. Rather than installing a battery in the camera, it came
as part of the film cartridge.
To promote the format, Polaroid gave it a more user friendly packaging
and released it as an OEM product as P100. It was later
complemented by the P500, which had a much higher capacity as it was
based on Lithium-manganese dioxide rather than Zinc-manganese dioxide.
The P100 and the P500 have the same form factor.
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These batteries were also available to other manufacturers of slimline
devices. Sinclair, for example, used it in 1983 for his iconic FTV-1
flat-screen pocket TV [3][4]. For AID it was the ideal battery for
the TX-873 and similar listening devices, as it didn't add to the tickness
of the device.
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The batteries that we got with the device, still hold some charge, but
that charge disappers as soon as it is connected to the transmitter.
They were manufactured in the 1980s or 90s and are no longer in production.
For demonstration purposes it would be nice to have a suitable replacement.
Perhaps we can empty one of the existing packs and use it to accomodate
a thin rechargeable Li-ion battery pack as an alternative.
To be continued ...
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- Batteries dead; no longer for sale
- Foam in storage case disintegrating
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Device Flat-pack covert surveillance transmitter Purpose Overhearing of conversations from within a concealment, Anti-corruption Model TX-873 Manufacturer AID Year 1986 (est.) Country USA Frequency 150-174 MHz 1 Channels 1 2 Output 200 mW Microphone Internal electret (Knowles) Power 6 V/DC Battery Polaroid P100, P500 Dimensions HWD: 7 × 250 × 96 mm Weight 80g (104 g with battery)
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On special order: 130-150 MHz or 216-200 MHz.
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The device featured here transmits at 168.360 MHz, which is a BOS
frequency used by the BND.
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Device Battery pack Purpose Slim-line package applications Type Disposable, Zinc-manganese dioxide 1 Model P100 Brand Polapulse, Polaroid Manufacturer Polaroid Year 1982-2001 Country USA Voltage 6 Vnom (6.3 V open circuit) Capacity 215 mAh (@100 mA) Duration ~ 2 hrs (@100 mA) Temperature -7°C to +130°C Dimensions HWD: 4.5 × 75 × 93.5 mm Weight 20 g Price USD 4.75
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Although the battery was not designed to be rechargeable,
people has successfully recharged them many times [2].
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Model P500 (Powerdex 6A1400, VP6-1400) Type Disposable, Lithium-manganese dioxide Capacity ~ 1400 mAh Duration ~ 8 hrs
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© Crypto Museum. Created: Sunday 06 September 2026. Last changed: Sunday, 06 September 2026 - 22:32 CET.
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