Non-Linear Junction Detector · NLJD
The Broom is one of the first Non-Linear Junction Detectors (NLJD), introduced by
in Corby (UK) in 1990, shortly after they acquired Security Research Ltd.
from Cray Defence Group in 1989 .
The Broom can radio bugs, even if they are not switched on, by
searching for the P-N junctions that are present in all electronic semiconductors,
such as diodes and transistors.
It was invented by WWII SOE veteran
who initially used it to find corrosion on airplanes.
The property of a non-linear junction (NLJ) is that, when a high-frequency signal
is aimed at it, it reflects harmonic frequencies of the input frequency.
By monitoring the 2nd harmonic frequency, the presence of a NLJ can be proven.
The Broom ECM has a built-in transmitter with extensive filtering, in order to
guarantee that it only transmits the base frequency of 888 MHz.
A separate built-in receiver is used to look for the 2nd harmonic of 888 MHz,
i.e. 1.776 GHz. Again, extensive filtering is needed to allow only the
1.766 GHz signal to be seen by the receiver.
The Broom ECM was battery powered and consisted of
two parts: the main unit and a broom-type TX/RX antenna.
By displaying the strength of the received signal on the LED bar at the
front panel of the main unit, the operator can determine the presence of
a non-linear junction. This might indicate the presence of electronic
components, such as a diodes, transistor, ICs (chips), etc.
A drawback of this method is that some naturally formed crystalline
structures, such as a rusty nail that got stuck somewhere inside a wall,
can also reflect the 2nd harmonic of the base frequency. The operator
therefore has consider that possibility as well.
He wouldn't be the first to needlessly knock down a wall...
One should always use
other types of bug detectors as well.
The Broom ECM was succeeded in 1995 by the
SuperBroom that did not suffer from this
problem. By looking at the ratio between the 2nd and the 3rd harmonic
of the base frequency, the operator can tell the difference between
a semiconductor (i.e. an electronic part) and a natural structure.
The Broom ECM only has a few controls and adjustments at its front
panel. At the left are three push buttons, marked TONE SELECT,
TUNE SELECT and BATTERY CHECK.
At the right are four adjustment controls: the audio volume,
the squelch (recommended to be set at zero), TUNE (for calibrating
the Broom after turning it on) and the transmitter output level
(adjustable between 60 and 600 mW ERP).
The Broom was invented by
an electronics engineer and WWII SOE veteran.
It was based on Bovill's wartime work on
a non-linear junction detector that was used to find corroded
parts on airplanes. In 1972, shortly after he became technical
director at Allen International Ltd. in Westminster (London, UK),
he started marketing the device as the bug finder we know today.
Alan International Ltd. had specialized in the development and sales
of spy gadgets, such as bugs, disguised microphones, etc. and
even supplied the Q-type gadgets for the James Bond movies for
many years. The company had an office and a showroom above a
bedding shop in Westminster (London, UK).
On 1 October 1973,
the company escaped a bomb attack, when a man tried to plant
five pounds of explosives. Luckily, Bovill's colleague
who was currently testing one of the company's
spy camera products, detected the suspicious figure and the
bomb didn't go off .
The police suspected the IRA, who was afraid that other
inventions of Bovill, such as the Photic Drive, might be used
After Allen International went bankrupt in 1974,
marketing of the Broom was taken over by Security Research Ltd.
of Guildford (UK), another company that was part of
CDI Holdings. According to
these companies were all front operations of MI6 .
The image on the right was taken from the book Weapons &
Equipment of Counter-Terrorism by Michael Dewar .
It was probably taken in 1975 and shows the original Broom
as it was sold by Security Research Ltd. Although the
receiver has changed over the years, the telescopic
broom stick with antenna at the end is still the same.
In 1999, at the age 88, Bovill demonstrated his invention in
the Channel 4 documentary The Walls Have Ears
that is now available on YouTube . It shows Bovill in the
shed behind his house, where he worked on other inventions
after his retirement.
Bovill passed away two years later.
➤ More about Charles Bovill
In some literature the Russians are credited for inventing the
Non-Linear Junction Detector (NLJD), as they came up with an equivalent
device around the same time as Charles Bovill did in 1972. However,
as Bovill already used the NLJD during WWII to find corroded parts on
airplaines, we assume that he is the original inventor.
It is quite possible though, that the Russians invented it completely
independently, or that they had information about Bovill's secret wartime work.
Battery12V DC, 3Ah, charge with 18V DC
FrequencyTX: 888-889 MHz
Output power30-300 mW (ERP)
DimensionsMain unit 310 x 235 x 70 mm, Antenna 210 x 190 x 65 mm
AccessoriesAntenna head, two interchangeable handles
ConnectorsBNC (TX), TNC (RX)
- Audiotel International Ltd., www.audiotel-int.com/history.html
Audiotel website, 18 June 2003. Retrieved via WayBack Machine.
- Audiotel International Ltd., Broom Operating Manual
Issue 2, 5 November 1992.
- Audiotel International Ltd., Principles of NLJDs
Electronic Espionage - Fact Sheet 9. 2002.
- Michael Dewar, Weapons & Equipment of Counter-Terrorism
ISBN 0-85368-841-9. 1987. p. 96.
- Channel 4, The Walls Have Ears
Fascinating Channel 4 documentary about The Spying Game - The Walls have Ears.
1999. Via YouTube. Interviews Glenn Whidden, Lee Tracey, Charles Bovill and others.
- Lee Tracey, Interview and personal correspondence
Crypto Museum. 23 May 2013.
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© Crypto Museum. Created: Wednesday 28 March 2018. Last changed: Monday, 29 November 2021 - 11:48 CET.