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Scanlock Broom ECM
Non-Linear Junction Detector · NLJD

The Broom is one of the first Non-Linear Junction Detectors (NLJD), introduced by Audiotel in Corby (UK) in 1990, shortly after they acquired Security Research Ltd. from Cray Defence Group in 1989 [1]. 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 Charles Bovill 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.
Broom ECM with broom-stick antenna

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.

Main unit (rear), telescopic stick and antenna (front)
Broom ECM with broom-stick antenna
Broom ECM antenna
Close-up of the antenna
Close-up of the connections on the antenna
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Main unit (rear), telescopic stick and antenna (front)
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Broom ECM with broom-stick antenna
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Broom ECM antenna
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Close-up of the antenna
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Close-up of the connections on the antenna

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 Charles Bovill, 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 Lee Tracey, who was currently testing one of the company's spy camera products, detected the suspicious figure and the bomb didn't go off [6].

The police suspected the IRA, who was afraid that other inventions of Bovill, such as the Photic Drive, might be used against them.

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 Lee Tracey, these companies were all front operations of MI6 [6].

The image on the right was taken from the book Weapons & Equipment of Counter-Terrorism by Michael Dewar [4]. 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.
The original Broom as it appeared in [4]

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 [5]. 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.

Technical Specifications
  • Battery
    12V DC, 3Ah, charge with 18V DC
  • Frequency
    TX: 888-889 MHz
  • Output power
    30-300 mW (ERP)
  • Dimensions
    Main unit 310 x 235 x 70 mm, Antenna 210 x 190 x 65 mm
  • Accessories
    Antenna head, two interchangeable handles
  • Demodulator
    AM, FM
  • Connectors
    BNC (TX), TNC (RX)
  1. Audiotel International Ltd.,
    Audiotel website, 18 June 2003. Retrieved via WayBack Machine.

  2. Audiotel International Ltd., Broom Operating Manual
    Issue 2, 5 November 1992.

  3. Audiotel International Ltd., Principles of NLJDs
    Electronic Espionage - Fact Sheet 9. 2002.

  4. Michael Dewar, Weapons & Equipment of Counter-Terrorism
    ISBN 0-85368-841-9. 1987. p. 96.

  5. 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.

  6. Lee Tracey, Interview and personal correspondence
    Crypto Museum. 23 May 2013.
Further information
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Crypto Museum. Created: Wednesday 28 March 2018. Last changed: Monday, 29 November 2021 - 11:48 CET.
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