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Technical Surveillance and Countermeasures

This section deals with equipment that has been used (or is currently being used) for finding clandestine transmitters, covert listening devices (bugs) and monitoring the frequency spectrum in general. Note that there will be overlaps with other categories on this website.

TSCM equipment on this website
Wire-line bug detector
Non-linear Junction Detector
Russian countermeasures receiver for the 100 MHz to 12 GHz frequency range
Mason surveillance receivers
Micro-Tel precision surveillance receivers
ACL SR-209 HF/VHF/UHF/SHF surveillance receiver
Scanlock series of TSCM receivers and other Audiotel equipment
Innovative add-on for the Scanlock Mark VB
Various bug detection devices made by Research Electronics International (REI)
Enhanced Omni-Spectral Correlator (bug finder)
Dating Ranger 2 bug tracer
Body worn record detector and bug detector
Portable RF probe monitor (bug finder)
Powerful hand-held bug tracer
Delta-V ECM
Cable checker for mains cables and telephone lines
Sadelco FS-3 field strength meter
NRP field strength indicator with built-in frequency counter
Dare CR-3000/C frequency counter with integrated field-strength indicator
Rohde & Schwarz FSH-3 portable spectrum analyzer
HE-100 directional antennas
ZAP Checker - field strength indicator
Optoelectronics Scout 40 automatic frequency counter 10 MHz - 1.4 GHz
Sensitive near-field test receiver 30 MHz - 2 GHz
Rohde & Schwarz EB-200 Monitoring Receiver 10 kHz - 3 GHz
Capri (7042) LW receiver, used by the Stasi for finding carrier frequency bugs
Kopchik aperiodic surveillance detection receiver
Non-linear Junction Detector · NLJD
A non-linear junction detector, commonly abbreviated to NLJD, is a countermeasures device for the detection of semi-conducting materials, such as the P-N barrier in electronic components like diodes, transtors and ICs. As most covert listening devices (bugs) are built from such parts, it is possible to discover their electronic circuits, regardless of whether the device is powered or not.

When a semi-conducting object is illuminated by an RF signal of a particular frequency f1, the non-linear properties of the object will cause harmonics of that frequency to be generated, in particular the 2nd and 3rd harmonics (f2 and f3). By examining the minute f2/f3 harmonic signals reflected by the object, it is possible to discover the object, even when it is hidden from sight.

Note that other semi-conducting materials, such as a rusty nail or an oxidised piece of metal, also generate harmonic frequencies and may there­fore cause an NLJD to generate a false positive.
Broom ECM with broom-stick antenna

For this reason, modern NLJDs often examine the ratio between the 2nd and the 3rd harmonic. With an electronic P-N junction, the second harmonic will generally be stronger than the third, allowing an electronic circuit to be discriminated from a rusty nail with reasonable certainty.

The NLJD was invented during WWII by Charles Bovill, who used it to discover corrosion below painted surfaces on airplanes. In 1972, shortly after Bovill had become technical director at Allen International Ltd. (Westminster, London, UK), the device was renamed Broom and was marketed as a bug-finder. The Broom was later marketed by Audiotel in Corby (UK) as the Scanlock Broom, succeeded in 1990 by the Scanlock Broom ECM and in 1995 by the Scanlock Super Broom.

Non-linear junction detectors on this website
Audiotel Scanlock Broom ECM
Non-linear Junction Detector
Known non-linear junction detectors (NLJDs)
  • 1972
  • 1990
    Scanlock Broom
  • 1995
    Scanlock SuperBroom
  • ?
    REI SuperScout
  • ?
    REI Eagle
  • ?
    REI Orion
  • ?
  • ?
    Boomerang 4
  • ?
  • 1985
  • ?
    Lornet 36
  • ?
As a countermeasure 1 against an NLJD, professional covert listening devices (bugs) of the US Central Intelligence Agency (CIA) were equipped from 1968 onwards with a so-called isolator. An isolator is a 3-port circulator of which the return port is terminated with a resistor, as shown in the image below. Any energy injected into the bug by an NLJD will be absorbed by the resistor, resulting in no (or very little) reflected energy. An example of such a CIA bug is the SRT-107.

A three-port circulator used as an isolator

Another very effective counter-countermeasure, is to plant a plethora of inexpensive diodes at locations that are likely to be swept with an NLJD. This will generate so many false positives that it masks the presence of the actual bug(s). This method was used by the Russians in the 1980s, during the construction of the (then ) new American embassy in Moscow. Diodes were mixed into the building's concrete, making the detection of real listening devices virtually impossible [2].

  1. As it is a countermeasure against a countermeasure device, it is also known as a counter-countermeasure.

  1. Wikipedia, Nonlinear junction detector
    Retrieved November 2018.

  2. Peter Grier, Cleaning the Bug House
    Air Force Managine. September 2012.
Further information
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Crypto Museum. Created: Saturday 24 December 2016. Last changed: Monday, 22 March 2021 - 13:37 CET.
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