The non-linear junction detector, or an NLJD, is a device that illuminates a small region of space with high-
frequency RF energy. Any "non linear junction" in the vicinity—for example, and particularly, the
p-n junction—will receive this energy, and because of the asymmetric response of the junction to an electric field, it will
mangle
Mangle can refer to:
* Mangle (machine), a mechanical laundry aid consisting of two rollers
* Box mangle, an earlier laundry mangle using rollers and a heavy weight
* Mangled packet, in computing
* Mangrove, woody trees or shrubs
* Name mangling, ...
it, re-emitting some of it on multiples of the illumination frequency (see
harmonic
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
). The detector has a sensitive
receiver tuned to these harmonics, as well as appropriate processing and displays to make their presence known to the user of the device.
Because the basis of almost all
semiconductor electronics is the p-n junction, an NLJD is correspondingly capable of detecting almost any
unshielded electronic device containing semiconductors, whether the electronics are actively powered or not.
In its basic form, an NLJD can also detect things that are not themselves electronic in nature, so the use of the device requires a modicum of skill and experience. For example, a rusty nail inside a wall can give a
false positive. For this reason, most modern NLJDs examine the ratio between the second and the third harmonic of the illumination frequency. When a true (electronic) p-n junction is encountered, the second harmonic will generally be stronger than the third.
History
The NLJD was invented by Charles Bovill during WWII. It was initially used 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 introduced as a device for finding inactive
covert listening devices (bugs). The Broom was later marketed by Audiotel in Corby (UK) as the Scanlock Broom, and was succeeded by the Scanlock Broom ECM and later the Scanlock Super Broom.
Similar devices are available from manufacturers in the US (e.g. Orion) and Russia (e.g
Lornet Star.
Countermeasures
As a
countermeasure against an NLJD, professional
covert listening devices (bugs) of the
Central Intelligence Agency were equipped from 1968 onwards with a so-called
isolator. An isolator is a 3-port
circulator
A circulator is a passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where an ...
of which the return ports is terminated with a resistor. 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 bug is the CIA'
SRT-107
A means to hinder isolating a non linear junction is to add inexpensive diodes to places where the NLJD is expected to sweep. This masks the true listening device against a field of false alerts when the many diodes are detected. Such a technique was used in the 1980s construction of the U.S. embassy in Moscow. Thousands of diodes were mixed into the building's structural concrete making detection and removal of the true listening devices nearly impossible.
[John Free, Naomi Freundlich, C.P. Gilmore]
"Bugging"
''Popular Science''. August 1987
See also
*
Covert listening device
References
External links
Non-Linear Junction Detector Review and TutorialNLJD Lornet Star 24s
GBPPR Non-Linear Junction DetectorHomebrew non-linear junction detector project.
{{DEFAULTSORT:Nonlinear Junction Detector
Detectors