The non-linear junction detector, or an NLJD, is a device that illuminates a small region of space with high-

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...

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 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 A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

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 A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test resul ...

. 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 Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

below painted surfaces on

airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spe ...

s. 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 device A covert listening device, more commonly known as a bug or a wire, is usually a combination of a miniature radio transmitter with a microphone. The use of bugs, called bugging, or wiretapping is a common technique in surveillance, espionage ...

s (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 A countermeasure is a measure or action taken to counter or offset another one. As a general concept, it implies precision and is any technological or tactical solution or system designed to prevent an undesirable outcome in the process. The fi ...

against an NLJD, professional

s (bugs) of the

Central Intelligence Agency The Central Intelligence Agency (CIA ), known informally as the Agency and historically as the Company, is a civilian intelligence agency, foreign intelligence service of the federal government of the United States, officially tasked with gat ...

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 wher ...

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

References

External links

Non-Linear Junction Detector Review and Tutorial

NLJD Lornet Star 24s

GBPPR Non-Linear Junction Detector
Homebrew non-linear junction detector project. {{DEFAULTSORT:Nonlinear Junction Detector Detectors

History

Countermeasures

See also

References

External links