Non Linear Juntion Detectors (NLJD)

A non-linear junction detector (NLJD) is a device which can be used to detect junctions between two materials exhibiting dissimilar electronic or electrical properties. Examples of non-linear junctions are semiconductor junctions (electronic devices) or junctions between dissimilar metals.

NLJDs usually comprise; a spectrally pure, high power RF transmitter, two very sensitive RF receivers, a control unit, a user interface and power supplies all housed in a robust screened enclosure which is connected to an antenna sensor.

Non Linear Junction Detectors (NLJDs) are widely used in counter surveillance operations and in electronic ordinance detection where concealed or buried electronic devices must be detected and located.

The non-linear semi-conductor junctions used in electronic components are present whether the device is powered on or off, this means that a non linear junction detector can be just as effective against dormant or inactive electronic devices.

NLJD operation

The NLJD operates by illuminating a target junction with energy at a spectrally pure fundamental RF frequency.

The non linear junction will essentially rectify the incoming fundamental RF signal and thus generate harmonics of the radiating signal.

Reflections or re-radiation from the non-linear junction can then be analysed using very sensitive receivers to determine the type of junction detected.

The reflections from the non-linear junction usually have principal frequency components at twice and three times the illuminating signal frequency (2nd and 3rd harmonics).

In general, a semiconductor junction will return predominantly second harmonics. Junctions between dissimilar metals will return higher levels of third harmonic or very similar levels of second and third harmonics, but predominantly third harmonics.

Defeating shielded electronics

In order for the NLJD to work the radiating signal must first enter the non linear junction and the generated harmonics must then re-radiate away from the junction.

For this reason well screened or buried electronic devices will be more difficult to detect than poorly or un-screened devices.

There are two main ways in which the NLJD can overcome improved screening;

Increased efffective radiated power (ERP) of the illuminating (transmitter) signal and increased receiver sensitivty.

A third, less obvious method, may be to vary the frequency of the illuminating RF signal to find a 'weak' spot in the screening.

The non-linear (2nd and 3rd order) nature of the target junction means that increasing the illuminating power will have more of an effect on detection range than increasing the NLJD sensitivity by the same (dB) amount.

In general what is required is high transmitter ERP coupled with good receiver sensitivity.

Technically this goal is very difficult to achieve, all NLJDs are constructed from electronic and metal components, hence they can appear as a valid target to themselves (self-detection).

The high power SuperBroom Advanced achieves the goal by combining a 10W ERP with an unprocessed sensitivty of -130dBm.