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Technical TSCM Blog: About Non Linear Junction Detectors

 
Radar

Conventional radar transmits a signal and receives an echo or radar signal return on the same frequency (fundamental frequency). It has the advantage of excellent range and resolution, but is subject to clutter and is generally unsuitable for ‘close-in’ or hidden object detection.

 Harmonic Radar

Harmonic radar is unlike conventional radar. Harmonic radar transmits on a fundamental frequency and its receiver is tuned to one or more harmonic frequencies that are produced by irradiated nonlinear junctions.

Harmonic radar is very useful for detecting hidden or obscured objects with nonlinear characteristics because the fundamental radar return or echo, frequency, is ignored and only the harmonic caused by the nonlinear junction is received. This process eliminates traditional radar clutter. Hence with harmonic radar it is possible to ‘see through’ objects such as clothing, floors, walls and soil, depending upon the frequency, power, and attenuation factors of the material the radar wave and resultant harmonic is traversing.

Harmonic radars are used by industry for detection of corrosion in metallic structures such as storage tanks and used by entomologists for tracking of specific targets such as tagged insects at distances of from 10 – 100 meters. The ‘tags’ in this case are generally small Schottky or switching diodes attached to a resonant dipole antenna. In the case of the ‘tagged’ insect, the harmonic radar’s illumination frequency is resonant to the frequency of the tag, and a large and fairly efficient RF transfer from space to the nonlinear diode occurs with a resultant large second harmonic being produced.

Most harmonic radar designed to date transmit on a single fixed frequency, receive only the 2nd harmonic, have high gain directional antennas and incorporate some form of microprocessor controller/display unit. The transmitter emits a signal (a short pulse or a continuous wave) at a frequency (e.g. f) and the receiver is tuned by the controller to receive a signal from the tagged object at a harmonic frequency (e.g. nf), wherein n is an integer.

Non-linear Junction Detector (NLJD).

An NLJD is an ‘application-specific’ form of harmonic radar that is used by a Technical Surveillance Counter Measures (TSCM) specialist who desires to search for and detect hidden clandestine electronic surveillance devices. The NLJD can be used for the detection of active, dead or idle, cell phones, micro-transmitters, digital recorders, amplifiers, microphones with amplifiers, etc. All NLJDs are currently designed and optimized for the search of clandestine electronic devices and tend to incorporate signal processing algorithms for reducing false alarms from non-electronic nonlinear junctions.

NLJDs are an indispensable tool for TSCM but they are complex tools and operating them effectively requires significant skill and training. The TSCM technician is looking for a hidden or planted technical surveillance device inside a room. To find a device using the NLJD, the TSCM operator "sweeps" the interior walls of the room requiring inspection. In sweeping, the TSCM operator moves the NLJD antenna slowly, in an up-down, left-right pattern at a stand-off distance of no more than 10 cm from the wall at a rate not exceeding 30 cm / sec. The NLJD operator only wants to know what is ‘in the office wall’ or in the ‘potted plant’ and not what is on the other side.  Thus his typical operational working range is around 6 – 12 inches and he uses primarily low power – sufficient to penetrate standard office walls. High power is used when penetration, into concrete floors, for example, is required.

The principle of operation of an NLJD is very similar to the harmonic radar previously described. Current art NLJDs typically transmit a signal around 900 MHz, with resultant 2nd and 3rd harmonic frequencies displayed/demodulated and analyzed by the NLJD operator.

NLJD Frequency selection is driven by availability of unlicensed spectrum. Thus older NLJDs typically use the 915 MHz ISM band, and more modern NLJDs are appearing using 2.4 and even 3.6 GHz. 

Nonlinear Conversion Efficiency

Conversion efficiency of the NLJD input signal to a harmonic signal is determined by the harmonic cross section of the nonlinear components in the technical surveillance device (TSD). The greater the cross section, the larger the amplitude of the radiated harmonic energy. The diode elements in a TSD have a harmonic power output curve that is exponential. However there is a ‘saturation limit’ that is reached after a certain level of irradiation. When a nonlinear junction reaches saturation, no further increase in harmonic amplitude occurs even if irradiation power is increased. In the extreme case of excessive irradiation the nonlinear junction ‘burns out’. This is observed, for example, when one places a box of nails in a microwave oven – there is significant observable ‘sparking’.

 The amplitude of the harmonic signal produced by a nonlinear junction is very strongly dependent on transferring power from the air to the nonlinear junction. This is influenced by the resonant frequency of the junction which is derived, in part, from the area and thickness of the junction. In practice we expect that no two TSDs exhibit similar impedances, and there is thus no single frequency capable of ‘fully-exciting’ or providing ‘maximum power transfer’ to a TSD.

 Resonance is directly related to frequency of illumination, therefore it is imperative for maximum detection probability (maximum excitation of the nonlinear junction) that the frequency of the NLJD be dynamic or tunable, given the wide impedance variations expected to be found in the TSD. 

The amplitude of the harmonic signals follows a square law relationship with regard to incident power level. For example, a 10 dB increase in input RF power results in a 20 dB increase in harmonic content. Thus, harmonic amplitude increases as the incident power level is increased. Clearly power transfer is important. High power is therefore necessary but there are practical technological, and safety limitations with extremely high power levels.

A modern 900 MHz NLJD does a pretty good job of detecting imbedded devices and is a critical part of any TSCM sweep. However, you simply can’t effectively perform a TSCM sweep and expect to find the newest, very small, GSM-based modern listening devices without an effective NLJD operating at 2.4 GHz. Unfortunately many TSCM professionals today are still using older examples of low power  NLJDs operating in the middle of the 800-900 MHz band. Not only does this invite interference problems with other electronics operating on the same frequency, the low power NLJDs are simply inadequate to penetrate reasonable distances and detect the very small modern threat.  To counter interference, an NLJD, at turn-on, should ‘listen’ for, then select a frequency clear of interference before transmission. It is absolutely imperative that the NLJD use Digital Signal Processing (DSP) algorithms to clarify and improve received signals.
 

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