Harmonic Radars Search for Hidden Devices

Harmonic radars have long held the promise of detecting devices at higher frequencies, although designing such systems involves tight control of internally generated harmonic and spurious signals. To aid in the detection of small, hidden electronic devices, researchers at the Sensor and Antenna Systems Group at the Informatics and Information Security Research Center of the Scientific and Technological Research Council of Turkey, Gebze, Turkey, presented the design and implementation of a portable harmonic radar.

This radar boasts transmit capability from 1.95 to 2.05 GHz and a receive range from 3.90 to 6.15 GHz, with the capability to receive signals as high as third-order harmonics. The portable system features +32-dBm transmit power and -103-dBm receiver sensitivity. The harmonic radar system can discriminate semiconductor targets from corrosive metals, as well as detect a semiconductor target as small as 1.85 × 1.85 cm at a distance of more than 50 cm.

Harmonic radar systems are also referred to as nonlinear junction detectors (NLJDs). They have been designed for the purpose of hidden electronic devices in walls and furniture that might have been implanted for use as surveillance devices, or “bugs.” An NLJD-based system is based on employing the nonlinearity of electronic devices because of the p-n junctions of their active devices, such as diodes and transistors. These semiconductor junctions will reradiate received high-frequency energy at double or triple the frequency, and those harmonic frequencies can be received by a sensitive receiver operating in the proper frequency range.

Unfortunately, harmonic radars are particularly susceptible to false alarms from internal system harmonic leakage, corrosive or junction metals in the scanned area, and harmonic reflectors in the scanned area. The researchers developed their system to operate at multiple transmit frequencies to reduce the occurrence of false alarms. The harmonic radar system includes transmit and receive antennas, transmitter and receiver circuits, a 10-MHz oven-controlled crystal oscillator (OCXO), a digitizer, and a central processing unit (CPU) to run software and a graphical user interface (GUI) for the radar.

The transmit and receive circuit blocks were well isolated to reduce the effects of signal leakage. Transmit and receive filters also contribute a great deal to the excellent performance of the harmonic radar system, with the transmit filter helping to reduce the level of harmonics emitted by the transmitter and the receive filter reducing the level of transmitted signals to the receiver. See “Getting the Bugs Out,” IEEE Microwave Magazine, November 2015, p. 40.