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Detection of Harmonic Micro-Doppler Signatures Using Passive RF Tags and Harmonic Radar

We present a new technique for detecting the motion of objects using a combination of harmonic radar, passive radio-frequency (RF) tags, and micro-motion radar signatures in cluttered environments. A harmonic radar transmitting at 2.5 GHz and receiving at 5 GHz is designed to excite a new passive harmonic tag consisting of two slot-dipole antennas tuned to the two radar bands. The tag receives the 2.5 GHz signal and uses a diode to generate a harmonic response at 5 GHz, which is retransmitted back to the radar. The motion of the tag manifests at the 5 GHz harmonic frequency through the addition of frequency sidebands, called micro-Doppler. We demonstrate the novel harmonic micro-Doppler system, which is able to detect the dynamic motion of harmonic tags in a realistic, cluttered environment. The tag motion is shown using the short time-frequency transform (STFT), demonstrating the ability to detect motion signatures in harmonic radar, and extending earlier works in tacking objects and activities classification.