Narrowband Passive RF Tags for Frequency-Selective Harmonic Doppler Radar Tracking

A design for frequency-selective radio frequency (RF) harmonic tags for harmonic Doppler radar tracking is presented. Harmonic RF tags receive an incident microwave signal and generate a retransmitted signal at a harmonic of the incident frequency, typically the second harmonic. Since environmental backscatter occurs at the fundamental frequency, the second harmonic can be easier to detect. Typical harmonic tags operate over a reasonably wide fractional bandwidth, and thus to detect multiple tags within the same field of view, additional circuitry must generally be added to the tag to differentiate the responses. We propose a different approach based on frequency selectivity implemented by the tag antenna. Based on narrowband split-ring printed antennas, the tags accept incident signals over a specified narrow fractional bandwidth, after which the second harmonic is generated by a diode and retransmitted with a standard dipole antenna. The narrowband filtering occurs at the fundamental frequency antenna, and thus with tags designed at adjacent narrowband frequency channels, no additional circuitry is necessary to differentiate multiple tags. Fabricated tags achieved fractional bandwidths below 0.5% at carrier frequencies below 2.5 GHz while maintaining gains above 2 dBi. The construction of the tag is simple and low-cost, requiring only a single component (a diode) on a printed antenna structure. We demonstrate the feasibility of simultaneously measuring the time-varying harmonic Doppler signature of multiple tags in the field of view by experimentally verifying the frequency selectivity of tags in motion using a 2.4/4.8 GHz harmonic Doppler radar.