Design of high-Q UHF radio-frequency identification tag antennas for an increased read range

In order to increase the radio-frequency identification (RFID) operation distance, read range analysis was performed based on a simple equivalent circuit of the ultra high frequency (UHF)-band passive RFID tag. The analysis shows that a tag with a large Q-factor leads to an increased input voltage in the tag chip and thus enhances the efficiency. Based on this analysis, a compact (37.5 mm 44 mm) RFID antenna employing inductive coupling between the radiation and feeding portions was designed. Simple adjustments of the two structural parameters of the antenna allowed for easy control of the antenna resistance and inductive reactance, from which a high Q-factor requirement could be readily satisfied. For a conjugate match to the tag chip impedance of 11-j127 Omega, a Q-factor of 11.5 was achieved. The designed RFID tag has a 3 dB radar cross-section (RCS) bandwidth of 6.5 , which is wide enough to handle the impedance detuning caused by the material properties of the attached objects. The designed RFID tag antenna was fabricated and a reading range test in an anechoic chamber was performed using two methods. When a measured RCS of the RFID tag was used, the detection distance was 9.8 m for a reader sensitivity of 65 dBm and 4.7 m for a tag sensitivity of 11 dBm. Using a commercially available tag chip, the range test resulted in 4.3 m, which is compatible with the derived range equations. The tag antennas having lower Q-factors (77+j100 Omega and 55+j155 Omega) have also been fabricated, and their measured read ranges were 2.6 and 1.1+m, respectively.