Independent Dual-Circularly Polarized Passive RFID Tag for Incident Angle Measurement

This study presents an independent dual-circularly polarized RFID tag and introduces a method for measuring the incident angle between the RFID tag and the reader. The tag, constructed with hybrid phase shift elements, possesses the capability to convert and independently control dual circular polarization. The reflection from left-handed circularly polarized (LCP) waves exhibits a higher monostatic radar cross section (RCS) than those from right-handed circularly polarized (RCP) waves within a certain angle range. The monostatic RCS difference between LCP and RCP depends on the tag incident angle, varying in magnitude as the angle changes. Therefore, the tag's incident angle can be obtained by comparing the measured difference with theoretically predicted values. A nine-element RFID tag was designed and fabricated, and its performance was validated through experiments. Experimental results confirmed significant distinguishability between the scattered LCP and RCP monostatic RCS patterns, aligning with simulated predictions. The proposed tag incident angle measurement method requires only a single RFID tag and a single reader. Additionally, the method is compatible with conventional positioning methods and can serve as an alternative method to enhance stability.