Matching Technique for an On-Body Low-Profile Coupled-Patches UHF RFID Tag and for Sensor Antennas

This paper introduces a novel impedance matching technique for extremely low-profile on-body UHF RFID tag antennas based on coupled shorted-patch antennas. The approach employs a novel arrangement of comb-notches perpendicular to the central radiation slot that excites the close higher order mode that affects the field distribution of the fundamental mode and sets the input impedance to the required complex values of UHF RFID chips over the range of 5-50 Ω for the real part and 100-200 Ω for the imaginary part, or directly to 50 Ω impedance. A set of parametric studies shows the flexibility of the proposed technique for achieving complex input impedances. To verify the proposed technique, we have developed and measured two antenna samples of relative size 0.3 × 0.17 × 0.0022λ₀. A first antenna is matched to 50 Ω, and is intended to be used as an on-body antenna sensor for mapping the received signal strength in applications of the European UHF RFID band. The second antenna operates as an RFID tag antenna with input impedance in Z_in = 22 + j195 Ω, and reaches a read range of 7.3 m.