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 Omega for the real part and 100-200 Omega for the imaginary part, or directly to 50 Omega 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 x 0.17 x 0.0022 lambda(0). A first antenna is matched to 50 Omega, 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 Omega, and reaches a read range of 7.3 m.