A compact passive two-bit metamaterial based phase-modulated chipless RFID tag at 2.4GHz is presented. The chipless RFID tag consists of a monopole antenna and CRLH delay lines operating in the left-handed (LH) region with distributed digital phase modulating sections. A Quadrature Phase Shift Key (QPSK) is implemented using the four phase modulating sections having unique input reflection coefficients corresponding to the four symbols. The dimension of the chipless tag with two phase modulating sections at 2.4GHz is 1.12λ_0×0.16λ_0×0.00648λ_0. A footprint reduction of 22% is achieved in comparison to the conventional chipless RFID tags. Measured and simulated results show that the proposed RFID chipless tag delivers the required performance of detection with four unique symbols with compact size.
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