Low-Cost Multiple-Bit Encoded Chipless RFID Tag Using Stepped Impedance Resonator

A novel compact multiple-bit encoded chipless RFID tag using a stepped impedance resonator (SIR) is proposed. The main advantage of SIR compared with other resonators is the independent control over the fundamental as well as first harmonic frequency by varying either the impedance ratio (K) or length ratio (α). The tag utilizes both the fundamental and the first harmonic frequency of the SIR to represent two bit information with a single resonator. The tag is capable of representing 2^2N number of bit combinations with N resonators. RFID tags are fabricated on C-MET LK4.3 (ε_r = 4.3 and tanδ = 0.0018) and RT Duriod (ε_r = 2.2 and tanδ = 0.0009) substrates. The structural information of the tag is encoded in the frequency spectrum, both in the magnitude and group delay of the backscattered signal. The data encoding capacity of the tag is enhanced by using the frequency shift coding technique. The proposed RFID tags have an operating range of 50 cm in the outdoor environment. Theoretical and numerical methods are used to verify the measured resonant frequencies of the tag.