| Summary: | For multi-resonator based chipless radio frequency identification (RFID) tags, a novel triple-mode resonator is designed by loading open-circuited stubs to a transmission line which is coupled to input and output ports. Since the proposed resonator provides independently controllable three resonant frequencies, three bits can be obtained by means of a single triple-mode resonator. The presence and absence of the open stubs results in eight states from a single triple-mode resonator. Therefore, N triple-mode resonators in different lengths can generate <inline-formula> <tex-math notation="LaTeX">$8^{N}$ </tex-math></inline-formula> different frequency codes. For this purpose, a multi-resonator circuit operating between 1.9 and 3.75 GHz is also developed by using 5 triple-mode resonators, so that 15 resonant frequencies can be obtained. Two conventional vertically/horizontally polarized monopole antennas are used to demonstrate the tag performance. Bit density and spectral efficiency of the designed tag are 29.04 bits/<inline-formula> <tex-math notation="LaTeX">$\lambda _{g}^{2}$ </tex-math></inline-formula> and 8.33 bits/GHz. Two chipless RFID tags having different frequency codes have been fabricated and tested in a good agreement with the predicted results. The designed tags have also been tested under various circumstances successfully.
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