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Wideband RFID tag antennas are highly desired for global applications. Moreover, the wideband feature makes tags less sensitive to the external detuning. This paper presents a circuit-based bandwidth-enhancement theory by applying the double-tuning technique to tag antennas. Conditions of the optimum double-tuning and the maximum bandwidth out of double-tuning are derived. Closed-form equations are provided to facilitate the bandwidth optimization and predication. Similarities and differences between the optimum double-tuned matching network and the equal-ripple two-stage bandpass filter have been discussed. In particular, a simplified design approach of double-tuned tag antennas is developed by examining the antenna impedance. An exemplary double-tuned tag antenna delivers a 10-dB bandwidth of 860~958 MHz on paper, and exhibits the material-insensitive feature to accommodate dielectrics of low-to-medium permittivity.