Dual-band bandpass filter using coaxial Stepped Impedance Resonators

This paper presents the theory of Stepped Impedance Resonator (SIR) and modifies traditional coaxial lines into coaxial SIRs for dual-band bandpass filter design. A simple feeding structure and an inter-resonator coupling structure have been demonstrated, which simplifies fabrication as well as providing flexibility of tuning. Two Chebyshev dual-band bandpass filters working in the GSM band (900/1750 MHz) were designed and simulated. The 2nd order dual-band bandpass filter has been fabricated and tested, realizing 1.23% and 1.07% fractional bandwidth at 906.5 MHz and 1772.5 MHz, respectively.