New dual-band open-loop resonator bandpass filters with controllable bandwidths and transmission zeros

This paper proposes a dual-mode dual-band open-loop bandpass microstrip filters. The open-loop is composed of two half-wave stepped-impedance resonators (SIR) with high-impedance short-circuited ends, which is equivalent to an inductor for providing coupling between the resonating modes and hence bandwidth control. Moreover, with the aid of source-load coupling which is employed in the second circuit, extra transmission zeros can be created to close the passband edges, greatly improving the performance in transition bands. The measured results show good agreement with the simulation.