Compact dual-band bandpass filter using multi-mode resonator of short-ended and open-ended coupled lines

A compact dual-band bandpass filter using a multi-mode resonator (MMR) is proposed. The MMR is composed of two pairs of short-ended coupled-lines, two pairs of open-ended coupled lines and four connecting transmission lines. The utilized structure enables creating five transmission zeros, resulting in controllable dual bands and wide upper-stopband suppression. By adjusting the related parameters of the MMR, a dual-band BPF is designed and modelled using a full-wave simulator. The two bands of the prototyped design are centred at 2.1 GHz and 3.6 GHz, with 3-dB fractional bandwidth of 15.6% and 9.2%, respectively. The overall size of the presented filter is only 0.2×0.2 guided-wavelength at the center frequency.