Compact Dual-band Bandpass Filter Using Coupled Lines Multimode Resonator

A quad-mode resonator is proposed for novel design of dual-band bandpass filters (DBBPFs) with good selectivity and compact size. The symmetrical resonator consists of a pair of coupled line sections (CLSs) and a transmission line section (TLS). Odd-/even-mode method is used to clarify the mode characteristics. Four excited resonant modes are applied to produce two passbands. The frequency ratio range can be adjusted by the CLSs, and the bandwidths can be controlled by the TLS. After installing a pair of U-shape coupling arms, a DBBPF with five transmission zeros ( $TZ$ s) around the two passbands are designed and fabricated. The measured results are in good agreement with the simulated results, demonstrating that the proposed filter is feasible in practical use.