Compact Balanced Dual-band Bandpass Filter Based on Slotline Zero-degree Feed and Stub-loaded Resonators

A novel balanced microstrip dual-band bandpass filter (BPF) with independently controllable frequencies, compact size, and high common-mode (CM) suppression is proposed in this paper. It is achieved by employing a pair of folded open stub-loaded resonators (OSLR) and a pair of slotline zero-degree feeds (SZDF). The SZDF is firstly introduced into microstrip-slotline feed structures to excite differential-mode (DM) and suppress CM. Meanwhile, the DM passbands are independent of the CM responses, which significantly simplifies the design procedure. The filter is simulated, fabricated and measured. Good agreement is observed between simulated and measured results.