Planar dual‐ and tri‐band bandpass filters using single improved ring resonator and simple feed scheme

A planar single improved dual‐mode ring resonator loaded by four mushroom‐shaped stubs based on a simple feed scheme is presented for design of dual‐ and tri‐band bandpass filters (BPFs). The simple pair of feed lines with source‐load coupling is used to not only feed the ring resonator but also generate the higher desired third passband when the length of feed lines changes. That is, a desired dual‐ or tri‐band BPF can be designed based on the same single‐ring resonator. For demonstration purpose, two experimental example filters have been fabricated and implemented. The simulated and measured results are presented and show good agreements.