Microstrip multi-band bandpass filters using a single improved ring resonator

A planar dual-mode ring resonator loaded by four mushroom-shaped stubs is presented for design of dual-band bandpass filter (BPF). The four mushroom-shaped stubs utilized in the inner area of the traditional dual-mode ring resonator can realize both compact size and controllable harmonic response of the filter to generate the second passband. Additionally, a novel pair of feed lines with source-load coupling is employed to not only feed the ring resonator but also generate the higher third passband when the length of feed lines increases. For demonstration purpose, two example filters (dual-band BPF and tri-band BPF) are implemented.