Planar microstrip tri-mode bandpass filter using center-stub-loaded spiral resonator

A new tri-mode banpass filter (BPF) using a half-wavelength spiral resonator center-loaded with two T-shaped open stubs has been presented. Analyzed by the odd- and even-mode method, an odd mode and two even modes of the proposed tri-mode resonator could be flexibly controlled by tuning the electrical length of the spiral resonators and T-shaped stubs. For demonstration purpose, a tri-mode filter example with passband centered at 1.8 GHz has been implemented.