Dual-band bandpass filter with wide upper stopband using quad-mode stepped impedance stub-loaded resonator

A novel quad-mode stepped impedance stub-loaded resonator is presented. The even-odd-mode method is applied twice to analyse the modal resonance characteristics due to its symmetry. All the four modes´ equivalent circuits are quarter-wavelength stepped impedance resonators, so the proposed resonator has a compact size and higher harmonic frequencies. Stub-to-stub coupling is introduced to split two identical modes and produce two transmission zeros. A dual-band bandpass filter operating at 1.5 and 2.4 GHz is designed using the proposed resonator. The centre frequency and bandwidth of each passband can be controlled by tuning the corresponding physical dimensions, and a wide upper stopband is achieved. Finally, the designed filter was fabricated and measured, and the measured results have good agreement with the simulated ones.