Compact tri-band bandpass filter with stub-loaded stepped-impedance resonator

A compact microstrip tri-band bandpass filter (BPF) based on two open stub-loaded multimode λ/4 stepped-impedance resonators (SIRs) with two separate coupling paths is investigated. A source-load coupling configuration is arranged to produce several extra transmission zeros for realising high-isolation performance. The centre frequencies of three passbands can be controlled by tuning the dimensions of two high impedance lines of the proposed open stub-loaded SIRs. The bandwidth of the three passbands could be conventionally controlled by adjusting the coupling strength of two coupling paths. The prototype of the BPF achieved insertion losses of 0.46, 1.05 and 1.4 dB at 0.82, 1.67 and 2.17 GHz, respectively. The measured results are in good agreement with electromagnetic simulation. The circuit only occupies 0.126λ_g × 0.108λ_g, where λ_g is the guided wavelength at a lower passband centre frequency.