A compact tri-band bandpass filter based on grounded tri-mode stepped-impedance stub-loaded resonator

In this work, a stepped-impedance stub-loaded resonator which exhibits tri-mode behaviour is presented. The proposed resonator is analyzed in detail and used to design a compact tri-band bandpass filter which operates at 1.8/3.5/5.2 GHz. The fabricated filter exhibits good selectivity, good transmission and return-loss characteristics, and a compact size of only 0.21λ
_g
× 0.11λ
_g
, where λ
_g
denotes the guided wavelength on the given substrate at the central frequency of the first passband.