Compact tri-mode microstrip filter with wide- stopband using asymmetric stub loaded resonators

A tri-band bandpass filter (BPF) using a novel asymmetric stub loaded resonator (ASLR) is presented. Firstly, the resonant behaviour of the ASLR is analysed theoretically. By properly selecting the electric length ratio of the ASLR, the centre frequencies could be designed at 1.9, 3.5, and 5.8 GHz, respectively. Furthermore, parallel coupled-lines feeding structure is employed to achieve harmonic suppression with wide-stopband. Finally, a tri-band BPF is designed and fabricated, and the measured results agree well with the simulations and verify the design principle.