Design of Multimode Tri-Band Bandpass Filter with Cross-Coupling Structure

This paper presents a compact, high-performance microstrip tri-band bandpass filter (BPF) based on a cross-coupling structure. By virtue of the structural symmetry, the evenodd mode analytical method based on the ABCD matrix theory is applied to characterize the transmission mechanisms and resonance conditions. Benefiting from the signal transmission between the dual paths, five transmission zeros are generated at 3.14, 3.56, 4.11, 5.55, and 5.61 GHz. Meanwhile, the center frequencies and fractional bandwidths of the three operating bands can be independently adjusted by tuning specific dimensions. The measured center frequencies are located at 2.50, 4.73 and 6.30 GHz, with corresponding 3 dB FBWs of 34.8%, 16.1% and 16.5%, respectively. The measured results show a good agreement with the Ansys HFSS full-wave EM simulations.