Bandpass filters with multiple transmission zeros using open/shorted stubs

Two novel bandpass filters with multiple transmission zeros using open/shorted stubs are presented in this study. By utilising shorted coupled lines and the open/shorted stubs, five and eight out-of-band transmission zeros can be realised for the two bandpass filters from 0 to 2f₀ (f₀ is the centre frequency of the passband). Five and eight transmission zeros can be adjusted freely by only changing the electrical length of the open/shorted stubs. Two planar bandpass filters (3-dB fractional bandwidths 7.44%, 3.88-4.18 GHz and 7.5%, 3.85-4.15 GHz) are designed and fabricated. The theoretical and measured results all show good in-band filtering performances and high selectivity.