Miniaturized micrsotrip high-selectivity bandpass filters with asymmetric frequency characteristics

A novel transmission line topology, using coupled-line and stepped impedance open-stub, is proposed for the design of compact bandpass filter. The topology produces high selective asymmetric bandpass response by realizing a transmission zero either lower side or higher side of the centre frequency. The overall size of the filter unit is compact, which is less than λ_g/4 × λ_g/4. Further due to the dual-mode like behavior, the filter exhibits low insertion loss. Design equations are derived explicitly based on lossless transmission line model analysis. Two filter prototypes operating at S-band having narrow bandwidths (<;10%) and having rejections of about > 30 dB at f₀±200 MHz are fabricated and demonstrated.