Design of compact quad‐band bandpass filter with good selectivity using two‐/tri‐section SIRs and stepped impedance inverters

Abstract

A compact quad-band filter with controllable central frequencies and bandwidths based on the $\lambda /4$ λ / 4 SIR, K inverters and multiple signal transmission paths technique is presented. Two synthesis design approaches and procedures for dual-band filter are firstly proposed, and then the investigations on two kinds of $\lambda /4$ λ / 4 SIRs, uniform and stepped impedance K inverters are carried out, respectively. Moreover, it is worth mentioning that the impacts of key design parameters, Ws 1 , Ls 1 , Rz , and α , on the impedance characteristics and electrical lengths of K inverters are numerically investigated. Based on the analysis, two demonstrations of dual-band filters are designed. One utilises the two-section $\lambda /4$ λ / 4 SIR and uniform impedance K inverter, and the other uses the tri-section $\lambda /4$ λ / 4 SIR and stepped impedance K inverter. As the cells of the quad-band filter, the two designs of dual-band filters employed as upper and lower sub filters, respectively, are combined based on the multiple signal transmission paths technique. The upper sub filter creates the second and third passbands and the lower sub filter forms the first and fourth passbands, respectively. The surface current distributions are also studied to verify the design strategy. Finally, a example is implemented, the measured and simulated results agree well.