Design of Wideband Bandpass Filters Using Parallel-Coupled Asymmetric Three Line Structures with Adjustment Elements

In this paper, three microstrip wideband bandpass filters having three, five and seven poles are designed by using parallel-coupled asymmetric three line structures. The proposed filter structures have parallel-coupled asymmetric line blocks having unequal strip widths and coupled each other with coupling gaps. Multi pole frequency responses are resulted from the cascaded parallel coupled transmission lines. The designed filters have adjustment elements as capacitive patches on asymmetrical three line blocks to enhance the control of in-band return loss levels. Approximate theoretical analyses of each filter are realized by introducing ABCD matrices. Three microstrip wideband filters are designed, analyzed and fabricated. Measurement results show good agreements with predicted results.