Design of Multiband Bandpass Inline Topology Filters Using Compact Multicoupled Line Structures

This article presents the synthesis and design of multiband bandpass filters (MBPFs) with inline topology. The coupled-line coupling structures are employed to achieve multichannel responses and a compact footprint. Two different approaches are developed in this research to implement coupled-line MBPFs based on frequency transformation methods. The first synthesis method is based on the direct mapping technique. According to this method, a quad-band bandpass filter (BPF) is theoretically synthesized and designed. The second proposed method is based on generalized ladder-type network analysis with frequency-invariant reactance elements (FIRs). It enables designing MBPFs with an arbitrary number of passbands. To validate this method, a sext-band MBPF is theoretically investigated, synthesized, and designed in microstrip technology as well. Very good in-band filter performance and very good band-to-band isolation are observed for the proposed MBPFs. These results show that the proposed synthesis methods and structures are attractive solutions to design MBPFs with coupled line technology.