Multi-band signal-interference planar bandpass filters based on stub-loaded transversal filtering sections

This paper addresses the application of bi-path stub-loaded in-parallel-transmission-line-type transversal filtering sections (TFSs) to the design of signal-interference microwave planar multi-band bandpass filters (BPFs). It is demonstrated that by inserting a ground-ended loading stub in one of the signal-propagation paths of their building TFSs, several performance benefits in multi-band planar BPFs can be gained. Among them, it is particularly noticed i) the creation of a higher number of passbands for a particular size of the TFS two-path loop, ii) DC-transmission supression, and iii) synthesis of wide-band dual-band bandpass filtering responses with broadly-separated passbands. A comparison of the stub-loaded TFS with its classic counterpart is provided. Two design examples consisting of three-stage quad-band and broad-band dual-band BPFs with widely-spaced passbands, along with measurements for the former one, are also presented.