Design of Balanced Diplexer Based on Hybrid Microstrip and Slotline Transition Structure With Intrinsic CM Suppression and Wide Stopband

This paper proposed a balanced diplexer featuring intrinsic common-mode (CM) suppression and a wide stopband. The proposed design incorporates three balanced hybrid microstrip and slotline transition structures (HMSTS) and two sets of channel bandpass filters (BPFs). Each HMSTS combines a U-shaped microstrip feedline with a stepped-impedance slotline, enabling inherent full-band CM suppression and simplifying the differential mode (DM) design process. In the channel filter design, E-shaped stub-loaded resonators (ESLRs) and single-mode C-shaped resonators (SCRs) are employed as non-resonant nodes, acting as virtual sources and loads for the BPFs. Each channel filter is configured as a fourth-order filter using folded U-shaped resonators (FURs), resulting in a compact layout for the diplexer. To achieve a wide stopband, the first spurious harmonic of the ESLR is designed to be well-separated from the harmonic of the FURs, effectively disrupting unwanted resonant paths. To validate the proposed design, a prototype balanced diplexer operating at 3.1 GHz and 3.8 GHz was designed, fabricated, and measured. The results demonstrate favorable DM and CM performance, with excellent agreement observed between simulated and measured.