Ultra-Miniaturization Design of Balanced Bandpass Filter Using Substrate-Integrated Coaxial Resonators

This study proposes ultra-miniaturized full- and half-mode balanced bandpass filters (BPFs) that incorporate four significantly size-reduced full and half substrate-integrated coaxial resonators (SICRs). The microstrips, in conjunction with the coupling slots, form the feeding structure of the filters. The proposed full- and half-mode SICR balanced BPFs exhibit center frequencies of 0.49 and 0.66 GHz, respectively, which are only 8.22% and 10.9% of the resonance frequency (5.96 GHz) of their substrate-integrated waveguide cavity (SIWC) counterparts. Sample full- and half-mode SICR balanced BPFs have been fabricated for measurement, and the results are consistent with the simulations. The common-mode rejection levels are greater than 50 and 48 dB, respectively, for the full- and half-mode configurations across a broad frequency range. The minimum in-band insertion losses (ILs) are approximately 1.7 dB for the full-mode configuration and 2.2 dB for the half-mode design.