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

ABSTRACT 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.