Dual-Mode Characteristics of Half-Mode SIW Rectangular Cavity and Applications to Dual-Band Filters With Widely Separated Passbands

The dual-mode characteristics of the half-mode substrate-integrated rectangular cavity (HMSIRC) are systematically analyzed for the first time and experimentally demonstrated by two planar substrate-integrated waveguide (SIW) dual-band bandpass filters (DBBPFs) with widely separated passbands. The mode spectrums in an HMSIRC and their constraint relationships are comprehensively investigated first to explore the realizable frequency ratio of the second mode (TE301 or TE102) to the first one (TE101). The maximum frequency ratio of 1.63 for TE301/TE101 and 1.72 for TE102/TE101 can be achieved considering the spacing between the second and third resonances. Subsequently, the formulas of determining the physical dimensions of the dual-mode HMSIRC are derived and validated. The factors impacting the unloaded quality factor Qu of the HMSIRC are also discussed and the square single-mode quarter-mode substrate-integrated rectangular cavity has been investigated. Two compact planar SIW DBBPFs, including a third-order direct-coupled one with frequency ratio of 1.5 based on TE101 and TE301 dual-mode HMSIRC, and a fourth-order cross-coupled one with frequency ratio of 1.7 based on TE101 and TE102 dual-mode HMSIRC, are synthesized and designed for verification.