Resonance characteristics of substrate-integrated rectangular cavity (SIRC) are systematically analyzed in this paper. Synthesis design techniques of substrate-integrated waveguide (SIW) dual-band bandpass filters (BPFs) and wide stopband BPFs are demonstrated on the basis of these characteristics. The design concept of dual-band BPFs is to add some extra single-mode SIRC resonators to dual-mode resonator filters to increase the degrees of freedom in extracting direct-coupling coefficients, while the external quality factors Qe and crosscoupling coefficients desired in two passbands can be realized simultaneously by determining proper offset position of the I/O ports and cross-coupling window, respectively. Consequently, all the design parameters including Qe and coupling coefficients Mi j required for both the passbands could be satisfied. The principle of wide-stopband BPFs is to select the constitutive SIRC resonators with identical fundamental frequency but staggered higher order ones to suppress multispurious peaks to low levels and narrow bandwidths. Then emphasis is placed on the rejection of TE202 mode, with its resonant frequency two times of TE101 for all SIRC resonators, by centered I/O ports and coupling windows. Several experimental prototypes of SIW dual-band BPFs and wide-stopband BPFs are designed and fabricated to validate these concepts. The measured results are in excellent agreement with the simulations.

A realization method, named the design parameters ratio technique, of substrate-integrated waveguide (SIW) dualmode dual-band bandpass filters (DM-DBBPFs) with widely controllable bandwidth ratios is presented when TE ₁₀₁ and TE ₂₀₁ modes in substrate-integrated rectangular cavities are exploited to dominate the first and second passbands, respectively. Emphasis is placed on filters design to simultaneously realize the design parameters including the external quality factors Q _e and coupling coefficients M _ij required for both passbands by determining proper offset positions of the feeding ports and coupling windows, which can easily be fixed by ratios of synthesized Q _e and M _ij for the two bands, respectively. Consequently, the fractional bandwidths (FBWs) of the two passbands can be allocated flexibly over a wide range of ratios, typically with the FBW ratio of 0.2-1.5. Three SIW DM-DBBPFs including a thirdorder direct-coupled one operating at 12 and 16 GHz with the identical absolute bandwidths, a third-order cross-coupled one operating at 12 and 15 GHz with the same FBWs, and a fourth-order cross-coupled one operating at 12 and 17 GHz with the FBW ratio of 0.6 are designed and fabricated to demonstrate the method.

A realization method of wide-stopband substrate-integrated waveguide (SIW) diplexers and dual-band bandpass filters (BPFs) with widely realizable frequency ratios is presented using modified intrinsic modes suppression technique. Design procedures of dual-band and wide-stopband characteristics for different frequency ratios are systematically summarized by sufficiently using the limited design degrees of freedom. The mixed dual-and single-mode cavities and the offset external coupling slots could help to flexibly control the dual-band frequencies and bandwidths. Meanwhile, the internal coupling windows could produce predictable wide-stopband performance when properly arranged at the positions of weakest electric fields of specific unwanted spurious modes determined by the frequency distributions for the required frequency ratio. Two SIW diplexers and an SIW dual-band BPF with frequency ratios of 1.225, 1.5, and 1.425 are synthesised and designed for demonstration, extending the stop-band up to 1.79f0 with better than 24 dB rejection level, 2.05f0 and 2.34f0 with better than 20 dB rejection level, respectively.

A realization method for compact diplexers using mixed dual-mode and single-mode resonators is presented based on an advanced coupling scheme. An example with one common dual-mode (TE₁₀₁ and TE₂₀₁) cavity and each channel of two single-mode (TE₁₀₁) cavities is demonstrated using substrate-integrated waveguide technology. The footprint of the diplexer is reduced compared to the traditional ones since the dual-mode cavity acts as a T-junction between the input port and the two channels and provides one pole for each channel filter. In addition, the fractional bandwidths (FBWs) of the two channel filters can be allocated flexibly based on the FBW design graph. A prototype centered at 12 and 14 GHz with respective FBWs of 3.25% and 3.75% is designed for demonstration.

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.

Substrate-integrated waveguide (SIW) triple-mode triple-band bandpass filters (BPFs) are synthesized, designed, and demonstrated for the first time based on a novel type of SIW triple-mode square cavities perturbed by centered cross-shaped metallized via holes. The resonant characteristics of the cavity are analyzed first to show the range of the realizable frequency ratios, and the design parameters corresponding to the specifications, i.e., the external quality factors and the internal coupling coefficients for the three passbands, can be specified flexibly within certain ranges by exploiting an advanced triple-mode coupling controlling technique. Two SIW triple-mode triple-band BPFs, including a third-order direct-coupled one operating at 13, 14, and 15 GHz and a fourth-order cross-coupled one operating at 11, 12, and 13 GHz, are synthesized, designed, fabricated, and measured for demonstration. Compact circuit sizes and excellent filtering performances have been achieved for the two prototypes.