Substrate Integrated Waveguide Bandpass Filters in Box-Like Topology With Bypass and Direct Couplings in Diagonal Cross-Coupling Path

Substrate integrated waveguide (SIW) bandpass filters in box-like topology based on combining single- and dual-mode cavities are presented. Two coupled TE
₁₀₁
-mode-based cavities are coupled to one square-shaped dual-mode (modes TE
₁₀₂
and TE
₂₀₁
) cavity to realize the box-like coupling scheme in which bypass and direct couplings can provide the diagonal cross-coupling path. Based on the characteristics of dual-mode SIW and multiple energy flow paths, the superior design flexibility in terms of TZ positioning has been realized and demonstrated. Finally, two asymmetrical response SIW filters with two lower TZs, a quasi-elliptic SIW filter with three TZs and a filter with linear-phase response are simulated, fabricated, and measured to demonstrate and verify the proposed structure.