Compact Dual-Mode SIW Bandpass Filters With Ultrawide Stopband Performance

This letter presents novel dual-mode bandpass filters (BPFs) based on a substrate-integrated waveguide (SIW) cavity loaded with slots, demonstrating small sizes and wide stopband performance. By etching <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i>-shaped split-ring slots on the top surface, the resonant frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{0}$</tex-math> </inline-formula>) of the fundamental mode transverse magnetic (TM)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> is remarkably reduced, thereby increasing the frequency ratios between the higher order modes and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode. The introduction of a metallized via enables the dual-mode operation by shifting the TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode closer to TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode while simultaneously moving TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode to approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5~f_{0}$</tex-math> </inline-formula>. CPW feeding with coupling slots is utilized to further enhance the stopband suppression. The operating principles are demonstrated through eigenmode solvers, facilitating the analysis and design of the filters. Two prototypes were fabricated and measured, achieving upper stopband rejections of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9.84~f_{0}$</tex-math> </inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13.6~f_{0}$</tex-math> </inline-formula>, both at a rejection level of 30 dB, thus effectively contributing to the suppression of out-of-band interference from adjacent wireless systems or internal nonlinear front-end modules.