Miniaturized Integrated Bandstop Filter With High In-Band Performance Based on Substrate Integrated Coaxial Line

This study introduces a high-performance and compact bandstop filter (BSF) designed using substrate integrated coaxial line (SICL) and stepped-impedance resonators (SIRs) in a multilayer printed circuit board. Miniaturization is realized by a sinuous SICL suspended in the center of the cavity and two meandered SIRs vertically stacked above and below it. The in-band response can be finely controlled by adjusting the vertical coupling between the low-impedance sections of the SIRs. An equivalent lumped circuit model with admittance inverters is provided to explain the underlying principles. The BSF is highly compact, measuring just 0.14 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> × 0.10 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> × 0.03 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub>, and features a surface-mountable design for easy integration into circuits.