Abstract
In this letter, high selectivity bandpass filters (BPFs) using folded stepped-impedance resonators (SIRs) are proposed based on substrate-integrated suspended line (SISL) platform. It is easy to change the position of transmission zeros (TZs) by controlling the strength of electric/magnetic coupling, separately. Also, the structure is compact with double-layer wiring and via holes. To validate the proposed method, two SIR electromagnetic coupling BPFs are implemented on FR-4 substrates. The rectangular coefficients (BW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{20\, \text{dB}}}$</tex-math> </inline-formula> /BW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{3\, \text{dB}}$</tex-math> </inline-formula> ) of the second-order and the fourth-order filters are only 2 and 1.3, respectively. In addition, air cavity, metal shielding wall, and substrate excavation methods also dramatically reduce the loss in substrates.
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