Low-Loss and Compact SISL Bandpass Filter Using Multilayer Stacked DLDCR

In this letter, a low-loss and compact substrate-integrated suspended line (SISL) bandpass filter (BPF) is designed by the improved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor double layers and dielectric cutout resonator (DLDCR). Compared to the classical SISL single-layer resonator, DLDCR exhibits lower dielectric loss. By incorporating multiple metalized vias in the DLDCR, the desired improved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor quasi-TEM mode remains minimally impacted, while the undesired resonant induced by the substrate-integrated suspended parallel stripline (SISPSL) mode is effectively moved to a frequency far away from the operating frequency of BPF. Meanwhile, to further reduce the dielectric loss associated with the resonator coupling, DLDCRs are vertically stacked on multiple boards to achieve air coupling. As a verification, a high-selectivity cascaded quadruplet (CQ) SISL BPF operating at 8.1 GHz using the low-cost FR4 dielectric is synthesized, simulated, and measured. The proposed BPF has the advantages of low loss, compact, low cost, and self-packaging.