Compact linear-phase bandpass filters with notched frequency response employing eighth-mode substrate integrated waveguide and dual-mode microstrip line structure

A series of fourth-order bandpass filters (BPFs) with compact footprints, broad bandwidth, high selectivity, linear phase and notched response are proposed. The sketch of these BPFs is in accordance with the combination of the eighth-mode substrate integrated waveguide (EMSIW) and dual-mode microstrip line (DMML) resonators. The DMML resonator is constructed by means of metallic via-holes and pad in the double-layered structures. Among them, the DMML resonator can provide two transmission poles. Besides, the cross coupling is introduced through the vertical slot or fan-shaped aperture in the dual-layered substrates. The coupling schemes of the proposed BPFs can introduce two transmission zeros in the vicinity of operating passband, and the coupled scheme also exhibits a relatively flat in-band group delay. Notched frequency responses have been obtained by exploiting the U-shaped resonator and folded-ring resonator embedded with the mixed EMSIW and DMML BPFs. To validate the conceptual design, the proposed BPFs are simulated, fabricated and measured. Both the simulation and measurement have reached to a satisfactory agreement.