Development of an Air-filled SIW Filter with Wideband Spurious Suppression

In this paper, we propose an air-filled substrate integrated waveguide (SIW) bandpass filter (BPF) to realize the wideband attenuation. The proposed filter has a direct-coupling structure via an iris. In the area where the TE103 and TE104 modes appear as electric fields of spurious modes are concentrated, the dielectric of the substrate is removed to make a hollow structure. The effective dielectric constant of the substrate decreases as a part of the substrate becomes hollow, and the spurious modes move to a higher frequency band. The designed BPF achieved −20 dB out-of-band rejection from 6.0 to 11.7 GHz. The hollow structure reduced insertion loss to −0.3 dB.