Substrate integrated waveguide bandpass filter with wide upper stopband

This communication presents the design of a Substrate Integrated Waveguide (SIW) Bandpass Filter (BPF) with wide stopband. The BPF is centered at 3.5 GHz with 0.5 dB equal ripple bandwidth of 4 % and offering a stopband extended up to 40 GHz. The FBW is so chosen that the variation of insertion loss remains within 1 dB over the 3.4 GHz – 3.6 GHz communication band. The proposed vertical integration uses the coupling through rectangular slots on the common plane of two SIW resonators. The slots are positioned to obtain required coupling in the TE₁₁₀ mode whereas at other higher order modes, couplings are minimum resulting a wide stopband. In measurement, the BPF offers minimum insertion loss of 2.55 dB at the center frequency of f₀ = 3.49 GHz and over 3.42 GHz – 3.59 GHz, it remains better than 3.55 dB. Over the mentioned frequency band, the measured return loss is better than 15 dB. The out of band rejection is at least 50 dB at (f₀ + 0.22) GHz and (f₀ – 0.2) GHz whereas in the upper stopband, minimum suppression is 40 dB over 3.7 GHz – 20 GHz and remains better than 30 dB up to 40 GHz.

Index Terms— Substrate Integrated Waveguide (SIW), Bandpass filter (BPF), harmonics.