Narrowband SIW-SSPP Hybrid Bandpass Filter With Compact Profile at Ka-Band

In this paper, a narrowband bandpass filter comprising substrate integrated waveguide (SIW) and spoof surface plasmon polaritons (SSPPs) is proposed, achieving miniaturization and low insertion loss at Ka-band. SIWs and SSPPs exhibit low and high cut-off responses, respectively, which can be combined for filtering functionalities. By designing I-shaped SSPP structures with large effective length, the cut-off frequencies of SIW and SSPPs can be tailed to converge at certain frequency for narrowband filter, where the SIW mode functions as a quasi-evanescent mode, and the mode impedance ramps fast. A sharp funnel-shaped transition is designed to connect SIW with large impedance and 50-Ω microstrip line for low insertion loss and compact profile. In addition, no transitional SSPP structures are desired in the proposed devices, which further shortens the profile dimension. To verify our design, a hybrid filter at 30 GHz is fabricated and characterized. The measured insertion loss is as low as 1.5 dB, the 3-dB bandwidth is as narrow as 3%, which shows good consistency with the simulation. The proposed SIW-SSPP device has a low profile of 0.70λ × 0.38 λ and may find important applications in frequency division multiplexing.