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 a certain frequency for the 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 a large impedance and 50- $\Omega $ microstrip line for low insertion loss and a 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, and 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 $0.70\,\,\lambda \times 0.38\,\,\lambda $ and may find important applications in frequency division multiplexing.
