Ka-Band Waveguide Bandpass Filters Using Double-Layer Grounded Spoof Surface Plasmon Polaritons

In this article, a new design method of waveguide bandpass filters (BPFs) using spoof surface plasmon polaritons (SSPPs) is proposed based on the double-layer grounded (DLG) technology. Two-layer metallic strips of the SSPP unit cells are coated on dielectric substrate and grounded to the rectangular waveguide (RWG) wall, where the substrate is inserted into the $E$ -plane of RWG. Due to the DLG structure, the capacitance coupling between two layers is introduced, and consequently the bandpass filtering responses are realized by the SSPP itself. For validation, two different types of Ka-band DLG-SSPP waveguide BPFs are presented and discussed. Their corresponding unit cells’ equivalent circuit models are extracted to clarify the filtering characteristics and transmission zero (TZ) generation. The electromagnetic (EM) simulation results show that the center frequency can be flexibly controlled by adjusting the groove length. Furthermore, in order to control the upper cutoff frequency independently, an improved structure of DLG-SSPP unit cell with central patch is also proposed. To demonstrate and verify the design feasibility, three examples of waveguide BPFs are fabricated. Good agreement between measured and simulated results show that the proposed idea will be a good candidate for the waveguide BPF design with compact size and flexibly tuning ability.