A Compact Low-Loss Bandpass Filter Using Spoof Surface Plasmon Polaritons in a Ridge Waveguide

This article investigates a spoof surface plasmon polariton (SSPP) first by etching the inverted T-shaped slot on the central ridge of the ridge waveguide (WG) for constructing a low-loss bandpass filter (BPF). To achieve flexible bandwidth, the upper and lower edges of the passband can be controlled independently by the SSPP and double gratings (DGs) on both sides of the ridge WG. Furthermore, the lower impedance of the ridge WG allows it to be directly connected to the coaxial connector to achieve a compact size. To validate the approach, an X-band ridge WG BPF is fabricated with a measured bandwidth of 7.9-12.2 GHz. The good agreement between simulation and measurement results demonstrates the method's advantages of small size, flexible controlling of bandwidth, and low insertion loss (IL) of 0.23 dB.