A Ridge WG SSPP Bandpass Filter With Lower Transmission Zero and Extended Upper Stopband

This article proposes a novel design method for introducing a lower transmission zero (TZ) in a ridge waveguide (WG) spoof surface plasmon polariton (SSPP) bandpass filter (BPF). Initially, the inverted T-shaped slots are etched on the central ridge to construct the SSPP and then design a wideband BPF, and the upper and lower cutoff frequencies of passband are determined by the SSPP and double gratings (DGs) on both sides of the ridge WG. To improve the rejection roll-off of the lower stopband, a pair of folded slots are etched at both ends of the ridge to form a quarter-wavelength ( $\lambda $ /4) resonance for introducing a TZ below the passband without increasing the size. Additionally, the stopband of the proposed SSPP filter can be extended by optimizing the DGs to suppress the higher order modes of the SSPP. To demonstrate this method, a ridge WG BPF was fabricated and measured. The measurement results indicate that the filter possesses favorable frequency selectivity and a wide stopband.