Compact Bandpass Filter Based on Interdigital Shorted Stepped-Impedance Stub SSPPs With Wide Stopband Rejection

In this letter, an interdigital structure is integrated into a shorted stepped-impedance stub (SSIS) and then a new interdigital SSIS spoof surface plasmon polaritons (SSPPs) unit with enhanced slow wave effect is constructed. Based on the proposed SSPPs structure, a compact bandpass filter (BPF) with wide stopband rejection is designed. Compared with the conventional rectangular SSPPs, the proposed interdigital SSIS SSPPs not only exhibit the bandpass property of fundamental mode and lower cutoff frequencies but also enable flexible control of the frequency spacing between high-order modes. By staggering the frequency bands of high-order modes of size-different SSPPs units, the spurious passbands can be suppressed and consequently a wide stopband is obtained. For demonstration, a compact BPF working at the center frequency ( $f_{0}$ ) of 2.3 GHz with stopband rejection up to $6.9f_{0}$ is fabricated and measured, whose overall size occupies only $0.61\lambda _{g} \times 0.10\lambda _{g}$ ( $\lambda _{g}$ : guided wavelength at $f_{0}$ ).