Design of a CPW-based SSPP band-pass filter with reflectionless notch

Abstract This work proposes a band-pass filter (BPF) with a reflectionless notch based on spoof surface plasmon polaritons (SSPPs), utilizing interdigital coupling structures and novel transmission line unit cells. This filter efficiently transmits signals within the 0.67 GHz −4.06 GHz frequency range. By analyzing the equivalent LC circuit of the novel transmission line unit cell, its dispersion relation is derived using microwave network theory, with a cutoff frequency of 4.11 GHz. By comparing its dispersion relation with that of the conventional transmission line unit cell, the miniaturization capability of the proposed unit cell can be verified. In the BPF, loading a zigzag groove onto the central transmission line can be equivalently represented as an interdigital coupling structure, generating a stopband in the low-frequency range near 0 GHz. By deriving and analyzing its S-parameters, it is shown that the bandwidth of the low-frequency stopband can be flexibly adjusted by modifying the geometric dimensions of the zigzag groove. Additionally, loading another set of interdigital coupling structures onto the transmission line generates a notch at its resonant frequency of 3.4 GHz. It is noteworthy that in this configuration, the interdigital coupling structures along with the central transmission line can be represented equivalently as a set of CPW antennas. At the resonant frequency, the atructure radiates signals into free space, forming a reflectionless notch. Based on the simulations, a physical filter was fabricated and tested, showing excellent agreement between simulations and measurements.