A CPW-Based Novel SSPP Reflectionless Low-Pass Notch Filter With Loaded Interdigitated Coupling Structure

This study proposes a novel spoof surface plasmon polariton (SSPP) transmission line unit based on a coplanar waveguide (CPW) design. The dispersion relationship is derived from its equivalent circuit, and a low-pass notch filter is designed accordingly, operating in the frequency range of 0 GHz to 4.45 GHz, effectively blocking the transmission signal at 2.4 GHz. The filter has the advantages of a compact size, high transmission efficiency, and compact structure. The mode transition from quasi-transverse electromagnetic (TEM) waves in CPW to transverse magnetic (TM) waves in the SSPP waveguide is achieved using a centrally symmetric linear gradient groove structure. For the first time within the scope of this study, we propose a novel method for forming notch filters that involves radiating signals of selected frequencies into free space to create notch, thereby eliminating the negative effects of the reflected signals at notch point. This study achieves signal radiation at 2.4 GHz to free space, forming a notch, by loading an interdigitated coupling structure on the backside of the filter. The relationship between finger length and resonant frequency enables the reconfigurability of the filtering band. To validate the accuracy of the proposed filter, a prototype filter is fabricated and tested in this frequency range. The measurement results are consistent with the analytical results.