Compact Slotline Bandpass Filter With Wide Upper Stopband Without Spoof Surface Plasmon Polariton Gradient Transitions

In this work, a compact bandpass filter based on slotline bent spoof surface plasmon polariton (SSPP) units loaded with metal patches is proposed. Through systematic analysis of the unit’s electric field distribution and impedance characteristics, it is found that the metal patch can adjust the equivalent dielectric constant of the entire unit, thereby modifying the unit’s impedance to achieve impedance matching. By loading SSPP units with slow wave effects into the microstrip-to-slotline transition structure, the size of the transition structure is successfully reduced. Combining the microstrip-to-slotline structure with the slotline bent SSPP unit loaded with a metal patch, a compact bandpass filter is designed. The filter eliminates the need for an SSPP gradient transition structure, with upper and lower cutoff frequencies independently controlled by the SSPP structure and the microstrip-to-slotline transition structure during design. To further improve out-of-band suppression, an SSPP low- pass structure is loaded onto the filter, leveraging its stopband effect to enhance the upper stopband suppression. Finally, the filter was fabricated and experimentally measured. The measured results demonstrate that the filter exhibits a passband from 2.4 to 9.0GHz with a fractional bandwidth (FBW) of 115.8%. The$\vert $S${}_{21}\vert $is higher than −0.83dB and$\vert $S${}_{11}\vert $within the passband is lower than −15.0dB, and$\vert $S${}_{21}\vert $in the range of 9.8–19.9GHz remains below −30dB. The filter exhibits excellent passband response and out-of-band suppression, with an overall compact size of only$0.475~\lambda _{g} \times 0.298~\lambda _{g}$. It holds significant application potential in microwave circuit systems such as microwave communication base stations, radar systems, and satellite communication terminals.