  In this paper, a compact rejection filter is proposed based on 2.5-D spoof
surface plasmon polariton (SSPP) structure and folded spilt-ring resonators
(SRRs). By embedding the folded SRRs into periodic grooves of the 2.5-D SSPP
transmission line, a rejection band is created in the ultra-wide passband,
resulting in pass-rejection-pass frequency characteristics. The rejection band
and the cut-off frequency of the filter can be adjusted independently by
changing the geometrical parameters of the 2.5-D SSPP rejection unit. Because
of the compact folding structure, the lateral size is reduced by 54% compared
with the conventional 2-D SSPP rejection filter.
