A Triband Second-Order Frequency Selective Surface

In this letter, we propose a triband second-order fre-quency selective surface (FSS). Each second-order bandpass re-sponse is realized by a three-layer patch-aperture-patch (PAP) structure. The PAP elements with different sizes are then arranged to a unit cell that is to be repeated periodically. Based on this tech- nique, an FSS unit cell with triband second-order response is de- signed. There is a respective second-order bandpass response for each operation band. The triband FSS has the merits of high selec-tivity and stable performance for incident angles within 30° and for different polarizations.