I. Introduction
Frequency selective surfaces (FSSs) are spatial filters for electromagnetic waves [1]–[3]. FSSs have been studied for many applications, such as antenna subreflectors, radomes, polarizers, and so on. FSSs are usually divided into two classes: metal patches printed on dielectric substrates and apertures realized on a metal plate [1]. In both cases, patches and apertures can be of arbitrary shape and they are placed in a 2-D periodic lattice [4]–[9].