Contents I. Introduction
Frequency selective surfaces (FSSs) have attracted much attention in the past few decades for their spatial filtering characteristic and have found extensive use as radomes, dichroic reflectors, absorbers, polarizers, and artificial electromagnetic materials, etc. [1]. The antenna–filter–antenna (AFA)-based FSSs are widely used due to their superior frequency selectivity and lower profile than conventional FSSs [2], [3]. Although the FSSs based on AFA module can provide good frequency selectivity while keeping low profile, the unit sizes of these FSSs are considerably large. For example, the periodicity of the designed FSS in [2] approaches , where is the wavelength in free space with respect to the central frequency in the operating band. Miniature FSSs are often favorable because small periodicities of FSS element will postpone the onset of grating lobe or trapped wave in thick supporting dielectric substrates and yield less distortion of elements to FSS structures with significant curvature such as radomes. Because of the interesting characteristics of space filling and self-similarity, the concept of fractal geometry was introduced to devise compact or multiband electromagnetic constructions [4], [5]. It was first utilized to design dual-band FSS in [6], where the self-similar configuration of Sierpinski dipole was adopted as an FSS element to achieve dual-band operation. After that, a variety of fractals were used for FSS engineering, among which the space-filling property was exploited to obtain size reduction of FSS element [7]–[9].
