Comment on ??An Overview of Three-Dimensional Frequency-Selective Structures?? [Letter to the Editor]

In the above paper (published in the IEEE Antennas and Propagation Magazine, 56, 3, June 2014, pp. 43-67), it was stated that the single-layer two-dimensional (2-D) frequency-selective surface (FSS) with dipole, ring, or square-loop elements exhibits a poor filtering response, and fails to meet the performance requirements of practical applications. On the contrary, various single-screen two-dimensional frequency-selective surfaces [1-6] were described with a sharp transition from in-band to out-of-band characteristics, which was also insensitive to the angle of incidence. Specifically, Holloway, et al. [1] presented the single-screen two-dimensional frequency-selective surface with split-ring resonator (SRR) elements, as depicted in Figure 1, having a sharp transition frequency response and nearly incident-angle-independent behavior up to 60°. Figure 2 shows the quasi-optical grids with a sharp transition performance and similar split-ring resonator elements printed on a one-mil thick Kapton sheet [2]. Finally, Figures 3a-3e illustrate the superior filtering performance of the single thin-screen two-dimensional frequency-selective surfaces with the fractal-type elements [3-6]. These low-mass, -cost, and -volume two-dimensional frequency-selective surfaces can be readily mounted in front of a large antenna aperture for a myriad applications in advanced and multifunctional radar or communication systems.