Transparent frequency selective surfaces with high optical transparency and wide bandwidth for 5 G mm-wave spatial filtering applications

This study presents a design method for a transparent frequency-selective surface (FSS) that combines high optical transparency (OT), enhanced frequency selectivity, and broad bandwidth (BW) performance, specifically for 5 G millimeter-wave spatial filtering applications. Transparent FSSs are designed to function as spatial filters in electromagnetic environments where visibility is required—such as on building windows, vehicle glass, drone surfaces, and radomes. However, most existing research has concentrated on lower frequency bands (e.g., S-band and X-band), with relatively little attention paid to the mm-wave range (K and Ka bands), which is essential for next-generation 5 G communications. To address this gap, a double-layer FSS (D-FSS) is proposed, composed of two spatially separated single-layer FSSs (S-FSSs). This configuration substantially improves frequency selectivity and achieves a wide fractional bandwidth of 23.1 %, spanning 24.5–30.9 GHz—covering the n257 and n258 bands used in 5 G mm-wave systems. Each S-FSS consists of narrow, symmetrically arranged metallic lines that provide both excellent optical transparency—84 % for the S-FSS and 70 % for the D-FSS—and strong angular stability. The proposed FSS structures demonstrate significant potential for improving signal coverage and transmission bandwidth in a variety of 5 G mm-wave applications.