# Frequency Selective Surfaces: Design, Analysis, and Applications
## 11 Discussion  
The review of frequency selective surfaces (FSSs) highlights several key findings. FSSs are planar structures composed of periodic metallic layers printed on dielectric substrates, with unit cells of varying geometrical shapes, including patches, loops, hexagons, convoluted shapes, and fractal geometries. Their frequency responses depend on these geometries and materials, categorizing them into low pass, high pass, band stop, and band pass filters. Recent research has focused on miniaturized FSSs with high selectivity, operating efficiently across multiple frequency bands. Innovations such as dual-band and multiband FSSs enhance wireless communication and mitigate electromagnetic interference. The incorporation of active components has enabled tunability and reconfigurability, with active FSSs featuring convoluted elements showing potential for dynamic control over frequency responses, paving the way for adaptive solutions in communication and sensing systems. Studies on FSSs on flexible and textile substrates suggest potential for conformal applications and cost-effective, environmentally friendly electromagnetic filters. Additionally, emerging research explores FSSs in liquid sensing and strain sensing, showcasing their adaptability for various applications beyond traditional electromagnetic uses. The findings indicate that FSS technology is advancing rapidly, driven by the need for more efficient, compact, and adaptable electromagnetic filtering solutions. The diversity in structural designs and the ability to integrate active components reflect a trend towards more dynamic and multifunctional FSS applications. Exploring flexible and unconventional substrates suggests a shift towards practical, real-world applications where flexibility and environmental considerations are crucial. Furthermore, the innovative use of FSSs in sensing applications demonstrates their potential beyond conventional filtering roles, indicating a broader scope for future research and development. The advancements in FSS technology have significant implications for various fields. Improved selectivity and miniaturization of FSSs can enhance the performance of wireless communication systems by providing more efficient filtering and reduced electromagnetic interference. The adaptability of FSSs for liquid and strain sensing opens new avenues for integrating these surfaces into structural engineering and environmental monitoring systems. The use of flexible and unconventional substrates for FSSs suggests potential for developing new materials and manufacturing processes, leading to cost-effective and sustainable solutions. Additionally, the application of FSSs in radomes and multiband reflector antennas demonstrates their value in defense and aerospace technologies, where efficient filtering and reduced radar cross-sections are critical. Despite the promising advancements, there are several limitations to consider. The design and realization of FSSs with desired frequency responses require precise control over unit cell geometries and materials, which can be complex and resource-intensive. The frequency response of FSSs can degrade under oblique incidence, affecting their performance in real-world applications where waves may not always be perpendicular. Incorporating active components into FSSs for tunability and reconfigurability presents integration challenges, particularly in maintaining stability and performance. Moreover, the performance of FSSs on flexible and unconventional substrates may be limited by the mechanical and electrical properties of the materials used. One of the key challenges in designing reconfigurable FSSs is solving optimization problems related to surface configuration. Reconfigurable FSSs must dynamically adjust their resonant frequency, polarization, and bandwidth to meet varying operational requirements. To achieve this, optimization algorithms such as genetic algorithms, particle swarm optimization, and differential evolution are commonly used.