# Frequency Selective Surfaces: A Review
## 5 Future Challenges and Potential Applications of FSSs
Importantly, most designers possibly may not be aware to check the dielectric profile, and instead would focus on changing other geometric parameters (typically inter-element spacing), without noticing the effect on the grating lobes, cross-polarization, and instability of the resonating frequency. It is challenging to sort the chaos that is created by such optimization computer program. Therefore, it isvery existent and a big problem for the designer to appropriately define parameters during the optimizing process about the good and bad result. 4. Accurate Analysis  
Generally, a good estimated FSS analysis yields very accurate response and is reliable, as predicted [2]. However, surprisingly this does not sometimes work in a few designs. The reason is that it is assumed the distribution of current on the array elements remains constant and only amplitude and/or phase vary with the incidence angle. On the contrary, in some cases, the odd-modes may alter the current distribution for oblique incidence angle, and so the BW is no more constant, thereby failing the analysis. A solution to this problem might be using compact array elements such that only the primary current distribution lies in the resonant area. However, there is a limitation to reduce the element size and inter-element spacing. Also, this may impact the flatness of the transmission curve's top. Therefore, a trade-off needs to be made between constant bandwidth and flat top of the frequency response. 5. Miniaturized Design  
Miniaturized FSS are required in some applications, especially in antenna radomes, because of flexibility requirement. In addition, large inter-element spacing and element dimensions may not be suitable in realizing curved surfaces [64]. Reduced unit cell size is enviable with fairly small electrical dimensions and higher angular stability of the FSSs. Therefore, the concept of the miniaturized array element has been evolved broadly, since they separate the operating bands from the grating lobes [62; 89]. Recently, various FSS miniaturization techniques have been utilized by different element shapes, Still, this challenge needs to be solved for many low-space and rigid applications. 6. Higher operating bandwidth  
A key challenge in FSSs design and theory is its limited operating BW, especially it is important to improve the BW for single layer FSS structures, as seen in Table 3 in this review. Although multilayer FSS and many schemes are presented in this regards, however, FBW is still not satisfactory: Meandered/convoluted/Interwoven, loop and fractal elements have been used to minimize the unit cell dimensions along with the thickness of the FSS, but these may not necessarily improve the BW. Even though multilayer build helps to achieve an optimal design, but attaining higher BW, especially at lower operating frequency is the foremost challenge for the FSS designers. 7. Fabrication challenges in 3D structures  
As compared to conventional FSS that is composed of 2D periodic array elements that experience poor frequency selectivity, slow roll-offs, and instability under different incident angles, recently reported 3D-FSS are innovative and alleviate the problems faced with 2D FSS. Although good frequency ranges of response have been demonstrated by these 3D structures, the real challenge related tothis advanced technology is the complexity of realization and fabrication. Extensive research is going on to beat this difficulty, such as in [112; 113; 131]. Ground-breaking implementations and superior performance in 3D FSS are predicted in the near future with broader bandwidth, in-band stable response, sharp cut-off, and wider out-of-band rejection. 8. Transmission through energy saving Glass  
Passband FSSs are designed to allow for the transmission of useful wireless communication signals through energy saving window glass in modern buildings with the key requirements of stable frequency response, and reasonably higher BW. However, FSS apertures need to be narrow as much possible for higher thermal insulation and offer a big challenge.