# Frequency Selective Surfaces: A Review
## 5 Future Challenges and Potential Applications of FSSs  
FSSs have been used in many applications, including reflectors, antenna radomes, radar absorbing materials, polarizers, and composite metamaterials. Although this research area is relatively old but enormously dynamic and fast growing, it bears a broad pedigree in applications as well as theory since numerous matured design and analysis tools have been developed. However, noteworthy problems related to theoretical and practical importance still to be solved, for example, the development of doubly curved FSS, non-periodic FSS. Generally speaking, required performance characteristics of an FSS are broader bandwidth, faster roll-off (sharp band edges), and stability with the variation of incident angles and polarizations of an incoming EM wave. It has been well identified from the earlier literature and in this review paper that the array element nature, its geometry, inter-element spacing, dielectric, and structure profiles are the defining structure parameters of an FSS [165]. Several critical challenges are to be addressed in successful designing of FSS structures. Choice of an appropriate FSS element, unit cell dimensions, substrate profile, and layer/iterations are of the major importance. Controlling these parameters to fit in demanding and compact applications has been a challenging task and it generally requires critical and good EM behavior analysis of an FSS. We perceive a number of challenging areas in significant exploration and practical applications of FSS where they might sharply impact the future trends include:  
1. Pragmatic FSS design  
In order to achieve a constant BW with the variation of incidence angle, low dielectric constant substrates and subwavelength thickness are required on the outer layers (for planar elements) [2]. However, currently, such low dielectric constant materials are not available, even if it is the case; they might not fit for mechanical reasons. An approach would be to engineer such demanding materials or find other appropriate solutions to this dilemma. 2. Design tools  
Another issue is linked with design tools. Full wave analysis techniques for all types of FSS geometries are very common nowadays as compared to two or three decades before. Therefore, it has become difficult to keep a record of such methods. Many commercial softwares are available providing accurate analysis with more reliability. However, periodic surfaces can be best analyzed with such analytical packages, because the structure is represented by a global unit cell with subwavelength dimensions. This limits the analysis for other types of complex or uneven FSS geometries including curved surfaces, tightly bound multiple FSS, and structures with non-periodicity [43]. It is not possible to apply approximation rules on such geometries that are commonly applied to the global unit cell. 3. Optimization Dilemma  
An abundance of approaches to optimization has been illustrated or put forward so far. Conversely, these should be utilized at the appropriate occasion and right place. No optimization process is followed in chapter 7 of ref. [2]. For-example, all of the results has been analyzed by examining each component of FSS. However, certain designs become questionable or difficult to optimize by calculations, as hundreds or thousands of test cases would require to be solved for different incidence angles and polarizations. A quick computer and program would be essential here. On the contrary, for immature designers, this may be the worst experience in real life scenarios. As an example, when the resonant frequency and the BW change all over in some FSS designs, it may be unsuccessful for even moderate variations of incidence angles and polarizations. The computer program might continue simulating in a loop without a good solution.