# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSS Based on Array Elements
#### 4.1.1 Basic Element Type FSSs  
In general, four basic types of FSS element groups have been classified [2] and are illustrated in Figure 5. This includes Group-1 (centrally connected or N-poles, such as dipole, tripole, square-spiral, Jerusalem crosses), Group-2 (looped shapes, e.g., circular, square, hexagonal loops), Group-3 (solid interiors or patch shapes of different shapes), and Group-4 (combinations of above all). Depending on the potential application, FSS designers select array element from any of these groups and/or use a combination of these as well. A good element should present a stable resonance response with the variation in incidence angles. Commonly, selecting a group-2 element with a larger loop area is beneficial and it enhances the bandwidth [2]. Simple circular loops, square loops to advance loaded elements with three or four legs have been utilized; especially hexagonal loop structures are useful for wideband applications. When circumference (perimeter) of the loop element becomes equal to a full wavelength, FSS reaches the resonance. It is found that an ample range of bandwidth can be achieved by changing the shape of these loop elements i-e loaded three or four-legged elements, which are narrow band, whereas hexagon loops are ultra wideband. Nevertheless, the bandwidth of almost all types of FSSs can be varied by altering the inter-element spacing, which must be small in terms of the wavelength. If the inter-element spacing is greater than half of the wavelength, it will result in the early onset of the grating lobes, pushing the main resonance towards downside with varying incidence angles. Figure 4: A taxonomy of FSSs as reviewed in this survey. Depending on the effective permittivity and with convincingly large inter-element spacing, such FSS resonates when the circumference is equal to a wavelength [2]. The bandwidth increases when the inter-element spacing is reduced. On the other hand, at a certain closer distance, this increases the inter-element capacitance as well, thereby pushing the resonant frequency towards the lower side. To overcome this discrepancy and regain the desired resonant frequency, the circumference of an element should be reduced. Group-1 stands next to loop family. With proper design of elements in this group, inter-element spacing can be reasonably reduced to experience extremely large bandwidth, for example, in case of gangbuster elements. This way, the inception of grating lobes is significantly delayed. Group 3 consisting of solid interior elements are mostly used in combination with complementary elements that are adjacent to it. Conventionally, the general attribute of FSSs is that they reach resonance at the desired frequency only when the patch or slot size in the unit cell becomes at least in the order of half of the wavelength. However, this limitation can be overcome by minimizing the electrical dimensions of the constituting element/inclusions at subwavelength level to ensure insensitivity to variations of incident angle and polarization. To facilitate such a requirement, FSS with miniaturized element is proposed by [PERSON] et al. [49], in which small metallic arrays are periodically printed in patch layer (capacitive mesh) on one side of a dielectric substrate. Wire grid patterns (inductive mesh) with equal subwavelength periodicity are arranged on the other side. Both capacitive and inductive layer in combination with the dielectric substrate, produce a parallel resonating LC-circuit. With such a structure arrangement, it is possible to organize a higher quantity of unit cells in a compact space [12], which is of practical importance for some sensitive low-spaced applications, like radomes. It is noted that an arbitrary phase front can efficiently be achieved for miniaturized unit cells, which can reduce the angular sensitivity of the incident plane wave.