# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Structure
#### 4.2.4 Three-Dimensional FSSs (3D FSSs)
A transmission curve with rectangular shape, sharp roll-off, and wider out-of-band rejection is obtained while five transmission poles within the passband (12.1 to 15.4 GHz) are observed. 3D printing technique is utilized to construct this uncommon structure with photo polymerization using ultraviolet light cured resin. The physical theory of this 3D FSS is evidently inferred with the explanation of current loop path to precisely recognize the passband, as depicted in Figure 12b. For such structures, surface waveguides allow the EM waves to pass by limiting the power along the metal surface instead of trapping the energy inside the cavity. This resolves the restriction of cut-off frequency and facilitates the miniaturization of unit cells. Ultimately, the strong coupling mechanism provides fast roll-off than any other multilayer FSS designs. Such 3D FSSs possess the benefits of flexibility in shape, reduced weight, free-standing, and integrated design in all types of radomes. The desired number of transmission zeros and/or poles at fixed frequencies can be realized by 3D FSS as compared to 2D FSS, by utilizing some specific count of resonators and/or propagation modes. Therefore, the filtering characteristics e.g., frequency selectivity, out-of-band rejection, can simply be improved by controlling the resonances of these added resonators. A general equivalent circuit model of a unit cell of 3D FSS is shown in Figure 13, including a number of resonators [112]. Here R\({}_{\rm{ai}}\) (i = 1, 2, \(\ldots\), N) represents resonators that link the input and the output ports, R\({}_{\rm{bj}}\) (j = 1, 2, \(\ldots\), N) and R\({}_{\rm{ck}}\) (k = 1, 2, \(\ldots\), N) are resonators attached only either to the input or to the output port, respectively. R\({}_{\rm{ai}}\) can present transmission zeros/poles at their resonating frequencies. R\({}_{\rm{bj}}\) and R\({}_{\rm{ck}}\) can achieve extra transmission zeros at the required frequencies, further improving the selectivity of a passband FSS. Figure 12: (**a**) Surface waveguide made of metallic frames. (**b**) Coupling of current through Three-Dimensional FSS (3D FSS) structure at transmission poles [111]. To overcome the problem of poor filtering in single resonant element based single layer 2D FSS (e.g., in [56]) exhibiting one transmission pole, an improved design of 3D broadband FSS with four poles passband response is presented in [112], in which each unit cell has two microstrip lines that are split by a horizontally laid long and thin metallic rectangular bar in the air region. Additionally, both of these lines are shorted to ground through a shorting via hole in the centre. The top line is printed on a substrate with high dielectric constant, while the bottom line is printed on a low dielectric constant substrate. The inserted metallic bar can block the air propagating paths while two propagating paths by two different substrates are still present there. Moreover, first resonance occurs due to resonators on the top substrate since the guided wavelength is shorter (high dielectric constant) as compared to the bottom substrate (low dielectric constant). Table 6 gives an overview of some of the 3D FSS reported in recent years and it is noticed that, although good ranges of frequency responses are exhibited by these 3D structures, the challenge that is linked with this advanced technology is the complication in manufacturing and realization. A lot of research is going to overcome this problem as recently a solution has been presented by [PERSON] et al. of using four-legged loop element, implemented by using via holes in a multilayer PCB geometry for stopband applications [113]. Superior performance and ground-breaking implementations are envisaged by future explorations in 3D FSS design scenarios.