# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.2 Active FSSs (AFSS)
The metallic elements of the conventional FSSs bear oxidization, corrosion, and reduced breakdown voltage, particularly in high temperature and high power applications. So, the development of novel types of FSSs without metallic composition is essential. Tunable FSSs are mainly based on liquid crystals [165; 166; 167], but the tuning ranges are comparatively limited, nevertheless still metallic screens are required to be embedded. High permittivity ceramic particles have been utilized as a basic unit cell to construct all-dielectric MM FSSs. The ceramic particles are of subwavelength size due to their high permittivity. Accordingly, the macroscopic EM properties of all-dielectric FSS can equivalently be illustrated by the effective permittivity and permeability, which can be tuned at will, and so the effective impedance of such FSSs can be designed at the required frequencies. The FSS in [168] composed of cross-shaped ceramic resonators exhibit different stopband responses with the change in orientation of ceramic resonators, as depicted in Figure 18b. The resonant frequencies of ceramic particles can be adjusted by varying the geometric parameters, to properly separate the two initial resonant modes and create two neighboring stopbands. The AFSS can toggle between a stopband (0.4 GHz) in the frequency range of 4.54-4.94 GHz (State 1) and a stopband (1.0 GHz) in range of 3.55-4.60 GHz (State 2). This easily reconfigurable FSS, just by rotating the structure, can be used in RCS reduction systems and reconfigurable antennas. Figure 18: (**a**) Top-view of AFSS with unit cell display. (**b**) Reconfigurable all-dielectric FSS with two states (1 and 2) and unit cell. EM shielding has gained revived interest due to emerging AFSS with varactors and PIN diodes as an efficient solution of this problem, so far, mostly proposed varactor controlled tunable AFSS are asymmetrical structures, and thus demonstrate sensitivity to polarization and experience small tuning range as in [163; 171]. AFSS structure in [172], offers numerous noteworthy novelties to solve the aforementioned challenges. Figure 19a shows the 2D array structure in which a periodic arrangement of circular metallic loops is printed on a dielectric substrate and each loop is coupled crosswise with four neighboring loops through varactor diodes (SMT). This makes the design fourfold symmetric, thus inherently tunable for all polarization and higher incident angles. With applied reverse bias voltage from 0 to 8 V of the varactor diode, the overall capacitance is varied from 38.08 to 1.72 pF, thereby shifting the resonant frequency smoothly from 0.54 to 2.50 GHz (FBW 152% at 0 V and 49.61% at 8 V), as seen from trend graph in Figure 19b. Although the performance of FSS for different polarizations and incidence angles is only evaluated by simulations without considering the horizontal bias lines and no measured results are presented after the addition of additional biasing. Furthermore, such a technique in AFSS reduces the resonance frequency due to higher junction capacitance as compared to passive FSSs. Such a wideband tunable AFSS finds potential application for EM shielding in different frequency bands (GSM, ISM, UNII, WLAN). The stopband performance of AFSS meant for shielding application is usually calculated by shielding effectiveness (SE) given by (11) [91], in which \(\mathrm{E_{T}}\) and \(\mathrm{E_{I}}\) are the transmitted and incident electric field components, respectively. \[\mathrm{SE_{dB}}=-20\times\log\left|\frac{\mathrm{E_{T}}}{\mathrm{E_{I}}}\right| \tag{11}\]