# Frequency Selective Surfaces: A Review
## 4 Types of FSSs
### FSSs Based on Applications
#### 4.3.2 Active FSSs (AFSS)
EM structural design of buildings is an important application in which propagation of EM wave is controlled by means of switchable AFSS absorber [157]. Similarly, different designs of tunable FSSs are presented so far. Figure 17 shows some AFSSs presented in different researches. Earlier, a tunable AFSS is designed in [158] by the use of a magnetically tunable ferrite substrate, while graphene-based tunable FSS has also been reported in [159; 160]. Ref. [161] illustrates the possibility of two FSS switching modes by using ferroelectric materials. Barium strontium titanate (BSTO)-magnesia compound is used for transmission/reflection switching by the shift in the transmission peak frequencies or reflection nulls. It is noticed that when a bias voltage is applied, a change in the dielectric constant of the ferroelectric substrate is observed. BSTO-alumina compound is suitably used for absorption/reflection switching. It is observed that a higher range of tunability is linked with the large values of dielectric constants. However, thin substrate layers facilitate in generating higher operating BW. Thick layers are to be employed for both switching modes, with large values of dielectric being constant to avoid smaller BWs. The drawback of implementing such AFSSs is exigent due to the complexity in controlling the conductivity of the surface layers. Generally, extra feeder lines are inserted within the AFSS array elements to control the impedance of the active components. However, such a biasing arrangement may significantly influence the performance of AFSS, for example, deviation in operating frequency, high insertion losses, and fallacious response. Besides the impact on performance, the bias network also demands additional cost and fabrication difficulties. A practical and efficient approach is to utilize FSSs configuration as an integrated component of the bias network to reduce the extra use of feeder lines within the element array, as done in [163]. Thus, realizing AFSSs with different polarizations without any extra bias circuitry within its array arrangement can be a very attractive scheme. Polarization is a salient characteristic of EM waves that is desired to be manipulated in controlling incident waves. A prospective useful alternative solution to earlier single featured AFSS has been given in [154], in which a novel multifunctional AFSS structure, concurrently exhibiting over one operating mode, is proposed at working frequency band of LTE 2.1 GHz. It consists of two orthogonal  
Figure 17: (**a**) A unit cell of the broadband tunable AFSS absorber with of its bias lines (**b**) Tunable metamaterial based Active Frequency Selective Structure (AFSS) with lumped capacitors [162] (**c**) Reconfigurable cross dipole-hash FSS with two states by using PIN diode. metallic layers with a thin dielectric spacer in between. PIN diodes are used for dual modes of EM switching and polarization selection (with On/Off states) and four working states (dual polarized passband for code 00, dual polarized shielding for code 11 and TE/TM wave selection for code 10, and TE/TM wave selection for code 01). Figure 18a shows the top view of proposed AFSS with the position of PIN diode. For such an arrangement, the resonating elements themselves perform the function of linking the PIN diodes and allow DC currents to pass through. Thus, no supplementary feeding lines are required within the AFSS array configuration, greatly diminishing the negative effect of the bias network, as in traditional AFSSs [151; 164]. RF/dc isolation is provided by RF chokes arranged on both top and bottom layers. This unique AFSS offers more liberty in manipulating EM waves especially for EM shielding, stealth technology, electrically configurable mechanisms, and scalability feature of periodic structures, which is applicable to the concept at microwave and millimeter wave regimes. Table 8 briefly shows some of AFSSs presented in recent researches along with their element types, configurations, and frequency ranges.