## 2 Fundamentals of Frequency-Selective Surfaces  
### The Concept of Frequency-Selective Surfaces  
FSSs can be designed to block or mitigate specific frequency bands while remaining transparent to others. In an extreme case, a building envelope composed entirely of metallic conductive plates would block all radio waves and offer no frequency selectivity. However, this approach is impractical due to the weight, invasiveness, and lack of benefits in terms of user comfort. Instead, by incorporating FSSs into a facade or interior partition, a building can achieve selective isolation of certain frequency bands used in radio communications, enhancing cybersecurity and reducing occupants' exposure to electromagnetic radiation. In addition, if energy harvesting circuits are integrated, it becomes possible to convert part of the blocked EM energy into electrical power, which could be used to support the building's operation [10; 11; 12; 13; 14; 15; 16; 17].  
The core concept behind FSS design is to create a basic unit cell that acts as a spatial filter and can be easily replicated in a rectangular matrix arrangement. FSSs are engineered to either reflect or transmit electromagnetic waves based on specific criteria such as frequency, polarization (the orientation of the electric field), and angle of incidence. The dimensions of the unit cell are often related to the wavelength (\(\lambda\)) of the incident wave, where \(\lambda=c/f\), (with \(c\) being the speed of light in m/s and \(f\) the frequency in Hz). This relationship allows for a standardized comparison of unit cell compactness, typically expressed in terms of electric dimensions. Although wavelength is a measure of distance (in meters), it is often used to define the electric dimensions of objects, serving as a practical unit of measurement. For example, a device may be sized in terms of a fraction or multiple of the wavelength.  
Figure 2 illustrates examples of unit cells from the four main design strategies: cross cell, patch cell, loop/ring, and meandered cell, as detailed in [9]. Each unit cell is tuned to a specific frequency band, or, in some cases, multiple frequency bands. In these diagrams, the dark color represents conductive (metallic) surfaces, while the white areas correspond to dielectric supports.  
Thus, the fundamental concept of an FSS involves the creation of a surface composed of conductive and dielectric elements. Depending on their designs and sizes, these elements either attenuate or block radio waves that attempt to pass through the surface. Typically, designers focus on creating a basic unit cell, which is then replicated in a rectangular matrix. Covering large areas by replicating this pattern is generally more practical than designing an entire surface in one piece. The design process relies on simulations that assume an infinite periodic structure of unit cells. This approach allows designers to optimize the geometrical dimensions of each element to tune the FSS to the frequencies that require attenuation. Several parameters are considered in this optimization, including the dimensions of the unit cell, the central frequencies to be blocked, the bandwidth around these frequencies, and the stability of performance with respect to incidence angle and polarization.