# Frequency Selective Surfaces: Design, Analysis, and Applications
## 2 Applications of FSSs
### Microwave and Millimeter-Wave Applications  
In microwave and millimeter-wave applications, controlling reflection is essential for reducing interference and signal degradation. Reflection, if uncontrolled, can cause multipath effects, where the same signal arrives at the receiver from different paths, resulting in destructive interference and reduced signal quality.  
FSSs can be designed to minimize reflection at specific frequency bands by carefully adjusting the periodicity and geometry of the elements (such as square patches, dipoles, and fractals). For instance, an FSS structure can be optimized to have a low reflection at 28 GHz (a common millimeter-wave band for 5G) by ensuring that the surface's elements resonate at this frequency, allowing for most of the signal to pass through the surface with minimal reflection.  
Recent developments in FSS design have focused on creating wideband FSS structures that maintain low reflection and return loss across multiple frequency bands. These structures are particularly useful in applications like radar and satellite communications, where wideband performance is critical. For example, a novel concentric ring FSS design achieved a return loss of \(-25\) dB across the 18 GHz to 40 GHz range, making it ideal for multiband communication systems [110].  
A low return loss is also a hallmark of a well-designed FSS, particularly in systems operating in the microwave and millimeter-wave ranges. Return loss affects how much of the signal energy is reflected back toward the source. A high return loss, or poor transmission, leads to power loss and reduced efficiency, making the system less reliable. This is especially critical in wireless systems where signal strength and efficiency are paramount.  
The reflection coefficient describes how much of the incident electromagnetic wave is reflected back by the surface. It is often used to assess how well the FSS rejects or transmits signals at specific frequencies.  
\[S_{11}=20\log_{10}\left(\frac{V_{\text{reflected}}}{V_{\text{incident}}}\right) \tag{3}\]  
where  
* \(V_{\text{reflected}}\) is the amplitude of the reflected wave.
* \(V_{\text{incident}}\) is the amplitude of the incident wave.  
The transmission coefficient represents how much of the incident wave passes through the FSS. It is crucial for evaluating the filtering efficiency of the surface at different frequencies.  
\[S_{21}=20\log_{10}\left(\frac{V_{\text{transmitted}}}{V_{\text{incident}}}\right) \tag{4}\]  
where  
* \(V_{\text{transmitted}}\) is the amplitude of the transmitted wave.
* \(V_{\text{incident}}\) is the amplitude of the incident wave.  
Return loss quantifies the power loss in the signal reflected back to the source and is closely related to the reflection coefficient \(S_{11}\). A lower return loss indicates that most of the signal is transmitted through the FSS.  
\[\text{Return Loss}=-20\log_{10}(|S_{11}|) \tag{5}\]  
For example, in satellite communications, a low return loss ensures that the signal transmitted from the ground station is efficiently directed toward the satellite with minimal loss. Similarly, in 5G systems, where millimeter-wave frequencies (e.g., 26 GHz, 28 GHz, 39 GHz) are used for high-data-rate transmissions, maintaining a low return loss allows for the system to provide a high throughput and stable connections over longer distances.  
In recent studies, researchers have achieved significant reductions in return loss using FSSs in millimeter-wave applications. For instance, in a study focused on 5G antennas, a multi-layer FSS was employed to reduce return loss at 28 GHz by 30 dB, demonstrating highly efficient signal transmission across a wide frequency range [110].