# Frequency Selective Surfaces: Design, Analysis, and Applications
## 2 Applications of FSSs  
FSSs have found a wide variety of applications, but conventionally, they have been used in radomes, multi-frequency reflector antennas, electromagnetic absorbers, polarization converters, etc. As shown in Figure 2 in a radome scenario, the FSS is employed to reduce the radar cross-section of the antennas, and it is transparent to a desired frequency band and reflective at other frequencies. An example of such an application is in aircraft, where a bandpass radome covers the antenna that is fixed at the tip. Radomes can be used in two ways in this situation: transparent mode and reflecting mode [28].  
FSSs have also been used in multiband reflector antenna applications [29; 30; 31; 32]. In such applications, the FSS is used as a sub-reflector, where it is designed to be reflective at a frequency band and simultaneously transparent at other bands. Hence, the sub-reflector allows for the application of multiple feeds and enables the multiple frequency state antennas. Different frequency feeds are positioned at real centers of the sub-reflector, and the single reflector geometry acts as a multiband antenna. A practical example is the high-gain antenna of the Voyager spacecraft that was designed in [29], where the FSS forms the sub-reflector that reflects at X-band and transmits the S-band as shown in Figure 3. In this antenna, the S-band is placed at the focal point of the reflec  
Figure 2: Radomes at the Cryptologic Operations Center, Misawa, Japan (photo courtesy of en. Wikipedia).  
located at the Cassegrain focal point. The single reflector architecture acts as a dual-band antenna reducing the mass, volume, and fabrication cost.  
Figure 4 shows the construction of an \(LC\) resonating surface incorporating an array of metallic strips, which are truncated at specific intervals to form capacitive junctions. The metallic grids are inductive and in combination with the capacitive gaps form a series \(LC\) resonator. The frequency response of the array can be tuned by adding lumped elements like varactor diodes.  
Recently, FSSs have been implemented using new materials such as conductive inks and transparent dielectric polymers. In such structures, the FSSs may be printed on walls and windows of libraries, prison cells, and theaters to block mobile phone signals, while simultaneously allowing emergency Terrestrial Trunked Radio (TETRA) services to operate. A simple FSS is used in the door of microwave oven and acts as a high-pass filter. The door blocks the 2.4 GHz microwaves inside and also allows the visible light to pass through the door. Table 1 presents typical applications of the FSS.  
Figure 4: An active LC array comprising metallic strips interrupted by gaps in a periodic fashion. The gaps can be loaded with varactor diodes to alter the gap capacitance [5].  
Figure 3: Cassini high-gain antenna (HGA) with a four-band FSS [2].  
The functionality of an FSS is highly dependent on the unit cell size and geometry. Thus, appropriate modeling and synthesis procedures should be developed for the efficient design of FSSs for different applications. The following section covers the properties of FSSs, i.e., element shapes, standard responses, configurations, angular stability, unit cell size, and analysis methods.