# An Ultra-Wideband Frequency Selective Rasorber with Low Infrared Emissivity
## 2 Analysis, Design and Experimental Verification
### Design of Infrared Emissivity Reduction  
As long as the temperature of all forms of matter is above absolute zero, they emit infrared radiation in proportion to their temperature. Infrared emissivity is defined as the ratio of infrared energy emitted by an object at a certain temperature to that which would be emitted by a perfect black body at the same temperature [35]. The emissivity of a black body is 1, so the infrared emissivity of a substance can be understood as the ratio of its emitted infrared energy to that of a black body at the same temperature. Therefore, reducing the infrared emissivity can effectively reduce the infrared emission of the target, which is an important means of infrared stealth technology. Due to the lower metal filling rate, the above-mentioned design of FSR obviously has a high infrared emissivity and therefore lacks infrared stealth capabilities. Below is the design for the low infrared emissivity of FSR. The thermal radiation power of an object can be quantified as:  
\[M=e\sigma T^{4} \tag{20}\]  
The infrared emissivity of the object is represented by \(e\), while \(\sigma\) denotes the Stefan-Boltzman constant, and T stands for the temperature. Therefore, in the same environmental conditions, infrared emissivity can be utilized to characterize the thermal radiation power of an object and serves as an indicator for measuring its infrared stealth capability. As is shown in Figure 14, ITO is considered a Drude medium in the infrared band [5], with an extremely low emissivity (0.09). However, in the radar band, it also exhibits a very high reflectivity. Inspired by FSS design, to achieve high microwave transmittance, ITO's nearly total reflection characteristic can be utilized as a substitute for metal as the unit  
Figure 13: S-parameters of FSR vary with \(\pm\)45\({}^{\circ}\) and \(\pm\)60\({}^{\circ}\) in TM polarization. cell pattern of FSS. The designed FSS has high transmittance in the microwave band and presents a first-order resonance band-stop characteristic in the infrared band, serving as an IRSL of the formerly designed FSR. Furthermore, according to the following equation, where t represents the fill factor of ITO. To ensure a low \(e_{IRSL}\), it is necessary to maximize the value of t as much as possible. \[e_{IRSL}=e_{ito}t+e_{sub}-e_{sub}t \tag{21}\]  
Based on the above discussion, a \(d_{i}\) = 300 nm thick ITO film with a sheet resistance of 6 \(\Omega\)/sq is printed on a \(d_{p}\) = 0.125 mm polyethylene glycol terephthalate (PET) substrate with \(e_{PET}\) = 0.9. The structure parameters in Figure 15 are presented as follows: \(p_{i}\) = 0.8 mm, \(l_{i}\) = 0.364 mm, \(w_{i}\) = 0.036 mm, and the relative permittivity of PET is 3.0 (1 \(-\) 0.06j). As indicated in Figure 15, this structure exhibits extremely high transmittance (nearly 99%) in the radar band. Furthermore, according to Equation (21), it can be inferred that \(e_{IRSL}\approx 0.23\). The IRSL is positioned 1 mm above the FSR to prevent electromagnetic interference, and both are surrounded by air as the intermediate medium. The unit cells of the them are cascaded at a ratio of 400:1 during simulation, as illustrated in Figure 16 for the structure and principle diagram and in Figure 17 for the simulation result. The results of simulation and calculation indicate that, after loading IRSL, the highest frequency of the absorption band of FSR decreases from 8.80 GHz to 8.01 GHz and the lowest frequency increases from 2.65 GHz to 2.66 GHz, with a slightly narrower bandwidth. Meanwhile, the transmission insertion loss increased from 0.02 to 0.045; however, the transmission band changed to 9.06-17.94 GHz. Therefore, it can be concluded that after loading IRSL, there is a slight modification between the absorption and transmission characteristics. Figure 14: The transmissivity and reflectivity of ITO in the IR band (**a**) and microwave band (**b**). Figure 15: The structure of IRSL (**a**) and its S-parameters in the microwave band (**b**).