# An Ultra-Wideband Frequency Selective Rasorber with Low Infrared Emissivity
## 2 Analysis, Design and Experimental Verification
### Experimental Verification  
The validity and accuracy of the IRSL-FSR and FSR designs mentioned above need to be verified through the following tests: (1) transmittance and reflectance tests in the microwave band (2-18 GHz); (2) infrared band (8-14 \(\upmu\)m) temperature test, with conversion to their infrared emissivity. The RS and FSS mentioned above were processed using printed circuit board technology. The metal material used was copper, and all dielectric substrates had a relative dielectric constant of 2.2, a loss tangent of 0.0009, and a thickness of 0.127 mm Rogers RT5880. The resistance model in the RS is a 0402 package with 160 \(\Omega\); the two layers of FSS were bonded together using a WL-PP280-045-50N semi-cured adhesive. To prevent the central collapse caused by the weight of RS, an 11 mm polymethacrylimide foam (PMI) was added between RS and FSS to form a component. In addition, considering the infinite size in FSR simulation and the finite size in actual machining, it is essential to minimize the truncation effect on the electromagnetic characteristics of the sample. Typically, engineering experience suggests that a noticeable truncation effect occurs when there are fewer than five unit cycles in both x and y directions but becomes negligible when there are more than 10 unit cycles. Therefore, a processed sample size of 300 mm \(\times\) 300 mm standard parts was shown in Figure 19, with 37 unit cycles in both x and y directions. Using magnetron sputtering technology, an ITO thin film with a thickness of 180 nm and sheet resistance of 6 \(\Omega/sq\) was etched on a PET substrate with a relative dielectric constant of 3 (1 - j0.06), resulting in a 100 \(\times\) 100 mm IRSL plate, as shown in Figure 10a. Subsequently, the IRSL was placed on a PET substrate with a thickness of 1 mm and cascaded with FSR to avoid electromagnetic interference between them. The IRSL was then placed on a 1 mm thick PET board and integrated with the FSR to mitigate potential electromagnetic interference. The IRSL-FSR sample is depicted in Figure 10b. Figure 19: The structure of RS (**a**) and FSS (**b**), size of the unit cell (**c**). The samples above have been measured below. For the transmissivity measurement, the \(S_{21}\) of FSR was tested using the free space test method. The experimental setup is illustrated in Figure 21a. In this configuration, pulse signals are transmitted via a transmitting horn antenna, received by a receiving horn antenna, and then analyzed using a vector network analyzer. During testing, the \(S_{21}\) of the sample was measured both with and without placement on the fixture, and the sample's \(S_{21}\) was determined by taking their difference. The \(S_{11}\) was determined using the bow frame test system. As depicted in Figure 21b, the sample and a metal plate of equal size were positioned at the center of the bow frame circle during testing, and the disparity between the two measurements was calculated using the time-domain gate function of the vector grid analyzer to derive the sample's reflectivity. The comparison between the experimental results and the simulation results is shown in Figure 22. It can be seen that the two are generally consistent but with slight differences. The main reasons for the errors are the parasitic parameters caused by manual soldering of chip resistors and inherent errors in the testing system. In conclusion, the experimental results validate the accuracy of the simulation design. Figure 21: Transmissivity measurement (**a**), reflectivity measurement (**b**). Figure 20: The structure IRSL (**a**) and IRSL-FSR (**b**). Furthermore, the infrared emissivity of the verified samples are measured using an infrared camera. The FSR sample and the IRSL-FSR sample are both heated to the same temperature (40\({}^{\circ}\)C).