## 4 Fabrication and Measurement  
To verify the characteristics of the designed absorber with a measurement, the metasurfaces are printed on a polyimide (PI) film and then added into a composite structure to produce test samples as shown in Figure 7,b. The printing process is accomplished with a commercial carbon paste in the well-known silk-screen printing technology. The thickness of the PI film is 50 \(\upmu\)m and the sheet resistance of the upper and lower metasurfaces is 40 ohm/sq. Figure 7: (a) Metasurfaces—top and bottom pattern, (b) Composite absorber. To validate the durability under harsh conditions, environmental tests were conducted with the proposed electromagnetic composite absorber based on the metasurfaces. For these environmental tests, the temperature and humidity tests were performed using the facility as shown in Figure 8a. As for the high- and low-temperature tests, the methods and procedures are based on Method 501.7, Procedure I according to the MIL-STD-810H. Also, the humidity test followed Method 507.6, Procedure II from the MIL-STD-810H. The detailed specifications of the performed experiment are shown in Table 1.  
Figure 8: Environment test (**a**) Set-up, (**b**) Temperature test results (solid: before, dotted: after), (**c**) humidity test results (solid: before, dotted: after). Figure 8b,c show the absorption rate before and after the environmental tests of the proposed absorber as a sub-array structure where the size of sample is 15 \(\times\) 15 cm\({}^{2}\). The procedure of the tests was as follows (Initial of the under-test sample measurement \(\rightarrow\) Temperature test \(\rightarrow\) Measurement \(\rightarrow\) Humidity test \(\rightarrow\) Measurement). The environmental tests were completed through the above process, and the measured results showed no significant difference in terms of performance before and after the tests. However, due to the small-sized test samples, the measured reflection coefficient results have an uncertainty because of the 15 \(\times\) 15 mm\({}^{2}\) size below 6 GHz range. This is because the sample was not enough to satisfy the area for the NRL method. So, the main focusing region of the reflectance results are above the 6 GHz region. As a result, the measured reflectance results tell us that the proposed composite absorber has good robustness for temperature and humidity variation equivalent to MIL-STD-810H. After conducting the environmental tests, to verify the lower band performance of the proposed EM absorber, the fabricated unit absorbers were arranged and measured in an anechoic chamber as shown in Figure 9.  
An NRL arch measurement system was used to measure the reflectance of the absorber, as shown in Figure 9. The measurement system consists of two wideband horn antennas (transmitter and receiver) as the bistatic RCS on an arch fixture and a test sample holder on the ground. The position of the horn antennas determines the incident or reflective angles of the electromagnetic waves with respect to the sample holder. The minimum incident angle is 7 degrees. The metallic plate for calibration and a test sample are placed and tested on the sample holder. The physical dimension of the sample holder is 60 by 60 in square centimeters. The measurement system can measure 1-40 GHz, and the standard complies with IEEE STD 149 [32]. The reflectance responses of the fabricated absorber with respect to four incident angles (7\({}^{\circ}\), 15\({}^{\circ}\), 30\({}^{\circ}\), and 45\({}^{\circ}\)) with the TE polarization were measured on an NRL Arch measurement system, and the results are shown in Figure 9. The measurement frequency range was from 2 to 18 GHz, and the polarization TE. As shown in Figure 10a, absorption of more than 90% can be seen in the target band from 2.2 to 18 GHz except for the 45\({}^{\circ}\) incidence angle. Even in the case of the 45\({}^{\circ}\) incidence angle, excellent performance is