## 2 Design of the Ultra-Wide Band Absorber
### Layer Configuration of EM Absorber  
An ultra-wideband EM absorber has a configuration of composite layers as shown in Figure 1. The absorber consists of the two thin metasurfaces of lossy conductive pixels, a GFRP layer, a CFRP layer, and several dielectric substrate layers. Design procedures of the pixelated metasurfaces are discussed in the next section. One of the metasurfaces, the lower layer, is inserted between the first and second dielectric layers. The other metasurface, an upper metasurface, is placed between the second and third dielectric layers. The first to third dielectric layers are made from a polymethacrylimide (PMI) form. The fourth dielectric layer, the GFRP layer, is added on the third dielectric layer to protect all structures from external forces. The thickness of the first and second dielectric layers is 5 mm, the third layer 2 mm, and the fourth layer 1 mm, respectively. The bottom of the overall structure is a high conductive CFRP layer serving as a reflector. The total thickness of the structure on the reflector was chosen to be 13 mm on excluding the protective fourth and third layers, the actual thickness is 10 mm up to the upper metasurface, which is equivalent to 0.06 \(\lambda_{0}\) at the lowest target frequency, 2 GHz, where \(\lambda_{0}\) is the wavelength in a free space. Since the dielectric constant of the form is similar to that of the free space, the proposed absorber has physically a very thin thickness. The complex relative permittivity of the PMI form was assumed to be 1.107 - j0.016, and the CFRP layer 4.6 - j0.253.  
From now on, the reason for the layer configuration in Figure 1 of the proposed absorber is discussed as follows. First, the proposed design was based on durability and environmental resistance, therefore the absorber includes robust composite materials. The multi-layered absorber of PMI form, GFRP, and CFRP underwent an autoclave process at 125 \({}^{\circ}\)C. Second, a practical aspect was considered to fabricate the composite materials such as the protective layer, the form substrate, adhesive materials, etc. in order to be directly applied to stealth warships and aircrafts. All materials were purchased from well-known suppliers. Third, a multi-layered structure was proposed to achieve broadband characteristics. The principle of the operating mechanism in the ultra-wideband is as follows and described in Figure 1a. An incident wave on the top of the composite absorber interferes with the wave transmitting through the pattern of the bottom layer after being reflected backwards by the reflective layer. Because the phase shift of the wave is \(\pi\) (180\({}^{\circ}\)) when it is reflected from a reflector, destructive interference can occur when there exists a phase difference of \(\pi\) between the incident and reflected waves. While the EM waves are reflected from the metasurfaces or are transmitted through them, the phases of waves can be changed abruptly by interacting with the metasurfaces. As a result, the phase condition for destructive interference can be achieved by designing the patterns of the metasurfaces optimally under the configuration of the total structure. Based on the proposed composite absorbers' multi-reflection as a backbone concept, the proposed absorber works and satisfies the absorption performance in the target frequency band as one composite structure.