## 2 Absorber Design  
### Configuration  
The unit cell configuration of the proposed EM absorber is shown in Figure 1. The top layer of the unit cell is the same variation as in the SRR structure. The SRR configuration is chosen as it can be designed symmetrically, and the number of resonances can be straightforwardly added by the number of resonators. The top and bottom layers are comprised of an ITO film, with a surface resistance of 15 ohm/m\({}^{2}\) and 10 ohm/m\({}^{2}\), respectively. They are coated on a polyethylene terephthalate (PET) with \(\varepsilon_{r}\) = 3.2 and \(tan\delta\) = 0.003, and a thickness (\(t\)) of 0.175 mm. The target resonant frequency of the unit SRR can be expressed as:  
\[f=\frac{c}{4\cdot 2r_{n}\cdot\sqrt{\frac{\varepsilon_{r}}{2}}} \tag{1}\]  
where \(r_{n}\) is the radius of each SRR (\(n\) = 1, 2, 3, and 4), \(c\) is the velocity of the light, and \(\varepsilon_{r}\) is the relative permittivity of the substrate [32]. Therefore, the resonant frequency of the unit cell, and thus the absorber, can be adjusted and calculated with its diameter. The resonant frequency of the SRR unit with the largest (\(r_{4}\) = 7.5 mm) and smallest diameter (\(r_{1}\) = 1.5 mm) is 5 GHz and 25 GHz, respectively. The quality factor for each resonance could be lower than that of normal dielectrics due to the higher loss of the ITO films, resulting in wider operating frequencies, even though only four different SRR sizes are in place. In other words, additional rings would expand the lowest operating frequency to even lower, at the expense of the increased size of the unit cell. The ITO film coated at the bottom works as a ground plane with lower resistivity. As can be seen from Figure 1a, the proposed SRR takes a symmetrical form, and it makes the absorber insensitive to the polarizations and incidence angle of the incoming EM wave. The top and bottom layers are separated by a 3 mm air gap (\(t_{d}\)). The thickness of the absorber is 3.35 mm, or 0.171\(\lambda\) (\(2t+t_{d}\)).  
The frequency response of the absorber is calculated using the full-wave electromagnetic simulator, CST Studio Suite, when the proposed unit cell structure is that of an infinite array. The absorbance (\(A\)) is calculated as:  
\[A=1-R-T\left(\text{dB}\right) \tag{2}\]  
where the reflection coefficient \(R\) is \(S_{11}\), and the transmission coefficient \(T\) is \(S_{21}\), with Port 1 and Port 2 being the input and output faces of the absorber, respectively, as shown in Figure 1b. Theoretically, a perfect absorbance is achieved when \(R\) and \(T\) are both zero [20]. The simulated \(A\), \(R\), and \(T\) of the proposed absorber are plotted in Figure 2, with the TM mode (or z-polarized) of the incident wave in the normal direction. It is found that absorbance above 90% is achieved across the frequency range of 7.2 GHz to 27 GHz. The fractional bandwidth is 115.8%, with a center frequency of 17.1 GHz where the maximum absorbance of 99% is observed. At a low level of transmittance in the operating frequencies, it is confirmed that the bottom layer, made of a relatively lower resistivity than the top part, works as a ground plane. The working frequency range includes part of the C-band, X-band, Ku-band, and K-band.