## 3 Experimental Validation  
A prototype of the absorber is built with a dimension of 150 \(\times\) 150 \(\times\) 3.35 mm\({}^{3}\) that is composed of a 10 \(\times\) 10 array of the proposed unit cells, as shown by the photos in  
Figure 6: Simulated absorbance. (**a**) For various polarizations (TEM mode), (**b**) for various incidence angles (TE mode), and (**c**) for various incidence angles (TM mode). Figure 7. The top and bottom ITO layers are separated by a supporter printed from a stereolithography (SLA) 3D printer using photopolymer standard resin from Formlabs. This physically provides the air gap shown in Figure 1b. As shown in Figure 7, the supporter is printed to have the same 3 mm thickness as the air gap, and only a minimal area of the ITO-deposited PET is attached to the supporter to minimize any possible changes stemming from the material characteristic of the supporter. The SRR patterns made of ITO are etched on a PET film using a laser ablation technique, which is the physical removal of materials by collecting a laser pulse. In this fabrication, a pulse laser with a 1064 nm wavelength and 60 \(\upmu\)m pulse size with 15 W output power was used. The side view is shown in Figure 7b. Photos of the measurement setup are shown in Figure 8. With the built prototype in front of the commercial pyramidal absorbers, as shown in the photos, its performance was measured using horn antennas with an operating frequency of 1-18 GHz and 18-27 GHz, respectively, to test the entire wideband absorbance frequencies of the proposed design from 7.2 GHz to 27 GHz. Note that the horn antenna was placed close to the proposed absorber to present clear photos of the measurement setup, but they were separated by 2 m when carrying out the measurement to satisfy the far-field condition. As shown in Figure 8a, the absorption rate according to the polarization angle \(\phi\) was measured by rotating the proposed absorber by 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\). This realizes the case described in Figure 6a. The measurement setup in Figure 8b shows the setup for measuring the absorbance according to the incidence angle of the TE mode (i.e., the case of Figure 6b). The same two horn antennas are positioned based on the difference in the angle of \(\theta\) according to the protractor on the ground. The measurement is then carried out by varying \(\theta\) from 0\({}^{\circ}\) to 30\({}^{\circ}\) by 15\({}^{\circ}\). The TM mode measurement, in the case of Figure 6c, is made by rotating the horn antennas by \(\phi\) = 90\({}^{\circ}\). A copper plate is measured first, instead of the proposed absorber, for use as a reference. It is then subtracted from the measurement result of the proposed absorber. In this manner, any possible noise and destructive influence from the measurement environment can be excluded. In Figure 9, the measured results are plotted with dots. The simulation results from Figure 6 are re-plotted as a reference. It can first be observed that the measured absorbance is not sensitive to the polarization and incidence angles because of the high symmetry of the proposed structure, such as the computed expectations for all measurements. In Figure 9a, the measurement results also reveal a high absorption rate of over 90% from 6.5 GHz to 27.3 GHz, and a fractional bandwidth of 123.1% with a reference central operating frequency of 16.9 GHz, which is slightly wider than for 7.2 GHz to 27.0 GHz (115.8%), as noted in the simulation. A deviation in the bandwidth and frequencies lower than 5 GHz may be caused by a loss in the built prototype due to the glue used in attaching the PET and the supporter, and the dielectric loss of the supporter; neither were considered in the simulations. Nevertheless, the measured results show a wide-operating bandwidth, as designed, with an insensitive characteristic to the incidence angles and polarizations. Figure 8: Measurement setup. (**a**) For various polarizations, and (**b**) for various incidence angles (TE and TM modes). Figure 9: Measurement setup.