## 3 Results and Analysis
### Simulation Results  
The absorbed EM energy, the energy distribution on the load, and the harvesting efficiency of the metasurface at a normal incidence were analyzed. It can be seen from Figure 2a that the metasurface had the maximum absorption efficiency at 5.8 GHz. The absorption and harvesting efficiencies were 96% and 91%, respectively. Only 5% of the energy was dissipated in the substrate and metal, and most of the energy was harvested by the resistor, which indicates that the metasurface can effectively collect EM energy. Figure 2b shows the harvesting efficiency of the TE and TM polarizations, which were basically the same. The arbitrary polarization wave could be deconstructed into two orthogonal polarizations of TE and TM waves so that a similar efficiency would be obtained for an arbitrary polarization at the normal incidence. Thus, the metasurface was insensitive in polarization. As shown in Figure 2c, the harvester performed well with low cross polarization, measuring below \(-\)20 dB at 5.8 GHz, which means the energy could be harvested simultaneously and separately by these two orthogonal polarizations. Figure 1: Topology of the proposed metasurface and unit cell structure. The absorption principle can be explained by the impedance matching between the free space and the metasurface interface. The effective parameters were extracted from the complex scattering parameters by the standard retrieval procedure [3]. As shown in Figure 3, the extracted permittivity and permeability at the resonance frequency were (4.42-j63.4) and (4.44-j44.60), respectively. The normalized impedance (\(\mathrm{z}=\sqrt{\mu/\varepsilon}\)), compared with that of the free space (377 \(\Omega\)), was (0.84-j0.01) \(\Omega\). Thus, the metasurface was a good match with the free space, and reflections at the metamaterial interface would be very small. The operation mechanism of the metasurface can be analyzed by the equivalent circuit approach. The transmission line equivalent circuit of the proposed metasurface is shown in Figure 4. Part 1 represents the incident plane wave from the free space, which had a wave impedance of 377 \(\Omega\). Part 2 is for the metasurface. C\({}_{1}\) and L\({}_{1}\) represent the total capacitance between each of two sectors and the total inductance of the eight sectors, respectively. C\({}_{2}\) and L\({}_{2}\) represent the capacitance between the unit pattern and the metallic ground and the inductance of the metal via near the center, respectively. R is the harvesting load, and C\({}_{3}\) represents the capacitance between the via and the ground. The substrate between the unit pattern and the ground can be modeled as a transmission line, with a characteristic impedance of 377/\(\sqrt{\varepsilon_{r}}\) = 231 \(\Omega\). The value of each lumped element in the equivalent circuit could be estimated using a microstrip line model that estimates the total inductance and capacitance [29]. Optimized using Advanced Design System (ADS) software, the reflection coefficient of the equivalent circuit, with a comparison to that of the metasurface in HFSS, is shown in Figure 5a. The resonance frequencies of the two models both occured at 5.8 GHz, which agrees well with the experiment results. Figure 5b,c show the trend of the reflection coefficient changing with the value of R. The minimum reflection was achieved with a value of 50, and the reflection increased when the value shifted from 50 in both models. The similar trend further demonstrates the consistency of the two models. It should be noticed that the coupling effects  
Figure 3: Extracted effective material parameters of (**a**) permittivity, (**b**) permeability, and (**c**) impedance. Figure 2: (**a**) Energy distribution efficiency versus frequency. (**b**) Harvesting efficiency versus frequency for transverse electric (TE) and transverse magnetic (TM) polarized waves at the normal incidence. (**c**) Cross polarization versus frequency.