# Electromagnetic Power Harvester Using Wide-Angle and Polarization-Insensitive Metasurfaces
## 3 Experimental Verification  
In this section, a 9 \(\times\) 9 MS energy harvesting array has been fabricated to verify the performance, as shown in Figure 9. The experimental scheme is illustrated in Figure 10. The MS energy harvester placed in the far field of the horn antenna that excited by a signal generator at the power level of 14 dBm, to ensure that the electromagnetic wave incident to the MS structure is a plane wave excitation [11]. The power of the load resistance on the MS structure is tested by using a spectrum analyzer. Because each cell of the 9 \(\times\) 9 MS array has a harvesting port to collect energy at the same time, the power measurement for all of the ports is almost impossible. The central unit can represent other units in a large array without edge effects. Therefore, a good estimation of the power harvesting can be measured. Based on the analysis and discussion above, we only test energy harvesting efficiency of the center element, and the overall efficiency of array can be calculated by multiplying the efficiency of the center element by the number of array elements [13].  
Figure 8: Simulated surface current distribution on the square-ring with different incidence angles (a) \(\theta\) = 30\({}^{\circ}\); (b) \(\theta\) = 60\({}^{\circ}\).  
Figure 10: Experimental setup for measuring the power harvesting efficiency of MS structure.  
Figure 9: Photograph of the fabricated MS structure array on (a) front view (b) back view.  
The energy collection efficiency is given by:  
\[\eta=\frac{P_{RF}}{P_{MS}}\times 100\% \tag{2}\]  
where \(P_{RF}\) is the total time-average power harvested by the port of central element (measured by the spectrum analyzer). \(P_{MS}\) is the total time-average power incidence on the center cell of the MS by the standard gain horn antenna.  
Figure 11 shows the measured energy collection efficiency of the fabricated 9 \(\times\) 9 MS energy harvesting array in different angles of TE and TM oblique incidence from the same distance. It can be seen that the collected energy efficiency is more than 80% in the TE normal incidence at 2.58 GHz. When the incident angle is changed to \(\theta\) = 60\({}^{\circ}\), the energy collection efficiency in this case is still 81%, but the operational frequency will shift to 2.7 GHz. In the TM polarization, we can see that the maximum collection efficiency is 85.1% and the resonant frequency is 2.57 GHz when the incident angle \(\theta\) is 0\({}^{\circ}\). When the incident angle changes to \(\theta\) = 60\({}^{\circ}\), the resonant frequency of maximum efficiency shifts to 2.69 GHz, and the collection efficiency is 82.6%. It is verified that the proposed MS structure has a good performance on the incident electromagnetic wave with arbitrary polarization. The energy harvesting efficiency and the operational frequency that was obtained from the measurement and the numerical analysis are different by reason of typical fabrication tolerances and measurement error. Moreover, the fabricated array is a 9 \(\times\) 9 array when compared to infinite periodic array in the simulation. So, the measurement efficiency is lower than the simulation and the operating frequency have a slight offset, but the energy collection efficiency remains above 80%. The frequency shift can be adjusted using self-adaptive tuning circuits. The good agreement of the simulation with the experiment results verifies the practicability and the effectiveness of the proposed MS structure.