# Electromagnetic Power Harvester Using Wide-Angle and Polarization-Insensitive Metasurfaces
## 2 Design of the MS Energy Harvester  
In the energy harvesting system, the rectenna is the most critical part [15; 16; 17]. It can be divided into energy harvester (e.g., antenna) and rectifier circuit. As the primary energy collector, it receives electromagnetic RF energy, which is converted to DC power through a rectifier circuit. The energy harvesting using MS should meet the following two conditions: (a) The electromagnetic wave energy can be captured by the MS structure as much as possible; (b) The captured energy into the MS structure can effectively be transferred into the rectifier circuits (matching load). This paper builds a two-dimensional energy harvesting array that is composed of sub-wavelength square-ring MS, and each small unit can be equivalent to LC resonance circuit. The incident wave is stimulated to produce surface current on the surface of the MS structure. Similar to the antenna-based coaxial feed, we take a via hole in the unit with an appropriate place and then lead the surface current to the port that is connected to the resistive load [11]. The MS energy collector can capture electromagnetic wave energy and transfer to the load, rather than dissipate in the structure. The designed MS structure  
Figure 1: Power spectrum distribution measured in the campus of Xidian University, China. is shown in the Figure 2. The center working frequency is set to 2.5 GHz (LTE/WiFi). The upper layer is the square-ring array, and the middle is the low-loss dielectric layer and the bottom layer is the metallic ground. The energy harvesting ports are set at the corner of each ring. Using ANSYS HFSS to establish the numerical simulation model, the boundary condition of the unit cell is set as periodic boundary conditions (PBC), which is combined with the Floquet ports to calculate the infinite array model. The port excitation of the incident wave power is set to 1 W. The dielectric substrate is F4B that has relative permittivity of 2.65 and a dielectric loss tangent of 0.001. The dimensions of the square-ring element are as follows: \(p=15.7\) mm, \(l=15\) mm, \(w=0.8\) mm, and \(t=3\) mm. Figure 3 shows that the calculated efficiency of the designed MS array for various load resistance at the operating frequency of 2.5 GHz. It can be seen that the highest efficiency of the operating frequency can be simultaneously obtained when the load resistance is 500 \(\Omega\). It should be noted that this matching load value is a local optimal solution. Other parameters, such as MS structure size, dielectric constant, and thickness of substrate, etc., will also affect the matching load value and the operating frequency. By changing these parameters, we can flexibly design the matching load value of rectifier circuit. This port is located in the diagonal direction of the square-ring element. Note that we can flexibly change the working frequency band, matching the load value by changing the value of each parameter of the MS structure. Figure 3: Calculated efficiency of the designed MS array for various values of load resistance at the operating frequency of 2.5 GHz. Figure 2: The square-ring resonator metasurface (MS) structure for ambient power harvesting. When compared to the periodic dimension of the previous works, such as the SRR element, is about 1/4 operating wavelength at 5.8 GHz [7], and the G-CSRR element is about 1/3 operating wavelength at 5.55 GHz [13], and the WG-CSRR element is about 1/5 operating wavelength at 5.6 GHz [14], the proposed design in this paper is about 1/8 operating wavelength at 2.5 GHz, which is more miniaturization. Note that the proposed closed square-ring resonant ring rather than the split-ring form from existing literature is adopted in this paper. In spite of the angles and polarizations of the incident wave, two symmetrical annular currents in the same direction excited by an incident wave on the surface of the closed-ring can channel to the collection port.