**2.2 Metasurface characteristics**

The metasurface is usually constructed using locally resonant unit cells in the deep subwavelength scale. For realized a compact metasurface and compromise between magnetic field enhancement, an array of 5 × 5-unit cells is chosen in this work. Each unit cell is a single-side planar 5-turn spiral with loaded a capacitor in order to resonate at the desired frequency as shown in **Figure 3(a)**. A loading capacitor of 117 pF is used to resonate at 11.88 MHz. It can be seen in **Figure 3(a)**

#### **Figure 2.**

*Configuration of the proposed defected metasurface. (a) LTLR and (b) LTSR. The defected positions are numbering from No. 1 (center) to No. 6.*

**153**

*A Defected Metasurface for Field-Localizing Wireless Power Transfer*

that the resonant frequencies of 117 pF and 98 pF loadings are 11.88 MHz and 12.95 MHz, respectively. The frequency response of the 98 pF loading is also shown and compared because this configuration is designed for the selective created cavity. The resonant frequency of the cavity is slightly higher than the uniform unit cell. Then, the effective permeability of the proposed MTS can be obtained by using the CST simulation [33] with extracting the S-parameters of the proposed unit-cell as shown in **Figure 3(b)**. Since the WPT system is based on magnetic field coupling when an incident EM wave with a magnetic field is perpendicular to the plane of the MTS, the MTS obeys the frequency dispersive Lorentz model to produce an effective negative permeability. At 13.56 MHz, the effective permeability of both capacitor-loading achieves the negative permeability. To adjust the resonant fre-

*Metasurface characteristics (a) resonant frequency and (b) effective permeability.*

To compare the transfer efficiency (η) of all configurations, we use the magnitude of |S21|, which can be easily measured using a vector network analyzer (VNA) in experiments. By using the |S21| and |S11|, the function of the transfer efficiency

ence cases and the LTLR with the defected MTS (**Figure 2(a)**) is shown in **Figure 4**. Compared with the free space, the magnitude of S21 is increased when a uniform or a defected MTS is used. At 13.56 MHz, the uniform MTS case has a maximum power transfer. It can be observed that a case of free-space case (without MTS) and the uniform MTS have only a single peak. The magnitude of S21 for the free-space case (|S21| = 0.24) is remarkable to lower compared with the case of the uniform MTS (|S21| = 0.69). When the defected unit cell is placed in each position, the magnitudes of |S21| are separated into two or more peaks, called *frequency splitting*, which its mechanism is totally different from the two-coil system as a function of separation distance between coils. In a two-coil system, generally, when the distance between transmitting and receiving coils is closer and smaller than a threshold value, it creates two frequency splitting due to the magnetic over-couplings. Hence, many research efforts have been developed the system performance against frequency splitting using optimizing and compensation methods such as non-identical resonant coil [34]. The frequency splitting phenomenon of the defected MTS occurs when non-identical resonant unit cells. It can be explained in terms of Fano

), however, when the network is matching at both ports, the

. A comparison of simulated |S21| for the four refer-

quency, it can be changed to the series capacitor [18].

2

**3. Results and discussion**

transfer efficiency equals |S21|

on η = |S21|

**Figure 3.**

2 /(1-|S11| 2

*DOI: http://dx.doi.org/10.5772/intechopen.95812*

*A Defected Metasurface for Field-Localizing Wireless Power Transfer DOI: http://dx.doi.org/10.5772/intechopen.95812*

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

tangent (tan δ) of 0.025.

**2.2 Metasurface characteristics**

field, the uniform MTS is placed between the transmitting and receiving coils as shown in **Figure 1(c)** and **Figure 1(d)**. All transmitting coil, receiving coil and unit cell are designed on FR-4 material with a dielectric constant (εr) of 4.3 and loss

Then, to study the performance of the proposed defected metasurface on the WPT system, a system configuration is shown in **Figure 2**. It is composed of three parts including a transmitting coil, a defected MTS and a receiving coil. For the LTLR cases, the separation between the transmitting-to-metasurface and receivingto-metasurface is the equal to 240 mm, so the distance from the transmitting to receiving coils is the total 480 mm as shown in **Figure 2(a)**. As we mentioned previous section, users prefer to able to freely location the receiver. A model of freely multiple receiver locations shows in **Figure 2(b)**. This configuration is not only freely movement the receiver, but also like misalignment between transmitter and receiver when the location is positioned on No. 2 to No. 6, so the efficiency deviates from the center (No. 1). Then, the effects of localization field have been extensively studied when the small receiving coil is placed closer to the defected MTS as shown in **Figure 2(b)**. The distance between the small receiving coil and the defected MTS is 40 mm, whereas the transmitting side is keeping the same. Numbers (No. 1 to No. 6) on unit cells are positioned of the cavities or hotspots formed in the defected MTS. The defected unit cell formed cavity is designed with different resonant frequencies from the uniform MTS. The resonant frequency of the defected unit cell is higher than the uniform one, which can be tuned by using the series chip capacitors. We examine the effects of the positions on the defected MTS using an EM simulator and measurement. The results will show and discuss in the following sections.

The metasurface is usually constructed using locally resonant unit cells in the deep subwavelength scale. For realized a compact metasurface and compromise between magnetic field enhancement, an array of 5 × 5-unit cells is chosen in this work. Each unit cell is a single-side planar 5-turn spiral with loaded a capacitor in order to resonate at the desired frequency as shown in **Figure 3(a)**. A loading capacitor of 117 pF is used to resonate at 11.88 MHz. It can be seen in **Figure 3(a)**

*Configuration of the proposed defected metasurface. (a) LTLR and (b) LTSR. The defected positions are* 

**152**

**Figure 2.**

*numbering from No. 1 (center) to No. 6.*

**Figure 3.** *Metasurface characteristics (a) resonant frequency and (b) effective permeability.*

that the resonant frequencies of 117 pF and 98 pF loadings are 11.88 MHz and 12.95 MHz, respectively. The frequency response of the 98 pF loading is also shown and compared because this configuration is designed for the selective created cavity. The resonant frequency of the cavity is slightly higher than the uniform unit cell. Then, the effective permeability of the proposed MTS can be obtained by using the CST simulation [33] with extracting the S-parameters of the proposed unit-cell as shown in **Figure 3(b)**. Since the WPT system is based on magnetic field coupling when an incident EM wave with a magnetic field is perpendicular to the plane of the MTS, the MTS obeys the frequency dispersive Lorentz model to produce an effective negative permeability. At 13.56 MHz, the effective permeability of both capacitor-loading achieves the negative permeability. To adjust the resonant frequency, it can be changed to the series capacitor [18].
