**2.1 System configuration**

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

categorized into two types: inductive coupling and magnetic resonance coupling [6]. Usually, the inductive coupling technique is lowering efficiency and shorter distance compared to the magnetic resonance method. The magnetic resonance method is based on the resonant coupling with two-same frequency resonant circuits while it is interacting weakly with other off-resonant frequency. Therefore, many studies have been proposed the techniques to improve efficiency and operating distance [7–10]. Moreover, a near-field WPT system is using magnetic field coupling and uses in high power applications, so electromagnetic fields (EMFs) accordingly increase. Hence, leakage EMF emitted from WPT system should follow the feasible guideline of ICNIRP (International Commission on Non-Ionizing Radiation Protection (ICNIRP) [11]. It can be damaging human bodies and surroundings. To meet safety requirements below a certain standard level, most of the previous works have been proposed using ferrite and metallic shields [12–14]. The ferrite-based shielding is moderately effective, but it is very heavy weight and normally used in low-frequency. While the metallic-based shielding is flatter and more lightweight, the reflected wave can degrade the overall efficiency of the WPT system. Both transfer efficiency and EMF leakage are concerned in modern WPT. Unfortunately, each proposed technique can solve only one by one. Recently, the use of electromagnetic metamaterials (MTM)/metasurface (MTS) for WPT systems have been proposed [15–19]. Metamaterials are engineered composites that exhibit unusual electromagnetics properties, which are not found in natural materials. Usually, implemented MTMs/MTSs are always used a uniform, which all unit cell is identical. Unfortunately, the most property of the uniform MTM/MTS is negative permeability, which can enhance only the transfer efficiency with evanescent wave amplification, hence increasing transfer efficiency. Consequently, many research efforts have been improved the transfer efficiency and decreased leakage electromagnetic fields (EMF) simultaneously. Recently, MTM/MTS have been proposed for enhancing the transfer efficiency and decreasing EMF leakage [20–25]. To solve both efficiency and safety problems, the non-uniform [18] and hybrid MTM/MTS techniques [22] have been proposed, however, these are suitable for the size of transmitting and receiving coils are comparable. Another method is using magnetoinductive wave (MIW), which is supported by MTM/MTS composed of inductively coupled electrically small resonators and created by inter-element couplings [26–29]. The MIWs can be constructed in one, two and three dimensions. Usually, however, the efficiency is fluctuation due to the standing MIW, which depends on the receiver position [27]. Moreover, the two-dimension MIWs have more complex

In practical various applications, a receiving side such as charging portable or implantable devices is usually smaller than a transmitting side. Moreover, users prefer to be able to freely location the receiving devices. When the receiver size is reduced to the transmitter, the efficiency rapidly decreases either with or without the MTM/MTS. This is because power distribution is spread out and unrefined above the charging surface; that is the transfer efficiency is variation when a receiving device position is misaligned, which is deviated from the concentric position with respect to a transmitter [19]. To refine and control the magnetic field by focusing the fields into the small regions, the non-uniform and defected MTSs with modified field localization technique have been proposed [20, 25, 30, 31]. The defected MTS is formed by the cavities on MTS. To create a cavity, a defected unit cell at the small receiving coils' positions can be designed and controlled by tunning the resonant frequency using external capacitor. The defected unit cell can be locally modified for magnetic field control at subwavelength scale. In addition, it does not change the macroscopic parameters including negative permeability. Unlike the previous approach [20, 25, 30], the receiver location is placed at center of

**150**

dispersion [27, 29].

A common WPT system consists of a transmitting coil and a receiving coil. Firstly, we investigate four configurations of the conventional WPT systems with and without uniform metasurface: (i) large Tx and large Rx coils (LTLR) without metasurface, (ii) large Tx coil and small Rx coil (LTSR) without metasurface, (iii) LTLR with uniform metasurface, and (iv) LTSR with uniform metasurface as shown in **Figure 1(a)** to **Figure 1(d)**, respectively. These four configurations are the reference configurations to compare with the proposed defected metasurface. The large size of coils is compared with the unit cell size of an MTS. For uniform MTS, all unit cells are identical, and its property is the negative effective permeability. The transmitting and receiving coils are used planar four-coils WPT system configuration and the operating frequency is 13.56 MHz band, which is an ISM (Industrial, Scientific and Medical) band. The large transmitting coil composes of the feed/load coil and three-turns planar resonator coil with a trace width of 3 mm and 17 mm. The gap between the adjacent loop of the resonator coil is 5 mm with the largest diameter of 240 mm. The radius of the feed/load coil is 48.5 mm. Due to the requirement of ISM bands; it is vital to fix the resonant frequency of WPT system. To resonate at a desired frequency of 13.56 MHz, a 68 pF chip capacitor is connected in series. The details are shown in **Figure 1(a)**. For the purposes of comparison, two different sizes of the receiving coil are examined. The first size of the receiving coil is identical to the large transmitting coil and the second one is smaller than the transmitting coil about five times, which the largest diameter is 60 mm. It is a two-turn planar coil and loaded with two capacitors at the driving loop coil and resonant loop coil. The detailed design of the two-turn planar coil is similar to our proposed in [32]. To enhance the efficiency and focus the magnetic

#### **Figure 1.**

*Configuration of the conventional WPT system. (a) large Tx and large Rx coils (LTLR) without metasurface, (b) large Tx coil and small Rx coil (LTSR) without metasurface, (c) LTLR with uniform metasurface, and (d) LTSR with uniform metasurface.*

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 tangent (tan δ) of 0.025.

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.
