**1. Introduction**

Nowadays, wireless technologies are important on our life societies. It is not only on emerging wireless communication systems e.g., 5G, WiFi6, but also on wireless power transfer (WPT) systems. A WPT system can deliver the electrical energy from one to another across the air gap without the need for wires or exposed contacts. An example of the most commonly used WPT is the charging systems of mobile phones and electronics gadget devices. Recently, iPhone12 has been released and all models feature wireless charging [1]. Its power is up to 15 W with the most up-to-date *Qi-* standard [2]. However, most of the mobile wireless charging is not truly wireless because a phone needs to touch a charging station. Another application of WPT technology is an electric vehicle (EV) charging station [3–5], which requires high power and without touching the charging station. The required power is up to a few kilowatts. Although the two systems seem different, both share the same technique, which are used in a near-field WPT. The near-field WPT can be

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 dispersion [27, 29].

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

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**Figure 1.**

*(d) LTSR with uniform metasurface.*

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

**2. System configuration and metasurface characteristics**

the MTS. There are different mechanisms to create the cavity on the MTS. In [30], the cavity is created by Fano-type interference, while the hybridization bandgap is used in [20] and MIW is applied in [25]. Motivated by these observations, we modify the cavity mode concept to create a new defected metasurface (MTS) for enhancing transferred efficiency and reducing the electromotive force (EMF). The MTS is based on a defected unit cell at the desired receiving position formed a twodimensional cavity with configuration of the conventional WPT system. Besides, a free space case, a conventional uniform MTS and the proposed defected MTS have

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

*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* 

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

been studied and compared.

**2.1 System configuration**

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

the MTS. There are different mechanisms to create the cavity on the MTS. In [30], the cavity is created by Fano-type interference, while the hybridization bandgap is used in [20] and MIW is applied in [25]. Motivated by these observations, we modify the cavity mode concept to create a new defected metasurface (MTS) for enhancing transferred efficiency and reducing the electromotive force (EMF). The MTS is based on a defected unit cell at the desired receiving position formed a twodimensional cavity with configuration of the conventional WPT system. Besides, a free space case, a conventional uniform MTS and the proposed defected MTS have been studied and compared.
