**4. Strongly coupled magnetic resonance WPT**

Strongly coupled magnetic resonance refers to inserting intermediate resonators with a high-quality factor (Q) in the transmission path between transmitter and receiver as revealed in **Figure 16**, these intermediate resonators are used to emphasize the transferred magnetic power. This technology is categorized as mid-range WPT. In 2007, a group of researchers at the Massachusetts Institute of Technology proposed an experiment using a strongly coupled magnetic resonance technique [20].

**Figure 16.** *Strongly coupled magnetic resonance WPT.*

**Figure 17.** *Setup of MIT researchers group experiment [20].*

They effectively powered a light bulb wirelessly using a power source located 2 m away from the light bulb. They obtained a power transfer efficiency of about 40%. The experiment is demonstrated in **Figure 17**, the intermediate resonators are self-resonant.

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**Figure 18.**

**Figure 19.**

**Figure 20.**

*conductive shorting wall [23].*

*3-D strongly coupled magnetic resonance WPT [22].*

Recently, several authors [21–27], have utilized from the strongly coupled magnetic resonance scheme to enhance the transmission properties of WPT systems. Barreto et al. [26] proposed a conformal strongly coupled magnetic resonance system for range extension by using U-loop as an intermediate resonator as shown in **Figure 18**. It provides a high transfer efficiency reach 70% at a transfer distance equal to the diameter of the U-loop (48 cm). Also, this WPT system can maintain efficiencies greater than 60% regardless of the angular position of the receiver around the U-loop. A multilayer

*(a) Geometry of a printed spiral coil, (b) two layers using conductive shorting wall, and (c) three layers using* 

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

*Conformal strongly coupled magnetic resonance system [26].*

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

**Figure 18.**

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

They effectively powered a light bulb wirelessly using a power source located 2 m away from the light bulb. They obtained a power transfer efficiency of about 40%. The experiment is demonstrated in **Figure 17**, the intermediate resonators are self-resonant.

**132**

**Figure 17.**

*Setup of MIT researchers group experiment [20].*

**Figure 16.**

*Strongly coupled magnetic resonance WPT.*

*Conformal strongly coupled magnetic resonance system [26].*

**Figure 19.**

*(a) Geometry of a printed spiral coil, (b) two layers using conductive shorting wall, and (c) three layers using conductive shorting wall [23].*

**Figure 20.** *3-D strongly coupled magnetic resonance WPT [22].*

Recently, several authors [21–27], have utilized from the strongly coupled magnetic resonance scheme to enhance the transmission properties of WPT systems. Barreto et al. [26] proposed a conformal strongly coupled magnetic resonance system for range extension by using U-loop as an intermediate resonator as shown in **Figure 18**. It provides a high transfer efficiency reach 70% at a transfer distance equal to the diameter of the U-loop (48 cm). Also, this WPT system can maintain efficiencies greater than 60% regardless of the angular position of the receiver around the U-loop. A multilayer

#### **Figure 21.**

*(a) Conventional four-coil system with the transmitter/receiver coils outside the resonators. (b) Wideband four-coil system with the transmitter/receiver coils at the center of resonators [24].*

#### **Figure 22.** *S21 versus frequency [24].*

resonator is discussed in [23], where extra layers of printed spiral coils are inserted in the transmitter/receiver resonators to enhance the Q factor and power transfer efficiency. Conductive shorting walls are employed for the connection between the

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negative (DNG),

**Figure 23.**

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

WPT systems are proposed in [28–35].

ε

**5. WPT utilizing meta-surface structures**

negative (ENG), and

multilayer resonators as illustrated in **Figure 19**. Liu et al. [22] reduced the misalignment sensitivity of strongly coupled WPT systems by applying two orthogonal coils

Using strongly coupled magnetic resonance WPT systems leads to getting a high quality factor (Q ). Nevertheless, this also results in limiting the system bandwidth. Therefore, Zhou *et al.* proposed a wideband strongly coupled magnetic resonance WPT system [24] to overcome the shifting problems of the resonance frequency that occurs in some practical applications, this, in turn, alleviates the decline in the efficiency caused by this shift in the resonant frequency. **Figure 21** shows the proposed technique, the transmitter and receiver coils are fixed at the center of their corresponding intermediate resonators. In this manner, the leakage of magnetic flux can be mitigated, and the bandwidth is broadened as shown in **Figure 22**. Broadband and multi-band WPT system using conformal strongly coupled magnetic resonance technique is introduced in [25]. A multi-band can be obtained by using multiple pairs of loop resonators with various dimensions to resonate at different frequencies, for example, in **Figure 23**, the source loop and load loop are placed between two resonators (resonator 1 and resonator 2). Each resonator resonates at a different resonance frequency to give a dual-band WPT. The broadband operation can also be achieved by merging between the resonance frequencies, this can be obtained using different values of the capacitance of the loop resonators or use resonators with size near each other. Many designs for multi-band and wideband

Metasurface structures are also used to boost the PTE by confining the magnetic field in a narrow channel between transmitter and receiver by combing the evanescent waves from the Transmitter and redirect them into receiver direction due to the negative relative permeability characteristics of some kinds of the metamaterial surfaces. Metamaterials are artificial periodic structures that have negative reflective index characteristics. Metamaterials are classified into three types depending on the polarity of the relative permeability and relative permittivity of the structure: double

µ

The inductive, resonance inductive, and strongly coupled magnetic resonance WPT systems rely on the magnetic field coupling between the transmitter and receiver. Thus, the MNG metamaterial category is used with WPT. When the magnetic field travels from the transmitter coil and incident on a metamaterial with MNG, the

negative (MNG), as shown in **Figure 24**.

together in a 3-D model instead of using planar coils as shown in **Figure 20**.

*Configuration for a dual-band conformal strongly magnetic coupling [25].*

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

**Figure 23.**

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

resonator is discussed in [23], where extra layers of printed spiral coils are inserted in the transmitter/receiver resonators to enhance the Q factor and power transfer efficiency. Conductive shorting walls are employed for the connection between the

*(a) Conventional four-coil system with the transmitter/receiver coils outside the resonators. (b) Wideband* 

*four-coil system with the transmitter/receiver coils at the center of resonators [24].*

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**Figure 22.**

*S21 versus frequency [24].*

**Figure 21.**

*Configuration for a dual-band conformal strongly magnetic coupling [25].*

multilayer resonators as illustrated in **Figure 19**. Liu et al. [22] reduced the misalignment sensitivity of strongly coupled WPT systems by applying two orthogonal coils together in a 3-D model instead of using planar coils as shown in **Figure 20**.

Using strongly coupled magnetic resonance WPT systems leads to getting a high quality factor (Q ). Nevertheless, this also results in limiting the system bandwidth. Therefore, Zhou *et al.* proposed a wideband strongly coupled magnetic resonance WPT system [24] to overcome the shifting problems of the resonance frequency that occurs in some practical applications, this, in turn, alleviates the decline in the efficiency caused by this shift in the resonant frequency. **Figure 21** shows the proposed technique, the transmitter and receiver coils are fixed at the center of their corresponding intermediate resonators. In this manner, the leakage of magnetic flux can be mitigated, and the bandwidth is broadened as shown in **Figure 22**. Broadband and multi-band WPT system using conformal strongly coupled magnetic resonance technique is introduced in [25]. A multi-band can be obtained by using multiple pairs of loop resonators with various dimensions to resonate at different frequencies, for example, in **Figure 23**, the source loop and load loop are placed between two resonators (resonator 1 and resonator 2). Each resonator resonates at a different resonance frequency to give a dual-band WPT. The broadband operation can also be achieved by merging between the resonance frequencies, this can be obtained using different values of the capacitance of the loop resonators or use resonators with size near each other. Many designs for multi-band and wideband WPT systems are proposed in [28–35].
