A Comprehensive Review of Midrange Wireless Power Transfer Using Dielectric Resonators

Magnetic resonant coupling (MRC) is one of the techniques that are widely used in wireless power transfer (WPT) systems. The technique is commonly used for enhancing distance while maintaining power transfer efficiency (PTE). Many studies have investigated new technologies to extend the distance of MRC while maintaining high PTE values. The most promising technique to date in MRC is the addition of a resonator between the transmitter and the receiver coil. The implementation of the resonator varies based on different designs, sizes, and material types, although the outcomes remain unsatisfactory. By introducing dielectric material resonators, PTE can be improved by lowering the ohmic loss which becomes a problem on conventional resonators. This study presents a general overview on the use of dielectric material as a resonator in MRC WPT technology and its technological development. The basic operation of MRC WPT is summarized with up-to-date technical improvements related to dielectric material as a resonator in the field of WPT. An overview of the current limitations and challenges of this technique is also highlighted in this study.


Introduction
e magnetic resonant coupling wireless power transfer (MRC WPT) technology uses two electromagnetic systems with the same resonance frequency to transfer power wirelessly at a targeted distance. Generally, both electromagnetic systems can excite strong magnetic resonance if the natural resonance frequency is the same despite the weak coupling at a targeted distance [1]. Prior to magnetic resonance coupling evolution, the most popular and typical technology utilized was inductive coupling (IC). e concept of transferring power over the air was initially founded by Nikola Tesla, leading the experiments and ideas surrounding this concept [2]. Despite its preference, IC could only transfer power within a short distance as compared to MRC as the system efficiency was typically affected by the coil's ohmic resistance and misalignment [2]. In 2007, a researcher from the Massachusetts Institute of Technology (MIT) proved an efficient nonradiative power transfer over a distance of up to 8 times the radius of the coils using a 4-coil system of strongly coupled magnetic resonance that transferred 60 Watts of power with an efficiency of more than 40% and a distance up to 2 meters [3,4].
Owing to the increasing demand for wireless power transfer systems over the past few years, studies on the application of WPT have also increased significantly. ere is a major focus on power transfer efficiency (PTE) as it drops quickly when the separation between the coils increases or when the coils become misaligned [5,6]. Since consumer applications require extremely efficient end to load wireless power transmission, it is critical to consider methods that achieve maximum efficiency. To attain optimal efficiency, several approaches have been investigated and proposed in previous studies [5][6][7][8][9][10][11][12][13]. ese approaches include impedance matching and control strategies using microcontroller techniques which include the quality (Q) factor and coupling (k) coefficient control, coil structure, and misalignment [14]. e adaptive impedance matching approach was previously introduced [5,[7][8][9][10][11][12][13] to improve PTE. Unlike automatic impedance tuning control, the disadvantage of adaptive tuning is the use of the varactor diode in the circuit which introduced additional losses. is reduces the total efficiency of the system. Previous studies have shown that efficiency optimization is achieved by varying the coupling (k) coefficient between the coils [5,[10][11][12][13]. is approach was performed by manually adjusting the distance between the coils to achieve a suitable coupling coefficient (k) at a targeted resonant frequency to ensure optimal efficiency. Although the approach yielded good results, the manual adjustment was not a feasible option for its practical implementation.
For the coil structure, several designs ranging from flat printed structured coil (PSC) [15][16][17] to helix [18][19][20][21], square [22][23][24], and 3D [25,26] designs have been implemented. Designs ranging from as simple as a circular structure to other complicated designs have been adopted although the efficiency remained below the targeted value. In addition to the inefficient performance, its incompetence as a working device due to the bulky and complex design requires further improvement. For misalignment issues [14], horizontal and angular (azimuthal) misalignment are the two main types of misalignments that can occur. Several attempts have been made to solve this issue by variation of coil structure [27][28][29] and use of multiple resonators [20]. Variation of the coil structure to address the misalignment issue reduces the target efficiency, and the implementation of multiple resonators is impractical as more than one resonator is used and placed between the transmitter and receiver coils.
Issues pertaining to the MRC WPT system are related to high ohmic losses in the conventional resonator. When the ohmic loss is high, the Q factor automatically decreases, thus reducing the system performance to "hold" the energy for a longer time. Consequently, energy will deteriorate fast when the coils are separated and when there is a misalignment between the coils.
A review of dielectric material as a resonator in the MRC WPT system is presented in this study. e purpose of using a dielectric resonator is to reduce the ohmic loss in the conventional resonator. Studies in the field of material sciences [30,31] have shown that dielectric material can increase the Q factor through the permittivity characteristic exhibited in the dielectric material. Previous review paper published in 2010 [31,32] presented an extensive review on dielectric material and design. erefore, this paper aims to discuss fundamental elements of dielectric material related to WPT and up-to-date developments over the past few years.

Working Principle of WPT Systems
WPT systems are classified into two categories based on their transmission technique: far-field and near-field transmissions. Radiative electromagnetic fields are used in far-field procedures, while nonradiative electromagnetic fields are used in near-field techniques. Near-field approaches, such as inductive coupling (IC), magnetic resonant coupling (MRC), and capacitive coupling, are the most prevalent and popular WPT techniques (CC). Table 1 shows a general comparison of the near-field WPT. e basic operation principle of the WPT system consists of 2 coils: (a) the transmitter at the source and (b) the receiver at the output. Figure 1 shows the block diagram of the basic operation in the WPT system.
For MRC, the basic 2-coiled WPT system consists of two or multiple resonators (usually two) placed between the transmitter and receiver coil, generally known as coils. Figure 2 illustrates the basic operation of the MRC system. e coils and resonators must resonate at the same frequency to achieve optimal efficiency although the distance is varied or misaligned. e MRC topology is represented by applying the reflected impedance method, in which the compensated capacitances can be determined based on different network topologies such as series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) [33]. Based on reflected impedance method, SS is the only topology that is independent of the coupling coefficient (k) (changing k does not affect the system) and the load condition since the reflected reactance is equal to zero on the primary side [33]. e SS topology is presented using the circuit analysis as shown in Figure 3. e characteristics of the magnetic transmitter are obtained by utilizing Kirchhoff's voltage law (KVL) in the first phase of the calculations for the circuit. e impedances of the transmitter and receiver circuits are represented by the following expressions: where the mutual inductance, M, depends on the inductances L 1 and L 2 of the transmitter/receiver coils and the 2 International Journal of Antennas and Propagation    coupling coefficient k through the expression shown as follows: Each circuit excites identical mutual inductance on its neighbor which is observed as a feedback mechanism between the two circuits. e net effect is that each circuit modulates the impedance of its neighbor through the coupled magnetic flux and the consequent mutual inductance. At resonance, the reactance of each of the impedances Z 1 and Z 2 is identical to zero, and the resonance equations are obtained as follows: At resonance, the two circuits work in near-perfect transfer harmony as indicated by the equation as follows: Equation (4) guides the operating frequency of the power transfer system. e inductors and capacitors used do not have to be of the same value. e most important characteristic is that the resonant frequency should be the same for both circuits. e power transfer relationships are the same as in the nonresonant two-coil systems based on the following equations: At resonance, the power factors in each case become one where the input and output power are calculated from the following equations: erefore, the power transfer efficiency is defined as the ratio of the output power to the input power as follows: e reflection coefficient, S 21 , and radiation pattern parameters (E-field and H-field) are measured as an outcome of the WPT system performance. e transmitter, receiver coil, and dielectric resonator can be realized by independent equivalent RLC circuits as stated above, with each responding to a certain resonant frequency. e equation as follows is calculated for S 21 (theoretical) which compromises the value of the Q factor to prove that the Q factor affects the WPT efficiency.
Equations (11) and (12) are used to calculate the reflection coefficient in the form of percentage based on the simulation and experimental results. To evaluate the overall WPT system performance, the PTE is calculated using the following equation: e Q factor value is related to an individual reactive component, and it relies on the calculated frequency, which is the resonant frequency of the circuit. e Q value for an inductor with series of loss resistance is the total Q of resonant circuit utilizing that inductor and capacitor as follows: Here, R L must be reduced to a minimum value to increase the value of Q. erefore, a suitable method to reduce R is by adding a resonator that has a good coupling with the transmitter and receiver coils. Many types of resonators have been adopted, with a range of various designs and wire types and even the use of multiple resonators such as copper coil and metamaterial [34]. Despite using various resonator types and designs, the achieved results were still below the target as indicated in the condition as follows: According to [35], the efficiency deteriorates fast and is less than 50% if the ratio of the distance, D, between the coils is bigger than the coil diameter, d m . When condition (14) occurs, the PTE drops significantly. Figure 4 show the representation of D and d m in MRC system.
Previous studies have noted this condition and aimed to increase the distance and PTE without comparing it with the coil diameter. Table 2 highlights several examples of studies showing that the D/d m ratio is lower than 1, thus indicating that the coil diameter is larger than the distance. e findings in [35] are supported whereby the PTE is shown to deteriorate when the distance is the same as or higher than the coil diameter value. e main concern lies in producing high PTE without considering the coil size. Subsequently, the system has a big coil but a lower effective distance of PTE. For practical applications, apart from high PTE and longer effective distance, the overall system design should be compact and able to adapt to at least a small misalignment between the coils. erefore, an alternative solution proposed for this issue is to replace the conventional resonator in the MRC system with other types such as the use of dielectric material. Studies employing the dielectric resonator in WPT are relatively scarce, and new insights can be unraveled to enhance the overall MRC WPT system.

Dielectric Resonators
Dielectric is a poor conductor material but can sustain electrostatic fields. If the movement of current within opposite electric charge poles is retained to a minimum and the electrostatic lines of flux are intact, the electrostatic field will be able to store energy. Most dielectric materials exist as a solid form [32] such as porcelain or ceramics, mica, glass, plastics, and oxides of various metals. Some liquids and gases also act as good dielectric material. Dry air is an excellent dielectric, while distilled water is a fair dielectric. On the other hand, vacuum is one example of an efficient dielectric.
Special and crucial characteristic of dielectric property is its capability to sustain an electrostatic field while releasing minimal energy in terms of heat. Reducing the dielectric loss made the dielectric material more efficient, as the proportion of energy lost as heat is minimal [36]. Another consideration is the dielectric constant which represents the extent to which a substance concentrates the electrostatic lines of the flux. Substances with a low dielectric constant include vacuum and dry air, while moderate dielectric constants include ceramics, distilled water, paper, mica, polyethylene, and glass. High dielectric constants are derived from metal oxides. High dielectric permittivity polarizes more in response to an applied electric field as compared to a material with low permittivity, thereby storing more energy in the material [37]. erefore, high dielectric permittivity is favorable option for the WPT system to increase PTE and the distance as well as to reduce misalignment issues. e operating frequency for dielectric resonators depends on the WPT system as previous studies have shown that lower dielectric permittivity (1-100) is often suitable and produces a higher frequency system, whereas higher permittivity (100 and above) is suitable for the kHz and MHz systems. WPT is widely known to operate in MHz range; hence, a dielectric with permittivity of around 1000 is preferable.
By using dielectric material with a suitable dielectric permittivity, it is possible to create a low loss resonator. Low loss resonator exhibits a high refractive index, which resulted in lowering electromagnetic wave movement. In WPT, this characteristic introduces stronger magnetic resonance in the dielectric resonator, thus leading to stronger resonances and higher efficiencies [32,34]. By using dielectric resonator in WPT, magnetic dipoles are generated due to the interactions between the transmitter and receiver with the source excitation. To increase the transfer efficiency, the quality factor, Q, must be higher than 12. Hence, to obtain a high Q value, the R L value based on (10) must be very small. To reduce R L , the resonator is selected and coupled with the transmitter and resonator. e disadvantage of resonators selected in previous studies was that the R L value was still high, thus resulting in a low Q factor as the coil-type resonators have ohmic resistance. erefore, the dielectric resonator is used as there is no additional ohmic resistance in the dielectric material and it is thus considered as a higher order mode of magnetic transfer [37,38]. e signal reflections and refractions based on boundary conditions and material properties in the dielectric were used to assess the quality factor for the system. e quality factor of the dielectric resonator depends on the dielectric size and dielectric constant value [38]. e echoes formed inside the dielectric can generate a higher order harmonic of fundamental frequency for input excitation. ese echoes are known as higher order modes. At any instance, these echoes are a weighted sum or superposition of all the modes present inside the dielectric and thus are termed as dominant modes [37]. Fundamental modes have the lowest resonant frequency, while higher order modes have the highest frequency. e implementation of the dielectric resonator in the WPT system drives the resonant frequency to the higher order mode. e basic MRC operates at the electric dipole (ED) mode, but when the dielectric resonator is used, other modes known as the magnetic dipole (MD) and magnetic quadrupole (MQ) modes occur [32,34,39]. e radiation loss is possibly reduced at a higher mode, thus increasing the system efficiency. Each frequency mode is different in shapes and magnitudes with respect to their magnetic fields. Figure 5 shows the differences between the E-and Hfield for MD and MQ as depicted in [39].
Studies have revealed that the WPT efficiency operating at MD mode not only was more efficient but also has higher resistant to the arbitrary orientation of the transmitter and receiver. e higher order modes exist when the value of the refractive index is higher, resulting in reducing ohmic resistance [39]. erefore, operating at the MD or MQ mode as   International Journal of Antennas and Propagation opposed to the ED mode produces more efficient WPT system due to the reduction in ohmic and radiation losses. e theories on magnetic and electric dipole are essential for the understanding of resonant modes in WPT when adopting dielectric material as a resonator. An interesting feature of the dielectric resonator is that it has different radiating modes that are similar to electric and magnetic multipoles known as dipole, quadrupole, and octupole. e radiation pattern can be predicted accurately without using any complicated calculations for a regular shaped dielectric resonator [38].
For example, in the cylindrical dielectric resonator, the TE 01δ mode exists and radiates like a magnetic dipole aligned along its axis as shown in Figure 6. e TE 011+δ radiates as an axial magnetic quadrupole [40]. Equally, the TM 01δ and TM 011+δ modes radiate as an axial electric dipole and a quadrupole, accordingly. e basic form of the radiated fields for the TE and TM modes is independent of the dielectric constant related to the resonator material. e unique characteristic of the dielectric resonator to produce various radiation patterns from different excitation radiation modes has not been investigated and appears to be interesting. For instance, the higher order mode HEM 21δ of a dielectric resonator, known as magnetic quadrupole mode, has the capability of sustaining magnetic fields that are less exploited in research related to WPT. Figure 7 shows the differences between the TM 01δ and HEM 21δ modes.

Inferences from Literature Review Studies
e manipulation and application of dielectric resonators in WPT began in 2013. However, their potential was hardly explored due to difficulties in producing high permittivity dielectric resonators. Inconsistent and unreliable fabrication processes were some of the challenges faced by researchers for their practical application. Nevertheless, the potential of dielectric resonators in enhancing WPT systems can be further explored, thus enabling the identification of new materials. A summary of publications reported from 2013 to 2019 is shown in Figure 8.
A WPT system based on ceramic material as a dielectric resonator was investigated in [42] as shown in Figure 9. In this study, Nishikawa and Ishizaki used a round shaped ceramic dielectric in their research and avoided the use of conventional resonators as conventional resonators have ohmic losses due to unloaded impedance. e researchers focused on the Q factor, particularly with emphasis on the microwave band frequency. e resonant frequency was set to 2.4 GHz and considered to be quite high for the MRC operating frequency. e dielectric material permittivity and PTE distance were not mentioned in the study.
In 2014, Hotta et al. used water as a dielectric to investigate the efficiency of MRC [43] as in Figure 10. ey compared the use of conventional resonators with dielectric resonators.
e results revealed that the conventional method produced a 5 to 10% increase in efficiency due to the low permittivity of water.
In another study [44], the authors used water or aqueous solution as a dielectric material, but the dielectric permittivity varied from 80 to more than 2000. e results based on the eigenmode analysis indicated that the higher the permittivity, the higher the Q factor. is observation supports the notion that the low permittivity dielectric observed in [43] is less efficient as compared to the conventional resonator. erefore, high permittivity is more suitable to be implemented in the WPT system. e use of higher dielectric permittivity was suggested in [39] as in Figure 11, in which ceramic dielectric with a permittivity of 80 was incorporated into the design. With a higher Q value, the radiation loss is minimized, and the efficiency of the system is increased. e basic principle of the system can be obtained from the Mie Scattering eory which specifies that the charges bound inside the dielectric resonator interact with an external magnetic field as different orders of magnetic multipolar modes can be formed at different frequency. erefore, the magnetic dipole (MD) mode is excited at the frequency of 210 MHz while the magnetic quadrupole (MQ) mode is excited at higher frequency of around 300 MHz.
ese two frequencies were recognized as the operating frequencies with an efficiency of nearly 90% at a 5 cm distance. e efficiency related to (14) was still low as the resonator dielectric was 8 cm and the sphere resonator design was big and bulky, thus deemed unsuitable for practical purposes.
In a follow-up study, the same authors improved the WPT system as proposed in [46,47] using a dielectric cylinder resonator with colossal permittivity. e dielectric resonator was made from ceramics with a high permittivity of around 1000.
e results were quite promising as the resonance frequency shifted from GHz to MHz, thus enabling the WPT operation. e simulation and experimental results were verified with a 50% efficiency which could be maintained within the separation distance of 16 cm (3.8 radius ratio with respect to resonator size) as in Figure 12 and Figure 13.
e effect of using low and high permittivity dielectric resonator in the WPT system was investigated using ceramics based on Ba, Sr, and TiO 3 solution doped with Mg. Previous studies were only based on the simulation using the CST software by locating the magnetic distribution within the WPT system. In 2018 [34], the working prototype was designed and validated for the WPT system using metasurface WPT for mobile charging applications. Figure 14 illustrates the proposed system, where the PTE increased tremendously to 80% at a 25 cm distance.
In addition, several studies [51][52][53][54] investigated the dielectric resonator effect in the WPT system. A recent study [35] showed that the WPT system operated at a quadrupole mode is more efficient and more resistant with different orientation of the transmitter and receiver placement. is finding affirms that the use of dielectric resonators can increase the WPT PTE and effective distance as well as reducing the misalignment issue. e effect of the dielectric design or shape was investigated using the dielectric strapping resonator [51] as in Figure 15(a) and dielectricloaded multimode split cavity resonator (SCR) [52] as in Figure 15(b). e results were quite promising although further investigation of the implementation is required for its practical use. In another study [53], as in Figure 16(a), dielectric resonators from EXXELIA TEMEX E5080 which are composed of the oxide composition of B m , S m , and T i material with a permittivity of 78 were employed. e authors indicated that this ceramic material was quite difficult to source as a ready-made resonator, and it was not easy to fabricate the resonator with a specific permittivity value. As in Figure 17 dielectric resonator is used layer by layer between the transmitter and receiver coil.
As previously indicated, misalignment in the MRC is another issue that must be addressed. ere are two main types: lateral (horizontal) misalignment and angular (azimuthal) misalignment [56]. Two authors highlight the issue of misalignment in dielectric resonators for MRC WPT in [53,57]. Both authors discussed misalignment on the lateral and angular axes. Table 3 shows a summary of previous studies on misalignment using dielectric material.
In contrast, Table 4 summarizes the previous studies on misalignment using conventional resonator. e findings for both types of misalignments using dielectric resonator are fairly good in comparison to previous studies such as [27,[58][59][60][61][62]. In a prior research, the authors used a different coil, resonator, and tracking technique to decrease the influence of misalignment. A precise comparison of performance cannot be done since the idea of misalignment has been defined variously in prior research and is application dependent. For example, in the case of an electric vehicle, lateral misalignment is considered since the charging pad is fixed beneath the vehicle, preventing azimuthal misalignment.
As shown in Table 5, the form of the dielectric varies across designs, and the size is identical to that of the transmitter and receiver coils. e shape of the dielectric has no discernible influence on the overall system's performance, and according to [65], the dielectric constant is the most critical factor. e greater the dielectric constant is, the longer the dielectric can retain magnetic fields; however, the frequency is shifted to a lower value. As demonstrated in [53], the parametric research employed dielectric constants ranging from 1000 to 2000; however, due to the inaccessibility of the needed dielectric constant, the authors employed 78 dielectric values for experimental verification. e frequency was increased from hundreds of MHz to GHz, which reduced the effective distance (to only a few millimetres) since the system wavelength decreased as the frequency was increased. According to Song et al., the initial research used a spherical shape dielectric to investigate parametric effects on the dielectric constant of a WPT system starting in 2016 [48] and shifted to a slimmer dielectric in 2019 [34] Figure 6: Nature of radiation for different modes of the cylindrical dielectric resonator. M is magnetic dipole and P is electric dipole [40].
International Journal of Antennas and Propagation ceramics shape because it is more suitable for implementation and commercialization in real applications.
Comparison of the performance of MRC with a dielectric resonator and a conventional resonator is shown in Tables 6 and 7. As shown in the tables, the D/d m ratio is greater when a dielectric resonator is used, indicating that the dielectric resonator has a substantial influence on the performance of the MRC. On a more positive note, research into dielectric resonators may be expanded and investigated further. Figure 7: View of the electric and magnetic field intensity using HFSS eigenmode method. (a) TM 01δ ; (b) HEM 21δ modes [41]. Water-filled acryl base a a Figure 10: Design of the WPT system as in [43]. R d θ ε Figure 11: Design of the WPT system as in [39,45].
N L a Figure 14: Design of the WPT system as in [34].
International Journal of Antennas and Propagation 9   Figure 17: Design of the WPT system as in [54].

Conclusion
Owing to the high potential of the WPT system, many industry practitioners are eager to adopt this system in their applications. Various studies have been performed to solve the problems related to the WPT system, particularly concerning the distance of PTE as well as misalignment issues. e manipulation of dielectric resonators previously used for antenna applications for high frequency has been adopted in the WPT system to provide a better solution for the current issues. However, the fabrication of the dielectric resonator with targeted permittivity is difficult to source, and    the cost of the dielectric material is high. erefore, a suitable design for the WPT system integrated with dielectric material is required to ensure its practical implementation.

Data Availability
Data are available from the corresponding author on request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.