Investigation of Improved Methods in Power Transfer Efficiency for Radiating Near-Field Wireless Power Transfer

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Introduction
Research and engineering application for WPT technology has been achieved much attention since the first WPT experiments were carried out by Tesla in the early 20th century [1,2].Research in wireless power transmission (WPT) technologies can be carried out by electromagnetic radiation in the radiating far-field region [3][4][5][6][7][8] and can by resonant coupling [9][10][11][12][13][14][15][16][17][18] or inductive coupling [19][20][21][22][23][24][25][26] techniques in the coupled mode near-field region.However, it is not very much that researchers' attention centers on the radiating near-field region [27][28][29][30].Compared to the radio waves transceiver in the far-field, WPT in the radiating nearfield region has less dissipated energy and obtains higher PTE.And compared to inductive coupling techniques, WPT in the radiating near-field region can transfer farther by efficient electromagnetic radiation.Using resonant coupling for WPT in the coupled mode near-field region, very high PTE can be achieved.However, the distance for the coupled mode nearfield region is very short in terms of wavelength.And not only the optimal load impedance [15][16][17], but also the resonant frequency and the input impedance of the transmitter are influenced by antenna spacing [11,12,18].It makes difficulty for design of an optimal scheme in the coupled mode nearfield region [17].
Following the coupled mode region is the radiating nearfield region, where the two antennas are weakly coupled and the PTE decreases rapidly with distance.The authors of [30] used spherical mode theory to derive a theoretical upper bound for wireless power transfer between two antennas in free space.It is pointed out that high antenna radiation efficiency is needed to maximize the power transfer in this region.In [27], two highly efficient folded cylindrical helix (FCH) dipole were used to demonstrate the PTE bound in [30].Two transmitting and one receiving antennas were investigated in [28] in this region.The authors of the paper found that a stable region in PTE can be created for sufficiently close spacing between two transmitters, in contrast to the monotonic decay of the single-transmitter case.In addition, the authors of [29] investigated the PTE regarding lossy dielectric materials effects on WPT based on numerical simulations and experimental measurements.
In the present paper, firstly we design a metamaterialinspired highly efficient electrically small antenna.The proposed antenna is shown to be well matched to a 50 Ω source.The purpose of this design is to obtain high PTE in radiating near-field region with the 50 Ω optimal load impedance.In the following sections, introduction of an antenna as a retriever to a classical WPT system, phase difference, and voltage ratio adjusting methods are proposed successively.Finally, two WPT schemes are constructed by combining the proposed methods to improve the PTE in varying degrees.The performance of the proposed antenna and WPT systems are characterized with FEldberechnung für Körper mit beliebiger Oberfläche (FEKO).

Efficient Electrically Small Antenna Design
We first define the power transfer efficiency for the usual WPT network in two cases: simultaneous conjugate matching and mismatching at feeding port of transmitting antenna: where PTE mat is the power transfer efficiency in the case of simultaneous conjugate matching, and PTE mis is in mismatching case.P R L is the power dissipated in the load R L of the receiving antenna, P in is the input power accepted by the transmitting antenna, and P T is the total input power, including accepted and reflected power by the transmitting antenna.
The feasibility of near-field WPT over a certain number of distances using two electrically small antennas was demonstrated [11,12,[27][28][29][30].When the WPT system performs in the radiating near-field region, it requires high antenna radiation efficiency to maximize the PTE between transmitting antenna and receiving antenna.In addition, without a matching network a traditional electrically small antenna is known for it is poorly matched to a given 50 Ω source.Thus, PTE mis decreases badly because there is a high reflective lossy at the feeding port of transmitting antenna of WPT system.
The so-called metamaterials has been utilized to realize the design of highly radiant efficiency electrically small antenna [31][32][33].The metamaterial-based efficient electrically small antennas have been conceptualized with structures constructed from ideal double negative (DNG) or single negative (SNG) media.For example, an electrically small electric dipole and a loop antenna radiating in the presence of, respectively, an isotropic, homogenous lossless, and dispersive electrically small epsilon negative (ENG), and a mu-negative (MNG) spherical shell have been shown theoretically to produce a radiating element that is impedance matched to a specified source to obtain an efficient electrically small antenna system [31,32].The electrically small ENG (MNG) metamaterial spherical shell provides the necessary capacitance (inductance) to produce the matching mechanism.In [33], an efficient and electrically small antenna based on a three-dimensional (3D) structure was presented.This antenna is designed to be resonantly driven by a semicircular loop fed through a finite-sized ground plane with a coax feedline.For its large ground plane, the 3D antenna system is not suitable for wireless power transfer.
We present an efficient and electrically small antenna based on a circular loop that acts as an electrically small magnetic dipole.The loop is fed with the power source and surrounded by the other parasitic loop terminated in capacitive impedance, which served as negative magnetic permittivity (MNG) material.Figure 1 shows configuration of the proposed antenna made of copper wires.The copper conductivity value for this design is 5.8E7 Siemens/m.The outer loop efficiently captures and resonantly magnifies the magnetic flux generated by the inner loop that is driving it.A properly designed radius of outer loop and the lumped element capacitance can provide a capacitive element resonantly that matched to the highly inductive electrically small inner loop antenna.By varying the parameters of the proposed antenna, we can obtain the optimum dimensions of the copper wire and capacitance for 50 Ω input resistance and high radiation efficiency.Figure 2 plots the input impedance of the proposed antenna with respect to one variable under the other four fixed parameters (five parameters are showed in Figure 1).It can be seen that the radius of the inner loop has a major impact on the input impedance of radiating element but has impact faintly to the resonant frequency (Figure 2(a)).Actually, increasing the radius of the inner circular loop enhances the resonant coupling of the inner driving loop to the outer parasitic loop and thus enhances the resulting radiation resistance of the antenna.The outer loop termed with capacitor provides the major capacitance of this antenna scheme.The resonant frequency moves to lower frequency region with the increasing radius of the outer loop; meanwhile, the input resistance decreases (Figure 2(b)), that is, because the inductive capacitance turns down and the resonant coupling between the two loops goes into a decline.It is reasonable that resonant frequency moves down as increasing capacitance (Figure 2(d)).It is also found that when the diameters of conductive wire or numbers of inner loop are changed, the affected parameters are only the input resistance (Figures 2(c) and 2(e)).It is very useful for us to tailor the input impedance of antenna system to match to the feedline.Table 1 shows the final design parameters of the proposed antenna for WPT. Figure 3 shows the complex input impedance and reflection coefficient ( 11 ) values for this antenna.It is clear from Figure 3 that the antenna for WPT is ant-resonant and matched to the feedline at about 140.8 MHz. Figure 4 presents that the radiation efficiency is 90.34% at 140.8 MHz.

Design Specifications for Efficient Power Transfer
3.1.Retriever-Based Scheme.In this section, a retriever-based system composed of proposed antennas shows in Figure 5(a).It consists of three antennas: a transmitter, a receiver, and a retriever.Only one power source is used to feed the transmitter of this system.The receiver is first fixed at a certain distance  from the transmitter, and then the distance   is optimized to achieve the maximum PTE.In this manner we can obtain the optimal   for maximum efficiency PTE mat at each distance  of the receiver from the transmitter.The PTE mat is calculated with a 50 Ω resistive load placed on port 2 of receiver.The frequency of maximum efficiency as a function of distance   for different values of separation  is plotted in Figure 6.It can be seen that the frequency for maximum efficiency is only affected by  when the distance   is long enough.This is reasonable since the effect of retriever grows fainter in the far distance.
In the state   beyond 1.7 m, it deviates upward from the 140.8MHz resonant frequency of an isolated antenna when  comes closely below 0.6 m.However, the frequency for maximum efficiency offsets obviously under the influence of shortened distance   .The transfer efficiency is calculated at the frequency shown in Figure 6 for various distance   and the corresponding spacing .These results are illustrated in Figure 7.As demonstrated in this figure, the efficiency decays monotonously with the distance  between receiver and transmitter.For a certain , the efficiency fluctuates feebly until it reaches a steady value with the incremental distance   and can achieve a maximum value at an optimal spacing   .This phenomenon is caused by phase difference between source field of transmitter and inductive field of retriever.In other words, when the fields from the transmitter arrive at the location of the receiver, differing from the phase of fields from the retriever by integral multiple of 360 ∘ , it is an inphase condition and gives a relative maximum.Conversely, a minimum is obtained at an out-phase condition.Figure 8(a) shows the frequency and distance   for maximum efficiency PTE mat extracted from Figures 6 and 7 against separation .
For the sake of comparison, we also calculate the efficiency of a classical WPT system made up of two proposed antennas (shown in Figure 5(b)).These results are illustrated in Figure 9.The frequency for maximum efficiency PTE mat against separation  is shown in Figure 8(b).A relatively obvious improvement in the transfer efficiency can be seen in short distance .For instance, the growth of PTE mat from classical system to retriever-based scheme is 7.3% (from 75.8% to 83.1%), 5.2% (from 41.6% to 46.8%), and 1.86% (from 12.3% to 14.16%) for  at 0.2 m, 0.4 m, and 0.6 m, respectively.

Phase Difference Adjusting for WPT System under Two
Transmitters.In this section, another power source is fed to the retriever in Figure 5(a).This setup, which is shown in Figure 10, consists of two transmitters and one receiver.The separation   between two antennas served as transmitters is fixed at 0.8 m.The following simulation results based on  2 / 1 = 1 and load resistance   = 50 Ω as the receiver are moved between the transmitters from 0.2 m to 0.6 m.The operating frequency is drawn in Figure 11.From this picture, it can be seen that the frequency decreases with increasing  and varies in a small scale with dynamic phase difference for a certain .The obtained power transmission efficiency as a function of phase difference for various distance  is shown in Figure 12.This figure shows that a relative maximum efficiency can be gained at optimum phase difference for a certain .The amount of efficiency variation changes with various phase difference by small degrees at close distance .However, it displays a wide range of changes with various phase difference when the receiver converges towards the middle position ( = 0.4 m) of the two transmitters.Numerical examples show that the efficiency changes are 8.2% (from 75.7% at 162 ∘ to 67.5% at 320 ∘ ) and 59.9% (from 59.9% at 0 ∘ to 0 at 180 ∘ ) for  at 0.2 m and 0.4 m, respectively.By this method, the calculated PTE mat across distance  is shown in Figure 14.For the distance  from 0.2 m to 0.4 m, the PTE mat result is achieved in the conditions of phase difference illustrated in Figure 12.When the receiver moves closer to transmitter 2 (0.4 m ≤  ≤ 0.6 m), it is obtained by role reversal between the two power sources.The optimum frequency and phase difference for maximum PTE mat plotted in Figure 14 is shown in Figure 13.For comparison, the efficiency of a classical WPT system is also  plotted in Figure 14.From 0.2 m to 0.35 m, owing to high lossy of power transmitted from transmitter 2, the PTE mat of phase difference adjusting scheme is lower than classical WPT system.Starting from approximately 0.35 m, the proposed method has an obvious advantage over the way of classical power transfer.

Voltage Ratio Adjusting for WPT System under Two Transmitters.
We also use the WPT system shown in Figure 10.Two antennas served as transmitters with a separation   fixed at 0.8 m.The following simulation results are based on the condition of optimum phase difference investigated in the foregoing section and load resistance   = 50 Ω as the receiver is moved between the transmitters from 0.2 m to 0.6 m.The operating frequency of efficiency is drawn as in Figure 15.The transfer efficiency is calculated at the frequency shown in Figure 15 for various voltage ratio and the corresponding spacing .These results are illustrated in Figure 16.This figure shows that a relative maximum efficiency can be gained at optimum voltage ratio for a certain .The amount of efficiency variation changes with various voltage ratio by small degrees at close distance .However, it displays a wide range of changes with various voltage ratio when the receiver converges towards the middle position ( = 0.4 m) of the two transmitters.Numerical examples show that the efficiency changes are 5.3% (from 80.99% at 0 to 75.7% at 1) and 20.8% (from 59.9% at 1 to 39.1% at 0) for  at 0.2 m and 0.4 m, respectively.The calculated PTE mat across distance  is shown in Figure 18.For the distance  from 0.2 m to 0.4 m, the PTE mat result is achieved under the conditions of  2 / 1 , phase 2 − phase 1.When the receiver moves closely to transmitter 2 (0.4 m ≤  ≤ 0.6 m), it is obtained by role reversal between the two power sources.The optimum frequency and voltage ratio, extracted from Figures 15 and 16, and phase difference for maximum PTE mat are shown in Figure 17.It is observed that the frequency for maximum PTE mat decreases gradually as the receiver approaches to the middle position of distance   between the two transmitters, whereas the voltage ratio for maximum PTE mat increases progressively.PTE mat of the two WPT systems mentioned above are also plotted in Figure 18.It is clear that the use of retriever, phase difference, and voltage ratio adjusting not only allows one to overcome the monotonic decay in efficiency observed in classical WPT system but also increases efficiency across the whole range of separation .

Integrated Utilization of the Proposed Methods for Efficient Power Transfer
In this section, two systematic frameworks for WPT based on the proposed methods are constructed.One is that phase difference and voltage ratio between the two sources are optimized to obtain maximum PTE mat on the premise of optimum   of retriever-based system, that is, the retriever, shown in Figure 5(a), served as another transmitter, which another source fed with in this scheme.The configuration is shown in Figure 19(a).The processes operate in order as Figure 20 shows the PTE mat of this WPT scheme in solid line marked as case 3. Figure 21 shows the corresponding conditions of this scheme for maximum PTE mat .From Section 3.3, it is worthwhile to note that the maximum PTE mat can be obtained under  2 / 1 = 1, phase 2 − phase 1 = 0, and  in the middle of   .The other WPT system is therefore constructed with   = 2, phase 2 − phase 1 = 0, and  2 / 1 = 1 to obtain the maximum PTE mat .The system schematic is shown in Figure 19(b).Figure 20 shows the PTE mat of this WPT scheme in dot-dashed line marked as case 4. Figure 22 shows the corresponding operating frequency of this scheme for maximum PTE mat .For comparison purposes, the classical WPT system, retriever-based system and system with two transmitters fixed at   = 0.8 m, phase difference, and voltage ratio optimized for each separation  that have been discussed separately in the previous paragraph are also considered here.In Figure 20, the transfer efficiency of them is plotted with dotted line (marked as case 1), double-dot-dashed line (marked as case 2), and dashed line (marked as case 5), respectively.
Case 3 performs a little better than case 2 in the far distance region, which starts from approximately 0.4 m.This is because the optimum   is only conducive to retrieve power from transmitter 1 and unfavourable for use of power Phase difference (degree) from transmitter 2. As shown in Figure 21, The power from transmitter 2 holds zero from 0.2 m to 0.3 m (actually, case 3 becomes case 2 in this situation).The proportion of power from transmitter 2 progressively increased for maximum PTE mat from 0.3 m to 0.6 m.The efficiency in case 4 is more efficient than that in the first three cases and even higher than case 5 during the first half of the distance .For the advantage of role reversal between the two power sources, case 5 performs well in the whole region.optimum frequency for maximum efficiency); case 2: retrieverbased system (single source; optimum frequency and   for maximum efficiency); case 3: WPT system with two transmitters based on the optimum   of retriever-based system (two sources; optimum frequency, distance   , phase difference and voltage ratio for maximum efficiency); case 4:   = 2 (two sources; optimum frequency, phase 2 − phase 1 = 0 and  2 / 1 = 1 for maximum efficiency); case 5:   = 0.8 m (two sources; optimum frequency, phase difference, and voltage ratio for maximum efficiency).

Conclusion
This paper investigates several methods used to achieve efficient WPT over a radiating near-field region.Power transfer efficiency of the retriever-based system demonstrates  some improvement compared with the classical system in the whole separation .The performance of the WPT system with two transmitters based on the optimum   of retrieverbased system is nearly the same as that of the retrieverbased system in most region except for far separation region (from 0.5 m to 0.6 m).The proposed WPT system with two transmitters under condition of   = 2, phase 2 − phase 1 = 0, and  2 / 1 = 1 performs better than the classical and the first two proposed ones.Using phase difference, voltage ratio adjusting,and role reversal between the two transmitters, we propose a WPT system under fixed separation   (  = 0.8 m) to achieve a relatively high efficiency over the whole region and to overcome the monotonic decay of efficiency seen in classical WPT systems.

Figure 1 :
Figure 1: Antenna geometry and detailed specification of each design variable.(a) 3D view and (b) 2D lateral view.

2 Figure 4 :
Figure 4: The radiation efficiency values of proposed antenna for WPT against frequency.

Figure 5 :Figure 6 :
Figure 5: System schematic of retriever-based WPT.(a) WPT system with retriever, that is, retriever-based system.(b) WPT system without a retriever, that is, the classical WPT system.

Figure 7 :
Figure 7: Efficiency of retriever-based WPT system at frequency shown in Figure 6 against   for different .

Figure 8 :
Figure 8: Optimum conditions for maximum efficiency PTE mat against separation  (a) retriever-based system and (b) the classical WPT system.

Figure 9 :
Figure 9: Efficiency of the considered WPT systems.Two different systems are considered: a WPT system with retriever and distance   optimized for each separation  (solid line); the classical WPT system shown in Figure 5(b) (dashed line).

Figure 10 :
Figure 10: System schematic of two transmitters and one receiver.

Source 2 Figure 11 :Source 2 Figure 12 :
Figure 11: Operating frequency of PTE mat against phase difference for different .

1 Figure 15 :Figure 16 :
Figure 15: Operating frequency of PTE mat against voltage ratio for different .

Figure 17 :Figure 18 :
Figure 17: Optimum phase difference, voltage ratio, and frequency for maximum efficiency PTE mat against separation .

Figure 21 :
Figure 21: Optimum   , phase difference, voltage ratio, and frequency of case 3 for maximum efficiency PTE mat against separation .

Figure 22 :
Figure 22: Optimum frequency of case 4 for maximum efficiency PTE mat against separation .

Table 1 :
The design parameters of the proposed antenna.
Figure 3: The complex input impedance response and  11 reflection coefficient values of proposed antenna for WPT against frequency.The reflection coefficient values are obtained using a  ch = 50 Ω transmission line feed.