1 Introduction

Wireless power transmission technology has been pursued by numerous researchers during the past several decades.In short-range wireless power transmission applications (that is,when the distance between a wireless power transmitter and a wireless power receiver is on the order of millimeter or centimeter),the inductive coupling technique is considered a highly efficient and cost-effective approach

.Because the inductive coupling technique takes advantage of the non-radiative magnetic field,its operating frequency is typically below 1 MHz.In recent years,some research efforts have been made on using the radio-frequency band to accomplish efficient short-range wireless power transmission

.Though the radio-frequency wave tends to be radiative,it is possible to achieve strong coupling between a pair of resonant structures such that the radiation is minimized.The radio-frequency short-range wireless power transmission systems (such as those demonstrated in Refs.[3–5]) appear competitive with the inductive coupling systems in terms of compactness and efficiency.A potential advantage associated with radio-frequency coupling is that it may enable broadband simultaneous information and power transfer.Since the inductive coupling technique relies on low frequencies,it does not support wireless communications with high bit rates.In contrast,the radio-frequency band (on the order of 100 MHz or several GHz,for instance) can be leveraged to accomplish high-speed wireless communications in addition to wireless power transmission.

This paper proposes a compact microstrip loop resonator configuration with lumped capacitive loading for short-range wireless power transmission.The overall physical dimensions of the proposed loop resonator are as small as 3 cm by 3 cm.Simulation and measurement results demonstrate the power transmission efficiency is greater than 80%with a power transmission distance smaller than 5 mm via the strong coupling between two loop resonators around 1 GHz.Experimental data also show that the power transmission performance is insensitive to various geometrical misalignments.Also,the numerical and experimental results of this paper reveal a bandwidth of more than 50 MHz within which the power transmission efficiency is above 80%.As a result,the proposed microstrip loop resonator has the potential to accomplish efficient wireless power transmission and high-speed (higher than 10 Mbit/s)wireless communication simultaneously.

The rest of this paper is organized as follows.The proposed microstrip loop resonator is described and some simulation results are presented to demonstrate the high wireless power transmission efficiency of the proposed resonator in Section 2.The short-range wireless power transmission performance of the proposed microstrip loop resonator is verified by experimental results in Section 3.Finally,Section 4 concludes the paper.

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2 Design of Microstrip Loop Resonator for Wireless Power Transmission

As illustrated by Figs.1 and 2,a microstrip loop resonator is designed for wireless power transmission over a short distance.Fig.1 depicts the top view of a circular loop conductor printed over a printed circuit board.The loop has a radius of

and width of

.The printed circuit board has a thickness of

,its substrate has

ε

as the dielectric constant,and the bottom side of the board is covered by a solid conducting ground plane.The loop is fed by a co-axial probe through the substrate.A lumped capacitor

is mounted across a gap over the loop.The angle between the gap and the feed point is

with respect to the loop center.Apparently,the loop structure can be modeled as an inductor.The loop structure and the lumped capacitor jointly lead to a resonant circuit.The resonant frequency can be adjusted by the loop’s geometry and/or the value of

.In practice,the dependence of the loop resonator’s performance on angle

is weak,as the capacitor’s physical size is much smaller than the wavelength.Wireless power could be transmitted between two loop resonators separated by distance

,as shown in Fig.2.The two loop resonators in Fig.2 are assumed to be identical to each other.Strong coupling between them is anticipated around the resonant frequency.The parameter

in Fig.2 denotes the lateral misalignment between the transmitting loop resonator and receiving loop resonator;in other words,the two loop resonators are aligned with each other when

=0.

When the lateral misalignment

between the two loop resonators varies,the simulated power transmission efficiency data are displayed in Fig.5,with

fixed as 0.5 pF and

fixed as 4 mm.When

is as large as 1 cm,the maximum power transmission efficiency drops to 50%.It is noted that the radius of the loop resonators is 1 cm.Thus,the power transmission performance associated with the proposed loop resonators appears quite insensitive to the lateral misalignment.

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In the simulation results in Fig.3,the two loop resonators are assumed to be perfectly aligned with each other geometrically.When there is an angular misalignment

between them,the simulated power transmission efficiency data are plotted in Fig.4.Because the proposed loop resonator has revolutionary symmetry,the power transmission performance is insensitive to the angular misalignment,as evidenced by Fig.4.

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With the parameters tabulated in Table 1,the wireless power transmission configuration in Fig.2 is simulated.The simulation results of power transmission efficiency are shown in Fig.3,with three different values of the loading capacitance

.Specifically,the simulated power transmission efficiency is obtained from the scattering parameter

between the power transmission port (Port 1) and power reception port (Port 2) specified in Fig.2.When

=0.5 pF,the simulated power transmission efficiency is greater than 95% over a 50-MHz frequency band,which may accommodate wireless communications with a bit rate higher than 10 Mbit/s.The resonant frequency drops when

increases,as anticipated.

Overall,our simulation results indicate that 0.5 pF is the optimal capacitance value for

.Specifically,with

=0.5 pF and with insignificant geometrical misalignments,it seems always possible to achieve power transmission efficiency greater than 80%when the distance

is smaller than 5 mm.

3 Experimental Results

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The performance of the proposed loop resonators in this paper is compared with that of the scheme in Ref.[6] which is based on inductive coupling.The loop resonators in this paper and the scheme in Ref.[6] are both proposed for shortdistance wireless power transmission applications.The loop resonators in this paper have a simpler physical configuration than the coils in Ref.[6],and thus are easier to fabricate.The loop resonators in this paper operate around 1 GHz with a bandwidth of more than 50 MHz,and as a result,can accommodate wireless communications with a data rate higher than 10 Mbit/s.The scheme in Ref.[6],in contrast,operates at 107 kHz and thus does not support high-speed wireless information transfer.The radius of the loop resonators in this paper is 1 cm;when the wireless power transmission distance is 5 mm,the wireless power transmission efficiency is higher than 80%;when the lateral misalignment is 1 cm,the wireless power transmission efficiency drops to 50% approximately.The radius of coils in Ref.[6] is 6 cm;when the wireless power transmission distance is 3 cm,the wireless power transmission efficiency is slightly below 80%;when the lateral misalignment is 6 cm,the wireless power transmission efficiency drops to 50% approximately.In summary,the loop resonators proposed in this paper have the potential to enable high-speed wireless communications while offering wireless power transmission performance comparable with the inductive coupling technique in Ref.[6].

When

=0.5 pF and

=4 mm,the power transmission efficiency is measured with four different values of lateral misalignment

.The measurement results in Fig.9 are consistent with the simulation results in Fig.5:the proposed loop resonator configuration can tolerate large lateral misalignment with respect to the size of the loop resonators.

When

=0.5 pF,the power transmission efficiency is measured with four different values of angular misalignment

.The measurement results in Fig.8 are consistent with the simulation results in Fig.4:the power transmission performance is insensitive to the angular misalignment.

Fig.10 shows the measured power transmission efficiency data for various values of distance

,with

=0.5 pF and

=0.Our measurement results reveal that the maximum power transmission efficiency is always greater than 80% when

is smaller than 5 mm.

Based on the simulation results presented in the previous section,the microstrip loop resonators with the parameters in Table 1 are fabricated and tested.Each loop resonator is printed over a printed circuit board with the dimension of 3 cm × 3 cm.The substrate material of the printed circuit boards is Arlon 25N (with

ε

=3.38 and loss tangent of 0.002 5).Two photos of the fabricated prototype are shown in Fig.6.The proposed loop resonator configuration is compact and low-cost.The scattering parameter |

| is measured by a network analyzer manufactured by Radiasun Instruments with model number AV3620A.

Fig.7 shows the measured power transmission efficiency data with three different values of loading capacitance.The maximum power transmission efficiency for

=0.5 pF,1 pF,and 1.5 pF is 88.1%,81.5%,and 75%,respectively.The measured power transmission efficiencies in Fig.7 are lower than the simulated data presented in Fig.3,which we believe is because certain loss is not included in the simulations,such as the loss of the dielectric substrate,printed conductors,and loading capacitor.As a result of the loss not taken into account by the simulations,the bandwidths in Fig.7 are greater than those in Fig.3.For instance,the bandwidth associated with

=0.5 pF is about 60 MHz in Fig.3 but becomes 85 MHz in Fig.7.

4 Conclusions

A compact microstrip loop resonator with lumped capacitive loading is proposed for short-range wireless power transmission around 1 GHz.The overall physical dimensions of the proposed loop resonator are as small as 3 cm by 3 cm.The simulation and measurement results demonstrate the power transmission efficiency is greater than 80% with a power transmission distance smaller than 5 mm.The experimental data also show that the power transmission performance associated with the proposed loop resonators is insensitive to the angular misalignment and can tolerate a large lateral misalignment with respect to the loop size.The numerical and experimental results reveal a bandwidth of more than 50 MHz within which the power transmission efficiency is above 80%.As a result,the proposed microstrip loop resonator has the potential to accomplish efficient wireless power transmission and high-speed (higher than 10 Mbit/s) wireless communication simultaneously.

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