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A New Resonator for High Efficiency Wireless. Power Transfer. Majid Manteghi. Bradley Department of Electrical and Computer Engineering. Virginia Tech.
A New Resonator for High Efficiency Wireless Power Transfer Majid Manteghi Bradley Department of Electrical and Computer Engineering Virginia Tech Blacksburg, Virginia, USA [email protected]

Abstract—This paper presents a new resonance structure for high efficiency near-field coupling wireless power transfer. This design can be directly matched to the frontend circuitry without any extra matching components. The new coupler uses a single turn wide strip resonator, which has been proven has the best coupling efficiency, as a resonator. The full-wave simulation results shows 50% coupling efficiency for a distance three times larger than the diameter of the coupler.

I.

INTRODUCTION

The efforts to transfer electromagnetic power without using a conducting transmission line was initiated by Henrick Hertz [1]. Further efforts to the field of wireless propagation were made by Nikolai Tesla in 1901 with the project known as the Wardenclyffe Tower. Tesla envisioned the Earth as a charge moving in free space, and theorized its use as a conductor for two resonant bodies. It can be deduced that this was the first attempt at high power propagation over large distances [2]. Most of the recent works on the wireless power transfer using near-field coupling are based on Tesla’s design [3]. Electromagnetic energy couples to a high-Q resonator and a second high-Q resonator, which is strongly coupled to the first resonator, receives the power and deliver it to the load through a wire or wireless coupling [4]. The main challenge here is the design of a high-Q resonator to minimize the loss and maximize the coupling. The resonator usually consist of a multi-turn inductor in addition to its matching circuitry. The matching circuitry also includes inductors. Inductor design in general is a challenge in high frequency regime due to the parasitic capacitors and ohmic loss. A single turn resonator is presented in this paper which does not need any extra inductors in the matching circuitry which helps to realize a high efficiency high-Q resonator. II.

ELECTRICALLY-COUPLED LOOP ANTENNA (ECLA)

Electrically-Coupled Loop Antenna, ECLA, was introduced as a small magnetic antenna to represent a dual for the Planar Inverted-F antenna, PIFA [5]. A miniaturized version of ECLA is used as a near-field coupler for this work. A picture of ECLA is shown in figure 1. ECLA, in essence, is a loop antenna which is excited through a combination of capacitive couplings which can be matched to a 50Ω load without any extra

978-1-4673-5317-5/13/$31.00 ©2013 IEEE

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Fig. 1 Prototyped Electrically-Coupled Loop Antenna, ECLA.

matching circuitry. A schematic of the antenna is illustrated in figure 2. The parameters W, L, and h are critical to tune the antenna to a particular frequency. In addition, the capacitance between the loop and the ground plane (ts , Ws, and Ls) can change the resonant frequency for a given W, L, and h. There is no dielectric involved with ECLA’s structure and the part which carries the electric current can be chosen wide enough to minimize the ohmic loss. The size of the feeding head, Wp, and its distance to the loop, tp, are responsible for scaling the input impedance. To keep the electric field small at the near zone, it is essential to choose the circumference of the loop (2L+2h) smaller than λ/4. The smaller the antenna versus λ, the lower electric field intensity and higher magnetic field intensity in the near zone. III.

SIMULATION RESULTS

A miniaturized ECLA performs like a magnetic coupler due to its high electric current density at its resonance. Two identical ECLA with the dimensions of: L = h = 300 mm, W = 60 mm , Ls = 150 mm and Ws = 60 mm are used for this

AP-S 2013

One can change the resonant frequency of the structure by changing the antenna’s size. Two different distances has been simulated for this paper: D1 = 600 mm and D2 = 1800 mm. The s12 for D1 and D2 are plotted in figures 3-a and 4-a, respectively. The magnetic field intensity at the resonant frequency is plotted in figures 3-band 4-b as well. These figures show that for a distance three times larger than the diameter of the coupler the s12 = -2.91 dB can be achieved. 0.00 -5.00

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Fig. 2 Electrically-Coupled Loop Antenna: a) 3D view, b) magnified feeding point. c) Side view. d) Magnified feeding point.

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(b) Fig. 4 a- s12 between two resonator while their distance is six times larger than their diameter. b- Magnetic field intensity at the resonant frequency

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REFERENCES [1]

[2] [3]

[4] [5] (b) Fig. 3 a- s12 between two resonator while their distance is three times larger than their diameter. b- Magnetic field intensity at the resonant frequency

experiment. The resonant frequency has been tuned at 31 MHz.

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Hertz Heinrich [Rudolph] 1857-1894. [from old catalog] and D. E. Jones, Electric waves; being researches on the propagation of electric action with finite velocity through space. London,: and New York, Macmillan and co., 1893. N. Tesla, "Patent 1119732: Apparatus for Transmitting Electrical Energy," US Patent Office, 1902. D. W. Williams. (2011). Optimization of near field coupling for efficient power transfer utilizing multiple coupling structures. Available: http://scholar.lib.vt.edu/theses/available/etd05172011-145609/ A. Karalis, J. D. Joannopoulos, and M. Soljacic, "Efficient wireless non-radiative mid-range energy transfer," Annals of Physics, vol. 323, pp. 34-48, Jan 2008. M. Manteghi, "Electrically-Coupled Loop Antenna as a dual for the Planar Inverted-F Antenna," Microwave and Optical Technology Letters, p. Submitted, 2013.