Journal of Electrical and Computer Engineering

Volume 2016, Article ID 2136923, 11 pages

http://dx.doi.org/10.1155/2016/2136923

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

^{1}School of Computer, Hefei Normal University, Hefei 230601, China^{2}Hefei University of Technology School of Instrument Science and Opto-electronics Engineering, Hefei, China

Received 31 March 2016; Accepted 14 July 2016

Academic Editor: Jit S. Mandeep

Copyright © 2016 Hesheng Cheng and Huakun Zhang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A metamaterial-inspired efficient electrically small antenna is proposed, firstly. And then several improving power transfer efficiency (PTE) methods for wireless power transfer (WPT) systems composed of the proposed antenna in the radiating near-field region are investigated. Method one is using a proposed antenna as a power retriever. This WPT system consisted of three proposed antennas: a transmitter, a receiver, and a retriever. The system is fed by only one power source. At a fixed distance from receiver to transmitter, the distance between the transmitter and the retriever is turned to maximize power transfer from the transmitter to the receiver. Method two is using two proposed antennas as transmitters and one antenna as receiver. The receiver is placed between the two transmitters. In this system, two power sources are used to feed the two transmitters, respectively. By adjusting the phase difference between the two feeding sources, the maximum PTE can be obtained at the optimal phase difference. Using the same configuration as method two, method three, where the maximum PTE can be increased by regulating the voltage (or power) ratio of the two feeding sources, is proposed. In addition, we combine the proposed methods to construct another two schemes, which improve the PTE at different extent than classical WPT system.

#### 1. 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 [3–8] and can by resonant coupling [9–18] or inductive coupling [19–26] techniques in the coupled mode near-field region. However, it is not very much that researchers’ attention centers on the radiating near-field region [27–30]. Compared to the radio waves transceiver in the far-field, WPT in the radiating near-field 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 near-field region is very short in terms of wavelength. And not only the optimal load impedance [15–17], 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 near-field region [17].

Following the coupled mode region is the radiating near-field 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 metamaterial-inspired 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).

#### 2. 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 is the power transfer efficiency in the case of simultaneous conjugate matching, and is in mismatching case. is the power dissipated in the load of the receiving antenna, is the input power accepted by the transmitting antenna, and 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, 27–30]. 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, 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 [31–33]. 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.