Analysis of Antenna Structure for Energy Beamforming in Wireless Power Transfer

The technology to send a magnetic field in a particular direction, known as energy beamforming, has been recently introduced as a magnetic field shaping technology in nonradiative wireless power transmission. In general, one of the most efficient conditions for energy beamforming is that the magnetic fields induced by each antenna should be synthesized to head the same direction. To synthesize the magnetic fields induced at each antenna, interference by mutual inductance that can occur between transmitting (TX) antennas should be minimized. In addition, energy should not be exchanged between the TXs, otherwise it lowers the transmission efficiencies of TX and receiving antenna. In this paper, we present an optimal antenna structure that minimizes the mutual inductance between two TX antennas. First, we have analyzed the mutual inductance between TX antennas that have asymmetric sizes with different antenna lengths and arrangement angles. The directivity of the magnetic field vector is also investigated through an experimental analysis of an antenna structure. Finally, it has been verified that the optimal TX antennas for energy beamforming should be symmetric, which means that all the length of antennas are same and disposed perpendicular to each other. The experimental results show that the deviations of magnetic field directivity for symmetric and asymmetric antennas are 0.045 and 0.355, respectively, which shows that the symmetric structure shows 8.2 times larger consistency over the asymmetric structure.

I. Introduction

Nonradiative wireless power transfer (WPT), which uses magnetic field in the near-field region, is being researched with great interest as a promising technology to improve wireless charging efficiencies for various electronic devices such as mobile, wearable devices, and electric cars [1], [2]. The researchers of magnetic resonant coupling WPT system have recently introduced various research results including the following items: 1)

conversion (ac–dc/dc–ac/dc–dc, etc.) technology for enhancement of transmission efficiency [3];

2)

antenna structure with high quality factor (Q) [4];

3)

technology to expand the transmission distance of repeaters [5], [6];

4)

adaptive impedance-matching technology and resonance frequency tracking technology to maintain the transmission efficiency with varying the transmission distance [7], [8];

5)

3-D antenna structure technology for enhancement of free positioning [9]–[11].

As specified in several previous studies for WPT, the primary concern is the improvement of power transmission efficiency, expanding transmission distance with free device positioning.