Gunyoung Kim

Metamaterial-Based Zeroth-Order Resonant Antennas for MIMO Applications

A compact (0.26×0.05 λ0 at 2.27 GHz) metamaterial-based zeroth-order resonant antenna system consisting of epsilon-negative (ENG) and mu-negative (MNG) structures is proposed. Although the spacing between the ENG and MNG antennas is only 0.09 λ0 , the isolation is relatively high (27.6 dB at 2.27 GHz). Furthermore, the envelope correlation is only 0.015. The radiation efficiencies of the proposed two radiators are also very high (90% on average).

Synthesis of Bulk Medium with Negative Permeability Using Ring Resonators

This paper presents simple expressions for the effective permeability of bulk metamaterial consisting of ring resonators (RRs) or split ring resonators (SRRs) based on the convenient geometrical factors of the structure compared with wavelength. The resonant frequency dependence of the medium permeability, including loss effects, is analyzed in detail. Inverting the analysis equations, useful design (or synthesis) equations are derived for a systematic design process with some examples. This paper may particularly be useful for the design of a bulk metamaterial with a specific negative relative permeability at a desired frequency. The loss of metamaterials consisting of RRs (or SRRs) is also analyzed over a wide frequency band from 10 MHz to 10 THz.

Wireless Power Transmission between Two Metamaterial-Inspired Loops at 300 MHz

Based on a provided closed-form wireless power transmission (WPT) efficiency formula, which may be used for any value of load, we have analyzed the WPT efficiencies between two metamaterial-inspired loop antennas in various aspects. Due to the modeling based on low frequency circuit theory, the provided formula at the center resonant frequency has been found to be accurate until when the distance between the two loop antennas increases to 15 cm (about λ0/6 at 300 MHz). When the two loops get closer, the resonant frequency has been found to split into two in theory, simulations, and measurements. The EM-simulated and measured efficiencies at new resonant frequencies are 60.9 % and 46.3 %, respectively, at d=15 cm. With two extra rings around the loops, the maximum efficiency is enhanced to 93.7 % at d=15 cm. The effect of the additional two rings is about 30 %.

Analysis of magnetically coupled wireless power transfer between two resonators based on power conservation

Based on an equivalent circuit in which the necessary elements for a magnetically coupled wireless power transfer (WPT) system are considered, its overall characteristics are formulated and analyzed. In particular, the WPT efficiencies are conveniently expressed as a function of a figure of merit and a load deviation factor, re-emphasizing the use of an optimal load for the maximum efficiency. Some useful closed-form formulas for extraction of circuit parameters and coupling coefficients are also presented. The theoretical and EM-simulated WPT efficiencies are shown to be in good agreement, validating the circuit modeling and presented formulation.

Effects of Metamaterial Slabs Applied to Wireless Power Transfer at 13.56 MHz

This paper analyzes the effects of a metamaterial slab (or a practical “perfect lens”) with negative permeability applied to a two loop magnetically coupled wireless power transfer (WPT) system at 13.56 MHz, based on theory, full-wave electromagnetic- (EM-) simulations, and measurements. When using lossless slabs with ideal negative permeability in EM-simulations, the WPT efficiencies have been found to be enhanced close to 100% due to the magnetic field focusing. For the case of using a realistic slab made of ring resonators (RR) with (s: slab width, d: distance between the transmitting and receiving loops), the WPT efficiency has been found to significantly decrease to about 20%, even lower than that of a free space case (32%) due to the heavy power absorption in the slab. However, some efficiency enhancement can be achieved when is optimized between 0.1 and 0.3. Overall, the significant enhancement of efficiencies when using a lossless slab becomes moderate or only marginal when employing a realistic slab.

Alternative Expressions for Mutual Inductance and Coupling Coefficient Applied in Wireless Power Transfer

Alternative analytic expressions for the mutual inductance (Lm) and coupling coefficient (k) between circular loops are presented using more familiar and convenient expressions that represent the property of reciprocity clearly. In particular, the coupling coefficients are expressed in terms of structural dimensions normalized to a geometric mean of radii of two loops. Based on the presented expressions, various aspects of the mutual inductances and coupling coefficients, including the regions of positive, zero, and negative value, are examined with respect to their impacts on the efficiency of wireless power transmission.

Investigation of isotropicity for 3-D metamaterial bulk structure

In this paper, we investigate the isotropicity of a 3-D metamaterial bulk structure consisting of ring resonators (RR) and thin wires based on the modeling of them using the definition of magnetization and polarization. The presented 3-D metamaterial bulk structure is shown to be synthesized to have the effective permeability and permittivity of near -1 at 8 GHz. Its refractive index is also about -1. The computed Brillouin dispersion diagram also showed that the proposed structure is almost near isotropic. The electrical length of the unit cell, examined based on the Brillouin zone, showed the deviation of about 20%.

Extraction Solution for the Coupling Coefficient at the Magnetically Coupled Wireless Power Transmission

This paper presented the extraction solution for the coupling coefficient at the magnetically coupled wireless power transmission(WPT) system through the analysis of its equivalent circuit considering the loss. The conventional extraction solution using coupled mode theory is generalized employing the extracted solution considering the load resistance. Consequently, the measuring process of extracting coupling coefficient becomes convenient since the even/odd mode analysis is not necessary. Furthermore, the coupling coefficient obtained from the induced extraction method was in excellent agreement with the coupling coefficient obtained using the ratio of magnetic flux passing through the two loops. The extraction of the accurate coupling coefficient at the magnetically coupled WPT is an essential work to analyze and optimize the WPT system.

Effects of metamaterial slab with negative permeability applied to magnetically-coupled wireless power transfer system

Effects of a metamaterial slab with negative permeability applied to a magnetically coupled two-loop wireless power transfer system on efficiencies are analyzed. Particularly, the maximum efficiencies are examined as a function of s/d (s: slab width, d: distance between two loops) and the slab loss tangent. They are evaluated in terms of coupling coefficients (k), effective Q-factors of the loop, and optimum loads for maximum efficiencies, considering the slab losses. Using the meta-slab, some enhance-ment of transfer efficiencies can be achieved when s/d ≈0.2 even for the case of considerable loss tangent of 0.2.

Design of Wideband Thin Absorber Using Resistive Cross-Shaped Surface Structures

This paper presents a design method for thin but wideband absorbers using resistive sheets. Its equivalent circuit consists of a series RLC resonant circuit and a short-terminated transmission line. Based on this equivalent circuit, we presented the three conditions for an electromagnetic absorber which has a thickness less than a quarter wavelength and wide absorption bandwidth at center frequency. By using an root-finding algorithm, the equivalent resistance, capacitance, and inductance of the absorbers are obtained. These equivalent circuit values for the absorber surface can be realized by a 2D periodic cross-shaped structure which has required surface resistance. Using the design method, we have designed the absorber which has 18.75 mm(67.5° electrical length) thickness and 90 % absorption bandwidth of 116 % bandwidth at 3 GHz.