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In this work, we demonstrate an approach for local reduction of the electric field amplitude of the transmitted radio-frequency signal in ultrahigh-field magnetic resonance imaging (MRI). We excite a suitable combination of three coupled dipole hybrid resonances composing a single transmit antenna array element. Using numerical optimization, we designed a feeding network for three coupled dipoles placed over an electromagnetic phantom mimicking a human body. This network of discrete elements provides the appropriate amplitudes and phases of three dipole currents excited by a single input port. It allows controlling the electric field distribution in the vicinity of the antenna. Our goal was to obtain a minimum of the electric field at the given relatively small depth inside the phantom, where body implants are typically located while keeping a tolerable level of the magnetic field toward the phantom’s center. We designed and manufactured a three-dipole antenna prototype optimized for MRI of the human body at 7 T (proton Larmor frequency of 298 MHz). The experimental validation showed a 40 dB reduction of the electric field amplitude at a depth of 4 cm compared to a conventional single-dipole antenna. The coupling network can be rearranged to target different depths. Therefore, a principle of electric field minimization at a controllable position inside the body has been shown, which may be useful for designing transmit MRI antennas with improved safety of implants. [ABSTRACT FROM AUTHOR]

A novel approach for designing an element of an arrayed radio frequency coil for ultrahigh field magnetic resonance imaging (UHF MRI) is presented. The purpose of this approach is to achieve the dual-band transceiver regime in a compact array of magnetic antennas. Our work qualitatively develops the concept of the so-called high-impedance coil (HIC), combined with the adequate interfacing circuitry, which offers the decoupling of the HICs in the array due to the very high self-impedance compared to the mutual impedance. This concept has not been previously thought applicable to dual-band transceiver arrays. We show that, by slightly modifying only the cable shield, we achieve a dual-band transceiver high-impedance coil (dual-band TRHIC). The needed modification implies two asymmetric gaps granting two useful eigenmodes to the cable loop. The resonant excitation of these modes will allow the magnetic resonant scanning of both hydrogen and Phosphorus in the 7 T dc magnetic field. To verify our ideas, we simulated and measured a single TRHIC in both transmitting and receiving regimes and similarly studied two linear arrays made of two and three proposed TRHICs. The method of asymmetric gaps, in our opinion, allows one to engineer the targeted multifrequency operation of arrayed TRHICs decoupled at these frequencies. [ABSTRACT FROM AUTHOR]

Phased antenna arrays of dipoles are widely used in ultrahigh field (UHF) magnetic resonance imaging for creating the controllable radio frequency (RF) magnetic field distributions in a human body. Due to safety and imaging quality reasons each individual channel of the array should be decoupled—electromagnetically isolated from the others. The required number of channels is large and in some techniques the dipole antennas should be located in the close proximity of the human body. Their ultimately dense arrangement leads to a strong mutual coupling and makes the conventional decoupling structures inefficient. This coupling needs to be suppressed without a significant distortion of RF fields in the imaged area. In this work, we propose and study a novel decoupling technique for two UHF transceiver on-body dipole antennas. The decoupling is performed by a periodic structure of five parallel resonant wires referred to as a metasurface (MS). In contrast to conventional decoupling with a single resonant wire, the MS decoupled by means of excitation of a higher order coupled mode of the wires, which field is highly confined. The main advantage is a low distortion of the RF-field in the central region of the body. [ABSTRACT FROM AUTHOR]

Conventional wireless power transfer systems consist of a microwave power generator and a microwave power receiver separated by some distance. To realize efficient power transfer, the system is typically brought to resonance, and the coupled-antenna mode is optimized to reduce radiation into the surrounding space. In this scheme, any modification of the receiver position or its electromagnetic properties results in the necessity of dynamically tuning the whole system to restore the resonant matching condition. It implies poor robustness to the receiver location and load impedance, as well as additional energy consumption in the control network. In this paper, we introduce a new paradigm for wireless power delivery based on which the whole system, including transmitter and receiver and the space in between, forms a unified microwave power generator. In our proposed scenario, the load itself becomes a part of the generator. Microwave oscillations are created directly at the receiver location, eliminating the need for dynamical tuning of the system within the range of the self-oscillation regime. As a proof-of-concept demonstration, we experimentally show that, such self-oscillating wireless power delivery systems can provide robust operation against changes in the environment or in the location of the load. The proposed concept has relevant connections with the recent interest in parity-time symmetric systems, in which balanced loss and gain distributions enable unusual electromagnetic responses. [ABSTRACT FROM AUTHOR]

In this communication, we show that broadband enhancement of radiation from subwavelength sources can be achieved by using a new type of metamaterials—irregular random wire media. In contrast to samples of aligned metal wires, which give a narrow-band enhancement at Fabry–Perot resonances, here, the broadband enhancement is achieved due to the internal structure of the sample. Next, in contrast to lenses and hyperlenses, the proposed structure offers this enhancement without enlargement of the radiating aperture. [ABSTRACT FROM PUBLISHER]

Hyperlens is a metamaterial structure initially proposed for magnifying imaging of subwavelength scattering objects. Here, we show that a wire-medium (WM) hyperlens can be used also for significant enhancement of radiation from small sources to free space and that this enhancement holds in the ultra-wide frequency band. We investigate how the divergence of metal wires modifies the radiated power of a dipole, comparing with an array of parallel wires, and investigate the impact of other design parameters. Next, we implement the optimized hyperlens in the microwave range and verify the theoretical results experimentally. We believe that this work will help to introduce and develop hyperlenses for a number of applications where broadband radiation enhancement of subwavelength sources is needed. [ABSTRACT FROM PUBLISHER]

In this paper, we propose, design, theoretically study, and experimentally test a simple periodical array which provides perfect absorptivity when one of its surfaces is illuminated and controllable reflectivity for illuminations of the other side. The proposed structure does not contain any ground plane and relies on the bianisotropic properties of the unit cells forming the absorbing layer. The absence of the ground plane makes the new design suitable for applications where the absorber should not block transmission outside the absorption band. The proposed structure is realized as an array of nonidentical conducting patches imprinted on the two sides of a thin dielectric slab. This ultra-thin ($\sim\lambda/150$, where $\lambda$ is the operational wavelength) metasurface absorber is optimized, fabricated, and tested. The test results confirm nearly perfect absorption (from one direction) and controllable reflection (from the other direction) at the resonance frequency, as well as partial transparency outside of the absorption band. [ABSTRACT FROM PUBLISHER]

Bloch analysis in conjunction with a full-wave simulation is used in order to design a composite right/left-handed (CRLH) transmission line (TL). We analyze the parasitic components introduced by loading elements and inter-cell coupling, and present details to the compensation required in order to maintain the balanced state of the periodic TL. The method is shown to yield a useful tool for the design of CRLH periodic TLs up to millimeter-wave frequencies for structures with arbitrary metallization. The performance of the method is shown numerically at 26 and 77 GHz. Experimental verification is done at 26 GHz, comparing radiative properties, dispersion, and attenuation constant of a through-broadside scanning leaky-wave antenna. [ABSTRACT FROM PUBLISHER]

The angular and polarization stability of the effective magnetic wall behavior is critical for successful applications of high-impedance surfaces, especially in low-profile antennas. We suggest an approach to realization of a high-impedance surface with unique magnetic-wall resonance for all incidence angles and for both TE-and TM-polarizations of incident plane waves. In order to achieve this goal we apply the concept of a grounded uniaxial material slab as a realization of a HIS surface and find the effective material parameters required for the unique parallel resonance of the surface impedance. We show that these parameters can be realized with a racemic arrangement of quasi-planar resonant chiral particles. The analytical model of the structure agrees well with full-wave numerical simulations of particular realizations. The maximal deviation of the magnetic-wall resonance frequency in the simulated example is smaller than 0.2% for both polarizations and for the incidence angles varying from 0 to 85 degrees. To our knowledge, this is a qualitatively significant improvement of previously obtained results. [ABSTRACT FROM PUBLISHER]

We address the question of the optimal choice of loading material for antenna miniaturization. A new approach to identify the optimal loading material (dielectric or magnetic) for maximization of bandwidth of resonant antennas is introduced. Instead of equivalent resonant circuits or transmission-line resonators, we use the analysis of radiation mechanism to identify the fields contributing mostly to the stored energy and determine the beneficial material type. The formulated rule is qualitatively illustrated using a dipole and a patch antenna, as well as a planar inverted-L antenna where the conventional analysis of circuit or a transmission-line resonator leads to incorrect conclusions. Guidelines are presented for miniaturizing different antenna types. [ABSTRACT FROM AUTHOR]

Low-profile antennas comprising a horizontal dipole above a high-impedance surface are analyzed. The emphasis of this paper is on the additional resonances of the radiating structure caused by surface waves propagating on the high-impedance surface. It is shown that such resonances can be favorably used for broadening the bandwidth of the antenna. The phenomenon is thoroughly modeled by exploiting a parallel between the HIS structure and a waveguide resonator. In the second part of the paper we discuss homogenized approaches for modeling the radiating properties of the antenna with emphasis to the phenomenon discussed in the first part. As it turns out, it is necessary to take into account the spatially dispersive properties of high-impedance surfaces, and most of the simplified models commonly used for analyzing high-impedance surface based antennas fail in predicting the discussed resonance mode. [ABSTRACT FROM AUTHOR]

A design for planar electromagnetic absorbers is presented. The performance of this absorber is maintained over a wide incidence angles and for both TE and TM polarization. The absorber is composed of a high-impedance surface comprising an array of patches over a grounded lossy dielectric slab perforated with metallic vias (wire medium). The main contribution of the paper is to demonstrate and practically use the presence of an additional resonance when the plasma frequency of the wire medium in the dielectric substrate is close to the original resonance of the high-impedance surface. The presence of the vias between FSS and the ground plane is discussed both for the case of a high-permittivity absorber and for a low permittivity one, through the derivation of simple and efficient analytical expressions, specifically derived for the problem at hand. We show that the presence of the vias influences the oblique incidence TM absorption, and when properly designed, the insertion of the vias into the absorber structure results in a bandwidth enlargement and higher absorption performance. [ABSTRACT FROM AUTHOR]

Simple analytical formulas are introduced for the grid impedance of electrically dense arrays of square patches and for the surface impedance of high-impedance surfaces based on the dense arrays of metal strips or square patches over ground planes. Emphasis is on the oblique-incidence excitation. The approach is based on the known analytical models for strip grids combined with the approximate Babinet principle for planar grids located at a dielectric interface. Analytical expressions for the surface impedance and reflection coefficient resulting from our analysis are thoroughly verified by full-wave simulations and compared with available data in open literature for particular cases. The results can be used in the design of various antennas and microwave or millimeter wave devices which use artificial impedance surfaces and artificial magnetic conductors (reflect-array antennas, tunable phase shifters, etc.), as well as for the derivation of accurate higher-order impedance boundary conditions for artificial (high-) impedance surfaces. As an example, the propagation properties of surface waves along the high-impedance surfaces are studied. [ABSTRACT FROM AUTHOR]

The effect of artificial magnetodielectric substrates on the impedance bandwidth properties of microstrip antennas is discussed. We review the results found in the literature and then focus on practically realizable artificial magnetic media operating in the microwave regime. Next, a realistic dispersive behavior of a practically realizable artificial substrate is embedded into the model. It is shown that frequency dispersion of the substrate plays a very important role in the impedance bandwidth characteristics of the loaded antenna. The impedance bandwidths of reduced size patch antennas loaded with dispersive magnetodielectric substrates and high-permittivity substrates are compared. It is shown that unlike substrates with dispersion-free permeability, practically realizable artificial substrates with dispersive magnetic permeability are not advantageous in antenna miniaturization. This conclusion is experimentally validated. [ABSTRACT FROM AUTHOR]

The work presented in this paper concerns a theoretical study on frequency selective surfaces (FSS) with application to artificial magnetic conductors or high-impedance surfaces (HIS). Current realizations of HIS are based on a planar FSS at the interface of a metal-backed dielectric slab either including vertical vias or not. A stable resonance was found for the case of series-resonance grids without vias in the slab. The resonance turns out to be unique in theory for all angles of incidence and both polarizations of plane waves illuminating the HIS. It was shown that vias destroy the stabilization effect and introduce a frequency shift. The analytical model was validated by HFSS simulations. [ABSTRACT FROM AUTHOR]

The influence of material coverings on the antenna quality factor is investigated for radiators formed by thin electric or magnetic line sources. It is shown that thin uniform and syms metric layers of arbitrary passive materials (including Veselago, left-handed, or double-negative materials) cannot help to overcome the bandwidth limitations imposed by the amount of energy stored in the antenna reactive field. Alternative possibilities offered by complex composite materials in the antenna design are identified. [ABSTRACT FROM AUTHOR]

We report free space transmission and the first reflection measurements of a composite double negative (DNG) metamaterial, also known as a left-handed material (LHM). The metamaterial composes of the split-ring-resonators and discontinuous thin wires. Very high transmission values of the metamaterial are observed within a frequency range for which both effective permeability and permittivity are expected to be negative. [ABSTRACT FROM AUTHOR]

Presents information on a study that featured electromagnetic excitation of two-dimensional arrays of bianisotropic particles by plane waves. Formulation of the problem; Evaluation of interaction dyadics; Example of collective polarizability dyadics for arrays of omega particles; Conclusions.

A correction to the article "Simple and Accurate Analytical Model of Planar Grids and High-Impedance Surfaces Comprising Metal Strips or Patches," by Olli Luukkonen and colleagues is presented.