Electric Fleld Research Articles

DUAL-BAND TERAHERTZ METAMATERIAL ABSORBER WITH POLARIZATION INSENSITIVITY AND WIDE INCIDENT ANGLE

This paper presents the design, simulation and mea- surement of a dual-band terahertz metamaterial absorber with polarization-insensitivity and wide incident angle. The unit cell of the metamaterial consists of top resonator structures and low metal- lic ground plane, separated by an isolation material spacer to realize both electric and magnetic resonances. The physical mechanism of dual-band absorption and the sensitivity to the polarization direction and incident direction of the EM wave are theoretically investigated by simulating the x-component and normal component electric fleld distribution, current distribution on ERRs and metallic ground plane, and distribution of power ∞ow and loss at the resonance frequencies as well as difierent modes EM waves, based the FDTD calculated method, respectively. The results show that the absorber is not only correctly coupling to the incident electric fleld and magnetic fleld, but also can trap the input power into speciflc positions of the devices and ab- sorb it, besides insensitivity to the polarized angle and incident angle. Moreover, the experiment demonstrates that the absorber achieves two strong absorptions of 82.8% and 86.8% near 1.724 and 3.557THz.

ACHIEVABLE TRANSVERSE CYLINDRICAL ELECTROMAGNETIC MODE

The system of Maxwell equations with an initial condition in a vacuum is solved in a cylindrical coordinate system. It derives the cylindrical transverse electromagnetic wave mode in which the electric fleld and magnetic fleld are not in phase. Such electromagnetic wave can generate and exist in actual application, and there is no violation of the law of conservation of energy during the electromagnetic fleld interchanges.

REFLECTION AND TRANSMISSION OF ELECTROMAGNETIC WAVE DUE TO A QUASI-FRACTIONAL-SPACE SLAB

A new method is introduced to construct a slab that has electric flelds with propagation properties which are equivalent to a fractional-space wave equation in two-coordinate system. While its magnetic flelds have propagation properties which are equivalent to the complementary fractional-space wave equation. Analytical forms for the re∞ection and transmission coe-cients of this slab are derived. Results of these re∞ection and transmission coe-cients show that such quasi-fractional-space slab has spatial and frequency selectivity properties.

CONCENTRATION OF THE SPECIFIC ABSORPTION RATE AROUND DEEP BRAIN STIMULATION ELECTRODES DURING MRI

During Magnetic Resonance Imaging (MRI), the presence of an implant such as a Deep Brain Stimulation (DBS) lead in a patient's body can pose a signiflcant risk. This is due to the fact that the MR radiofrequency (RF) fleld can achieve a very high strength around the DBS electrodes. Thus the speciflc absorption rate (SAR), which is proportional to the square of the magnitude of the RF electric fleld, can have a very high concentration in the near-fleld region of the electrodes. The resulting tissue heating can reach dangerous levels. The degree of heating depends on the level of SAR concentration. The efiects can be severe, leading to tissue ablation and brain damage, and signiflcant safety concerns arise whenever a patient with an implanted DBS lead is exposed to MR scanning. In this paper, SAR, electric fleld, and temperature rise distributions have been found around actual DBS electrodes. The magnitude and spatial distribution of the induced temperature rises are found to be a function of the length and structure of the lead device, tissue properties and the MR stimulation parameters.

RADIATED EM FIELDS FROM A ROTATING CURRENT-CARRYING CIRCULAR CYLINDER: 2-DIMENSIONAL NUMERICAL SIMULATION

The radiated electromagnetic (EM) flelds from a rotating current-carrying circular cylinder were numerically simulated in two dimensions using the method of characteristics (MOC), and the numerical results were presented in this paper. To overcome the di-culty of the grid cell distortion caused by the rotating cylinder, the passing center swing back grids (PCSBG) technique is employed in collaboration of MOC in a modifled O-type grid system. In order to have clear demonstration of radiated EM flelds, the circular cylinder is set to be evenly divided in radial direction into an even number of slices that are made of perfect electric conductor (PEC) and nonelectric nonmagnetic material, alternatively. The surface current is assumed to have a Gaussian proflle and to ∞ow uniformly along the axial direction on the PEC surface. The radiated electric and magnetic flelds around the cylinder were recorded as functions of time and reported along with the corresponding spectra which were obtained through proper Fourier transformation. Several fleld distributions over the whole computational space are also given.

ELECTROMAGNETIC ANALYSIS OF DISPERSION AND FORM FACTOR CHARACTERISTICS OF A DISC-LOADED CYLINDRICAL WAVEGUIDE FOR A GYRO-TWT AMPLIFIER

A disc-loaded cylindrical waveguide structure for its potential use in a gyro-TWT amplifler operating in the TE mode is analysed for its electromagnetic behaviour. A modal matching technique is used to develop a dispersion relation in order to study the shape of the dispersion curve for wide device bandwidth. The band-pass characteristic of the structure has also been explored. The structure dispersion characteristic has been numerically plotted over a wide range of structure parameters to appreciate the role of individual parameter on the dispersion behaviour of the structure. It has been found that the disc-hole radius may be decreased and the structure periodicity increased for widening the device bandwidth. To validate the theoretical considerations, the structure is also simulated using commercially available simulation software \CST Microwave Studio in order to observe the penetration of the travelling electric flelds in difierent regions of the disc-loaded structure. A good agreement (< 1%) has been achieved between the analytical and simulation results of dispersion characteristics. The analysis is extended further to derive the form factor which plays a very important role in the optimization of the electron-beam position for maximum interaction between RF wave and electron beam. Maximum coupling of the cyclotron wave to the resonant waveguide mode is found to be at the ratio of hollow beam radius to waveguide wall radius (rb=rW = 0:44) for TE01, (rb=rW = 0:39) for TE02 and (rb=rW = 0:35) for TE03 mode. This article has been removed from the website because it has been found to violate plagiarism rule of our journal.

SAR DISTRIBUTION IN A BIO-MEDIUM IN CLOSE PROXIMITY WITH RECTANGULAR DIELECTRIC RESONATOR ANTENNA

In this paper, the simulation and experimental studies of SAR distribution in a bio-medium situated very close to a rectangular dielectric resonator antenna (RDRA) in C-band of microwave frequencies are reported. The simulation study has been carried out using CST Microwave Studio simulation software. The experimental distribution has been obtained using two 50› L-shaped and straight probes and Agilent make 3Hz{50GHz spectrum analyzer. The experimental results for SAR distribution are compared with simulated results. The use of wireless portable devices is growing at fast rate throughout the world. These devices are often used in the vicinity of human body. The continuous growth of wireless portable devices has forced the worldwide researchers to study the interaction between the electromagnetic waves emanating from the device antenna and the human body. The human body, being lossy, absorbs certain amount of electromagnetic radiation generated from portable wireless device situated in its vicinity. This necessitates the evaluation of the performance of antenna in terms of input VSWR, return loss and bandwidth of antenna in presence of bio-medium on one hand and the estimation of the rate of electromagnetic energy absorption, known as speciflc absorption rate (SAR) in the body tissues on the other (1,2). The electric fleld induced and hence SAR within the human body

MAGNETIC FIELD AND CURRENT ARE ZERO INSIDE IDEAL CONDUCTORS

We prove a theorem on the magnetic energy minimum in a system of perfect, or ideal, conductors. It is analogous to Thomson's theorem on the equilibrium electric fleld and charge distribution in a system of conductors. We flrst prove Thomson's theorem using a variational principle. Our new theorem is then derived by similar methods. We flnd that magnetic energy is minimized when the current distribution is a surface current density with zero interior magnetic fleld; perfect conductors are perfectly diamagnetic. The results agree with currents in superconductors being conflned near the surface. The theorem implies a generalized force that expels current and magnetic fleld from the interior of a conductor that loses its resistivity. Examples of solutions that obey the theorem are presented.

ENHANCEMENT OF A MICROWAVE RADIOMETRY IMAGING SYSTEM'S PERFORMANCE USING LEFT HANDED MATERIALS

Aiming at the enhancement of a non invasive Microwave Radiometry Imaging System's (MiRaIS) attributes, Left Handed Materials (LHM) with negative permittivity and negative permeability simultaneously, have been utilized. The optimization of the system focusing properties is being theoretically explored, implementing a semi-analytical Green's function technique and difierent matching structures. In the framework of this analysis the head is modeled by a double layered cylinder while a dielectric cylindrical layer consisting of LHM is placed on the surface of the human head model with a view to achieve focusing improvement inside the brain. Numerical code executions have been conducted for two difierent operating frequencies (0.5GHz and 1.0GHz) and for matching layers of various values of thicknesses and electromagnetic properties. The numerical results for the electric fleld distribution inside the head model, presented in this paper, verify that the LHM can provide an increased sensitivity of the system focusing properties and thus improve its overall performance.

PROTECTION OF NAVAL SYSTEMS AGAINST ELECTROMAGNETIC EFFECTS DUE TO LIGHTNING

This study investigates possible lightning threats to naval crafts, especially those sailing in the shallow waters of tropical oceans where thunderstorms prevail throughout the year and Far-East Asian region where dangerous positive lightning is a signiflcant characteristic in winter thunderstorms. It is empathized that sea water acts as nearly a perfect conductor thus lightning electromagnetic transients propagate over the sea with almost zero attenuation of amplitude and high frequency components intact. The ratio between the peak electric flelds at 5km from the lightning channel, after flelds propagate over dry soil and over sea water is 0.75. The ratio between the peak electric fleld derivatives under the same conditions is 0.1. Such small ratios are observed in the magnetic flelds and their time derivatives as well. Apart from the conductivity, the topological irregularities of the plane over which propagation takes place also contribute to further attenuation of flelds and their time derivatives. This makes marine naval systems more vulnerable to lightning induced efiects than their ground-based counterparts. The paper discusses in detail the lapses of existing naval standards in the defense of electrical and electronic systems against both direct lightning currents and induced efiects of nearby lightning. Consequently we propose the development of a dedicated standard for the lightning protection of naval systems, with the inclusion of several signiflcant recommendations specifled in this paper.

A QUASI-STATIC THEORY FOR DIELECTRIC-COATED THIN-WIRE ANTENNA STRUCTURES

Analytical investigations of the problem of dielectric- coated thin-wire antenna structures have invariably focused on the physics of developing appropriate models for the dielectric insulation on the thin-wire conductors that serve as antenna for the structure. These include the frequency domain moment-method-based approaches in which the dielectric insulation is replaced by equivalent volume polarization currents; and the time-domain analysis based on the 'equivalent radius' concept. An earlier paper gave a physical interpretation to the frequency-domain solutions to suggest that the volume polarization currents derive from an equivalent static charge distribution, which excites an essentially radially-directed quasi-static fleld, conflned to the region associated with the dielectric insulation. It is the main objective of this paper to investigate the veracity of the claims made in open literature as they concern the physics of the model for the dielectric insulation in terms of the electric fleld excited in the dielectric region. And to that end, simulation experiments were carried out, using a commercial Transmission Line Matrix (TLM) Method code, with which the characteristic features of the radial and axial components of the electric fleld within the dielectric region were investigated. The simulation results obtained from the experiments suggest that the fleld in question is not only of the quasi-static variety, but that it is also characterized by an axial component that meets the

CIRCULAR SLOT ANTENNAS USING L-SHAPED PROBE FOR BROADBAND CIRCULAR POLARIZATION

Novel circularly polarized antennas with a circular radiating aperture for broadband characteristics are presented in this paper. The vertical and horizontal components of the L-shaped probe are separated and placed at the front and back side of the substrate. The antennas are excited by a microstrip line which is connected to the vertical component of the L-shaped probe and electromagnetically couples the signal to the horizontal component of the L-shaped probe. A novel concept of placing stub in the slot of a planar antenna, by observing the electric fleld vector behaviour in the slot, is proposed to enhance the axial ratio (AR) bandwidth by around 10%. Unidirectional patterns can be obtained by having a cylindrical cavity of height ‚g=4 behind the antenna and is efiective when no stubs are placed in the slot. A < 3dB AR bandwidth of 39.5% with cavity and 41.18% without cavity but with stub in the slot is obtained in simulation and the results well match with the measurement.

A RIGOROUS FOREST SCATTERING MODEL VALIDATION THROUGH COMPARISON WITH INDOOR BISTATIC SCATTERING MEASUREMENTS

In this paper, we present numerical simulations and indoor bistatic scattering measurements on scaled targets. The targets are vertical and/or tilted dielectric parallelepipeds representing the main forest elements (tree-trunks and primary branches) at VHF and low- UHF frequencies. They are placed above an aluminum circular plate to simulate a ∞at ground. The measurements have been conducted in the anechoic chamber of the \Centre Commun de Ressources Micro- ondes (CCRM) in Marseille, France. A 3D forest scattering model using a Method of Moments (MoM) is deployed to simulate the electric flelds scattered by these targets. Two radar geometric, azimuthal and zenithal, bistatic conflgurations with special attention to the specular direction have been considered. Simulation results and experimental data are confronted for both V V - and HH-polarizations in order to evaluate the accuracy of our model. We have obtained a very good agreement between theoretical and experimental scattered flelds for both the magnitude and phase.

OPTICAL ANTIREFLECTION OF A MEDIUM BY NANOSTRUCTURAL LAYERS

This work examines re∞ection of a light from a semi- inflnite medium which is modifled with an ordered monolayer of spherical nanoparticles placed on or under its surface. We derive analytical expressions for the electric flelds within and outside such structures and verify them with help of strict numerical simulations. We show that nanoparticles layer acts as an imaginary zero-thickness surface having complicated non-Fresnel re∞ection coe-cients with wavelength dependent phase shift. It is shown that such monolayers may reduce re∞ection relative to re∞ection from a pure substrate surface. We derive and analyse a zero-re∞ection condition in the simple intuitive form. It is shown that a single layer of nanocavities near the medium-vacuum interface may increase the transparency of a dielectric medium to values close to 100% in a wide wavelength range.

IMPROVING THE PERFORMANCES OF A HIGH TC SUPERCONDUCTING CIRCULAR MICROSTRIP ANTENNA WITH MULTILAYERED CONFIGURATION AND ANISO TROPIC DIELECTRICS

The moment method technique has been improved to in- vestigate the scattering properties of high Tc superconducting circular antennas with anisotropic substrate in multi-layered conflguration. In this method, the electric fleld integral equation for a current element on a grounded dielectric slab of inflnite extent was developed by basis functions involving Chebyshev polynomials. An improved analytical model is presented taking into account anisotropic substrate, super- conducting material for the circular patch and multilayered structure. To validate the theoretical results, an experimental study has been performed for a perfectly conducting circular patch on a single layer, with and without air gap. Good agreements were obtained between our theory and measurements. Efiects of temperature and thickness

UNIFORM PLANE WAVE REFLECTION FROM PEC PLANE EMBEDDED IN A NONLINEAR MEDIUM

Re∞ection from a perfect electric conductor (PEC) plane of inflnite dimensions embedded in a second order nonlinear medium is studied. The re∞ected wave has two parts, one due to linear behavior, the other due to nonlinear behavior of the medium. The expressions of the re∞ection coe-cients for parallel and perpendicular polarization cases are obtained. The reduction of re∞ection coe-cient to a linear medium case is also reported. Dependence of the said coe-cients on incident electric fleld intensity and the angle of incidence is also plotted.

DESIGN OF BROADBAND CIRCULAR POLARIZATION TRUNCATED HORN ANTENNA WITH SINGLE FEED

This paper presents the design of a broadband circular polarization truncated horn antenna with single feed. It does not require any complex feeding structure and uses only a coaxial feed extended with a simple electric fleld coupling probe. The corners of the horn are truncated to generate circular polarization modes, and a broad axial ratio bandwidth which is insensitive to the probe feed dimension is achieved. Simulated and measured results of an S band truncated horn antenna are presented. The antenna has a broad 3dB axial ratio bandwidth of 26% with aperture e-ciency of 60%.

AXISYMMETRIC ELECTRIC FIELD CALCULATION WITH ZONAL HARMONIC EXPANSION

The electric potential and fleld of an axially symmetric electric system can be computed by expansion of the central and remote zonal harmonics, using the Legendre polynomials. Garrett showed the usefulness of the zonal harmonic expansion for magnetic fleld calculations, and the similar radial series expansion has been widely used in electron optics. In this paper, we summarize our experience of using the zonal harmonic expansion for practically interesting axisymmetric electric fleld computations. This method provides very accurate potential and fleld values, and it is much faster than calculations with elliptic integrals. We present formulas for the central and remote expansions and for the coe-cients of the zonal harmonics (source constants) in the case of general axisymmetric electrodes and dielectrics. We also discuss the general convergence properties of the zonal harmonic series (proof, rate of convergence, and connection with complex series). Practical considerations about the computation method are given at the end. In our appendix, one can flnd many useful formulas about properties of the Legendre polynomials, various derivatives of the zonal harmonic functions, and a simple numerical integration algorithm.

ELECTROMAGNETIC FIELDS IN A CIRCULAR WAVEGUIDE CONTAINING CHIRAL NIHILITY METAMATERIAL

Propagation of electromagnetic flelds and power in a circular waveguide containing chiral nihility metamaterial is studied. Space inside the waveguide is divided into two circular regions. One region contains chiral nihility metamaterial while other region is of free space. Two cases of the waveguide, in this regard, are considered for analysis. For the case of perfect electric conductor (PEC) waveguide, there is no net electric fleld and power propagation in chiral nihility region of the guide whereas both flelds and power exist in non- nihility region (which is free space in our cases) of the guide. For perfect electromagnetic conductor (PEMC) waveguide, both electric and magnetic flelds exist in the chiral nihility and non-nihility regions.

EVALUATION OF ELECTROMAGNETIC FIELDS ASSOCIATED WITH INCLINED LIGHTNING CHANNEL USING SECOND ORDER FDTD-HYBRID METHODS

Evaluation of electromagnetic flelds caused by the lightning channel is an appealing topic in order to consider the indirect efiects of lightning on the power lines. A common assumption for the calculation of electromagnetic flelds at the observation point is a vertical lightning channel, but the fact is that in reality the lightning channel is seldom vertical on the ground surface. In this study, the electromagnetic flelds due to inclined lightning channel at various observation points with difierent angles and with respect to the image of lightning channel on the ground surface were explored. This study also proposes general equations that can estimate the electric flelds due to inclined lightning channel through the 2nd FDTD method. The proposed method supports the notion of vertical lightning channel while the channel angle with respect to z-axis is assumed to be zero. This method was validated through the data gathered from flve flelds: three at a close distance from inclined lightning channel and two at intermediate distances from vertical lightning channel. Similarly, due to inclined lightning channel, the efiects of geometrical and current parameters on the electromagnetic flelds are considered. This study substantiates difierent coupling models with FDTD structure directly at the time domain without a need for extra converters.