Dimitrios P. Chrissoulidis

Induced EM field in a layered eccentric spheres model of the head: plane-wave and localized source exposure

The induced electromagnetic (EM) field in a layered eccentric spheres structure is determined through a concise analytical formulation based on indirect mode-matching (IMM). The exact analytical solution is applied to a six-layer model of the head. This model allows for eccentricity between the inner and outer sets of concentric spherical layers which simulate brain and skull, respectively. Excitation is provided by a nearby localized source or by an incident plane wave. The numerical application provides information about the total absorbed power, the absorption in each layer, and the spatial distribution of the specific absorption rate (SAR) at frequencies used by cellular phones. The effects of excitation frequency, eccentricity, exposure configuration, and antenna-head separation are investigated.

Radio-wave exposure of the human head: analytical study based on a versatile eccentric spheres model including a brain core and a pair of eyeballs

A versatile eccentric-spheres model of the human head is used to investigate radio-wave absorption. Numerical results, obtained by use of an exact analytical solution, are presented for the total, percentage, and gram-specific absorption. Interest is mainly in the brain and in the eyes of an adult or an infant head. Our model comprises a host sphere and several spherical inclusions, all concentrically stratified with respect to their own center. Any number of inclusions and any number of concentric layers for the host sphere and each one of the inclusions can be considered. Excitation is provided either by a plane-wave or by a nearby electric dipole. The analytical solution is obtained by use of the indirect-mode matching method. The theory of this paper and the accompanying computer code constitute a versatile tool for analytical studies of cellular-phone interactions with the human head. Specific absorption rate maps in a horizontal cross section of the head model manifest the existence of hot spots in the eyes and near the center of the brain.

Indirect mode-matching solution to scattering from a dielectric sphere with an eccentric inclusion

The analytical formulation of electromagnetic wave scattering from an eccentrically stratified dielectric sphere is greatly simplified through the indirect mode-matching technique. The resulting exact solution is the most compact available and hence the least prone to analytical or numerical errors. After some checks we present a comparison between our solution and two previous solutions that were obtained through direct mode matching. Our numerical investigation is focused on an acrylic sphere with an eccentric cavity. All four elements of the scattering matrix are available, and specific information about the possibility of detecting the scatterer’s internal asymmetry is given.

Study of interactive scattering by clusters of spheres

A compact extended Mie solution to electromagnetic scattering from a cluster of spheres is obtained through indirect mode matching. Our interest is focused on interactions among member spheres, as manifested in multiple scattering from the cluster. We therefore define and investigate the interactive-backscattering cross section of the cluster and the interaction length of member spheres. Numerical results are presented to probe the effect of sphere spacing as well as the effect of look direction and incident polarization on interactive backscattering from silicate clusters. Moreover, we present a brief extinction and absorption study of a soot particle near a cloud droplet.

Electromagnetic-wave scattering by a sphere with multiple spherical inclusions

An exact solution to the problem of electromagnetic-wave scattering from a sphere with an arbitrary number of nonoverlapping spherical inclusions is obtained by use of the indirect mode-matching technique. A set of linear equations for the wave amplitudes of the electric field intensity throughout the inhomogeneous sphere and in the surrounding empty space is determined. Numerical results are calculated by truncation and matrix inversion of that set of equations. Specific information about the truncation number pertaining to the multipole expansions of the electric field intensity is given. The theory and the accompanying computer code successfully reproduce the results of other pertinent papers. Some numerical results [Borghese et al., Appl. Opt. 33, 484 (1994)] were not reproduced well, and that discrepancy is discussed. Our numerical investigation is focused on an acrylic sphere with up to four spherical inclusions. This is the first time that numerical results are presented for a sphere with more than two spherical inclusions. Interesting remarks are made about the effect that the look direction and the structure of the inhomogeneity have on backscattering by the acrylic host sphere.

Dyadic Green's function of a sphere with an eccentric spherical inclusion.

An exact, analytical solution to the problem of point-source radiation in the presence of a sphere with an eccentric spherical inclusion has been obtained by combined use of the dyadic Greens function formalism and the indirect mode-matching technique. The end result of the analysis is a set of linear equations for the vector wave amplitudes of the electric Greens dyad. The point source can be anywhere, even within the aforesaid nonspherical body, and there is no restriction with regard to the electrical properties in any part of space. Several checks confirm that this solution obeys the energy conservation and reciprocity principles. Numerical results are presented for an electric Hertz dipole radiating from within an acrylic sphere, which contains an eccentric spherical cavity.

EM-wave scattering by an inclined strip grating

The characteristic spectral response of an inclined strip grating, either free-standing or embedded in a dielectric slab, is quite sensitive to variations of the cant angle. It is an attractive frequency-selective surface which allows considerable design flexibility through the cant angle and/or the refractive index of the host dielectric. The inclined strip grating is analyzed through an indirect mode-matching technique based on Greens second theorem. The analytical approach is simple and numerically efficient. Calculations focus on the effect of cant angle on the magnitude of all propagating Floquet harmonics on both sides of the structure. Interesting features of the scattering behavior are discussed. >

Electromagnetic-wave scattering by an eccentrically stratified, dielectric cylinder with multiple, eccentrically stratified, cylindrical, dielectric inclusions

A cylinder with an arbitrary number of cylindrical inclusions, all eccentrically stratified, parallel, and infinite in length, may serve as generic model of several bodies, natural or artificial. The electromagnetic field within such a complex cylindrical structure, as well as the scattered wave, is determined in this paper by use of the indirect mode-matching method. A linearly polarized, plane wave, normally incident upon the stratified cylinder, is used as excitation. The resulting analytical solution is exact in the sense that no approximation is used in any part of the analysis, except for the truncation of multipole expansions for the electric- field intensity. Application is made to a model of the human leg. Interest is in the effect of osteoporosis on the tangential field at the perimeter of the gastrocnemial. Appropriate values for the frequency, polarization, and angle of incidence of the excitation are determined and appropriate points on the perimeter of the leg are proposed for measurements of the aforesaid effect.

Dyadic Green's function of a cluster of spheres.

The electric dyadic Greens function (dGf) of a cluster of spheres is obtained by application of the superposition principle, dyadic algebra, and the indirect mode-matching method. The analysis results in a set of linear equations for the unknown, vector, wave amplitudes of the dGf; that set is solved by truncation and matrix inversion. The theory is exact in the sense that no simplifying assumptions are made in the analytical steps leading to the dGf, and it is general in the sense that any number, position, size and electrical properties can be considered for the spheres that cluster together. The point source can be anywhere, even within one of the spheres. Energy conservation, reciprocity, and other tests prove that this solution is correct. Numerical results are presented for an electric Hertz dipole radiating in the presence of an array of rexolite spheres, which manifests lensing and beam-forming capabilities.

Miniaturization of microstrip patch antenna for wireless applications by use of multilayered electromagnetic band gap substrate

The effect of a two-layer, periodic, electromagnetic bandgap (EBG) substrate on the performance of a dual-polarization, square, patch antenna is investigated in this paper. The antenna is printed in a gap area of the uppermost surface of the EBG substrate, disturbing by its presence the periodicity and the symmetry of that surface. Finite difference, time domain methods have been used for the simulations and a prototype of the proposed structure has been made in our laboratory in order to verify the simulated results. The proposed antenna resonates at 2415MHz and it is 24% smaller in area than a square patch with the same type of feedlines and the very same resonance frequency, which is printed on an ordinary, reference substrate. By ¿ordinary¿ it is implied that the reference substrate is a stack of two layers having the same dielectric constant and thickness as those of the proposed two-layer, EBG substrate. The decrease in frequency is 45.2% with respect to a square patch of the same size on the reference substrate. Still, the bandwidth of the proposed antenna increases by 12%, despite the narrowband nature of the quarter-wavelength matching circuits at both inputs, and it presents higher isolation between the two ports.