Scattering by a cylindrical dielectric shell with inhomogeneous permittivity profile

The scattering by a plane wave incident on an infinitely long cylindrical shell is derived using the boundary-value method. The cylindrical shell is assumed to have an inhomogeneous dielectric profile in both the radial and azimuthal directions or a circular cylindrical coordinate system. The validity of the solution is verified by comparison with the method of moments. Different types of inhomogeneities are investigated and the resulting echo width as well as the scattered field pattern indicate significant variations which could be used to improve the radiation characteristics of antennas and the reradiation behaviour of scattering objects.     

Radar cross sections of dielectric or plasma coated conducting spheres and circular cylinders

Radar cross sections for a variety of spherical and cylindrical scatterers having homogeneous dielectric or plasma shells are obtained by using both the exact boundary value solutions and approximate, semi-empirical methods based on physical principles. The plasma is assumed to have the macroscopic properties of a lossless dielectric with a permittivity less than that of free space. A superposition approximation for the radar cross section of a dielectric coated conducting body is obtained by considering the scattered field to be the phasor sum of two principal components, the field scattered by the air-dielectric interface and the field scattered by an equivalent conducting body which differs from the actual body because of the lens action of the shell. This approximation yields very good agreement with the exact solutions for both spherical and cylindrical dielectric clad scatterers with radii in the Rayleigh region and in the resonant region, and for bistatic scattering as well as for backscatter. The echo area of a conducting sphere with nonconcentric spherical dielectric shell calculated by means of the superposition approximation is in excellent agreement with experimental measurements, thus demonstrating the validity of this method in a case for which the exact solution cannot be obtained.     

Scattering by a dielectric cylinder of arbitrary cross section shape

The theory and equations are developed for the scattering pattern of a dielectric cylinder of arbitrary cross section shape. The harmonic incident wave is assumed to have its electric vector parallel with the axis of the cylinder, and the field intensities are assumed to be independent of distance along the axis. Solutions are readily obtained for inhomogeneous cylinders when the permittivity is independent of distance along the cylinder axis. Although other investigators have approximated the field within the dielectric body by the incident field, we treat the total field as an unknown function which is determined by solving a system of linear equations. In the case of the dielectric cylindrical shell of circular cross section, this technique yields results which agree accurately with the exact classical solution. Scattering patterns are also presented in graphical form for a dielectric shell of semicircular cross section, a thin homogeneous plane dielectric sheet of finite width, and an inhomogeneous plane sheet. The effects of surface-wave excitation and mutual interaction among the various portions of the dielectric shell are included automatically in this solutiom     

Radiation characteristics of a dielectric disk-loaded circularcylindrical waveguide

The radiation pattern of a circular cylindrical waveguide that is loaded inside with periodically spaced dielectric disks is derived by considering the region between two disks as a medium with anisotropic permittivity and using Schelkunoff's equivalence principle. The far-field electric field components and their relationship to cross-polarization ratio are determined. The variation of the latter quantity with the size, spacing, and dielectric constant of the dielectric disks is studied     

Analysis of microstrip wraparound antennas using dyadic Green's functions

The radiation pattern of microstrip wraparound antennas was obtained here using a theory based on dyadic Green's functions for concentric-cylindrical layered media. The dielectric layer that is usually neglected as a first-order approximation was considered here. An asymptotic expression for the dyadic Green's function that takes into account only the space wave is first obtained. Radiation patterns for various radii, permittivities, and thicknesses of the dielectric layer of a microstrip wraparound antenna were obtained using as a source a uniform annular magnetic current obtained by means of a cavity model with conducting magnetic walls. The calculated values of the percent pattern coverage decreases as the thickness and the permittivity of the dielectric layer increase. The influence of the dielectric layer is more pronounced for radiation direction near that of the axis of the cylindrical surface. It is also shown that the radiation patterns at a frequency of 2.0 GHz are not much dependent on the diameter of the antenna for values from 3 to 120 in.     

Electromagnetic imaging for complex cylindrical objects

Electronic imaging of complex cylindrical objects with arbitrary cross sections was investigated, assuming an incident wave upon both penetrable inhomogeneous dielectric cylinders and perfectly conducting cylinders with known shape, and external measurements of the scanned field. By properly processing the scattered held measurements, the dielectric permittivity distribution of the scanned object can be reconstructed. A theoretical formulation was based on proper arrangement of the incident field directions resulting in a set of integral equations derived and solved by the moment method and the unrelated illumination method. Numerical results demonstrate the capability of the imaging algorithm. Good reconstruction results were obtained even in the presence of additive random noise. In addition, noise effects on the reconstruction results were investigated.     

An improved calculation procedure for the radiation pattern of acylindrical leaky-wave antenna of finite size

A method is given for calculating the radiation pattern of a finite-size cylindrical leaky-wave antenna structure consisting of a single source near a planar surface that supports leaky-wave propagation. The method accounts for the finite radius of the planar surface by subtracting the radiation from that part of the leaky wave that lies beyond the physical surface, from the exact radiation pattern corresponding to an infinite planar surface. Experimental results obtained for a leaky-wave structure consisting of multiple dielectric layers above a ground plane confirm the improved accuracy of the method, particularly for the part of the radiation pattern away from the peak     

Multiple scattering of EM waves by dielectric spheres located in the near field of a source of radiation

An analysis of multiple scattering of electromagnetic (EM) waves by two loss-free dielectric spheres with radii greater than a wavelength and located in the bear field of a source of radiation is presented. The incident field is expressed in terms of spherical vector wave functions (SVWF). Translational and rotational addition theorems are employed to express the SVWF of the incident field in the coordinate system associated with the dielectric scatterer. Numerical computations are performed for obtaining the amplitude and phase patterns of fields multiply scattered by two loss-free dielectric spheres, whose centers are located on the boresight axis and in the nearfield of an open-ended circular cylindrical waveguide excited in its dominant mode. Numerically computed results show good agreement with measured results obtained from a systematic experimental study on forward scatter performed in theX-band.     

Numerical scattering analysis for two-dimensional dense randommedia: characterization of effective permittivity

A new numerical method for determining effective permittivity of dense random media in two dimensions is presented. The core of the method is to compare the average scattered field of a random collection of scatterers confined within an imaginary boundary with the scattered field from a homogeneous dielectric of the same shape as the imaginary boundary. The two-dimensional (2-D) problem is aggressively studied to provide insight into the dependence of the method's convergence on particle size, boundary shape, and boundary dimension. A novel inverse scattering method is introduced based on the method of moments (MoM), which greatly reduces the computation time and increases the flexibility of the procedure to analyze a variety of geometries. Results from this 2-D method may be used directly to compare with theoretical methods for determining effective permittivity such as the Polder-Van Santen (1946) mixing formula or field techniques such as the quasi-crystalline approximation     

Scattering from an anisotropic cylindrical dielectric shell

The electromagnetic scattering from an anisotropic cylindrical dielectric shell is formulated by using the wave functions for anisotropic media and the boundary-value method. The cylindrical shell is assumed to be infinite in length, and it is illuminated by a plane wave or a cylindrical wave from a line source. The problem is two-dimensional and the solutions to both types of polarization (TE and TM) are presented. Numerical results for the effects of various geometrical and electrical parameters on the bistatic radar cross section are presented.     

Metamaterial-based efficient electrically small antennas

A metamaterial paradigm for achieving an efficient, electrically small antenna is introduced. Spherical shells of homogenous, isotropic negative permittivity (ENG) material are designed to create electrically small resonant systems for several antennas: an infinitesimal electric dipole, a very short center-fed cylindrical electric dipole, and a very short coaxially-fed electric monopole over an infinite ground plane. Analytical and numerical models demonstrate that a properly designed ENG shell provides a distributed inductive element resonantly matched to these highly capacitive electrically small antennas, i.e., an ENG shell can be designed to produce an electrically small system with a zero input reactance and an input resistance that is matched to a specified source resistance leading to overall efficiencies approaching unity. Losses and dispersion characteristics of the ENG materials are also included in the analytical models. Finite element numerical models of the various antenna-ENG shell systems are developed and used to predict their input impedances. These electrically small antenna-ENG shell systems with idealized dispersionless ENG material properties are shown to be very efficient and to have fractional bandwidths above the values associated with the Chu limit for the quality factor without any degradation in the radiation patterns of the antennas. Introducing dispersion and losses into the analytical models, the resulting bandwidths are shown to be reduced significantly, but remain slightly above (below) the corresponding Chu-based value for an energy-based limiting (Drude) dispersion model of the permittivity of the ENG shell.     

An E-plane ferrite loaded simple waveguide radiator

Experimental investigations were conducted to examine the effect of dielectric loading on a ferrite loaded waveguide radiator (notch antenna). The antenna was made by symmetrically tapering an X-band waveguide. A cylindrical ferrite post was disposed at the apex of the notch and it was biased with a dc magnetic field. The antenna was tested for several notch angles namely 140/spl deg/, 120/spl deg/, 90/spl deg/, 80/spl deg/, 60/spl deg/ while ferrite was loaded with several dielectric sleeves, such as wood, polymer, pyrex. The dielectric loading with high permittivity material is found to give better radiation and scanning characteristics. The radial thickness of the dielectric ring is observed to affect the radiation.     

Broadband cylindrical dielectric resonator antenna excited by modified microstrip line

The impedance bandwidth of a high permittivity cylindrical dielectric resonator antenna excited by a microstrip line was significantly improved by modifying the feed geometry. The 10 dB return loss bandwidth is enhanced from 12 to 26% without much affecting the gain and other radiation properties of the antenna. Good agreement has been observed between the predicted and measured results.     

Procedures for noninvasive electromagnetic property and dosimetrymeasurements

Five different formulations are presented that can be used to determine local values of the complex permittivity, electric field, polarization current density, and rate of energy deposition, within an inhomogeneous dielectric object of arbitrary shape. These formulations were obtained from the matrix equations for moment-method solutions of the electric field integral equation. Implementation of these formulations requires knowledge of the shape of the object and the incident electric field, and measurements of the scattered electric field at a number of points external to the object. The use of a new type of model, with antenna arrays for measurements, has caused a substantial improvement in matrix conditioning. Results of numerical simulations using 21-cell models with high dielectric contrast (bone/muscle) suggest that all five formulations have sufficient accuracy to warrant experimental testing using cylindrical scatterers with transverse magnetic polarization     

Resonance transformation of the field of a linear source into multipole radiation with the help of a cylindrical metamaterial layer

A hollow circular metamaterial cylinder with a negative permittivity and a negative permeability is considered. The 2D problem of excitation of the cylinder by a filament source is numerically investigated. It is found that, when the relative permittivity and permeability are close to minus unity, high-Q resonances exist in hollow cylinders of an electrically small diameter. It is shown that, when the cylindrical wave source is situated inside the cavity, omnidirectional radiation is transformed at the resonance frequency into multipole radiation with a large number of identical lobes in the pattern. Near- and far-field patterns are calculated. The influence of loss on the resonance characteristics is investigated.     

Experimental investigation for wideband perforated dielectric resonator antenna

A wideband perforated dielectric resonator antenna (PDRA) is presented. The effective permittivity of the dielectric resonator is altered by drilling holes into a circular ring lattice inside the DRA. The PDRA is equivalent of having an annular ring with lower permittivity outside the cylindrical disk, resulting in enhanced impedance bandwidth. The measured bandwidth of a prototype PDRA with relative permittivity 10.2 is 26.7% (S/sub 11/<-10 dB).     

A numerical solution to full-vector electromagnetic scattering bythree-dimensional nonlinear bounded dielectrics

This paper deals with electromagnetic scattering by nonlinear dielectric objects. In particular, a numerical approach is developed that is aimed at determining the distributions of the electromagnetic field vector inside a three-dimensional nonlinear, inhomogeneous, isotropic scatterer illuminated by a time-periodic incident electric field vector. An integral-equation formulation for the full-vector scattering problem is considered, and the nonlinear effect is taken into account by introducing equivalent sources and a Fourier-series representation. A system of integral equations (for each harmonic vector component and for the static term) is obtained that includes the internal electric field distribution as the unknown. After discretization, the solution is reduced to solving an algebraic system of nonlinear equations. Some preliminary numerical results are reported concerning scatterers that exhibit a specific (quadratic) dependence of the dielectric permittivity on the total electric field. The harmonic components of the scattered electric field outside the objects are also computed     

An alternative solution of the scattering from an anisotropic cylindrical dielectric shell

A method based on the approximate wave functions for anisotropic media and the mode-matching approach is developed to solve the problem of the electromagnetic scattering from an anisotropic cylindrical dielectric shell. The cylindrical shell is assumed to be infinite in length, and it is illuminated by a plane wave or a cylindrical wave from a line source. The problem is two-dimensional and the solutions to both types of polarization (TE and TM) are presented. The validity of this solution is verified by comparing the numerical results with those in literatures and the previous calculations based on the exact wave functions for anisotropic media. Numerical results show the higher computational efficiency of the present method for bounded anisotropic media.     

Corrugated spherical antenna

The radiation characteristics of a corrugated metallic sphere with an azimuthal slot are studied. For the purpose of analysis, a dielectric coating equivalent to the corrugation on the surface is assumed. Expressions for the equivalent relative permittivity, input admittance, gain and radiated far field, based on the boundary-value approach, are presented. For a spherical antenna of k0a = 4.9087 with corrugation corresponding to an equivalent dielectric coating of relative permittivity 8.36, it is shown that the increase in gain over the uncorrugated antenna is 6.15 dB. The above antenna was fabricated and the experimental pattern obtained is compared with the theoretical result.