Imaging of objects buried in a finite-sized dielectric background

The authors propose a simple algorithm to reconstruct the image of objects embedded in a finite-sized dielectric background. The algorithm is based on the concept that an undistorted image may be reconstructed if the true range profiles of the embedded object can be obtained. Neglecting the phenomena of multiple reflections and the bending effect of wave propagating in the dielectric, they propose a simple algorithm to compensate the electrical length of the wave traveling in the dielectric. Although the assumptions made are not precise, nevertheless, experimental results show that the proposed method is very time-efficient, and is effective in reconstructing a recognizable image of the object embedded in a background having moderate dielectric constant and smooth air-dielectric boundary shape     

On waves in an anisotropic plasma imbedded in a moving dielectric medium

The governing equations for electromagnetic fields in an anisotropic plasma imbedded in a moving dielectric medium are obtained. Examination of the wave and dispersion equations yields information on the elliptically or linearly polarized waves which may exist when the external magnetic field is parallel or perpendicular to the direction of motion, respectively.     

Electromagnetic resonances of immersed dielectric spheres

The complex natural resonances (CNR) for lossless dielectric spheres in a lossless dielectric medium are investigated. Significant differences between the external and internal resonances are presented. The external resonances are related to the external creeping waves and the internal resonances to the internally reflected waves. The internal resonances are more important in practice because of their smaller damping factors. A simple physical interpretation for predicting the resonance behavior of a general dielectric sphere is obtained     

A surface wave interpretation for the resonances of a dielectric sphere

The calculated radar and bistatic cross sections of dielectric spheres exhibit numerous resonances when plotted versus frequency. These resonances may be related to the excitation of electromagnetic eigenvibrations of the sphere, with resonance frequencies calculable from a characteristic equation. It is shown that the resonances may be viewed as originating from families of circumferential (surface, or creeping) waves that are generated during the scattering process; at each eigenfrequency of the sphere, one of these surface waves matches phases after its repeated circumnavigations around the sphere, with the ensuing resonant reinforcement leading to the given scattering resonance. This mechanism explains the existence of electromagnetic eigenvibrations of a general smooth dielectric object; for the case of a sphere, it is shown that the surface waves suffer a phase jump ofpi/2at each of their two convergence points. We calculated numerical values of the eigenfrequencies of dielectric spheres, and obtain dispersion curves for the phase velocities of the surface waves.     

Effect of dielectric jacket on a buried travelling-wave antenna

Transmission line theory has long been used to analyze the performance of dielectric-coated buried antennas. However, at UHF the ground cannot be assumed to be highly conducting and a new approach is necessary. A method is presented to predict the input impedance and pattern of a dielectric-coated traveling-wave antenna buried in lossless ground.     

Near-field detection of buried dielectric objects

The plane-wave scattering-matrix method is used to compute the response of a detector to a buried dielectric scatterer. The Born approximation is used to derive the scattering matrix for scatterers of small dielectric contrast, but the general theory is not limited to such cases. Specific numerical results are generated for a UHF dipole detector swept over a buried dielectric cube. The maximum response is obtained when the detector is located at the soil interface, and the response decays rapidly with the detector height. The sweep curves are symmetrical in the horizontal direction and have a null when the detector is directly over the object. An experimental curve for a free-space environment has the same qualitative features     

Electromagnetic detection of buried dielectric targets

Scattering from subsurface objects is studied using the T-matrix method. The scatterer is represented by a dielectric ellipsoid, and the source is assumed to be an electric dipole in a borehole. This approach accommodates rotations of the scatterer relative to the source and also permits relaxation of the two-dimensional constraint inherent in some models. In addition, it provides a framework within which a large number of subsurface scattering problems can be studied. To illustrate how this approach can be applied, the fields scattered from highly eccentric ellipsoids are found. In addition, polarization results demonstrate that a method based on the polarization of the scattered field may provide a more robust diagnostic of scatterer location than methods using only a single field component. An experiment based on a scale model of a subsurface tunnel is described. Results from the analytic model compare favorably with those obtained from the experiment     

Effective imaging of buried dielectric objects

Ultrawide-band CW radar have shown good potential for remote imaging of surface laid or shallow buried landmine-like objects. However when sensing fields near to the ground, a number of factors including direct antenna coupling, air/ground coupling, receiver noise floor, and effective dynamic range conspire to degrade or mask the scattered return, resulting in a loss of signal magnitude, range accuracy, and range resolution. The present paper addresses the latter problems using a near-field detection methodology and providing a thorough experimental and systems analysis of the signal-to-clutter issues. The method proves advantageous in sensing weak echoes in the vicinity of the sensor from objects of low dielectric contrast with their environment. To enhance image processing gains, an effective space frequency synthetic aperture technique applicable to the two-media problem is outlined. The algorithm is straightforward and robust enough to be implemented on compact GPR systems operating in real time. The remaining problem of object identification will not be addressed here. To demonstrate the utility of these combined techniques, field experiments, over different frequency bands, were conducted and their results are reported     

Electromagnetic field of a horizontal electric dipole buried in a medium covering one-dimensionally anisotropic medium

The electromagnetic fields of a horizontal electric dipole buried in a medium covering one-dimensionally anisotropic medium are studied. There are three media, one-dimensionally anisotropic medium covered with a dielectric under the air. The electromagnetic field components are complex because of the multiple reflections from the up and down boundaries. The electromagnetic field components between air and one-dimensionally anisotropic medium are given, the trapped surface waves and lateral waves along the dielectric-anisotropic medium boundary are computed. The results have some practical applications in the communication in sea or lake above one-dimensionally anisotropic earth or sediments.     

Complex natural resonances of conducting planar objects buried in adielectric half-space

A scheme is presented to incorporate a mixed potential integral equation (MPIE) using Michalski's “formulation C” with the method of moments (MoM) for analyzing the scattering of a plane wave from conducting planar objects buried in a dielectric half-space. The robust complex image method with a two-level approximation is used for the calculation of the Green's functions for the half-space. To further speed up the computation, an interpolation technique for filling the matrix is employed. While the induced current distributions on the object's surface are obtained in the frequency domain, the corresponding time domain responses are calculated via the inverse fast Fourier transform (FFT). The complex natural resonances of targets are then extracted from the late time response using the generalized pencil-of-function (GPOF) method. The authors investigate the pole trajectories as they vary the distance between strips and the depth and orientation of single, buried strips. The variation from the pole position of a single strip in a homogeneous dielectric medium was only a few percent for most of these parameter variations     

Optical resonances in dielectric microspheres for transverse magnetic wave

Optical resonances for Transverse Magnetic (TM) wave based on Whispering Gallery Modes (WGM) have been presented with comprehensive mathematical formulations. The present study is characterized both theoretically and experimentally by considering the fact that the size parameter of the dielectric microspheres is very large at optical wavelengths. An asymptotic expression has been developed based on electromagnetic theory for large size parameter. The developed expressions for optical resonance condition of TM wave are very simple and can accurately characterize resonances in dielectric microspheres. The theoretical development is mathematically robust and significantly less complicated than existing approaches based on quantum physics presented in the literatures. The theoretical result of size parameter for consecutive morphology dependent resonance (MDR) peaks is validated by experimental data. The comparisons are shown to be very accurate for large size parameters.     

Electromagnetic scattering from a dielectric cylinder buried beneath a slightly rough surface

An analytical solution is presented for the electromagnetic scattering from a dielectric circular cylinder embedded in a dielectric half-space with a slightly rough interface. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the rough interface and the. buried cylinder. First-order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. The derivation includes both TM and TE polarizations and can be easily extended for other cylindrical buried objects (e.g., cylindrical shell, metallic cylinder). Several scattering scenarios are examined utilizing the new solution for a dielectric cylinder beneath a flat, sinusoidal, and arbitrary rough surface profile. Results indicate that the scattering pattern of a buried object below a slightly rough surface differs from the flat surface case only when the surface roughness spectrum contains a limited range of spatial frequencies. Furthermore, the illuminated area of the incident wave is seen to be a critical factor in the visibility of a buried object below a rough surface.     

Effective dielectric constant of a medium with spherical inclusions

The Maxwell-Garnett theory is frequently used to predict the effective, or the average, dielectric constant of a mixture composed of spherical inclusions embedded in a host medium. This effective medium theory assumes that the volume fraction occupied by the spherical inclusions is small and that the size of the inclusions is small compared to the wavelength. Experiments using controlled samples have shown that the Maxwell-Garnett theory is applicable up to an inclusion volume fracton of 0.2. At higher volume fractions, the effective dielectric constant appears to be dependent on the inclusion sizes     

Electromagnetic scatter from small dielectric gradients in a continuous medium

A new approach is presented to the formulation and understanding of the problem of electromagnetic scatter from a slightly perturbed continuous medium. The problem is expressed in terms of periodic functions instead of conventional power series. From this formulation, specular properties are observed that are not obtainable from older considerations. The approach was verified with a laboratory experiment using microwave transmission lines with length-dependent, slowly varying dielectric cross sections. Further experiments were carried out using backscatter from a radar designed to detect clear air turbulence. The primary result shows that a turbulent medium can be replaced for the purposes of scatter calculation by a structure of finite periodicities because the medium acts as a lumped insertion loss, the stopping characteristics being determined by a spectral scale in the turbulence. The formulation defines the limits of knowledge about the medium and shows that, although it is not unique, some knowledge of the structure is obtainable. Insight is gained into the work needed to understand and interpret remote probe experiments.     

Radiation characteristics of a source in a thin substrate mountedover a dielectric medium

The radiation pattern of a line source is calculated for the case in which the source is lying on the top or the bottom surface of a lossless dielectric substrate that is mounted on the top of semi-infinite dielectric medium. It is found that in both cases the pattern along the interfaces has a null; that the pattern in the upper semi-infinite medium has a single lobe; and that the pattern in the lower semi-infinite medium has many lobes, the number of which varies with the substrate thickness. In both cases, the power radiated into the lower medium is more than that radiated into the upper medium. Applications of this calculation to remote sensing, microstrip antenna technology, and antenna arrays are discussed     

Capacitance computations in a multilayered dielectric medium usingclosed-form spatial Green's functions

An efficient method to compute the 2-D and 3-D capacitance matrices of multiconductor interconnects in a multilayered dielectric medium is presented. The method is based on an integral equation approach and assumes the quasi-static condition. It is applicable to conductors of arbitrary polygonal shape embedded in a multilayered dielectric medium with possible ground planes on the top or bottom of the dielectric layers. The computation time required to evaluate the space-domain Green's function for the multilayered medium, which involves an infinite summation, has been greatly reduced by obtaining a closed-form expression, which is derived by approximating the Green's function using a finite number of images in the spectral domain. Then the corresponding space-domain Green's functions are obtained using the proper closed-form integrations. In both 2-D and 3-D cases, the unknown surface charge density is represented by pulse basis functions, and the delta testing function (point matching) is used to solve the integral equation. The elements of the resulting matrix are computed using the closed-form formulation, avoiding any numerical integration. The presented method is compared with other published results and showed good agreement. Finally, the equivalent microstrip crossover capacitance is computed to illustrate the use of a combination of 2-D and 3-D Green's functions     

Subnanosecond pulse propagation in a Goubau line immersed in a lossy dielectric medium

The distortion and attenuation of a subnanosecond pulse propagating along a Goubau line immersed in a lossy dielectric medium is considered in the letter. The pulse response characteristics of this line have been analysed by using a fast Fourier transform algorithm. The results of this investigation have important implications in the design of high-resolution guided-wave radar systems.     

A technique for solution of Maxwell's equations in a moving dielectric medium

A simple technique is presented for converting a known solution for the electric and magnetic vector fields in a dielectric medium at rest into the corresponding fields in a moving dielectric medium. The technique combines methods presented by Tai [1] with a scaling procedure developed by Clemmow [2]. Tai's work reduces the moving medium problem to the solution of Maxwell's equations in a uniaxial medium, and Clemmow's procedure enables one to convert a known solution in an isotropic medium to the corresponding solution in a uniaxial medium. Thus by first solving for the fields in the medium at rest, then following Clemmow's procedure to obtain the fields in Tai's uniaxial medium, and finally applying Tai's reasoning, one may easily obtain the solution of Maxwell's equations in the moving medium.     

Calculation of resonances of microwave resonator loaded with inhomogeneous dielectric layer

A procedure for calculating resonance frequencies of a rectangular cavity loaded with an inhomogeneous layer is given. Calculations are based on a previous modification of the theoretical approach which, to the authors' knowledge, has not been verified numerically or experimentally. The numerical results presented illustrate both the effect of the dielectric inhomogeneity on the resonance frequencies, and the errors that are made if the correction to the original theory is not made.<>