Showing papers in "IEEE Transactions on Antennas and Propagation in 1995"

A perfectly matched anisotropic absorber for use as an absorbing boundary condition

Abstract: An alternative formulation of the "perfectly matched layer" mesh truncation scheme is introduced. The present scheme is based on using a layer of diagonally anisotropic material to absorb outgoing waves from the computation domain. The material properties can be chosen such that the interface between the absorbing material and free space is reflection-less for all frequencies, polarizations, and angles of incidence. This approach does not involve a modification of Maxwell's equations and is easy to implement in codes that allow the use of anisotropic material properties.

A GTD-based parametric model for radar scattering

Abstract: This paper presents a new approach to scattering center extraction based on a scattering model derived from the geometrical theory of diffraction (GTD). For stepped frequency measurements at high frequencies, the model is better matched to the physical scattering process than the damped exponential model and conventional Fourier analysis. In addition to determining downrange distance, energy, and polarization, the GTD-based model extracts frequency dependent scattering information, allowing partial identification of scattering center geometry. We derive expressions for the Cramer-Rao bound of this model; using these expressions, we analyze the behavior of the new model as a function of scatterer separation, bandwidth, number of data points, and noise level. Additionally, a maximum likelihood algorithm is developed for estimation of the model parameters. We present estimation results using data measured on a compact range to validate the proposed modeling procedure. >

Improved PO-MM hybrid formulation for scattering from three-dimensional perfectly conducting bodies of arbitrary shape

Abstract: The method of moments (MM) represents a suitable procedure for dealing with electromagnetic scattering problems of arbitrary geometrical shape in the lower frequency range. However, with increasing frequency both computation time and memory requirement often exceed available computer capacities. Therefore a current based hybrid method combining the MM with the physical optics (PO) approximation suitable for three-dimensional perfectly conducting bodies is proposed in this paper. The hybrid formulation allows a substantial reduction of computation time and memory requirement, while the results are in reasonable agreement with those based on an application of the MM alone. Further improvement can be achieved for flat polygonal parts of the scattering body by a heuristic modification of the PO current density taking into account the effects of edges. As opposed to the physical theory of diffraction (PTD), no additional electric and magnetic line currents along the edges are necessary. >

On solving first-kind integral equations using wavelets on a bounded interval

Abstract: The conventional method of moments (MoM), when applied directly to integral equations, leads to a dense matrix which often becomes computationally intractable. To overcome the difficulties, wavelet-bases have been used previously which lead to a sparse matrix. The authors refer to "MoM with wavelet bases" as "wavelet MoM". There have been three different ways of applying the wavelet techniques to boundary integral equations: 1) wavelets on the entire real line which requires the boundary conditions to be enforced explicitly, 2) wavelet bases for the bounded interval obtained by periodizing the wavelets on the real line, and 3) "wavelet-like" basis functions. Furthermore, only orthonormal (ON) bases have been considered. The present authors propose the use of compactly supported semi-orthogonal (SO) spline wavelets specially constructed for the bounded interval in solving first-kind integral equations. They apply this technique to analyze a problem involving 2D EM scattering from metallic cylinders. It is shown that the number of unknowns in the case of wavelet MoM increases by m-1 as compared to conventional MoM, where m is the order of the spline function. Results for linear (m=2) and cubic (m=4) splines are presented along with their comparisons to conventional MoM results. It is observed that the use of cubic spline wavelets almost "diagonalizes" the matrix while maintaining less than 1.5% of relative normed error. The authors also present the explicit closed-form polynomial representation of the scaling functions and wavelets. >

Design and performance of a microstrip reflectarray antenna

Abstract: Microstrip reflectarray antennas present an alternative to conventional directive antennas in that they are flat, inexpensive, easy to install and manufacture, conformal to the mounting surface, easy to package, and they possess high power and beam steering capabilities. For the first time, a comprehensive, experimentally verified design procedure for the microstrip reflectarray is presented. In this design procedure, the microstrip reflectarray has been shown to be an effective linear array capable of beam switching and dual polarization. The design procedure also allows the array to be fed from a feed-horn offset at any given angle to the plane of the array. The design procedure is well suited for CAD programs. >

A study of wavelets for the solution of electromagnetic integral equations

Abstract: The use of wavelet basis functions for the efficient solution of electromagnetic integral equations is studied. It has previously been demonstrated that the use of wavelets for expansion and testing functions produces a sparse moment-method matrix. Here, this effect is examined and analyzed in terms of the radiation/receiving characteristics of the wavelet basis functions. The limitations of wavelets as an efficient solution technique are discussed, and a comparison is made to other fast algorithms. >

An iterative physical optics approach for analyzing the electromagnetic scattering by large open-ended cavities

Abstract: A formulation based on the high frequency asymptotic principles of physical optics is developed for analyzing the scattering by relatively arbitrary open-ended waveguide cavities containing complex interior terminations. A magnetic field integral equation (MFIE) is obtained for the equivalent currents on the interior cavity walls and is solved using an iterative physical optics (IPO) algorithm which iteratively applies physical optics to account for multiple reflections inside the cavity. The number of iterations required for convergence is related to the expected number of important reflections. The IPO method is more approximate than a matrix solution of the MFIE, but it is quite accurate for electrically large cavities and is much more efficient. Numerical results are presented which demonstrate the convergence and accuracy of the method by comparison with modal reference solutions. >

A generalized diffraction synthesis technique for high performance reflector antennas

Abstract: Stringent requirements on reflector antenna performances in modern applications such as direct broadcast satellite (DBS) communications, radar systems, and radio astronomy have demanded the development of sophisticated synthesis techniques. Presented in the paper is a generalized diffraction synthesis technique for single- and dual-reflector antennas fed by either a single feed or an array feed. High versatility and accuracy are achieved by combining optimization procedures and diffraction analysis such as physical optics (PO) and physical theory of diffraction (PTD). With this technique, one may simultaneously shape the reflector surfaces and adjust the positions, orientations, and excitations of an arbitrarily configured array feed to produce the specified radiation characteristics such as high directivity, contoured patterns, and low sidelobe levels, etc. The shaped reflectors are represented by a set of orthogonal global expansion functions (the Jacobi-Fourier expansion), and are characterized by smooth surfaces, well-defined (superquadric) circumferences, and continuous surface derivatives. The sample applications of contoured beam antenna designs and reflector surface distortion compensation are given to illustrate the effectiveness of this diffraction synthesis technique. >

Multiple-polarization microstrip reflectarray antenna with high efficiency and low cross-polarization

Abstract: This paper presents the study and prototype demonstration of the concepts of antenna focusing, cross-polarization reduction, and multiple-polarization capability of a planar microstrip reflectarray antenna. A square patch with two equal microstrip delay lines connected to its two orthogonal feeding points is used as the antenna element of the planar reflectarray. The length of the delay lines, which varies from patch to patch, is schematically designed to focus the plane wave to the feed point. The measured overall efficiency of the prototype antenna is above 50% in the normal operating band, and in some frequency ranges the efficiency has reached 70%. The experimental patterns show that the measured cross polarization due to special arrangement of the delay lines is quite low at the direction of the main beam. X-polarization and Y-polarization measured data show that the antenna is suitable for multiple-polarization applications (dual linear and dual circular). Surprisingly, the antenna has achieved approximately greater than 7% of gain bandwidth (-3 dB gain drop). These results demonstrate the feasibility of such an antenna for radar and communication system applications. >

Conjugate gradient method applied to inverse scattering problem

Abstract: A new reconstruction algorithm for diffraction tomography is presented. The algorithm is based on the minimization of a functional which is defined as the norm of the discrepancy between the measured scattering amplitude and the calculated one for an estimated object function. By using the conjugate gradient method to minimize the functional, one can derive an iterative formula for getting the object function. Numerical results for some two-dimensional scatterers show that the algorithm is very effective in reconstructing refractive index distributions to which the first-order Born approximation can not be applied. In addition, the number of iterations is reduced by using a priori information about the outer boundary of the objects. Furthermore, the method is not so sensitive to the presence of noise in the scattered field data. >

On the calculation of potential integrals for linear source distributions on triangular domains

Abstract: Present analytical formulas for the singular field contributions of linear source distributions on triangular domains. The formulas can be used for near-field calculations of electric and magnetic source currents, especially for the calculation of method of moments (MOM) matrix elements due to the solution of surface integral equations. For MOM self-coupling terms involving the scalar Green function of free space, the authors present expressions comprising analytical solutions of both the source integrals and the testing integrals.

Time-domain uniform geometrical theory of diffraction for a curved wedge

Abstract: A time-domain version of the uniform geometrical theory of diffraction (TD-UTD) is developed to describe, in closed form, the transient electromagnetic scattering from a perfectly conducting, arbitrarily curved wedge excited by a general time impulsive astigmatic wavefront. This TD-UTD impulse response is obtained by a Fourier inversion of the corresponding frequency domain UTD solution. An analytic signal representation of the transient fields is used because it provides a very simple procedure to avoid the difficulties that result when inverting frequency domain UTD fields associated with rays that traverse line or smooth caustics. The TD-UTD response to a more general transient wave excitation of the wedge may be found via convolution. A very useful representation for modeling a general pulsed astigmatic wave excitation is also developed which, in particular, allows its convolution with the TD-UTD impulse response to be done in closed form. Some numerical examples illustrating the utility of these developments are presented.

Efficient computation of generalized scattering matrix for analyzing multilayered periodic structures

Abstract: An efficient technique is proposed for the computation of the generalized scattering matrix (GSM) of a dielectric interface with periodic metallizations. The technique is based on a spectral domain moment method, but assuming multiple incident Floquet-harmonics and computing the GSM directly. Also, some symmetry properties are exploited, and the whole GSM is computed with similar computer effort as that required for a single scattering coefficient. The technique has been applied to the analysis of periodic surfaces involving rectangular and arbitrarily-shaped metallizations using entire- and sub-domain basis functions, respectively. Losses in both dielectric layers and metallizations have been included in the formulation. Multilayered periodic structures are analyzed in a very flexible and efficient way by cascading iteratively the GSM of each interface with or without metallizations considered as building blocks. Numerical results have been provided for different multilayered structures, and a good agreement with other experimental and theoretical data has been obtained. The proposed technique is very appropriate for the analysis of composite structures when the separation between interfaces is small, and therefore higher-order Floquet-harmonic interaction cannot be neglected. >

A Monte-Carlo FDTD technique for rough surface scattering

Abstract: A Monte-Carlo finite-difference time-domain (FDTD) technique is developed for wave scattering from randomly rough, one-dimensional surfaces satisfying the Dirichlet boundary condition. Both single-scale Gaussian and multiscale Pierson-Moskowitz surface roughness spectra are considered. Bistatic radar cross sections are calculated as a function of scattering angle for incident angles of 0, 45, 70, and 80 degrees measured from the vertical. The contour path FDTD method is shown to improve accuracy for incident angles greater than 45 degrees. Results compare well with those obtained using a Monte-Carlo integral equation technique. >

Propagation in linear dispersive media: finite difference time-domain methodologies

Abstract: Finite difference time-domain (FDTD) methodologies are presented for electromagnetic wave propagation in two different kinds of linear dispersive media: an Nth order Lorentz and an Mth order Debye medium. The temporal discretization is accomplished by invoking the central difference approximation for the temporal derivatives that appear in the first-order differential equations. From this, the final equations are temporally advanced using the classical leapfrog method. One-dimensional scattering from a dielectric slab is chosen for a test case. Provided that the maximum operating frequency times the time step is small and that the wave is adequately resolved in space, as shown in the error analysis, the agreement between the computed and exact solutions will be excellent. The attached data, which are associated with the four pole Lorentz dielectric and the five pole Debye medium, verify this assertion. >

Monte-Carlo simulations of large-scale problems of random rough surface scattering and applications to grazing incidence with the BMIA/canonical grid method

Abstract: Scattering of a TE incident wave from a perfectly conducting one-dimensional random rough surface is studied with the banded matrix iterative approach/canonical grid (BMIA/CAG) method. The BMIA/CAG is an improvement over the previous BMIA. The key idea of BMIA/CAG is that outside the near-field interaction, the rest of the interactions can be translated to a canonical grid by Taylor series expansion. The use of a flat surface as a canonical grid for a rough surface facilitates the use of the fast Fourier transform for nonnear field interaction. The method can be used for Monte-Carlo simulations of random rough surface problems with a large surface length including all the coherent wave interactions within the entire surface. We illustrate results up to a surface length of 2500 wavelengths with 25000 surface unknowns. The method is also applied to study scattering from random rough surfaces at near-grazing incidence. The numerical examples illustrate the importance of using a large surface length for some backscattering problems. >

Theory and experiment of an aperture-coupled hemispherical dielectric resonator antenna

Abstract: A hemispherical dielectric resonator (DR) antenna using aperture coupling for excitation is studied both theoretically and experimentally. The reciprocity method is used to formulate the problem. The exact magnetic field Green's function due to the equivalent magnetic current in the slot is derived rigorously, and it is presented in a form which can be evaluated very efficiently. Moment method is used to solve the magnetic current from which the input impedance of the antenna configuration is obtained. The equivalent radius of the slot is used so that simple formulae developed for the cylindrical dipole can be applied directly. The effects of the slot's length, the slot's position, and the slot's width on the broadside TE/sub 111/ mode input impedance are studied, and reasonable agreement between theory and experiment is obtained. >

Photonic wideband array antennas

Abstract: Presents an introduction to the optical control of array antennas by using fiber optic links for remote control and a photonic time shift network for wide instantaneous bandwidth. An overview of the development of a wideband conformal array designed for airborne surveillance radars is given. The paper covers the system design and the performance of an L-band (850-1400 MHz) M-element array controlled by photonics. Packaging techniques of the photonic components and the array aperture are discussed. The wideband performance of the system is highlighted. A nano-second impulse response has been measured to demonstrate a 50% instantaneous bandwidth (550 MHz, 30 cm range resolution) for target ID and imaging. A built-in signal injection technique based on time-domain impulse measurement was used to calibrate the wideband components in the time-shift beamforming network. >

Stacked annular ring dielectric resonator antenna excited by axi-symmetric coaxial probe

Abstract: A stacked annular ring dielectric resonator (DR) antenna composed of commercially available dielectric resonators and exited by axi-symmetric coaxial probe is studied computationally using the finite-difference time-domain (FDTD) method. The numerical results are checked against experimental measurements, and good agreement is observed. It is demonstrated that the introduction of air-gaps between the two DR's and also between the driven DR and the ground plane can improve the impedance bandwidth of the antenna significantly. The effects of the air-gaps thickness on the return loss and the bandwidth are discussed. The far-fields are also calculated by applying the electromagnetic field equivalence principle. The antenna produces an end-fire radiation field similar to that of an electric monopole. >

Direction finding in phased arrays with a neural network beamformer

Abstract: Adaptive neural network processing of phased-array antenna received signals promises to decrease antenna manufacturing and maintenance costs while increasing mission uptime and performance between repair actions. We introduce one such neural network which performs aspects of digital beamforming with imperfectly manufactured, degraded, or failed antenna components. This paper presents measured results achieved with an adaptive radial basis function (ARBF) artificial neural network architecture which learned the single source direction finding (DF) function of an eight-element X-band array having multiple, unknown failures and degradations. We compare the single source DF performance of this ARBF neural network, whose internal weights are computed using a modified gradient descent algorithm, with another radial basis function network, Linnet, whose weights are calculated using linear algebra. Both networks are compared to a traditional DF approach using monopulse.

A Ka-band circularly polarized high-gain microstrip array antenna

Abstract: In this article, the development of a circularly polarized microstrip array with 28 dBic of gain at 32 GHz is presented. Two primary objectives of this development are minimizing the microstrip array's insertion loss and maintaining a reasonable frequency bandwidth (3%). The parallel/series feed technique for the array's power distribution circuit and the sequential rotation method for the element arrangement are employed to meet these objectives. >

Single and double folded-slot antennas on semi-infinite substrates

Abstract: Design guidelines are presented for single and double folded-slot (DFS) antennas on a dielectric half-space. The DFS antenna consists of two folded-slot elements which are separated by a half-wavelength and are fed from one side using a coplanar waveguide transmission line. A space-domain integral equation technique, based on the free-space Green's function, has been used to compute the resonant circumference and self- and mutual impedances of single folded-slots on various substrates. In addition, the input impedance and radiation patterns of DFS designs are included. Specific examples are shown for air, quartz, and silicon substrates, and measured data for DFS antennas on air and silicon substrates is provided to validate the calculated results.

Terrain-based propagation model for rural area-an integral equation approach

Abstract: A terrain-based propagation model for vertically polarized radio waves is described, based on the field integral equation for a smooth surface. The model is simplified from a 3D integral equation model to a one-dimensional integral equation by assuming that the surface is magnetically perfectly conducting (a soft surface) with no transverse variations. By assuming no back scattering, the integral equation is turned into a simple integral. The method is tested numerically with known solutions. The integral equation model is also applied to actual terrain profiles at four frequencies (144, 435, 970, 1900 MHz), and the results are compared with results from the Hata model and measurements. The standard deviation (STD) of the error is a growing function of frequency, from 3 to 9 dB. >

A 3-D hybrid finite element/boundary element method for the unified radiation and scattering analysis of general infinite periodic arrays

Abstract: We present a rigorous frequency domain variational 3-D electromagnetic formulation for the general nonself-adjoint infinite periodic array problem. The hybrid method described combines the vector finite element and Floquet boundary element techniques. It is general in the sense that it is applicable to infinite periodic arrays of the open or aperture-types. It is thus effective for modeling both the scattering and radiation performance of diverse FSS, absorber, and phased-array structures. The technique accurately handles arbitrarily complicated 3-D geometries, lossy inhomogeneous media and internal as well as external excitations. These analyses can be applied to general skewed grids under arbitrary scan and polarization conditions. >

The two-sheet capacitive Jaumann absorber

Abstract: Many Jaumann absorber designs rely on purely resistive sheets spaced /spl lambda//4 apart, and the designs are based on the requirement that the voltage reflection coefficient and its derivatives vanish at the center frequency. The thickness of the absorber can be reduced and its bandwidth considerably expanded if the sheets are allowed to have capacitive reactances in addition to pure resistance. The bandwidth can be expanded even more by imposing different design criteria. We illustrate those advantages by invoking what we call a double-notch design process for a simple two-sheet absorber. >

Two-dimensional hybrid element image reconstruction for TM illumination

Abstract: A Newton's iterative scheme for electromagnetic imaging is proposed for reconstruction of the dielectric properties of inhomogeneous, lossy bodies with arbitrary shape. The algorithm is based on a finite element (FE) representation which is coupled to a boundary element (BE) formulation for the forward solution of the electric fields; together these are termed the hybrid element method. It utilizes FE discretization of only the area of interest while incorporating the RE method to match the conditions of the homogeneous background region extending to infinity. This paper presents image reconstruction for the 2D TM polarization case where two classes of dielectric distributions are studied which demonstrate the flexibility of this method along with some of the difficulties associated with larger imaging problems. >

An efficient implementation of surface impedance boundary conditions for the finite-difference time-domain method

Abstract: An efficient way to implement the surface impedance boundary conditions (SIBC) for the finite-difference time-domain (FDTD) method is presented in this paper. Surface impedance boundary conditions are first formulated for a lossy dielectric half-space in the frequency domain. The impedance function of a lossy medium is approximated with a series of first-order rational functions. Then, the resulting time-domain convolution integrals are computed using recursive formulas which are obtained by assuming that the fields are piecewise linear in time. Thus, the recursive formulas derived here are second-order accurate. Unlike a previously published method [7] which requires preprocessing to compute the exponential approximation prior to the FDTD simulation, the preprocessing time is eliminated by performing a rational approximation on the normalized frequency-domain impedance. This approximation is independent of material properties, and the results are tabulated for reference. The implementation of the SIBC for a PEC-backed lossy dielectric shell is also introduced. >

Slot excitation of the dielectric disk radiator

Abstract: Dielectric disk radiators which are excited by a narrow slot in the ground plane of a microstrip line are investigated. The resonance frequencies of the dielectric disk for the HEM/sub 11/ mode are computed numerically in the complex frequency plane. From the later results, the actual resonance frequency and the Q-factor are obtained. The dielectric disk is made of a high dielectric constant ceramic material with /spl epsivsub r/=22. The radiation patterns and reflection coefficients are measured and presented for several slot lengths and dielectric disk dimensions. The radiation patterns are also computed assuming a magnetic current element, which models the slot and excites the HEM/sub 11/ mode. Good agreement is obtained between the computed and measured results. The results presented here also demonstrate the viability of this type of antenna, which has high dielectric constants an efficient radiator provided the proper mode is excited. >

Backscattering enhancement and clustering effects of randomly distributed dielectric cylinders overlying a dielectric half space based on Monte-Carlo simulations

Abstract: Backscattering enhancement can exist in volume-surface interactions where a double bounce can arise from one volume scattering and one surface scattering An important quantity to be determined in backscattering enhancement is the angular width The authors study backscattering enhancement of the volume-surface interaction by performing Monte-Carlo simulations of scattering by vertical dielectric cylinders overlying a dielectric half space using the Foldy-Lax multiple-scattering equations The results indicate that the angular width of backscattering enhancement for a scattering layer with a small optical thickness is of the order of the wavelength divided by the layer thickness giving an appreciable angular width of the order of 10-30 degrees that can be important for remote sensing applications The effects of clustering of scatterers leading to collective scattering and absorption effects are also studied >

Improvement of the PO-MoM hybrid method by accounting for effects of perfectly conducting wedges

Abstract: A correction of the conventional physical optics (PO) current close-to-perfectly conducting wedges based on an application of the uniform geometrical theory of diffraction (UTD) is presented. This improved PO current is used in a hybrid formulation in combination with the method of moments (MoM) to deal with three-dimensional scattering bodies of arbitrary shape. The accuracy of this hybrid method is demonstrated by some examples. As opposed to an application of the physical theory of diffraction (PTD), only surface current densities and no fictitious electric and magnetic line currents along the edges are involved which allows a uniform treatment of the MoM and the PO region by expressing the surface current density as a superposition of basis functions defined over triangular patches. >