Showing papers in "IEEE Transactions on Antennas and Propagation in 1991"

Direction finding in the presence of mutual coupling

Abstract: An eigenstructure-based method for direction finding in the presence of sensor mutual coupling, gain, and phase uncertainties is presented. The method provides estimates of the directions-of-arrival (DOA) of all the radiating sources as well as calibration of the gain and phase of each sensor and the mutual coupling in the receiving array. The proposed algorithm is able to calibrate the array parameters without prior knowledge of the array manifold, using only signals of opportunity and avoiding the need for deploying auxiliary sources at known locations. The algorithm is described in detail, and its behavior is illustrated by numerical examples. >

Inverse scattering: an iterative numerical method for electromagnetic imaging

Abstract: The authors propose a spatial iterative algorithm for electromagnetic imaging based on a Newton-Kantorovich procedure for the reconstruction of the complex permittivity of inhomogeneous lossy dielectric objects with arbitrary shape. Starting from integral representation of the electric field and using the moment method, this technique has been developed for 2-D (for TM and TE polarization cases) objects as well as for 3-D objects. Its performance has been compared with spectral techniques of classical diffraction tomography, the modified Newton method, and the pseudo-inverse method. >

A frequency-dependent finite-difference time-domain formulation for transient propagation in plasma

Abstract: Previous FDTD (finite-difference time-domain) formulations were not capable of analyzing plasmas for two reasons. First, FDTD requires that at each time step the permittivity and conductivity be specified as constants that do not depend on frequency, while even for the simplest plasmas these parameters vary with frequency. Second, the permittivity of a plasma can be negative, which can cause terms in FDTD expressions to become singular. A novel FDTD formulation for frequency-dependent materials (FD)/sup 2/TD has been developed. It is shown that (FD)/sup 2/TD can be applied to compute transient propagation in plasma when the plasma can be characterized by a complex frequency-dependent permittivity. While the computational example presented is for a pulse normally incident on an isotropic plasma slab, the (FD)/sup 2/TD formulation is fully three-dimensional. It can accommodate arbitrary transient excitation, with the limitation that the excitation pulse must have no zero frequency energy component. Time-varying electron densities and/or collision frequencies could also be included. The formulation presented is for an isotropic plasma, but extension to anisotropic plasma should be fairly straightforward. >

Near-field multiple source localization by passive sensor array

Abstract: The localization of multiple near-field sources in a spatially white Gaussian noise environment is studied. A modified two-dimensional (2-D) version of the multiple signal classification (MUSIC) algorithm is used to localize the signal sources; range and bearing. A global-optimum maximum likelihood searching approach to localize these sources is discussed. It is shown that in the single source situation, the covariances of both the 2-D MUSIC estimator and the maximum likelihood estimator (MLE) approach the Cramer-Rao lower bound as the number of snapshots increases to infinity. In the multiple source situation, it is observed that for a high signal-to-noise ratio (SNR) and a large number of snapshots, the root mean square errors (RMSEs) of both localization techniques are relatively small. However, for low SNR and/or small number of snapshots, the performance of the MLE is much superior that of the modified 2-D MUSIC. >

Millimeter-wave design of wide-band aperture-coupled stacked microstrip antennas

Abstract: K-band aperture-coupled stacked microstrip antennas are studied and numerical results based on the solution of integral equations solved in the spectral domain are presented. The effects of varying physical parameters of the structure are investigated with the goal of designing millimeter-wave wideband microstrip antennas. Antennas with different characteristics are then analyzed and compared with experimental data. Bandwidths in excess of 20% are obtained and applications to phased array antennas are discussed. >

A finite-difference time-domain near zone to far zone transformation (electromagnetic scattering)

Abstract: An efficient time-domain near-zone-to-far-zone transformation for FDTD (finite-difference-time-domain) computations is presented. The approach is to keep a running accumulation of the far-zone time-domain vector potentials due to the tangential electric and magnetic fields on a closed surface surrounding the scatterer at each time step. At the end of the computation, these vector potentials are converted to time-domain far-zone fields. Many far-zone bistatic directions can be included efficiently during one FDTD computational run. Frequency domain results can be obtained via fast Fourier transform. Wideband results for scattering from a perfectly conducting plate were obtained from a single FDTD computation transformed to the frequency domain, and compared with moment method results. This approach is significantly more efficient than computing many FDTD results using sinusoidally varying excitation if a wide frequency band is of interest. Coupled with recent advances in computing FDTD results for frequency-dependent materials, wideband results for far-zone scattering from targets including frequency-dependent materials can be obtained efficiently. >

Transient scattering by conducting surfaces of arbitrary shape

Abstract: The time-domain electric field integral equation (EFIE) is used along with the method of moments to develop a simple and efficient numerical procedure for treating problems of transient scattering by arbitrary shaped conducting objects. The conducting surface is modeled by planar triangular patches for numerical purposes. Because the EFIE is used in the solution procedure, the method is applicable to both open and closed bodies. the EFIE approach is applied to the scattering problems of Gaussian plane wave illumination of a flat square plate and sphere. Comparisons of surface current densities and far-scattered fields are made with previous computations and good agreement is obtained in each case. >

Analysis of the numerical error caused by the stair-stepped approximation of a conducting boundary in FDTD simulations of electromagnetic phenomena

Abstract: A rigorous analysis of the numerical error associated with the use of stair-stepped (saw-toothed) approximation of a conducting boundary for finite-difference time-domain (FDTD) simulations is presented. First, a dispersion analysis in two dimensions is performed to obtain the numerical reflection coefficient for a plane wave scattered by a perfectly conducting wall, tilted with respect to the axes of the finite-difference grid, under both transverse electric and transverse magnetic polarizations. The characteristic equation for surface waves that can be supported by such saw-tooth conducting surfaces is derived. This equation leads to expressions that show the dependence of the propagation constant along the boundary and the attenuation constant perpendicular to it on cell size and wavelength. Numerical simulations that demonstrate the effects predicted by the dispersion analysis are presented. >

Microstrip Yagi array antenna for mobile satellite vehicle application

Abstract: A novel antenna structure formed by combining the Yagi-Uda array concept and the microstrip radiator technique is discussed. This antenna, called the microstrip Yagi array, has been developed for the mobile satellite (MSAT) system as a low-profile, low-cost, and mechanically steered medium-gain land-vehicle antenna. With the antenna's active patches (driven elements) and parasitic patches (reflector and director elements) located on the same horizontal plane, the main beam of the array can be tilted, by the effect of mutual coupling, in the elevation direction providing optimal coverage for users in the continental United States. Because the parasitic patches are not connected to any of the lossy RF power distributing circuit the antenna is an efficient radiating system. With the complete monopulse beamforming and power distributing circuits etched on a single thin stripline board underneath the microstrip Yagi array, the overall L-band antenna system has achieved a very low profile for vehicle rooftop mounting, as well as a low manufacturing cost. Experimental results demonstrate the performance of this antenna. >

A finite-element-boundary integral formulation for scattering by three-dimensional cavity-backed apertures

Abstract: A novel numerical technique is proposed for the electromagnetic characterization of the scattering by a three-dimensional cavity-backed aperture in an infinite ground plane. The technique combines the finite element and boundary integral methods to formulate a system of equations for the solution of the aperture fields and those inside the cavity. Specifically, the finite element method is used to formulate the fields in the cavity region, and the boundary integral approach is used in conjunction with the equivalence principle to represent the fields above the ground plane. Unlike traditional approaches, the proposed technique does not require a knowledge of the cavity's Green's function and is, therefore, applicable to arbitrary shape depressions and material fillings. Furthermore, the proposed formulation leads to a system having a partly full and partly sparse as well as symmetric and banded matrix which can be solved efficiently using special algorithms. >

Angle and polarization estimation using ESPRIT with a polarization sensitive array

Abstract: It is shown how estimation of signal parameters via rotational invariance techniques (ESPRIT) may be used to estimate both the arrival directions and the polarizations of incoming plane waves with a uniform linear array of crossed dipoles. The ESPRIT algorithm exploits the invariance properties of such an array so that both angle and polarization estimates may be computed. Some typical examples showing the use of this method are presented. >

An L-shaped array for estimating 2-D directions of wave arrival

Abstract: A simple structured 2-D array, the L-shaped array, is presented. The L-shaped array consists of two uniform linear arrays (ULA) connected orthogonally at one end of each ULA. It is shown that the Cramer-Rao bounds (CRB) of the estimated wave directions based on the L-shaped array are about 37% smaller than those for the cross array. The CRB indicates the accuracy potential because it is the (reachable) lower bound on the variance of any unbiased estimate. An efficient maximum likelihood algorithm is developed utilizing the ULA structure inherent in the L-shaped array. >

A hybrid finite element method for scattering and radiation by microstrip path antennas and arrays residing in a cavity

Abstract: A hybrid numerical technique is presented for a characterization of the scattering and radiation properties of microstrip patch antennas and arrays residing in a cavity recessed in a ground plane. The technique combines the finite-element and boundary integral methods to formulate a system for the solution of the fields at the aperture and those inside the cavity via the biconjugate gradient method in conjunction with the fast Fourier transform (FFT). By virtue of the finite-element method, the proposed technique is applicable to patch antennas and arrays residing on or embedded in a layered dielectric substrate and is also capable of treating various feed configurations and impedance loads. Several numerical results are presented, demonstrating the validity, efficiency, and capability of the technique. >

Coplanar waveguide excitation of dielectric resonator antennas

Abstract: The circuit and radiation properties of a cylindrical dielectric resonator antenna excited by a coplanar waveguide feed are investigated. The coupling between the feed and the radiator was measured as a function of the position, dielectric constant, and height of the cylinder. The radiation patterns and resonant frequencies were also measured. These measured results are found to be in good agreement with theoretical calculations. >

On the design of maximally sparse beamforming arrays

Abstract: The problem of designing nonuniformly spaced arrays is formulated as one of constrained optimization in which the cost function is chosen to select the array with the minimum number of elements. The response of the array is controlled by a set of inequality point response constraints. It is shown that a suitable cost function for this problem is the l/sub p/ quasi-norm for 0 >

Design trades for Rotman lenses

Abstract: The foundation of a satisfactory Rotman lens design is geometric. The effects of seven design parameters (focal angle, focal ratio, beam angle ratio, maximum beam angle, beam port curve ellipticity, array element spacing, and focal length/ lambda ) on the shape, and on the geometric phase and amplitude errors of a Rotman lens are described. The advantage of beam port shaping to reduce phase error, and of pointing port horns at the opposite apex (instead of normal to the curve) to reduce off-axis beam amplitude asymmetries, are shown numerically. A design procedure for selecting these parameters is given, and a calculation of lens gain is presented. >

Reconfigurable arrays by phase-only control

Abstract: Due to space or cost reasons, a single array antenna can be required to radiate more than one pattern, each pattern being selected by an electronic control, in which only the phase can be modified. A synthesis method for such a problem that is able to determine both the common amplitude and the various phases in an integrated way is presented. Moreover, the approach is flexible enough to take into account additional constraints and allows an efficient implementation. Some test cases showing the effectiveness of the method are presented. >

Design of multioctave spiral-mode microstrip antennas

Abstract: The design of spiral-mode microstrip antennas with a bandwidth of 6:1 is demonstrated. The antenna has a small dissipative loss due to a resistive loading outside its active region. As a specific measure of bandwidth at microwave frequencies, the spiral-mode microstrip antenna gain is higher than that of the conventional loaded-cavity spiral over a 5:1 and if the spacing d between the microstrip and the ground plane is 0.25 in. or larger over the 2-12 GHz range. Cursory efficiency calculations show consistent improvement over a loaded-cavity spiral. A study of ground plane spacing showed that the antenna gain remains above the conventional spiral gain as d is reduced until it is as small as 0.1 in. >

Theoretical prediction of mean field strength for urban mobile radio

Abstract: Physical multipath propagation structure in urban areas is analyzed, showing ray-theoretical waves make a major contribution to a mean field strength on a street in a moderate-sized city. A computer-aided prediction system is used to calculate a mean field strength around an arbitrary receiving point on the ray-theoretical basis when building data are given. The prediction principle and the prototype prediction system are described. The predicted values are shown to exhibit appreciable agreement with the measured values, confirming the possibility of purely theoretical prediction. >

Simple approximate formulas for input resistance, bandwidth, and efficiency of a resonant rectangular patch

Abstract: Simple approximate formulas for the input resistance, bandwidth, and radiation efficiency of a resonant rectangular microstrip patch are derived. These formulas become increasingly accurate as the substrate thickness decreases. Because the formulas are derived from approximations of a rigorous Sommerfeld solution, they provide insight into the effect of the substrate parameters on the patch properties, in addition to providing approximate design equations. >

Optimal interpolation of radiated fields over a sphere

Abstract: An optimal sampling interpolation algorithm is developed that allows the accurate recovery of scattered or radiated fields over a sphere from a minimum number of samples. Using the concept of the field equivalent (spatial) bandwidth, a central interpolation scheme is developed to compute the field in theta , phi coordinates, starting from its samples. The maximum allowable sample spacing and error upper bounds are also rigorously derived. Several simulated examples of pattern reconstruction are presented, for both the cases of field and power pattern interpolation. The interpolation error, as a function of the retained sample number, has been also evaluated and compared with the theoretical upper bounds. The algorithm stability versus randomly distributed errors added to the exact samples is demonstrated. >

A frequency-selective surface using aperture-coupled microstrip patches

Abstract: A frequency-selective surface based on the aperture coupled microstrip is presented. This surface has a narrow bandpass response useful for building radomes with low out-of-band radar cross section (RCS). The analysis employs the spectral domain moment method. The design criteria and a simple lumped element model are derived and presented. Experimental data from a waveguide simulator are presented which confirm the theory. >

A new class of resonant antennas

Abstract: A new class of wire antennas called meander antennas is introduced as possible elements for size reduction. Efficiency is affected only by the ohmic losses in the wire, and cross polarization is negligible. An increase in the number of meander sections introduces less size reduction in return for an improved bandwidth. These antennas can be used to reduce the size of existing wire antennas such as Yagi-Uda antennas and log-periodic dipole arrays. A size reduction in the resonant length typically from 25-40% is obtained. More size reduction is obtained by decreasing the radius of the wire or increasing the separation of the folded arms. >

A slot design for uniform aperture field distribution in single-layered radial line slot antennas

Abstract: A single-layered radial line slot antenna (RLSA) is proposed for a direct broadcast from a satellite (DBS) subscriber antenna. It utilizes a radially outward-traveling wave and the aperture field is intolerably tapered if the uniform slots are used. Two novel techniques are applied. First, the slot design for uniform aperture field distribution is introduced. Second, the radial line is terminated by a matching spiral, in place of a dummy load. It suppresses reflection and radiates all the residual power in circular polarization. For a model antenna of 600-mm diameter, the efficiency of 65% is observed. >

A general polarimetric radar calibration technique

Abstract: A polarimetric radar calibration procedure is introduced and verified with experimental results. The procedure requires measurements of three known targets in order to determine the distortion matrices that characterize the effect of the measurement system on the transmitted and received waves. The scattering matrices for the known targets can be of any form, provided that a limited set of constraints is satisfied. A special case, wherein the transmit and receive distortion matrices are the transpose of each other, is considered. This case is useful for some single antenna systems and has the advantage that only two known targets are required. >

Superresolution techniques for time-domain measurements with a network analyzer

Abstract: Superresolution techniques for time delay estimation are proposed and applied to frequency-domain data measured with a network analyzer. A MUSIC (multiple signal classification) algorithm preprocessed by spatial smoothing is used. The spatial smoothing preprocessing is performed to destroy signal coherence, and the decorrelation performance is examined in detail. The expression which gives an individual response is given. Using this expression, it is possible to eliminate unwanted signals that appear as ripples in the frequency domain. Experimental results show that the frequency bandwidth required by the MUSIC algorithm to resolve distinct time-domain responses and eliminate unwanted signals is much narrower than that required by the FFT (fast Fourier transform). Thus, the MUSIC algorithm is applicable to the time-domain measurements with the network analyzer and has much higher resolution capability than the conventional FFT techniques. The MUSIC algorithm is one of the most promising methods of enhancing the accuracy of measurement for narrowband devices such as antennas. >

Null steering in phased arrays by controlling the element positions

Abstract: Null steering methods usually involve costly and complicated amplitude and/or phase control systems. A technique is presented for null steering based on the element position perturbations. The technique frees the phase shifters to be used solely for steering the main beam toward the direction of the desired signal. It also removes the limitations of the other techniques by independently steering the main beam and the nulls to arbitrary independent directions. This technique is also capable of obtaining sidelobe cancellation and wideband signal rejection. >

High frequency scattering from trihedral corner reflectors and other benchmark targets: SBR versus experiment

Abstract: A general method for calculating the radar cross section (RCS) from a three-dimensional target is described. The target is first constructed by using a solid-geometry-modeling computer-aided design (CAD) package. Following the shooting and bouncing ray (SBR) method, a very dense grid of rays is launched from the incident direction toward the target. Each ray is traced according to the geometrical optics theory including the effect of ray tube divergence, polarization, and material reflection coefficient. At the point where the ray exits the target, a physical optics-type integration is performed to obtain the scattered far fields. This method is tested using several simple examples involving interaction among plates, cylinders, and spheres. The theoretical results are generally in good agreement with measured data. >

A conjugate gradient-based algorithm to minimize the sidelobe level of planar arrays with element failures

Abstract: Element failures increase the sidelobe power level of an array. Reconfiguring the amplitude and phase distribution of the remaining elements can partially compensate for the failed elements and thus reduce the sidelobe level. An algorithm is obtained that yields the reconfigured distribution by minimizing the ratio of the average peak sidelobe power level to the power in the main beam, taking into account the defective elements. The minimization of this nonlinear function is carried out via a conjugate gradient method. The algorithm is applied to the synthesis of sum and difference patterns of planar arrays. >

Fast and accurate near-field-far-field transformation by sampling interpolation of plane-polar measurements

Abstract: An optimal sampling interpolation algorithm which allows the accurate recovery of plane-rectangular near-field samples from the knowledge of the plane-polar ones is developed. This enables the standard near-field-far-field (NF-FF) transformation, which takes full advantage of the fast Fourier transform (FFT) algorithm, to be applied to plane-polar scanning. The maximum allowable sample spacing is also rigorously derived, and it is shown that it can be significantly greater than lambda /2 as the measurement place moves away from the source. This allows a remarkable reduction of both measurement time and memory storage requirements. The sampling approach is compared with that based on the bivariate Lagrange interpolation (BLI) method. The sampling reconstruction agrees with the exact results significantly better than the BLI, in spite of the significantly lower number of required measurements. >