Showing papers in "IEEE Transactions on Antennas and Propagation in 1988"

A theoretical model of UHF propagation in urban environments

Abstract: The development is given of a model in which the rows or blocks of buildings are viewed as diffracting cylinders lying on the earth. When the buildings are represented as absorbing screens, the propagation process reduces to multiple forward diffraction past a series of screens. Numerical computation of the diffraction effect yields a power-law dependence for the field that is within the measured range. Accounting for diffraction down to street level from the roof tops gives an overall absolute path loss in good agreement with the average measured path loss. >

Reflection properties of the Salisbury screen

Abstract: The reflection properties of multiple electric/magnetic Salisbury screens have been studied. Analytical formulae are developed for the maximally flat design of two- and three-screen planar stackups that have taken into account appropriate values for the spacer materials used to separate the screens. The effect of curvature has also been assessed. It is shown that relatively large reductions in specular reflection are possible over sizeable bandwidths using relatively few Salisbury screens. Furthermore, these reductions are not completely destroyed if the incident wave is not normal to the surface, if the surface is curved or if there are errors in fabrication of the screen. >

Reduction of sidelobe and speckle artifacts in microwave imaging: the CLEAN technique

Abstract: Large random thin arrays provide a high angular resolution microwave images but are plagued with artifacts (false targets and target breakup or speckle) caused by high sidelobe levels. The CLEAN algorithm for reducing the sidelobe-induced artifacts is extended to the coherent radiation field and the theory placed on a quantitative basis. The CLEAN technique decomposes the received echoes of a coherent multiple-target scene by iterative cancellation of the largest target found. At each step, cancellation information is used to create a target image. The image includes target intensities, phases, and directions. The process is designed for an imaging instrument consisting of a random thinned array. A condition is derived which, when satisfied, guarantees that all proper targets will be preserved in the cleaned image and all false targets discarded. An algorithm involving moving thresholds is derived to extract the target coordinates. It is shown that targets much weaker than the sidelobe level can be detected and displayed without the hazard of artifacts. The target dynamic range and the image contrast can be increased by up to twice the signal-to-noise ratio per element. >

A leaky-wave analysis of the high-gain printed antenna configuration

Abstract: A leaky-wave analysis is used to explain the narrow-beam resonance-gain phenomenon in which narrow beams may be produced from a printed antenna element in a substrate-superstrate geometry. It is demonstrated that the phenomenon is attributable to the presence of both transverse electric and transverse magnetic-mode leaky waves, that are excited on the structure. Asymptotic formulas for the leaky wave are compared with the exact patterns to demonstrate the dominant role of the leaky waves in determining the pattern. Results are presented as a function of frequency, the scan angle, and the permittivity of the superstrate. >

Error analysis techniques for planar near-field measurements

Abstract: A combination of techniques is described for reliably estimating the magnitude of each error arising in planar near-field measurements. They include mathematical analysis, computer simulation, and measurement tests. There are three primary applications for these tests: in designing a measurement facility, the requirements of each part of the measurement system can be specified to meet a given level of accuracy; during actual measurements, the experimenter can identify, and reduce where necessary, potential sources of error in the measurement; and when a measurement has been completed, the estimated uncertainty in the measurement can be obtained with confidence and ease. The latter application has been used in many measurements to verify that the planar near-field technique produces high-accuracy results competitive with any other measurement technique. >

Comparison of three methods for the measurement of printed antenna efficiency

Abstract: Three methods for the measurement of antenna efficiency are evaluated: (1) the Wheeler cap method, (2) the radiometric method, and (3) the directivity/gain method. Each of these methods was used to measure the efficiency of four different printed antennas (three microstrip patches with various feeds, and an eight-element series-fed microstrip array). These methods and the experimental results which were obtained are discussed. >

Modeling electromagnetic wave propagation in the troposphere using the parabolic equation

Abstract: A computational method is described for predicting electromagnetic wave propagation in the troposphere using the parabolic approximation of the Helmholtz wave equation. The model represents propagation over a spherical, finitely conducting Earth and allows specification of frequency, polarization, antenna pattern, antenna altitude, and elevation angle. The method enables calculations to be performed using either ideal or measured refractivity profiles that vary in both altitude and range. A brief discussion of the theoretical formulation and computational implementation of the propagation model is presented, followed by examples that demonstrate various features. Example calculations include 3-GHz propagation over a calm sea in the presence of both range-dependent and range-independent surface-based ducts as well as in standard atmosphere conditions. Comparisons with two other propagation models are also discussed. >

Detailed FD-TD analysis of electromagnetic fields penetrating narrow slots and lapped joints in thick conducting screens

Abstract: The physics of electromagnetic-wave transmission through narrow slots and tapped joints in thick conducting screens is examined in detail by applying numerical models to compute both field distributions within the slots and joints, and fields transmitted to the shadow region. The primary modeling tool is the finite-difference time-domain (FD-TD) method, using a Faraday's-law contour integral approach to modify the basic FD-TD algorithm to properly model the slot physics, even when the slot gap width is much less than one space lattice cell. Finely sampled method-of-moments (MM) models are used to validate the FD-TD tool for relatively simple straight slots; FD-TD is then used to explore properties of more complicated lapped joints which are widely used for shielding purposes at junctions of metal surfaces. It is found that previously reported slot resonances occur in a more general sense for lapped joints as path-length resonances. In addition to greatly enhanced power transmission, path-length resonances can result in total fields within the joint exceeding the incident fields by more than one order of magnitude. Sample field distributions for this case are given. >

Theory and application of radiation boundary operators

Abstract: A succinct unified review is provided of the theory of radiation boundary operators. With the recent introduction of the on-surface radiation condition (OSRC) method and the continued growth of finite-difference and finite-element techniques for modeling electromagnetic wave scattering problems, the understanding and use of radiation boundary operators has become increasingly important. Results are presented to illustrate the application of radiation boundary operators in both these areas. Recent OSRC results include analysis of the scattering behavior of both electrically small and large cylinders, a reactively loaded acoustic sphere, and a simple reentrant duct. Radiation boundary operator results include the demonstration of the effectiveness of higher-order operators in truncating finite-difference time-domain grids. >

A new prediction method for tropospheric scintillation on Earth-space paths

Abstract: A method of predicting scintillation fading occurring on Earth-space paths is proposed that is based on the data obtained from a low-elevation 14/11 GHz wave propagation experiment at Yamaguchi, Japan. This model includes parameters representing meteorological conditions that have not been covered in existing models and can be applied to broad regions with varying climates. The prediction accuracy of the method is evaluated using experimental data collected at four sites in the western part of Japan, as well as data reported thus far on a worldwide basis. Although data used for evaluation are limited, the model shows good prediction accuracy for frequencies from 7 to 14 GHz and elevation angles from 4 degrees to 30 degrees . >

Near-field probe used as a diagnostic tool to locate defective elements in an array antenna

Abstract: Results of an experimental study are presented in which the near-field probe was used as a diagnostic tool to locate the defective elements in a planar array. The near-field data were processed not only to obtain the far-field patterns of the array under the test, but also to reconstruct the aperture field for diagnostic purposes. The backward transform enables the near-field probe to identify accurately aperture faults at a distance, free of interactions and couplings with the array elements. In practice, to recover the aperture field properly from the near-field distribution, the evanescent components in the computed far-field spectrum must be excluded from the inverse process with fast-Fourier-transform (FFT) techniques. For low-gain array antennas, a correction on the far-field spectrum is required to remove the contribution of the probe and the element factor before the inverse transform, strongly enhancing the resolution. >

A selective survey of computational electromagnetics

Abstract: Some of the tools used in computational electromagnetics are placed into perspective with respect to the different kinds of approaches that may be used and their computer-resource requirements. Particular attention is paid to numerical models based on integral and differential equations. After a brief background discussion, some of the analytical and numerical issues involved in developing a computer model are reviewed. Some practical considerations are included from the viewpoint of computer-resource requirements, followed by a discussion of some ways by which computer time might be reduced. The presentation concludes with a brief examination of validation and error checking. The emphasis throughout is on review and summarization rather than detailed exposition. >

An examination of the theory and practices of planar near-field measurement

Abstract: Using computer simulation, several fundamental issues in planar near-field measurement are examined. The results indicate that some of the prevailing views of practices regarding the evanescent modes, the sampling and filtering of data, and the selection of the location and directivity of the probe are incomplete or misleading. In particular, the merits of using smaller low-directivity probes in conjunction with a closer probe-to-antenna distance have been found to be unduly overlooked. >

Generation of wide-band data from the method of moments by interpolating the impedance matrix (EM problems)

Abstract: The method of moments (MM) requires computation of the impedance matrix at each frequency. Since the computation can be a very time-consuming process, its performance over a wide frequency range can require a prohibitive amount of CPU time. A method is described whereby the impedance matrix is computed at relatively large frequency intervals and then interpolated to approximate its values at intermediate frequencies. Basically, the method trades reduced computer CPU time for increased storage. >

Characteristics of the equilateral triangular patch antenna

Abstract: The results of a comprehensive analysis of this antenna based on the cavity model with a coaxial feed source are given. The theoretical formulas are given, and the characteristics obtained from them are presented, including radiation patterns, percentages of power radiated, total Q-factors, input impedances, and their variations with feed position. A number of disagreements with previously published results are noted. The theoretical and experimental results are compared. The results of the comparison indicate that the equilateral triangular patch can be designed to function as a triple frequency antenna. A method for correcting for the nonperfect magnetic wall is suggested. >

Comparison between computed and measured bandwidth of quarter-wave microstrip radiators

Abstract: A simple theory based on a cavity model is used to analyze a quarter-wavelength flat element, without a dielectric substrate, which is connected to the ground plane in one of its sides by a metallic wall. The bandwidth of the element is computed from this model. Very good agreement is obtained between the computed and the measured bandwidth for elements in the S-band. >

Application of hybrid finite element method to electromagnetic scattering from coated cylinders

Abstract: A hybrid finite-element method is applied to solve the electromagnetic scattering (for both E- and H-polarizations) from coated cylinders. The coating material can be nonhomogeneous and have permeabilities and permittivities different from their free-space values. Three numerical examples are presented to show the validity and versatility of the method: (1) bistatic scattering from a coated circular cylinder which verifies the formulation and the computer program, (2) backscattering from a coated rectangular cylinder, and (3) backscattering from an ogival cylinder with coated edges. Computations are compared with those obtained using moment-method codes and exact solutions when applicable. >

Resonance frequency of a rectangular microstrip patch

Abstract: The microstrip-patch resonant frequency problem is formulated in terms of an integral equation using vector Fourier transforms. Using Galerkins's method in solving the integral equation, the resonant frequency of the microstrip patch is studied with both Chebyshev polynomials and sinusoidal functions as basis functions. In the case of the Chebyshev polynomials, the edge singularity is included, but it is not important for convergence. Furthermore, the resonant frequency of the microstrip patch is ascertained with a perturbation calculation. The results for Galerkin's method and experiments are in good agreement. The perturbation calculation agrees asymptotically with Galerkin's method. With the aim of developing a computer-aided design formula, the solutions obtained by Galerkin's method are interpolated with a three-variable polynomial. >

Spectral dyadic Green's function formulation for planar integrated structures

Abstract: The problem of the complete determination of the dyadic spectral Green's function for an integrated planar structure with a grounded dielectric slab has been considered and solved in a rigorous way by using the spectral theory of the electromagnetic field. The reciprocity theorem and the geometrical symmetry of the structure have demonstrated the different roles played by the independent terms of the spectral Green's function in the evaluation of the electromagnetic characteristics of the grounded slab excited with a general source. Furthermore, an equivalent circuit representation of the structure, allowing a noteworthy simplification in the determination of the total power, has been obtained. These equivalent circuits and the derived spectral Green's function presented here can be used to analyze and design microstrip antennas of arbitrary shape with a general type of loading, such as matched or unmatched loads, parasitic, and shorting pins. >

Analysis of multilayered periodic structures using generalized scattering matrix theory

Abstract: The use of generalized scattering matrix theory is proposed as a fast, efficient approach for analyzing multilayer structures where in each layer is either a diffraction grating or a uniform dielectric slab, and all grating layers have the same periodicity. The overall scattering from the structure is determined by first evaluating a matrix of scattering parameters for each individual layer and then forming a scattering matrix for the entire structure by a procedure analogous to the cascading of networks in circuit theory. Higher-order spatial (Floquet) harmonics, including nonpropagating modes and cross-polarized fields, are taken into account as necessary. The approach is illustrated by computing the reflection coefficient of a multilayered resistive strip grating as a function of frequency. >

Eigenstructure based partially adaptive array design

Abstract: A procedure is presented for designing partially adaptive arrays having nearly fully adaptive performance under steady-state conditions. Theory predicts that the required adaptive dimension is less than or equal to the rank of the spatially/temporally correlated portion of the interference correlation matrix for arbitrary linearly constrained minimum-variance beamformers. Knowledge of the eigenstructure of the interference correlation matrix is required to implement a beamformer with this adaptive dimension. To avoid adaptive estimation of the eigenstructure, the eigenstructure of an averaged correlation matrix (which spans the interference scenarios of interest) is utilized, and the adaptive dimension is given by the rank of the averaged correlation matrix. Construction of a transformation for reducing adaptive dimension is addressed. Simulations illustrating the utility of the eigenstructure approach are provided. >

Two-port higher mode circular microstrip antennas

Abstract: For circular microstrip antennas, single-mode design techniques work well for the dominant mode (n=1, where n is the azimuthal dependence of the fields in the antenna), but this is not the case for zero-order (n=0) and higher order (n>1) modes, where a modal expansion is necessary. The design of higher order dominant mode circular microstrip antennas is addressed, with an example and measurements for the n=3 case. Measurements for the n=3 modes show reasonable agreement with theory in the forms of the impedance loci, but the frequency dependence is in error by the order of the bandwidth of the antenna. A design example is given for a roof-mounted circular patch for vehicular communications. The example indicates that an n=3 dominant mode patch can be used as a two-port antenna for diversity applications or with a 90 degrees phase shifter and adder as a single-port antenna for omnidirectional coverage with (essentially) circular polarization. >

Generalized formulations for electromagnetic scattering from perfectly conducting and homogeneous material bodies-theory and numerical solution

Abstract: A generalized E-field formulation for three-dimensional scattering from perfectly conducting bodies and generalized coupled operator equations for three-dimensional scattering from material bodies are introduced. A fictitious electric current flowing on a mathematical surface enclosed inside the body is used to simulate the scattered field, and, in the material case, a fictitious electric current flowing on a mathematical surface enclosing the body is used to simulate the diffracted field inside the body. Application of the respective boundary conditions lead to operator equations to be solved for the unknown fictitious currents, which facilitates calculation of the fields in the various regions, using the magnetic vector potential integral. The existence and uniqueness of the solution are discussed. These alternative operator equations are solvable using the method of moments. The numerical solution is simple to execute, rapidly converging, and general in that bodies of smooth but otherwise arbitrary surface, both lossless and lossy, can be handled effectively. Comparison of the results with available analytic solutions demonstrates the accuracy of the moment procedure. >

The bandwidth performance of a two-element adaptive array with tapped delay-line processing

Abstract: The bandwidth performance of a two-element adaptive array with a tapped delay line behind each element is examined. It is shown how the number of taps and the delay between taps affect the bandwidth performance of the array. An array with two weights and one delay behind each element is found to yield optimal performance (equal to that obtained with continuous-wave interference) for any value of intertap delay between zero and T/sub 90//B, where T/sub 90/ is a quarter-wavelength delay time and B is the fractional signal bandwidth. Delays less that T/sub 90/ yield optimal performance but result in large array weights. Delays larger than T/sub 90//B yield suboptimal signal-to-interference-plus-noise ratio when each element has only two weights. For delays between T/sub 90//B and 4T/sub 90//B, the performance is suboptimal with only two taps but approaches the optimal if more taps are added to each element. Delays larger than T/sub 90//B result in suboptimal performance regardless of the number of taps used. >

A novel principle for optimization of the instantaneous Fourier plane coverage of correction arrays

Abstract: A novel principle for the design of correlation arrays is introduced, based upon the maximization of the distance between samples. Simulated annealing is applied to the problem of finding solutions for moderate numbers of elements (up to 12). The resulting arrays have symmetric crystalline structures. >

Soft and hard horn antennas

Abstract: Horn antennas with soft and hard boundaries are analyzed. A soft boundary which exists in classical hybrid-mode horns gives zero field intensity at the wall. A hard boundary corresponds to a uniform field distribution over the horn aperture. Soft and hard horn antennas are compared with respect to directivity, sidelobes, and beamwidth. The dependency between the edge taper directivity, and sidelobes is also calculated based on the solution to the spherical hybrid modes in a conical horn with arbitrary wall impedances. This makes it possible to study how to compromise between directivity and sidelobes. Also discussed is how the different wall impedances may be realized, and some preliminary experimental work on hard horns is presented. >

Canonical sources and duality in chiral media (antenna arrays)

Abstract: The authors examine the characteristics of antenna arrays embedded in unbounded chiral media using the Green's dyadic for electric sources and the Green's vector for magnetic sources. The purpose is to bring to light the new characteristics of sources, both point and extended, which interact with this medium and to examine general characteristics of sources located in a medium with handedness. Very simple quality relations are presented that are characteristic of chiral media when the results are written in terms of the circular eigenmodes. Appropriate measures of chirality such as the chirality admittance and impedance and a dimensionless chirality factor are introduced as needed. It is shown that, in the far field, both point and extended sources, whether electric or magnetic, radiate two electromagnetic eigenmodes which are of opposing handedness. Sources that access only one of the eigenmodes of the medium are demonstrated. Several applications of the results and array performance in chiral media are noted. >

Phase-only synthesis of minimum peak sidelobe patterns for linear and planar arrays

Abstract: Minimization of the maximum sidelobe level for a given array geometry by phase-only adjustment of the element excitations is considered. Optimum phases are obtained by using a numerical search procedure to minimize the expression for the pattern sidelobe level with respect to the element phases. Results for both linear and planar arrays of equispaced elements are presented. The data suggests that optimum sidelobe level is a logarithmic function of array size, and optimum patterns have relative efficiencies that are typically somewhat greater than for comparable-amplitude tapered arrays. An analytic synthesis algorithm is presented for use on very large arrays for which the numerical search technique for the minimization of the sidelobe level is computationally impractical. This method produces patterns with characteristics similar to arrays synthesized using the numerical search method, i.e. relatively uniform angular distribution of energy in the sidelobe region, and generally decreasing maximum sidelobe level as the array size is increased. >

Rapid near-field antenna testing via arrays of modulated scattering probes

Abstract: Key results are summarized of efforts to significantly reduce the near-field measurement time by utilizing one- or two-dimensional arrays of modulated scattering probes in lieu of the single probe ordinarily used in conventional near-field measurement techniques. Results of analytical, numerical, and experimental investigations show that the modulated-scattering technique (MST) using arrays of hundreds or even thousands of modulated scattering probes can be used to map the complex near-field of antennas or scatterers in a few seconds or minutes. The results also strongly indicate that classical (nonmodulated) receiving/transmitting arrays can be adapted for rapid near-field data collection. Major factors affecting the accuracy and speed of probe arrays for near-field measurement are delineated and discussed. Experimental results obtained using laboratory prototype MST systems are also presented and discussed. >

Special problems in applying the physical optics method for backscatter computations of complicated objects

Abstract: The backscatter computation of complicated objects is carried by the physical optics (PO) method, known as the vector Kirchoff approximation. The object is described by a geometrical model using flat plates (panels). These panels can be nonperfectly conducting and multilayered. The PO solution for the scattering matrix of a single multilayered panel is evaluated in detail using the Fresnel reflection coefficients. An example of the computed reflection coefficient of a two-layered medium is presented. The phase integral of the PO method is solved analytically. The hidden-surface problem is discussed, and the procedure for the treatment of doubly reflecting panels is described. For an ideal conducting cube with additional surfaces that generate shadow and double-reflection effects, the computed radar cross section (RCS) is compared with measurements. Computational results of the RCS for nonperfectly conducting panels are given. >