Showing papers in "IEEE Transactions on Antennas and Propagation in 1987"

On the spatial bandwidth of scattered fields

Abstract: It is shown that the scattered fields are almost space bandlimited functions. The effective bandwidth W is introduced and evaluated for a very general scattering system, as well as the error made using functions bandlimited to w > W for representing the scattered field. The effective bandwidth is very simply related to the maximum dimension of the scattering system; the error drops to negligible values for modest increases of w compared to W , in the case of large scatterers. Important consequences of the above general results are finally stressed.

Radiation and scattering from a microstrip patch on a uniaxial substrate

Abstract: The problem of a rectangular microstrip antenna printed on a uniaxially anisotropic substrate is treated. The effect of anisotropy on the resonant frequency and surface wave excitation of the antenna is considered, and the radar cross section (RCS) of the antenna is calculated. The RCS calculation includes the effect of the load impedance (antenna mode scattering). Results for the resonant frequency of a patch on a uniaxial substrate are compared with measurements, and the RCS of a patch on an isotropic substrate is compared with measurements. The derivation of the uniaxial Green's function in spectral form, the associated moment method analysis for the input impedance and scattering of the microstrip patch, and the expressions for the far-zone fields of a source on a uniaxial substrate are presented.

Gain enhancement methods for printed circuit antennas through multiple superstrates

Abstract: Reciprocity and a transmission line model are used to determine the radiation properties of printed circuit antennas (PCA's) in a multilayered material configuration. It is demonstrated that extremely high directive gain may result at any scan angle, with practical materials, if the thickness of the substrate and multiple superstrate layers is chosen properly. This model is also used to analyze the radiation characteristics of printed circuit antennas in inhomogeneous substrates.

Calculation and experimental validation of induced currents on coupled wires in an arbitrary shaped cavity

Abstract: An efficient numerical technique is presented for the calculation of induced electric currents on coupled wires and multiconductor bundles placed in an arbitrary shaped cavity and excited by an external incident plane wave. The method is based upon the finite-difference time-domain (FD-TD) formulation. The concept of equivalent radius is used to replace wire bundles with single wires in the FD-TD model. Then, the radius of the equivalent wire is accounted by a modified FD-TD time-stepping expression (based on a Faraday's law contour-path formulation) for the looping magnetic fields adjacent to the wire. FD-TD computed fields at a virtual surface fully enclosing the equivalent wire are then obtained, permitting calculation of the currents on the wires of the original bundle using a standard electric field integral equation (EFIE). Substantial analytical and experimental validations are reported for both time-harmonic and broad-band excitations of wires in free space and in a high- Q metal cavity.

A new formulation of electromagnetic wave scattering using an on-surface radiation boundary condition approach

Abstract: A new formulation of electromagnetic wave scattering by convex, two-dimensional conducting bodies is reported. This formulation, called the on-surface radiation condition (OSRC) approach, is based upon an expansion of the radiation condition applied directly on the surface of a scatterer. Past approaches involved applying a radiation condition at some distance from the scatterer in order to achieve a nearly reflection-free truncation of a finite-difference time-domain lattice. However, it is now shown that application of a suitable radiation condition directly on the surface of a convex conducting scatterer can lead to substantial simplification of the frequency-domain integral equation for the scattered field, which is reduced to just a line integral. For the transverse magnetic (TM) case, the integrand is known explicitly. For the transverse electric (TE) case, the integrand can be easily constructed by solving an ordinary differential equation around the scatterer surface contour. Examples are provided which show that OSRC yields computed near and far fields which approach the exact results for canonical shapes such as the circular cylinder, square cylinder, and strip. Electrical sizes for the examples are ka = 5 and ka = 10 . The new OSRC formulation of scattering may present a useful alternative to present integral equation and uniform high-frequency approaches for convex cylinders larger than ka = 1 . Structures with edges or corners can also be analyzed, although more work is needed to incorporate the physics of singular currents at these discontinuities. Convex dielectric structures can also be treated using OSRC. These will be the subject of a forthcoming paper.

Analysis of longitudinal slots in rectangular waveguides

Abstract: An accurate computation of the admittance characteristics of longitudinal shunt slots in a rectangular waveguide is presented. A moment method with entire basis functions is used for solving the electric field in the slot. The waveguide wall thickness is accounted for by introducing higher order waveguide modes in the short waveguide connecting the inner and outer regions. Particular attention is given to the resonant length of the slot, which is a critical parameter in the design of waveguide slot array antennas. Computed resonant lengths compare very well with measured results.

Scattering from a microstrip patch

Abstract: A solution to the problem of plane wave scattering by a rectangular microstrip patch on a grounded dielectric substrate is presented. The model does not include the microstrip feed, and thus does not include the so-called "antenna mode" component of the scattering. The solution begins by formulating an electric field integral equation for the surface current density on the microstrip patch. The integral equation is solved using the method of moments. Computed data for the patch radar cross section (RCS) is found to be in close agreement with measurements over a broad frequency range. The microstrip RCS versus frequency consists of a number of large peaks which are identified as impedance or pattern factor resonance peaks.

Scattering center analysis via Prony's method

Abstract: High-frequency scattering can often be described in terms of scattering centers, and an understanding of the geometries which give rise to these centers is important in the area of radar cross section modification. Certain canonical geometries have been treated theoretically with asymptotic methods, but, in general, one must study the behavior of scattering centers empirically. Prony's method provides a way of extracting the locations (projected on the path of propagation) and weighting coefficients of scattering centers from the backscattered field as a fuction of frequency. It has been found to be superior to the conventional Fourier transform technique in resolution and dynamic range.

Bow-tie antennas on a dielectric half-space: Theory and experiment

Abstract: A new formulation is discussed for the rigorous calculation of the radiation pattern of a bow-tie antenna of finite length and infinitesimal thickness, placed on a lossless dielectric substrate. The analysis is based on a representation of the current density on the metal surface of the antenna as a sum of an imposed (quasistatic) term and a set of current modes with unknown amplitudes. Free-space fields that are expressed in terms of continuous spectra of symmetrized plane waves are matched to the current modes using the method of moments. The resulting set of equations are solved for the unknown current amplitudes. The calculations show that for increasing bow length the antenna impedance spirals rapidly to a value predicted by transmission line theory. The theory also shows that the E -plane pattern of a two wavelength, 60\deg bow-tie antenna is dominated by low-loss current modes propagating at the dielectric wavenumber. As the bow tie narrows, the loss of the modes increases, and the dominant wavenumber tends to the quasistatic value. Pattern measurements made at 94 GHz are shown to agree well with theoretical predictions. Measurements for a long-wire antenna, a linear array of bow-tie elements, and a log-periodic antenna are also presented.

Analysis of electromagnetic scattering from dielectric cylinders using a multifilament current model

Abstract: A moment solution is presented for the problem of transverse magnetic (TM) scattering from homogeneous dielectric cylinders. The moment solution uses fictitious filamentary currents to simulate both the field scattered by the cylinder and the field inside the cylinder and in turn point-matches the continuity conditions for the tangential components of the electric and magnetic fields across the cylinder surface. The procedure is simple to execute and is general in that cylinders of arbitrary shape and complex permittivity can be handled effectively. Metallic cylinders are treated as reduced cases of the general procedure. Results are given and compared with available analytic solutions, which demonstrate the very good performance of the procedure.

Dual-frequency stacked annular-ring microstrip antenna

Abstract: Experimental results of a dual-frequency microstrip antenna consisting of two stacked annular rings of outer radii 5 cm and inner radii 2.5 cm are presented. Fabricated on a Duroid substrate with relative permittivity 2.32 and thickness 0.159 cm, the separations of the two resonant frequencies range from 6.30-9.36 percent for the first three modes. The frequency separations can be altered by means of an adjustable air gap between the lower ring and the upper substrate.

Point-matched time domain finite element methods for electromagnetic radiation and scattering

Abstract: Direct time domain computation of Maxwell's differential equations will soon become a practical technique because of the availability of supercomputers. The principal methods used in time domain computations and the supporting theories are presented. The point-matched finite element method is chosen as the main feature of this presentation, which includes the discretization of equations, conforming mesh generation, dielectric and metallic interfaces, numerical stability and simulation of radiation conditions. Numerical results of scattering of Gaussian pulses are presented in time sequences.

A rigorous analysis of a microstripline fed patch antenna

Abstract: A rigorous solution to the problem of a rectangular microstrip antenna fed by a microstripline is presented. The currents on the feed line and the patch are expanded in a suitable set of modes, and a moment method solution is formulated in the spectral domain. Three special feed cases are treated: a patch fed at a radiating edge; a patch fed at a nonradiating edge; and a proximity coupled patch. Good results are obtained for the radiating edge fed case and the proximity coupled case. The nonradiating edge fed case results were not as good; reasons for this discrepancy are discussed. Calculations are compared with measurements for high and low dielectric constant substrates.

Backscatter analysis of dihedral corner reflectors using physical optics and the physical theory of diffraction

Abstract: Physical optics (PO) and the physical theory of diffraction (PTD) are used to determine the backscatter cross sections of dihedral corner reflectors in the azimuthal plane for the vertical and horizontal polarizations. The analysis incorporates single, double, and triple reflections; single diffractions; and reflection-diffractions. Two techniques for analyzing these backscatter mechanisms are contrasted. In the first method, geometrical optics (GO) is used in place of physical optics at initial reflections to maintain the planar nature of the reflected wave and subsequently reduce the complexity of the analysis. The objective is to avoid any surface integrations which cannot be performed in closed form. This technique is popular because it is inherently simple and is readily amenable to computer solutions. In the second method, physical optics is used at nearly every reflection to maximize the accuracy of the PTD solution at the expense of a rapid increase in complexity. In this technique, many of the integrations cannot be easily performed, and numerical techniques must be utilized. However, this technique can yield significant improvements in accuracy. In this paper, the induced surface current densities and the resulting cross section patterns are illustrated for these two methods. Experimental measurements confirm the accuracy of the analytical calculations for dihedral corner reflectors with right, acute, and obtuse interior angles.

On the optimum polarizations of incoherently reflected waves

Abstract: In radar imaging, the scattered waves are usually partially polarized. Accordingly, the concept of optimum polarization must be extended to the case of incoherent scattering where the scattered waves are partially polarized. Here, it will be shown that the Stokes scattering operator is the most suitable characterization of incoherent scattering. The problem of finding the polarization that would yield an optimum amount of power received from the scatterer is solved by assuming a knowledge of the Stokes scattering operator instead of the 2 \times 2 scattering matrix with complex elements. The advantage of this method is that it may be used to find the optimum polarizations for the case wherein the scatterers can only be fully characterized by their Stokes scattering operator (incoherent scattering) and the case wherein the scatterer can be fully characterized by the complex 2 \times 2 scattering matrix (coherent scattering). In this report, it will he shown that the optimum polarizations reported thus far in the literature, i.e., when the problem is solved by using a knowledge of the 2 \times 2 scattering matrix, form the solutions for a subset of a more general class of problems. When the solution of the problem is based on a knowledge of the Stokes scattering operator, it is found that for incoherent scattering six optimum polarizations can exist, whereas when the solution is based on the 2 \times 2 scattering matrix, the number of optimum polarizations is necessarily limited to four.

Scattering from a periodic array of free-standing arbitrarily shaped perfectly conducting or resistive patches

Abstract: A fast Fourier transform (FFT)-based iterative approach for computing the fields scattered by an infinite array of free-standing patches is presented. The method is capable of handling patches that are lossy and have arbitrary shape; it is useful for analyzing configurations that may not have been analyzed previously. Though a rectangular FFT is used, the formulation allows the study of the common triangular array periodicities. Results for various geometries are presented and are compared with existing results.

On the resonant frequencies of the triangular patch antenna

Abstract: The appropriate correction factor to use in the resonant frequency formula of an equilateral triangular patch antenna obtained from the cavity model with perfect magnetic walls is discussed.

Wire grid modeling of surfaces

Abstract: When a surface is numerically modeled with a wire grid, results are sensitive to the wire diameter. It is shown that the best accuracy is obtained when the wire satisfies the "same surface area" rule of thumb, for the canonical problem of scattering (or radiation) from an infinite circular cylinder. It is important to note that wires that are too thick are just as bad as wires that are too thin. It is also shown that the boundary value match between wires is not a reliable check on the validity of farfield results. Finally, data are given on the effect of wire spacing. Results are obtained from exact solutions of both the true problem and the wire grid model, thus isolating the effects of wire grid modeling per se.

Analytical method of a multilayered meander-line polarizer plate with normal and oblique plane-wave incidence

Abstract: Simple empirical formulas for perpendicular and parallel polarization susceptances for a meander-line grating plate are given. The numerical results compared favorably with experimental data and published data. Simple transmission-line model in terms of E -type mode and H -type mode for multilayered meander-line polarizer plate is presented for plane wave incidence at normal and oblique angles. Numerical results for design examples are given for practical application.

Scattering from cavity-backed apertures: The generalized dual series solution of the concentrically loaded E -pol slit cylinder problem

Abstract: The generalized dual series solution to the scattering of an E -polarized ( E -pol) plane wave from an infinite circular cylinder having an infinite axial slot and enclosing an infinite concentric impedance cylinder is presented. This solution explicitly exhibits the correct edge behavior, and it can handle cylinders that are either electrically small or large without special considerations. The angle of incidence is arbitrary. A variety of current, field, and cross-section results are presented. These are compared with the corresponding H -pol problem results to establish the polarization dependencies of the aperture coupling. It is also shown that effects corresponding to the presence of the interior cavity dominate all of the scattering data. In particular, the bistatic cross sections in either case and the current induced along an interior wire in the E -pol case exhibit new resonance features that are due to the cavity-backed nature of the aperture.

A comparison of time delay spread and signal level measurements within two dissimilar office buildings

Abstract: A detailed analysis of time delay spread and signal level measurements at 850 MHz was performed within a large office building. The results were compared with studies of a much smaller and dissimilar office building. The results were found to be substantially the same, despite the physical differences of the buildings themselves. This may be due to external geographical features. Overall worst-case root mean square (rms) time delay spreads in the buildings were 250 ns and 218 ns in the larger and smaller building, respectively. However, these values improved to under 100 ns when there was a good direct path between the antennas. Time delay spread was also shown to be independent of relative antenna polarizations of the transmitter and receiver, even on line-of-sight paths. Received signal levels were below -90 dB with reference to the level at 0.3 m separation, in the worst cases.

Microwave attenuation and cross polarization in dust storms

Abstract: Some of the electrical and mechanical properties of dust are discussed. An expression for optical visibility during storms is given in terms of the relative volume occupied by dust. Expressions for attenuation and phase shift constants for a medium with precipitating dust particles are derived in terms of visibility and wavelength for both vertical and horizontal polarizations. Expressions for the attenuation and cross polarization of circularly polarized waves are also derived. Computations show that linearly polarized waves suffer but little effect. Circularly polarized waves on the other hand are significantly cross polarized.

Limits and validity of the impedance boundary condition on penetrable surfaces

Abstract: It has been shown that application of the impedance boundary condition (IBC), also known as the Leontovich boundary condition, is effective in solving electromagnetic scattering problems involving electrically large, penetrable bodies. However, it is important to clarify the limits and range of validity of the IBC. For example, as our results show, the IBC is not valid on pure dielectric (lossless) surfaces. Two specific criteria are given to clearly define the range of the IBC validity on arbitrary smooth surfaces. These criteria are verified with numerical calculations and theoretical analyses for scattering by spheres and cylinders from both the exact boundary conditions and the IBC. The scattering calculations for prolate spheroids show that the criteria presented here also hold for nonspherical cases.

Electromagnetic scattering by stratified inhomogeneous anisotropic media

Abstract: An analytical formulation is presented for the computation of scattering and transmission by general anisotropic stratified material. This method employs a first-order state-vector differential equation representation of Maxwell's equations whose solution is given in terms of a 4 \times 4 transition matrix relating the tangential field components at the input and output planes of the anisotropic region. The complete diffraction problem is solved by combining impedance boundary conditions at these interfaces with the transition matrix relationship. A numerical algorithm is described which solves the state-vector equation using finite differences. The validation of the resultant computer program is discussed along with example calculations.

A microwave printed planar log-periodic dipole array antenna

Abstract: A microwave printed planar log-periodic dipole array (LPDA) antenna analysis and design procedure is presented together with the description of a planar feeding technique. The effects of the dielectric substrate on the antenna design are also discussed and taken into account. A numerical technique using the finite element method is employed to obtain the characteristic impedance and effective dielectric constant of the transmission lines involved, i.e., an unbalanced stripline and a balanced line having only two parallel strips. A 4-8 GHz printed LPDA antenna was manufactured and tested, showing very good radiation pattern and voltage standing-wave ratio (VSWR) values over the entire frequency range.

Weakly dispersive spectral theory of transients, part I: Formulation and interpretation

Abstract: Dispersive effects in transient propagation and scattering are usually negligible over the high frequency portion of the signal spectrum, and for certain configurations, they may be neglected altogether. The source-excited field may then be expressed as a continuous spatial spectrum of nondispersive time-harmonic local plane waves, which can be inverted in closed form into the time domain to yield a fundamental field representation in terms of a spatial spectrum of transient local plane waves. By exploiting its analytic properties, one may evaluate the basic spectral integral in terms of its singularities-real and complex, time dependent and time independent-in the complex spectral plane. These singularities describe distinct features of the propagation and scattering process appropriate to a given environment. The theory is developed in detail for the generic local plane wave spectra representative of a broad class of two-dimensional propagation and diffraction problems, with emphasis on physical interpretation of the various spectral contributions. Moreover, the theory is compared with a similar approach that restricts all spectra to be real, thereby forcing certain wave processes into a spectral mold less natural than that admitting complex spectra. Finally, application of the theory is illustrated by specific examples. The presentation is divided into three parts. Part I, in this paper, deals with the formulation of the theory and the classification of the singularities. Parts II and III, to appear subsequently, contain the evaluation and interpretation of the spectral integral and the applications, respectively.

Resolution of a controversy surrounding the Kirchhoff approach and the small perturbation method in rough surface scattering theory

Abstract: It is shown that the small amplitude limit of the mean backscatter cross section for a perfectly conducting random rough surface calculated from the first two terms in the iterative series solution of the surface current integral equation gives the same results as the small perturbation method. This demonstration resolves a long-standing controversy in rough surface scattering theory.

Analysis of interinjection-locked oscillators for integrated phased arrays

Abstract: The behavior of a system of coupled oscillators is shown to have potential applications in the generation of power for integrated phased antenna arrays. Nonlinear differential equations are derived to describe a system of oscillators coupled by an arbitrary frequency-dependent network. State-variable analysis of the linearized equations leads to closed-form solutions for one- and two-dimensional phased array systems. Experimental data for a VHF prototype system is presented, and practical considerations in system design are discussed.

The hat feed: A dual-mode rear-radiating waveguide antenna having low cross polarization

Abstract: The hat antenna is a new self-supported rear-radiating waveguide feed, related to existing feeds known as splashplate and waveguide cup antennas. An analytical model for the radiation pattern of the hat antenna, a model which includes the axial waveguide itself is presented. This model shows that by exciting the feed with two modes it is possible to use the waveguide constructively as one of the dominant radiating parts of the feed instead of having to live with it as an undesirable blockage effect. Thereby aperture efficiencies up to 81 percent and cross-polar sidelobes down to -30 dB are available. The results are confirmed by measurements on a practical model, but the bandwidth is narrow.

Extraction of the natural frequencies of a radar target from a measured response using E-pulse techniques

Abstract: A new scheme is introduced for extracting the natural resonance frequencies of a radar target from a measured response. The method is based on the E -pulse technique and is shown to be relatively insensitive to random noise and to estimates of modal content. Verification of the technique is accomplished by comparing the natural frequencies extracted from the measured responses of a thin cylinder and a circular loop with those obtained from theory. The applicability of the technique to low- Q targets is also demonstrated, using the measured response of a scale model aircraft.