Showing papers in "IEEE Transactions on Antennas and Propagation in 1984"

Abstract: A method for calculating the electromagnetic scattering from and internal field distribution of arbitrarily shaped, inhomogeneous, dielectric bodies is presented. A volume integral equation is formulated and solved by using the method of moments. Tetrahedral volume elements are used to model a scattering body in which the electrical parameters are assumed constant in each tetrahedron. Special basis functions are defined within the tetrahedral volume elements to insure that the normal electric field satisfies the correct jump condition at interfaces between different dielectric media. An approximate Galerkin testing procedure is used, with special care taken to correctly treat the derivatives in the scalar potential term. Calculated internal field distributions and scattering cross sections of dielectric spheres and rods are compared to and found in agreement with other calculations. The accuracy of the fields calculated by using the tetrahedral cell method is found to be comparable to that of cubical cell methods presently used for modeling arbitrarily shaped bodies, while the modeling flexibility is considerably greater.

Abstract: Formulas for the potentials due to uniform and Linearly varying source distributions defined on simply shaped domains are systematically developed and presented. Domains considered are infinite planar strips, infinite cylinders of polygonal cross sections, planar surfaces with polygonal boundaries, and volumetric regions with polyhedral boundaries. The expressions obtained are compact in form and their application in the numerical solution of electromagnetics problems by the method of moments is illustrated.

Abstract: The fundamental effects of superstrate (cover) materials on printed circuits antennas are investigated. Substrate-superstrate resonance conditions are established which maximize antenna gain, radiation resistance, and radiation efficiency. Criteria are determined for material properties and dimensions for which surface waves are eliminated and a radiation efficiency due to substrate-superstrate effects of e_{s} = 100 percent is obtained. Criteria for nearly omnidirectional \bar{H} -plane patterns and nearly omnidirctional \bar{E} -plane patterns are presented. Finally, a general criterion is given for choosing a superstrate to optimize efficiency for the important case of nonmagnetic layers with the antenna at the interface.

Abstract: Diffraction propagation over hills and ridges at VHF and UHF is commonly estimated using Fresnel knife edge diffraction. This approach has the advantage of simplicity, and for many geometries yields accurate results. However, since it neglects the shape and composition of the diffracting surface, it can in some cases yield results which are in serious disagreement with measurements. To remedy this, attempts have been made to approximate the diffracting hill or ridge by other shapes, most notably cylinders. These approaches have not been widely adopted, due in large part to their greater numerical complexity. In this paper it is proposed to apply wedge diffraction in the format of the geometrical theory of diffraction (GTD), modified to include finite conductivity and local surface roughness effects. It is shown that, for geometries with grazing incidence and/or diffraction angles, significant improvement in accuracy is obtained. Further, the GTD wedge diffraction form used is based on the Fresnel integral, so that it is only slightly more complex numerically than knife edge diffraction. Finally, the GTD includes reflections from the sides of the ridge (wedge faces), and can be extended to multiple ridge diffraction and three-dimensional terrain variations.

Abstract: A comprehensive study of infinite phased arrays of printed dipole antennas is presented, with emphasis on the scan blindness phenomenon. A rigorus and efficient moment method procedure is used to calculate the array impedance versus scan angle. Data are presented for the input reflection coefficient for various element spacings and substrate parameters. A simple theory, based on coupling from Floquet modes to surface wave modes on the substrate, is shown to predict the occurrence of scan blindness. Measurements from a waveguide simulator of a blindness condition confirm the theory.

Abstract: Calculation of mean field strength for urban mobile radio has been made on a ray-theoretical basis assuming an ideal city structure with uniform building heights. The result shows that building height, street width, and street orientation as well as mobile station antenna height are controlling propagation parameters in addition to the ordinary factors. The major theoretical characteristics agree approximately with experimental data including conventional empirical predictions. This suggests a way of theoretically predicting mean field strength in an urban area.

Abstract: Explicit expressions for equivalent edge currents are derived for an arbitrary local wedge angle and arbitrary directions of illumination and observation. Thereby the method of equivalent currents (MEC) is completed as a practically applicable theory of the electromagnetic high-frequency diffraction by edges. The derivation is based on an asymptotic relationship between the surface radiation integral of the physical theory of diffraction (PTD) and the line radiation integral of MEC, and the resulting expressions are deduced from the exact solutions of the canonical wedge problem.

Abstract: A method for increasing the bandwidth of microstrip patch antennas by incorporating two additional resonators which are gap-coupled to the radiating edges of a rectangular patch is described. A two-dimensional analysis using Green's function and segmentation method is used for analyzing the proposed antenna configurations. A bandwidth as large as five times a single rectangular patch is obtained in S -band. Changes in the radiation pattern over this wide bandwidth are discussed. Experimental results are in reasonable agreement with the analysis.

Abstract: A solution is presented for the problem of an infinite array of microstrip patches fed with short current elements. The input reflection coefficient is calculated versus scan angle in an arbitrary scan plane, and the effects of substrate parameters and grid spacings are considered. The scan blindness phenomenom is observed and discussed in terms of a forced surface wave response. Measurements from waveguide simulators confirm the theory for thin substrates.

Abstract: A novel class of probability distributions resulting from a compound Poisson process is found to correlate well with amplitude distributions of radar clutter returns spatially sampled from composite terrain. This class of distributions, derived from assumptions of random scattering phase and Poisson spatial distribution of elementary scattering sources, is specified by several physical and statistical parameters in its complete generality. These parameters are: 1) the number of scatterer types; 2) the average radar scattering cross section and the cross-sectional distribution of each different scatterer type; 3) the occurrence probability or the average scatterer size and spatial density; 4) the radar resolution area; and 5) the average background radiation as well as the radar internal noise power. Excellent fits of the theoretical clutter distributions to the measurement data are obtained by assuming a Rayleigh amplitude distribution for the elementary scatterer return for high grazing angle cases and a more general K -distribution for low grazing angle cases.

Abstract: Applying the Laplace transform, the exact distributed image current function is obtained for the classical Sommerfeld half-space problem with vertical magnetic current source. The resulting field integral is well behaved when the image current is situated in complex space. Unlike previous approximate images, the present theory is valid for any distance, height of the source, frequency, and half-space parameters. It is demonstrated that the present image theory reduces to the well-known dipole image at complex depth for large dielectric parameters of the half-space. Also, the reflection-coefficient method is obtained as a farfield approximation. Calculation of fields through exact image integration is seen to be simple and accurate and require modest computer capacity and time. In an appendix, some properties of the multivalued Green's function arising from a dipole source in complex space are also studied.

Abstract: The general Sommerfeld problem with both \epsilon and \mu discontinuous and a source consisting of arbitrarily oriented electric and/or magnetic dipoles at the same location is considered in terms of image theory. The problem is formulated with electric and magnetic fields instead of potential quantities resulting in a vector transmission-line interpretation of the Fourier transformed problem. The image sources are seen to be located in complex space expressable in terms of a certain basic image current function, which was encountered in part II of this paper on the vertical electric dipole problem. The horizontal electric/magnetic dipole image is solved and found to consist of both vertical and horizontal current components. The image concept is generalized to the most general three-dimensional sources. As a check, the well-known reflection coefficient method is obtained as the far-field approximation of the present theory.

Abstract: An iterative technique is developed to rigorously compute the electromagnetic time- and frequency-domain scattering problems. The method is based upon a wave-function expansion technique (this also includes the integral-representation techniques), in which the electromagnetic field equations and causality conditions are satisfied analytically, while the boundary conditions or the constitutive relations have to be satisfied in a computational manner. The latter is accomplished by an iterative minimization of the integrated square error. For the solution of an integral equation, it is shown how to obtain optimum convergence. Some numerical results pertaining to a number of representative problems illustrate the numerical advantages and disadvantages of the iterative method.

Abstract: A full-wave analysis for the problem of scattering frequency selective surfaces from (FSS) comprised of periodic arrays of cross dipoles and Jerusalem crosses is presented. The formulation is carried out in the spectral domain where the convolution form of the integral equation for the induced current is reduced to an algebraic one. The equation is then solved using the Galerkin's procedure applied in the spectral domain. A set of entire-domain type "junction basis functions," which, is demonstrated in this paper to be essential to account correctly for the discontinuous nature of the induced current at the junction of the cross, is included in the expansion for the unknown induced current. This analysis is computationally efficient, and its accuracy is verified by the agreement between the computed theoretical data and the experimental results reported by other authors.

Abstract: A generalized solution for a class of printed circuit antennas excited by a strip transmission line is presented. The strip transmission line may be embedded inside or printed on the substrate. As an example, microstrip dipoles electromagnetically coupled (Parasitically excited) to embedded strip transmission line have been analyzed accurately, and design graphs are provided for a specific substrate material. These graphs permit the establishment of a design procedure which yields the microstrip dipole length, overlap, offset, and substrate thickness with the goal of a desired input match for a given substrate material. The method accounts for conductor thickness and for arbitrary substrate parameter. Comparison with experiment shows excellent agreement.

Abstract: A millimeter-wave dual frequency circularly polarized feed for an offset reflector antenna is being developed for a portable satellite ground terminal. The two frequency bands are Q - and K -bands for transmit and receive, respectively. A compact feed design consisting of a single corrugated horn with two circular waveguide concentric openings at the horn throat is described. Good aperture efficiency and low sidelobes in both of the frequency bands are achieved.

Abstract: Approximate formulas are derived for the far field and gain of standard, open-ended, rectangular waveguide probes operating within their recommended usable bandwidth. (Such probes are commonly used in making near-field antenna measurements.) The derivation assumes first-order azimuthal dependence for the fields, and an E -plan pattern given by the traditional Stratton-Chu integration of the transverse electric ( TE_{10} ) mode. The H -plane pattern is estimated by two different methods. The first method uses a purely E -field integration across the end of the waveguide. The second, more accurate method approximates the fringe currents at the shorter edges of the guide by isotropically radiating line sources. The amplitude of the line sources is determined by equating the total power radiated into free space to the net input power to the waveguide. Comparisons with measurements indicate that for X -band and larger waveguide probes, both methods predict on-axis gain to about 0.2 dB accuracy. The second method predicts far-field power patterns to about 2 dB accuracy in the region 90\deg off boresight and with rapidly increasing accuracy toward boresight.

Abstract: The directive properties of antennas for transmission into a material half-space are investigated. In a practical situation, the antennas might be located in air with the directive transmission into the earth. The field of a general antenna over the half-space is expressed as a spectrum of plane waves. The integrals representing the field are evaluated asymptotically to obtain the "geometrical optics" field of the antenna, and this field is used to define quantities that describe the directive properties of the antenna (pattern function, gain, and directivity). Numerical results are presented for infinitesimal electric and magnetic horizontal dipole antennas in a dielectric half-space, region 1, with directive transmission into the adjacent dielectric half-space, region 2, and the ratio of permittivities \epsilon_{2}/\epsilon_{1} greater than one. The theory for the infinitesimal dipoles completely explains the directive properties previously obtained for the resonant circular-loop antenna over a material half-space. Measured field patterns and gains for dipole and loop antennas near an interface between air and fresh water are in good agreement with the theory.

Abstract: A new E -field solution is presented for the electric current and electric charge induced on a perfectly conducting surface illuminated by an incident electromagnetic field. This solution is a moment solution to the electric field integral equation on the surface. The expansion functions consist of a set of functions suitable for expanding the magnetostatic current and a set of functions whose surface divergences are suitable for expanding the electrostatic charge. The testing functions are similar to the expansion functions. With these expansion and testing functions, the new E -field solution works well with surfaces whose maximum dimension may be as small as 10^{-15} wavelengths or as large as a few wavelengths. Previous E -field solutions begin to deteriorate when the maximum dimension of the surface falls below a few hundredths of a wavelength. The new E -field solution is applied to a conducting circular disk and a conducting sphere.

Abstract: The equivalent of a Taylor pattern for continuous apertures is developed for discrete arrays. This array factor can be realized exactly, since it belongs to the appropriate class of patterns. A method for determining the element excitations is also presented. As an example, the pattern and excitations are calculated for a 41-element array with \bar{n} = 6 and a 25 dB design sidelobe level. Comparison is made with the patterns obtained by applying root matching and sampling of a continuous Taylor aperture.

Abstract: The electromagnetic scattering from partially or totally penetrable bodies of revolution (BOR) is formulated in terms of coupled Fredholm integral equations, solved by the method of moments (MM). The scatterers can have axial inhomogeneities, formed by dissimilar dielectric materials. The case of conducting bodies with axially discontinuous coatings is also treated. The penetrable regions can be lossy, characterized by complex permeability and permittivity. Boundary conditions are rigorously treated everywhere including the intersection of the various regions. The solutions are expressed in terms of combinations of two special matrices arising from the Galerkin technique. These solutions are implemented numerically for a class of generic axially inhomogeneous BOR scatterers. Numerical results given for various conducting/dielectric cylinder combinations using this formulation are compared with experimental data. For special cases where comparisons are possible, the present analysis replicates the results of the Mie theory.

Abstract: A technique for modeling wire objects interacting across or penetrating the planar interface which separates two half-spaces is described. The moment-method treatment is employed, based on the thin wire approximation to the electric-field integral equation, with the effect of the interface included via the Sommerfeld integrals. The computation time associated with evaluating the latter is substantially shortened by using an interpolation-based technique plus asymptotic field expressions. Although developed specifically for the wire problem, the procedure is also applicable, with slight modification, to modeling surface objects as well. Special care is taken to account for the charge discontinuity that occurs at the point a wire penetrates the interface. Example calculations are shown for a monopole antenna driven against ground stakes and simple ground screens, the fields of buried objects, and a simple electromagnetic pulse (EMP) simulator.

Abstract: A terrain sensitive propagation model based on the geometrical theory of diffraction (GTD) modified for finite conductivity and local surface roughness is applied to hilly terrain including multiple diffraction, with the results compared with measurements and the Longley-Rice model. Even though the approach used to calculate multiple-wedge diffraction is known to be invalid in certain situations, the comparison is favorable to the GTD model results.

Abstract: Complex anisotropic media can generally be described by a 6 \times 6 macroscopic constitutive tensor \hat{C} Using \hat{C} properties, a reaction integral formula is derived from which an anisotropic reaction theorem (modified reciprocity theorem) is developed Reduction of the \hat{C} medium into a reciprocal medium is discussed including tensor symmetry attributes and limiting cases The anisotropic reaction theorem is utilized to derive a zero reaction theorem, and then treated in relation to the moment method Mutual and self-impedance elements of a network are also derived in terms of reaction integrals, symmetry covered using the anisotropic reaction theorem, and impedance elements related to moment calculations Use of spectral domain analysis is also covered

Abstract: Two types of modified dipole antennas, zigzag and meander-line types, are analyzed and the shortening ratios are calculated. A zigzag dipole antenna with a wire length of 0.58 wavelengths has a shortening ratio of 24 percent with a resonant resistance of 46 \Omega . A meander-line dipole antenna with a wire length of 0.70 wavelengths has a shortening ratio of 30 percent with a resonant resistance of 43 \Omega . It is found that the radiation patterns of these two types of antennas are similar to the radiation pattern of a conventional half-wave linear dipole antenna.

Abstract: The circular-disk microstrip antenna with an air gap between the substrate and the ground plane is analyzed using the cavity-model theory. A simple formula for the resonant frequencies is obtained. The internal fields, the radiation fields, the effective loss tangent, and the input impedance are derived when the antenna is provided with a coaxial feed. The theory predicts that the air gap does not alter the relative field patterns significantly but it provides a simple parameter by which the resonant frequencies of the antenna can be tuned. Measurements have been performed on a 5 cm circular-disk microstrip antenna with an air gap and good agreement between theory and experiment is obtained.

Abstract: Use is made of an existing technique to synthesize a sum pattern whose sidelobe peaks fit a prescribed envelope in a specified segment of the visible region. In terms of a polynomial representation of the pattern, certain roots are then displaced radially off the unit circle to achieve null-filling. The result is a shaped beam with low, equal-percentage ripple, and sidelobe heights in the nonshaped region that are individually controlled. The procedure is illustrated for a linear array, designed to produce a \csc^{2}\theta \cos\theta pattern.

Abstract: A single surface integral equation for problems involving electromagnetic scattering from homogeneous dielectric bodies illuminated by time-harmonic sources is developed via the equivalence principle. The equation is formulated in terms of an equivalent electric current defined at the body surface. When allowed to radiate in a homogeneous medium having the material parameters of the exterior medium of the original problem, the electric current solution to the integral equation produces the correct scattered electric and magnetic fields external to the body.

Abstract: Sidelobe nulling at symmetric locations in linear array patterns can be accomplished with phase-only weight control if no restriction is placed on the magnitude of the phase perturbations. Nonlinear programming techniques can be used to calculate the required phases. Several examples are presented.

Abstract: The determination of the scattered and transmitted transient electromagnetic waves produced by a uniform dielectric body is reduced to the solution of a singular integral equation of the first kind for one tangential vector field defined on the surface. All derivations are carried out within the heuristic approach to Green functions and delta functions. The electric and magnetic fields are expressed in terms of the sources, initial values, and the boundary values by means of the Green function for the scalar wave equation. The appropriate integral equation is derived, and the integrals for the scattered and transmitted fields are given. The simpler problem of scattering of scalar waves is developed first. Formulas for the scattering of monochromatic fields are also given in the scalar and electromagnetic cases when transmitted fields do not vanish.