Showing papers in "Electromagnetics in 1994"

Analysis of Probe-fed Circular Microstrip Patches Backed by Circular Cavities

Abstract: In this article, a full-wave moment method solution for probe-fed circular microstrip patch antennas backed by circular cavities is presented. The antenna performance in both scattering and radiation cases is considered. Results are compared to those of conventional probe-fed circular patch antennas.

Electromagnetic Scattering from Microstrip Patch Antennas and Spirals Residing in a Cavity

Abstract: A new hybrid method is reviewed for the analysis of the scattering by conformal antennas and arrays comprised of circular or rectangular elements. In addition, calculations for cavity-backed spiral antennas are given. The method employs a finite element formulation within the cavity and the boundary integral (exact boundary condition) for terminating the mesh. By virtue of the finite element discretization, the method has no restrictions on the geometry and composition of the cavity or its termination. Furthermore, because of the convolutional nature of the boundary integral and the inherent sparseness of the finite element matrix, the storage requirement is kept very low at 0(n). These unique features of the method have already been exploited in other scattering applications and have permitted the analysis of largo–size structures with remarkable efficiency. In this paper, we describe the method’s formulation and implementation for circular and rectangular patch antennas in the presence of lumpe...

Analysis of Resistively Loaded, Printed Spiral Antennas

Abstract: A method of moments formulation for the current on arbitrarily shaped planar conductors in layered media is utilized to study the characteristics of printed spiral antennas. This algorithm permits the use of distributed and lumped impedance loads on the conductor. Also, it takes advantage of the discrete rotational symmetry exhibited by certain structures, thus reducing the matrix storage requirements. Numerical results are presented for two-arm and four-arm spiral antennas. Resistive loading along the outer portions of the spiral is used to eliminate reflected current contributions. For conductor-backed printed spirals, the presence of the ground plane reduces the amount of mode 1 radiation, allowing for the radiation of higher order spiral modes. Increasing the number of the spiral arms is shown to alleviate this problem by increasing the order of the first contributing higher order mode.

Two-Arm Slot Spiral Antenna

Abstract: The derivation process of a pair of H-type integral equations is reviewed for a system composed of curved wires and slots. The integral equations are characterized by two kernels Π and Π#, each being expressed in closed form. The integral equations are applied to a two-arm slot spiral antenna fed by two wires. Each feed wire is not in contact with the edge of the slot. The magnetic current distribution on the slot spiral, the radiation pattern, and the axial ratio are presented. The 3-dB axial ratio bandwidth is calculated to be 18%.

RCS Computation of Coax-Loaded Microstrip Patch Antennas of Arbitrary Shape

Abstract: A space-domain mixed-potential integral equation approach is applied in conjunction with the method of moments to compute the radar cross-section (RCS) of coax-loaded microslrip patch antennas having arbitrary or irregular shapes. The effects of Lhe substrate—which may be electrically thick and may consist of any number of planar, possibly uniaxially anisotropic dielectric layers, backed by a ground plane—are rigorously incorporated in the analysis by means of the vector and scalar potential Green's functions. The latter are expressed in terms of the voltages and currents on transmission line analogs of the layered medium, associated with TM and TE partial fields. The current distribution on the microstrip patch is approximated using vector basis functions defined over triangular elements and the coax probe current is expanded in terms of piecewise-lincar subdomain basis functions. A simple probe-to-patch attachment mode, compatible with the triangular element model of the microstrip patch, is us...

Time Domain Scattering from Dielectric Bodies

Abstract: We investigate the stability and accuracy of a time dependent, surface integral equation method to calculate the scattering from a dielectric body subjected to a transient incident electromagnetic field. The integral equation is solved using time marching, and an averaging method is used to ensure the numerical stability of the solution. The accuracy of the method as the dielectric parameters and pulse width are varied is discussed.

Design of Multi-Arm Multi-Mode Spiral Antennas for Directional Beams Using Equivalent Array Concept

Abstract: Multi-arm spiral antennas can be considered as antenna arrays where the spiral arms constitute the array elements. The proximity of the arms over the full length of the spiral and their direct coupling make such arrays unique in the sense that they operate with very strong mutual coupling. As a consequence, the array performance is dominated by the array configuration, rather than the array elements. In particular, the phase mode concept that is used successfully for the analysis of circular arrays can be used also for investigation of these arrays. For symmetric multi-arm spirals the phase mode excitation is also symmetric and used with Butler matrix type feeds in applications such as the direction finding, where the beam rotation and scanning is achieved with an aid of inter-mode phase shifts. In such applications, however, mostly the first two phase modes are utilized and mode purity has been the dominant requirement, rather than the antenna gain. In this paper the concept of phase modes is re...

Effective Permittivity and Permeability of a Fibers Grating

Abstract: A diffraction problem for a grating consisting of thin parallel cylinders (fibers) is considered in the low frequency approximation. Relative permittivity and permeability are supposed to be constant in each fiber. An electric field vector of an incident plane wave is parallel to the fibers. Results of this approximation are compared with the field scattered by a homogeneous hypothetical layer. Thus, its relative effective permittivity (ee ), permeability (µe), refractive index (ηe), and impedance (z e) are determined. It has been shown that such an equivalent material is sometimes characterized by negative values of either Im(ee ) or Im(µe) (with the time dependence e−iωt), and may be interpreted as a medium with an internal generation.

Polarization Properties of Sprial Antennas: A Tutorial

Abstract: The basic polarisation properties of spiral antennas are considered by means of circularly polarised spherical waves (CPSW). At first, specific properties of the CPSW and their impact on the antenna polarisation are discussed and conditions for the radiation of pure circular polarisation are identified. Then, symmetrical multi-arm structures are introduced as a general class of antennas which permits an optimisation of the polarisation performance by a suitable control of the CPSW spectra. Finally, the specific CPSW spectra of planar spirals - one member of this class - are discussed. An explanation of the polarisation properties of planar spirals and some design guidelines are derived from this discussion.

Scattering from Microstrip Patch Antennas Using Subdomain Basis Functions

Abstract: The scattering properties of microstrip patch antennas are examined with a spectral domain moment method approach. The Galerkin solution presented uses subdomain basis functions to model the current distribution on the patch so that a space varying surface resistance on the patch may be included. The subdomain approach is also used to model circular and triangular microstrip patches by approximating the patch boundary with a rectangular grid. Both calculated and measured results are presented for a few representative cases.

Second-Mode Operation of an Archimedean Spiral Antenna Backed by a Conducting Plane Reflector

Abstract: A two-arm Archimedean spiral antenna operating in the second mode is investigated numerically and experimentally. The effects of the presence of the Infinite plane conducting reflector on the radiation characteristics are studied using an integral equation technique. A traveling-wave type current is realized due to the mutual coupling between the real and image spirals, provided the antenna height is appropriately chosen, in spite of the fact that in the absence of the reflector a standing wave is appreciable in the current distribution. The operating bandwidth of the spiral antenna is discussed in detail. Experimental results are also presented to verify the numerical results.

Analysis and Design of Conductor-Backed Square Archimedean Spiral Antennas

Abstract: An extensive parametric analysis of conductor-backed square Archimedean spiral antennas has been conducted numerically by the method of moments. The results show that, at a near quarter wavelength above the ground plane, the square spiral yields a 1:1.21 bandwidth of the boresight axial ratio less than 3 dB. It also demonstrates that the axial ratio of the square spiral is controlled not only by its outer perimeter but also by its arm length. Design charts on the selection among wire radius, arm spacing, and spiral ground-plane height are presented as general guidelines for the design of circularly polarized conductor-backed square Archimedean spiral antennas.

Resonant Frequencies in an Electromagnetic Cylindrical/Spherical Cavity with an Internal Off-Axis Small Dielectric Sphere

Abstract: Analytical expressions for the resonant frequencies in an electromagnetic cylindrical/spherical cavity with an off-axis inner electrically small dielectric sphere are derived, for both magnetic and electric modes. The walls of the cylindrical/spherical cavity are perfectly conducting. Cylindrical/spherical vector wave functions and related addition theorems, as well as expansion of cylindrical wave functions in terms of spherical ones, are used. Our results are useful in problems connected with resonant cavities containing small inhomogeneities, as well as in cases that we want to determine the permittivities or magnetic permeabilities of various materials, by measuring the resonant frequency shifts caused by the introduction of small samples of them, inside cavities. Graphical results for some of the lower order modes are given, for various values of the parameters and for both kinds of modes.

Radar Cross Section Computation of Microstrip Patches

Abstract: The paper presents a method for the computation of the radar cross section of microstrip patches. The approach is based on a combination of ideas derived from the mixed potential integral equation formulation for microstrip antennas, fast convolution algorithms and the biconjugate gradient iterative method. All operations are kept at a vector level and this renders the method capable of handling a large number of unknowns. Numerical results for a rectangular and a circular patch have been compared with measured results available in literature and a good agreement has been observed. Advantages and limitations are discussed.

Monofilar Archimedean Spiral Slot Antennas

Abstract: A monofilar Archimedean spiral is adopted as a circularly polarized slot radiator which is electromagnetically coupled to a feed line. Numerical analysis of the coupling between the slot and the feed line reveals that, when the slot arm length is properly chosen, a traveling-wave magnetic current is distributed along the slot, leading to the radiation of a circularly polarized wave. On the basis of this radiation mechanism, a spiral slot printed antenna fed by a triplate transmission line is experimentally designed to obtain a unidirectional radiation beam. It is found that an axial ratio of 0.6 dB is achieved, and that the spiral slot is matched to the transmission line with a VSWR of less than 1.5. An array antenna composed of 32 printed spiral slots is constructed, attaining a 5 % bandwidth for a 3-dB axial ratio criterion. The formation of several tilted beams is also demonstrated.

Theoretical and Experimental Determination of the Polarimetric Antenna Radar Cross Section

Abstract: The total radar cross section (RCS) of antennas is the vector sum of structural and antenna mode scattering. These RCS contributions can be described completely by their polarimetric scattering matrices. The paper introduces a new polarimetric network description for three and four port antennas. The total scattering matrix is separated into the individual contributions with a new measurement procedure, based on complex load variations. Two waveguide and two planar antennas are chosen for demonstration and verification. By load variation the antenna mode scattering can be manipulated in phase and amplitude. It can especially be steered to compensate for the structural and aperture contributions. The possibilities for RCS minimization under polarimetric aspects are shown to be limited in bandwidth, polarization and aspect angle.

Radiation and Scattering of a Square Archimedean Spiral Antenna Using FDTD

Abstract: A Finite-Difference, Time-Domain (FDTD) analysis of a square Archimedean spiral antenna is performed to calculate antenna gain and scattering patterns. The spiral is mounted in a large, conducting ground plane and is backed by an air-filled or absorber-filled cavity. The effects of resistive sheets placed over the spiral are also evaluated for both radiation and scattering. Total-field FDTD calculations are used to compute the gain patterns, while a specially-modified scattered-field approach for aperture antennas in infinite ground planes is used for the scattering results. Comparisons are made with calculations done with a Finite Element method and excellent results are obtained.

An Outward-Looking Differential Equation Formulation for Scattering From One-Dimensional Periodic Diffraction Gratings

Abstract: A formulation that combines the scalar Helmholtz equation with an integral equation radiation boundary condition is developed for electromagnetic scattering from heterogeneous geometries exhibiting one-dimensional periodicity. A finite element discretization is implemented using piecewise-linear interpolation functions. Numerical results are presented for the reflection and transmission coefficients associated with several diffraction gratings.

Approximate Calculation of Transmission Line Parameters and Field Distributions of Coplanar Electrodes in the Presence of a Buffer Layer

Abstract: The effect of a thin, dielectric buffer layer on coplanar electrode structures is examined. The buffer layer is placed between the metal electrodes and the dielectric substrate. It is seen to have a significant effect on the charge distribution, field distribution, and capacitance. A new approximate formula is derived for the charge in the presence of the buffer layer, which allows the fields to be calculated efficiently. In addition, closed form approximate expressions are derived for the capacitance. The expression for the charge is based on the observation that it's distribution in the presence of a buffer layer is almost the same as that for a microstrip The formulas are compared to results from a moment method solution for the problem

Integral Equations for the Induced EM Fields in Finite Heterogeneous Bodies with Arbitrary Electric and Magnetic Parameters

Abstract: Field integral equations of various forms governing the scattering of an incident EM wave by a finite heterogeneous body with arbitrary electrical and magnetic parameters are investigated and presented in this paper. Either dyadic or scalar Green's function is used to expressed the kernels of the integral equations. Numerical solutions of these integral equations are compared and advantages and disadvantages of various equations are discussed. The effect on the induced electric field in the body due to magnetic materials is also investigated.

Graphical Method in Studying Lossy Transmission Lines and Layered Absorber

Abstract: The locus of an impedance or admittance function for a lossy transmission line when plotted on a Smith chart is a spiral. In this paper, an alternative presentation is given by means of a new chart, designated as the A-B/R-X chart. The spiral is then replaced by a circular path and one linear path to relate the input impedance and the load impedance of a lossy line. Another graphical solution is given in the design of a multiple layered absorber placed in front, of a conducting plane to reduce the reflection.