Equivalent current excitation for an aperture antenna embedded in an arbitrarily shaped impedance surface

It is demonstrated, via the equivalence principle, that if the aperture fields are known, one can explicitly obtain the equivalent current excitation required for modeling an aperture in an arbitrarily shaped impedance surface, when the aperture surface is closed, by replacing it with an impedance surface. It is also demonstrated that the impedance condition over the antenna body and the closed aperture can be nonuniform and anisotropic and that different choices are possible for the value of the surface impedance of the surface that closes the aperture. It is expected that the procedures described will be most useful in approximately modeling aperture antennas embedded in nonplanar impedance surfaces where the aperture electric field is known.     

Equivalent network for an aperture in the center conductor ofmicrostrip line

The equivalent circuits for discontinuities in the form of a thin transverse slot and a circular aperture in the center conductor of a microstrip line are determined by a quasi-static analysis using equivalent dipole moments. The series reactance as a function of frequency is evaluated in terms of the geometrical parameters of the discontinuity. Data obtained from transmission measurements are presented to support the theory     

Equivalent surface impedance of an infinite periodic array of slot impedance loads based on a semicylinder cavity

A problem of finding the equivalent surface impedance of an infinite periodic array of slot impedance loads based on a semicylinder cavity. The problem is solved by the integral-equation method. As the numerical method for solving the integral equation, the Krylov-Bogoliubov method is used. The results of the numerical experiment are presented in the form of dependences of the impedance on the cavity radius, slot width, conductor width, and angle of electromagnetic-wave incidence. The angular dependences of the impedance are compared to the earlier results obtained for an array of rectangular grooves and a single impedance load based on a semicylinder cavity.     

Equivalent surface impedance of an infinite array of slot impedance loads designed on the basis of coupled rectangular domains

A 2D design of an infinite array of slot impedance loads (SILs) is considered. Each SIL is designed from two coupled rectangular domains connected through apertures with both each other and free space. The integral equation method is used for determination of the equivalent surface impedance (ESI). Dependences of the ESI value are investigated for different geometric dimensions to wavelength ratios. It is shown that the SILs under study can be used to obtain a reactive component of ESI having either positive or negative sign for the thickness of the impedance structure not exceeding one-quarter the wavelength.     

Equivalent circuit parameters of an aperture coupled open resonatorcavity

In this paper, we use modified form of Bethe's small hole coupling theory to compute equivalent circuit parameters of an aperture coupled open resonator cavity. The open resonator cavity is composed of spherical mirrors of circular cross section. The cavity is coupled to a rectangular waveguide by means of a common hole in the mirror and the shorted end wall of the rectangular waveguide. Closed form expressions have been obtained for the equivalent circuit parameters. Experiments conducted in the W-band frequency range show good agreement with theory when an experimentally estimated correction to the transmission coefficient is applied for the thickness of the coupling holes     

A new surface impedance function for the aperture surface of aconducting body with a dielectric-filled cavity

A surface impedance function (SIF) appropriate for use on the aperture surface of a conducting body with a dielectric-filled cavity, is presented. Unlike the usual SIFs that might be used on an aperture, this SIF takes into account not only the wave transmitted through the aperture but also the wave reflected from the inside of the cavity the shape of the aperture and cavity, and the polarization and direction of the incident wave. The SIF is derived heuristically from the series-reflection solution for a plane wave normally incident on an infinite flat conducting plate with a flat dielectric coating. The SIF was developed and used in a combined method of moments solution for the scattered fields due to an incident plane wave. This combined technique greatly reduces the number of current expansion coefficients to be determined using the method of moments and hence also reduces the number of impedance elements required for calculation in the method of moments. Application of the SIF in a combined method is illustrated for a two-dimensional object     

Asymptotic expressions for the surface currents induced on acylindrically curved impedance strip

The present paper is concerned with the derivation of the electric and magnetic surface currents induced on a cylindrical curved impedance strip. By considering the locality of the high-frequency diffraction phenomena the physical (-π,π) interval for the usual cylindrical polar angle is replaced by an abstract infinite interval (-∞,∞) whereby the related mixed boundary value problem is formulated as a “modified matrix Hilbert” problem. By using the Debye approximations for the Hankel and Bessel functions involved, the modified matrix Hilbert problem is first decoupled and then reduced to two pairs of simultaneous Fredholm integral equations of the second kind which are solved by iterations, The explicit expressions for the electric and magnetic surface current components attributable to the reflection, edge or surface diffractions of the incident field as well as to the edge reflections of these components themselves are obtained by evaluating the current integrals asymptotically. The results derived in the paper constitute also a rigorous proof for a conjecture made by Idemen on the reflections of the surface currents at the edges     

Equivalent surface impedance of a slot impedance load on the basis of a cavity with an equilateral triangular cross section

The problem of plane-wave excitation of an impedance load on the basis of a slot in an infinite perfectly conducting screen closed from one side by a cavity with an equilateral triangular cross section is considered. The problem is solved by the integral equation method. The numerical solution of the integral equation is performed by the Krylov–Bogolyubov method. For an H-polarized incident wave, the impedance as a function of the slot and cavity sizes and the angle of incidence of a plane wave is calculated.     

Equivalent currents for a ring discontinuity

Based on the geometrical theory of diffraction, equivalent filamentary currents are derived for a linear element of a surface singularity for general incidence and polarization. The currents are used to obtain an analytical expression for the bistatic field diffracted by a ring singularity, valid within the caustic region as well as at wide angles.     

Equivalent edge currents for arbitrary aspects of observation

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.     

Radiation from a slot in an impedance surface

The effects of a lossy-ground plane on the radiation from a slot are analyzed by using a Green's function of an impedance surface. Numerical results of the radiation field, radiated power, and loss power are presented for various lossy ground planes. In particular, the radiation field and the radiated power from a short slot are given by simple closed expressions. The analytical model of a slot in an impedance surface is validated by radiation field measurements     

Electrical impedance tomography using induced currents

The mathematical basis of a new imaging modality, induced current electrical impedance tomography (EIT), is investigated, The ultimate aim of this technique is the reconstruction of conductivity distribution of the human body, from voltage measurements made between electrodes placed on the surface, when currents are induced inside the body by applied time varying magnetic fields. In this study the two-dimensional problem is analyzed. A specific 9-coil system for generating nine different exciting magnetic fields (50 kHz) and 16 measurement electrodes around the object are assumed, The partial differential equation for the scaler potential function in the conductive medium is derived and finite element method (FEM) is used for its solution. Sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular value decomposition of the sensitivity matrix shows that there are 135 independent measurements. It is found that measurements are less sensitive to changes in conductivity of the object's interior. While in this respect induced current EIT is slightly inferior to the technique of injected current EIT (using Sheffield protocol), its sensitivity matrix is better conditioned. The images obtained are found to be comparable to injected current EIT images In resolution. Design of a coil system for which parameters such as sensitivity to inner regions and condition number of the sensitivity matrix are optimum, remains to be made.     

Equivalent surface representation for heterogeneous dielectriccylinders

The inner structure of a heterogeneous two-dimensional object interacting with an external electromagnetic field is completely accounted for by a pair of surface operators that yield the field components tangent to the outer surface exclusively in terms of a single unknown current density distributed on that same surface. Such an equivalent surface representation is invariant under rotation and translation, and is independent of external material and illumination     

Surface currents on impedance bodies of revolution

The surface currents induced by a plane wave axially incident on a rotationally symmetric body are determined by solving numerically extended form of Maue's integral equation. The relative surface impedance is independent of the azimuthal angle but may vary along the profile of the scatterer in any plane containing the axis of symmetry. Numerical results are shown for a sphere and a cone-sphere that are either perfectly conducting or perfectly absorbing. Apart from internal resonances, the computer code is found to provide accurate results well into the high-frequency region. A simple line-integral representation of the far field is given, and internal resonances are discussed for the backscattering radar cross section of a perfectly conducting and a perfectly absorbing sphere     

Input impedance of aperture coupled hemispherical dielectric resonator antenna

A hemispherical dielectric resonator (DR) antenna fed by a microstripline through an aperture on the ground plane is investigated both theoretically and experimentally. The broadside TE/sub 111/ mode input impedance of the antenna configuration is evaluated, and reasonable agreement between theory and experiment is obtained. The effects of the slot length and the slot width on the input impedance are studied and discussed.<>     

Effective impedance boundary conditions for an inhomogeneous thinlayer on a curved metallic surface

Effective impedance boundary conditions for an inhomogeneous thin layer coated on a perfectly conducting object are considered. The permittivity of the thin layer is inhomogeneous along both the normal and tangential directions. Explicit forms of the first- and second-order approximate impedance boundary conditions are derived first for a two-dimensional (2D) thin layer for the TE and TM cases. Numerical results are presented. The case of Maxwell's equations for a three-dimensional inhomogeneous thin layer is also considered     

Analysis of aperture blockage in reflector antennas by usingobstacle-located blockage currents

An improved method is presented to account for blockage effects in the analysis of reflector antennas. Commonly this is done by introducing shadows on the reflector surface according to the location of the obstacles when performing the physical optics integration. By using physical optics blockage currents located at the blocking obstacle instead of at the main reflector surface, the effect of the different locations in the axial direction is accurately accounted for. This can easily be included by a single phase factor in existing computer programs based on physical optics integration     

FDTD surface impedance model for coated conductors

A new finite-difference time-domain (FDTD) model of dielectric and conductive layers on conductive surfaces is developed. The model is based on surface impedance boundary condition, allowing the coating and the conductive backing to be removed from the computational space. The proposed model extends the previous FDTD models, which are formulated for coatings on perfect electric conductors (PEC), to coatings on more general dielectric and conductive materials. On the other hand, the model can also be regarded as a generalization of models designed for conductors without coatings. The present model accounts for the first thickness resonance of the layer by modeling the singularity of the tangent function appearing in the impedance function. The proposed model is numerically verified for normal incidence in the frequency domain and for varying oblique angles of incidence in the time domain by comparison with analytical results.     

Exact surface impedance for a spherical conductor

A spherically concentric model of the earth is employed to obtain expressions for the surface impedance matrix at the surface. It is shown the surface impedance is a function of both the earth's electrical parameter and the source field configuration. In some cases the latter dependence is of minor consequence.