Electromagnetic transmission through mesh covered apertures and arrays of apertures in a conducting screen

The problem of electromagnetic transmission through wire mesh covered arbitrarily shaped aperture or arrays of apertures (possibly covered by a thin lossy dielectric sheet) in a perfectly conducting ground plane is considered. The equivalence principle and image theory are used to derive an integral equation for the equivalent magnetic currents. The method of moments is utilized to solve the integral equation, with the aperture modeled by triangular patches. Numerical results are presented for transmission coefficients and transmission cross-section patterns for electrically small apertures.     

Radiation from a waveguide-backed aperture in an infinite ground plane in the presence of a thin conducting plate

Coupled integrodifferential equations are formulated for the general problem of radiation from a waveguide-backed aperture in an infinite conducting plane when a thin, finite, and arbitrarily oriented conducting plate is located in its proximity. The general equations are particularized for the special case of a rectangular aperture and a rectangular plate and solved numerically using the method of moments. Detailed data are presented and discussed for various quantities of interest.     

A Nonmodal Formulation for Electromagnetic Transmission through a Filled Slot of Arbitrary Cross Section in a Thick Conducting Screen

This paper considers the two-dimensional problem of electromagnetic transmission through a filled slot of arbitrary cross section in a thick perfectly conducting screen. The equivalence principle is used to divide the original problem into three isolated parts where postulated equivalent sources radiate into unbounded, homogeneous media. These equivalent electric and magnetic currents are chosen to ensure continuity of the tangential components of electric and magnetic fields at each aperture. An integral equation is written for each of the three parts with the equivalent currents as unknowns. The resulting set of coupled integral equations is solved by the method of moments. It is shown in the Appendix that this set of equations has a unique solution. The primary quantities computed are the equivalent magnetic and electric currents on each aperture and the electric current on the remaining portions of the slot cross section. These results are compared with those obtained from a modal solution, where the fields in the slot cross section are expressed in terms of parallel-plate waveguide modes.     

Coupling through narrow slot apertures to thin-wire structures

Electromagnetic coupling through a narrow slot aperture in an infinite ground plane is considered when a thin wire is present in the coupled half-space. The effect of the wire on coupling is determined. The slot has width and depth. The method of moments is applied to determine equivalent aperture currents and wire currents. Then coupling is calculated from the resulting fields. Results are presented for high and low Q slots and on and off resonance     

Electromagnetic coupling to a conducting wire behind an aperture of arbitrary size and shape

The electromagnetic coupling to a conducting wire behind an aperture in a plane conducting screen is analyzed. The aperture can be of arbitrary size and shape. The wire can be of finite length, with or without terminating loads, or of infinite length. The electric current on the wire and the equivalent magnetic current over the aperture region are calculated by the method of moments. An equivalent circuit for the effect of the aperture on the transmission line mode of the wire is derived.     

Electromagnetic transmission through narrow slots in thick conducting screens

The general formulas for electromagnetic transmission through an infinitely long slot in a perfectly conducting screen of finite thickness are specialized to the case of a narrow slot. The slot may be filled with a homogeneous isotropic material. A simple equivalent circuit for the narrow slot is developed. It is found that for certain screen thicknesses the slot becomes resonant, and exceptionally large transmission of energy may occur. In fact, as the slot width approaches zero, the transmission width at resonance becomes1/piwavelengths regardless of the actual slot width. The effect of loading the slot with a lossy dielectric is also considered. Computations are given to illustrate the validity of the equivalent circuit model and the accuracy of transmission characteristics computed from it.     

An approximate solution for E-polarization coupling into acoaxial cylinder with an infinite axial slot

An approximate solution is derived for determining the total current induced on a centrally located conducting wire within a two-dimensional axially slotted cylinder for the case of E-polarization. Its validity is tested against a full moment-method numerical solution of the describing integral equation. The derivation of the approximate solution is similar to the hollow-cylinder case presented by P.M. Morse and H. Feshbach (195) in that the same basic form for the aperture distribution is assumed. A rapidly converging series solution is developed which is shown to be valid for an electrically `narrow' slot (accurate for apertures up to one wavelength wide) over a rather broad frequency spectrum. Though the aperture must be electrically narrow, many practical coupling problems of interest can be accommodated. Useful insight into coupling phenomena is obtained by direct inspection of the solution, and some novel observations about the current response are made     

Stripline to microstrip line aperture coupler

Aperture coupling between stripline and microstrip line is analyzed as a function of the electric and magnetic polarizabilities of the aperture, using Bethe's small-aperture coupling theory. A coupling expression for the general case of an aperture in the common ground plane is derived. Coupling variations with aperture dimensions for the case of a thin slot, a hole, a diamond, a rounded end slot, and an ellipse are determined using McDonald's polynomial approximations for the electric and magnetic polarizability for small apertures. Experimental results for the case of a thin slot and a circuit aperture are also presented     

A method of moments analysis of electromagnetic coupling throughslots using a Gaussian beam expansion

A moment solution is presented for the problem of transverse electric (to the slot axis) electromagnetic coupling through a slot in infinitesimally thin, perfectly conducting screen separating two identical or contrasting half-space regions. The moment solution uses sets of properly shifted and modulated Gaussian elementary beams to expand the unknown equivalent magnetic current and a simple point-matching procedure for testing. The Gaussian expansion offers a numerical advantage in subsequent field calculations. Instead of integrating over surface currents when computing the fields, the proposed expansion allows the evaluation of the fields by summations of analytic terms exploiting the simple and well-understood propagation features of Gaussian beams. Sample computations are given and compared with a standard pulse-pulse Galerkin solution     

Electromagnetic radiation and scattering at the American Universityof Beirut

This paper presents the work carried out at the American University of Beirut on electromagnetic radiation and scattering, using the characteristic-modes theory. Several problems of various two-dimensional and three-dimensional geometrics have been considered: slots in a plane, slots in a conducting cylinder, a parallel plate-fed slot antenna, an aperture-fed waveguide, a rectangular aperture in a perfectly conducting plane, a cavity-backed aperture. The characteristic modes for each geometry were first computed, and the convergence of the solution was tested for all these problems. A combined characteristic-modes formulation, for the problem of electromagnetic coupling to conducting objects behind arbitrary apertures in a conducting plane, has been developed and used to solve many of these problems     

Resonant electromagnetic field coupled into a lossy cavity through a slot aperture

A new approach is proposed for determining the electromagnetic fields coupled from an incident plane wave into a lossy conducting cavity through a slot aperture under resonant conditions. Use is made of the duality between a slot and a conducting strip and of the equivalence between a strip and a wire. Simplified formulas are derived that are explicit and flexible. Determination of resonant fields inside a cavity is reduced to simple calculations. Numerical examples are given.     

Surface-wave phenomena in phased slot arrays with parasitic wirearrays

Blind spots observed in structures consisting of a slot array and parasitic straight wire arrays are investigated. Each blind spot is found to be associated with either a pattern null or the excitation of a surface wave on the corrugated structure consisting of the parasitic wires and conducting screen, with the slots shorted. The plane-wave-expansion technique is used to evaluate the coupling between arrays. It is shown analytically that for structures consisting of a single wire array, the blind spot can exist only at broadside, where there is a pattern null for the parasitic wires. For structures with two-wire arrays, such as an array of Clavin elements, a surface wave can be excited at a particular scan angle provided a certain uniformity condition in the structure is satisfied. The information presented is relevant to the incorporation of parasitic wire arrays for scan compensation     

A hybrid moment method solution for TEz scattering fromlarge planar slot arrays

This paper develops a hybrid moment method (MM) based numerical model for electromagnetic scattering from large finite-by-infinite planar slot arrays. The model incorporates the novel concept of a physical basis function (PBF) to reduce dramatically the number of required unknowns. The model can represent a finite number of slot columns with slots oriented along the infinite axis, surrounded by an arbitrary number of coplanar dielectric slabs. Each slot column can be loaded with a complex impedance to tailor the array's edge currents. An individual slot column is represented by equivalent magnetic scattering currents on an unbroken perfectly conducting plane. Floquet theory reduces the currents to a single reference element. In the array's central portion, where the edge perturbations are negligible, the slot column reference elements are combined into a single basis function. Thus, one PBF can represent an arbitrarily large number of slot columns. A newly developed one-sided Poisson sum formula is used to calculate the mutual coupling between the PBF and the slot columns in the presence of a stratified dielectric media. The array scanning method (ASM) gives the mutual coupling between the individual slot columns. The hybrid method is validated using both numerical and experimental reference data. The results demonstrate the method's accuracy as well as its ability to handle array problems too large for traditional MM solutions     

Stable modeling of arbitrarily oriented thin slots in the FDTD method

A subcell model for thin wires in the finite-difference time-domain (FDTD) method using modified telegraphers equations has been developed by Holland et al. Edelvik has previously presented an extension of their algorithm, which allows for arbitrarily located and oriented wires with respect to the Cartesian grid. This is important to be able to accurately model wires that cannot be aligned to the Cartesian grid, e.g., tilted wires and circular loop wires. Recently, a dual set of equations has been proposed for modeling of thin slots. In this paper, we show that using a similar algorithm as for thin wires we can also handle slots of arbitrary location in Cartesian planes. Previous thin slot models have been susceptible for instabilities. We show that a symmetric coupling between field and slot yields a stable time-continuous field-slot system and that the fully discrete field-slot system is stable under a generalized Courant-Friedrich-Lewy (CFL) condition. The proposed method is demonstrated for scattering from a finite-length slot in an infinite conducting wall and a shielding enclosure including a slot. The results are in good agreement with published experimental data.     

Solution of Some Electrostatic Potential Problems Involving Spherical Conductors: A Dual Series Approach

The solution of several electrostatic potential problems involving open spherical perfectly conducting shells is presented in the context of the dual series equations method. These equations are a consequence of mixed boundary conditions imposed on the electric potential and field. A solution is given for the dual series equations corresponding to two concentric spherical shells, one of which has a circular aperture. The aperture dipole moment, the capacity, and field penetration of some electrostatic devices are also calculated.     

Generalized Form of Telegrapher's Equations for the Electromagnetic Field Coupling to Buried Wires of Finite Length

In this paper, a generalized form of telegrapher's equations for electromagnetic field coupling to buried wires is derived. The presented approach is based on thin-wire antenna theory. The effect of a dissipative half-space is taken into account via the reflection/transmission coefficient approximation. The conductor losses can be taken into account via the surface impedance per unit length. The derived equations are treated numerically via the Galerkin–Bubnov indirect boundary element method. Numerical results are presented for induced current along the wire, and compared with transmission-line (TL) and modified TL (MTL) approximations, respectively, for the case of perfectly conducting electrode buried in a lossy medium. It is shown that the TL and MTL approximations can result in an inaccurate induced current distribution along the conductor at HFs and for shorter electrode lengths, respectively.      

Near-Field and Plane-Wave Electromagnetic Coupling into a Slotted Circular Cylinder: Hard or TE Polarization

An analytically tractable model is proposed in this initial study of the electromagnetic phenomena that control our ability to synthesize, by using a near-field source, the effect of plane-wave coupling through an aperture into the interior of a vehicle under test. An integral equation for the tangential electric field in the slot aperture of a perfectly conducting, infinitesimally thin-walled circular cylinder is solved using a basis set of Chebyshev polynomials that are properly weighted according to the static edge condition. The resulting matrix elements from a Galerkin procedure are computed to high precision upon extracting the logarithmic singularity of the kernel of the integral operator. Exact expressions for the matrix elements, in the form of rapidly convergent series of elementary terms, are constructed by isolating another logarithmic function of the aperture width. A minimization of the mean-square error between the true plane-wave response and that due to a near-field line-source establishes the optimal complex source strength of the near-field source     

Scattering from a cavity-backed circular aperture penetrated by awire

The problem of a wire penetrating a circular aperture in an infinite conducting screen and entering a circular cylindrical cavity is considered. An approximate method is developed for evaluating the fields outside the cavity for observation points located far from the aperture. Results are presented in the frequency domain which compare this far-field approximation to the method of moments solution. The presence of interior resonance features in the exterior fields is discussed     

Analysis of an antenna composed of arbitrarily located slots and wires

A technique of analyzing an antenna composed of arbitrarily oriented thin wires and slots in a planar conducting screen is presented. The analysis is based on the generalized Hallen's type integral equation with closed form kernel. A parametric evaluation is done for a configuration of slot and two bending wires (Clavin element). The results obtained are shown to be in close agreement with the experimental ones obtained by Clavin et al.     

Scattering from loaded grooves

Electromagnetic scattering from a two-dimensional groove recessed in an arbitrarily thick conducting screen is studied. The groove may be empty or loaded with a lossy material which may or may not completely fill the cavity. For the partially loaded groove, the filling material is assumed electrically dense so that the standard impedance boundary condition is applicable at the top surface of the material. Employing a full-wave analysis, integral equations are derived for the tangential components of the electric field over the aperture. It is shown that the equations are identical for both partially loaded and completely loaded (or empty) cases provided that the aperture admittance of the groove is treated as the equivalent admittance of the internal medium looking into the aperture, thus simplifying the integral equations. When the groove is completely filled by a dense material, the formulation reduces to that corresponding to a direct application of the impedance boundary condition over the aperture.