A doubly periodic moment method solution for the analysis anddesign of an absorber covered wall

A periodic moment-method solution for scattering from a doubly periodic array of lossy dielectric bodies is developed. The purpose is to design electromagnetic wedge and pyramidal absorbers for low reflectivity so that one can improve the performance of anechoic chamber measurements. The spectral-domain formulation and the moment-method volume polarization current approach are used to obtain the expressions for determining the scattering from a doubly periodic array of lossy dielectric bodies. Some wedge and pyramidal absorber configurations that have been designed, fabricated, and tested in the OSU/ESL compact range measurement facility are presented. By taking into account the complexity of real-world material structures, good agreement between calculations and measurements has been obtained     

Current on a long, thin wire in a chiral medium

The propagation of current on a thin, straight wire in an infinite chiral medium is examined by solution of the integral equation for an infinite wire and also from the moment-method solution for a long wire of finite length. The current on the infinite wire is shown to consist of three components: a discrete mode that decays exponentially and two continuous-spectrum components from branch cuts from the two chiral wavenumbers. The integral equation for a finite wire in the chiral medium is solved by the method of moments using a modified version of the numerical electromagnetics code (NEC). The moment-method solution is shown to be in close agreement with the modal solution for the infinite wire, providing validation for the numerical treatment     

An equivalent model for the analysis of electromagnetic scatteringfrom a long, lossy tether structure

An equivalent lossy wire model is introduced for the analysis of electromagnetic scattering from a long lossy tether structure, which is used to suspend a satellite from a shuttle in free space. The equivalent model incorporates the electrical characteristics of the uniform TM₀₁ mode of a coated wire configuration and the geometrical properties of a lossy wire configuration. By considering residue contributions and the resulting surface wave quantities alone, and by adopting two additional simplifications in the calculation of induced currents and scattered field, the equivalent lossy wire model is used to determine the effect of a long, lossy tether on communications between its two ends. It is shown that for tether lengths of up to 30 km with the primary source being located close to one end of a lossy, straight tether, the presence of the tether does not have a significant effect on the near-zone fields in the neighborhood of its `receiving end'     

An efficient formulation to determine the scatteringcharacteristics of a conducting body with thin magnetic coatings

Two new and efficient surface integral equations, derived from corresponding volume integral equations, are developed to calculate the scattering of electromagnetic (EM) waveform from an arbitrarily shaped conducting body coated with thin lossy magnetic film. Their numerical solutions by the method of moments (MM) for two-dimensional structures with full or partial coatings are presented. It is shown that the radar cross-section of a conducting body can be significantly reduced by coating it with a lossy magnetic film. To verify the validity and accuracy of the proposed formulation, another method based on the expansion of cylindrical harmonic functions with real arguments is also developed to calculate the scattering of a plane EM wave from an electrically large coated circular cylinder. The same problem was also solved by the proposed formulation, and excellent agreement between the two approaches was achieved. In addition, numerical results of the scattering from a rectangular coated cylinder is shown to be consistent with that obtained by a modified finite-difference-time-domain (FDTD) method     

Analytical antenna model for chiral scatterers: comparison withnumerical and experimental data

An analytical model for polarizability dyadics of small chiral conductive particles in free space or those embedded in a lossy material is presented and discussed. Chiral particles are modeled by a wire loop connected to two straight wire elements. The electromagnetic analysis is based on the replacement of the particles by two connected antennas representing the wire and loop portions. Analytical expressions for polarizabilities are given. For electrically small particles, a lumped-element equivalent circuit can be constructed and the polarizabilities can be expressed in terms of equivalent circuit parameters. It is shown that the wire-and-loop antenna model for scatterers satisfies the reciprocity condition and other basic physical requirements. Approximate analytical expressions are compared with numerical simulations and with the experimental data on reflection from single chiral particles, and the results are seen to be in good agreement. The model can be used in analytical modeling of chiral and omega composite materials     

Scattering by thin dielectric strips

A plane wave incident on a thin dielectric strip with infinite length is considered, letting the incident electric field vector be parallel with the edges of the strip. The field is expanded in the dielectric region as the sum of three plane waves (the forced wave and two surface waves). Thex-axis andy-axis propagation constants are known for each wave, and Galerkin's method is employed to determine the amplitudes of these waves. Finally, the far-zone scattered field is determined by considering the polarization currents radiating in free space. Numerical data are presented to illustrate the scattering properties of lossless and lossy dielectric strips as a function of the angle of incidence and the width of the strip. The calculations show excellent agreement with an earlier moment method using pulse bases and point matching.     

Range profiles and images of a loaded straight wire

The range profiles and images of a straight wire without and with lumped impedance loading are discussed and demonstrated. The scattering mechanisms of a straight wire are qualitatively analyzed using the method of moments. Plots of range profiles at different aspect angles show that surface traveling waves are important scattering mechanisms of a straight wire. The presence of traveling wave makes ringlike artifacts appear in the reconstructed images. It was found that lumped impedance loading can effectively distort the range profiles and microwave images of a wire scatterer. In addition, randomly varied reactive loading can generate random peaks in the range profiles     

Highrise Building Reradiation and Detuning at MF

An evaluation of the reradiation from highrise buildings in the MF broadcast band is presented. Scattered field measurements were made at MF on buildings of the order of one quarter wavelength in electrical height and at UHF on scale models. A computational model incorporating the effects of lossy ground and building materials was developed using moment-method techniques. Reduction of scattering is demonstrated, both for models and a full-scale building, using simple wire detuning stubs to modify the induced RF currents. The current distributions resulting in the decrease of scattering are described for a vertical stub like those used in power line detuning, and for two related new designs, a rooftop stub and the "umbrella" stub. The latter is shown to be the most effective, yielding scattered field reductions of the order of 6dB in fullscale tests.     

Approximate natural response of an arbitrarily shaped thin wirescatterer

The authors introduce an approximate but computationally single technique for calculating the natural current and backscattered field response of an arbitrarily shaped thin wire scatterer. A single one-term resonant current approximation is shown to yield excellent results for the natural frequencies of perturbed straight wires and circular loops. Comparison with the measured current response of a compound wire target validates the simple theory     

Convergence tests of several moment-method solutions

Matrix elements used in previously published two- and three-dimensional moment-method solutions are here used for arrays of large numbers of cells arranged to approximate a thin dielectric slab. Richmond's two-dimensional solutions for both transverse magnetic (TM) and transverse electric (TE) excitation and the three-dimensional solutions considered appear to approach the analytical solution in the limit of zero slab thickness.     

Shielding and scattering analysis of lossy cylindrical shells usingan extended multifilament current approach

An investigation of the multifilament current method's (MFCM) ability to solve electromagnetic scattering and/or absorption problems involving inhomogeneous cylindrical structures is presented. Dielectric cylinders of arbitrary cross section covered by multiple layers of lossy dielectrics are considered. Both cases of wave incidence, TM and TE, are treated. Like some other moment-method solutions, the MFCM experiences numerical difficulties when dealing with a medium characterized by a high loss tangent or large electrical conductivity. To overcome this problem, a new boundary condition based on an impedance matrix equivalent circuit approach that accounts for the curvature of the surfaces has been developed. This new impedance matrix boundary condition (IMBC) extends the MFCM capability to the analysis of two-dimensional (2-D) structures involving conducting layers with ohmic losses. The usefulness of this extended method is confirmed by the study of metallic shells for which a strong energy coupling with the incident electromagnetic (EM) field is demonstrated at their structural resonance frequencies     

Impedance, bandwidth, and Q of antennas

To address the need for fundamental universally valid definitions of exact bandwidth and quality factor (Q) of tuned antennas, as well as the need for efficient accurate approximate formulas for computing this bandwidth and Q, exact and approximate expressions are found for the bandwidth and Q of a general single-feed (one-port) lossy or lossless linear antenna tuned to resonance or antiresonance. The approximate expression derived for the exact bandwidth of a tuned antenna differs from previous approximate expressions in that it is inversely proportional to the magnitude |Z'/sub 0/(/spl omega//sub 0/)| of the frequency derivative of the input impedance and, for not too large a bandwidth, it is nearly equal to the exact bandwidth of the tuned antenna at every frequency /spl omega//sub 0/, that is, throughout antiresonant as well as resonant frequency bands. It is also shown that an appropriately defined exact Q of a tuned lossy or lossless antenna is approximately proportional to |Z'/sub 0/(/spl omega//sub 0/)| and thus this Q is approximately inversely proportional to the bandwidth (for not too large a bandwidth) of a simply tuned antenna at all frequencies. The exact Q of a tuned antenna is defined in terms of average internal energies that emerge naturally from Maxwell's equations applied to the tuned antenna. These internal energies, which are similar but not identical to previously defined quality-factor energies, and the associated Q are proven to increase without bound as the size of an antenna is decreased. Numerical solutions to thin straight-wire and wire-loop lossy and lossless antennas, as well as to a Yagi antenna and a straight-wire antenna embedded in a lossy dispersive dielectric, confirm the accuracy of the approximate expressions and the inverse relationship between the defined bandwidth and the defined Q over frequency ranges that cover several resonant and antiresonant frequency bands.     

Wire-junction matrix model for the TLM method

The additional capacitance and inductance introduced by a conducting wire can be modelled in the transmission line modelling (TLM) method by a special node. A symmetrical transmission-line structure is proposed for the modelling of wire junctions in a coarse mesh. The model is tested by calculating the response of a wire-cross scatterer and the RCS of a perfectly conducting plate simulated by means of a square wire mesh. Although, initially, TLM was not developed for solving wire structures, the results are in good agreement with the theoretical and moment-method solutions traditionally used for this type of structure     

Infinite planar arrays of arbitrarily bent thin wire radiators

A plane wave expansion moment method is presented for computing active impedances and current distributions of infinite planar arrays of thin wire radiating elements. The array lattices can be rectangular or triangular. The excitations can be plane waves or progressively phased voltage sources. Each radiating element, including feedline, can be any collection of bent thin wires. Results are given for arrays of straight dipoles with straight feedlines, straight dipoles with bent feedlines, and swept back dipoles (arms inclined with respect to the array planes) with straight feedlines. The experimentally observed phenomenon of array blindness as a consequence of feedline scattering is verified theoretically. The absence of this effect when the dipole arms are inclined with respect to the array plane also is verified.     

An implanted spherical head model exposed to electromagnetic fields at a mobile communication frequency

Can cellular phones and personal communication systems base station antennas affect the active or passive implantable medical devices adversely? Concerns over the possible harmful effects of nonionizing irradiaton upon implanted medical devices have been present for many years. Key issues to address are the questions of whether mobile phones have a detrimental effect on implants, and how the interaction of the handset with the body can be minimized in order to both alleviate public fears and improve handset antenna performance and new implant designs. This paper presents a thorough investigation of the scattering of an electromagnetic (EM) wave from a perfectly conducting implant (a cylindrical wire and a very thin cylindrical disk) of electrically small radius (of resonant length), embedded eccentrically into a dielectric spherical head model by a dipole antenna (0.4 wavelength) at 900 MHz. The dyadic Green's function (DGF) for spherical vector wave functions is employed. Analytical expressions for the scattered fields of an implant embedded head model is obtained. Numerical results from analytical expressions are computed for this problem and then compared with the results from the same model using the finite-difference time-domain, EMU-FDTD electromagnetic simulator. Good agreement is observed between the analytical results on the proposed method in comparison with the FDTD method.     

Scattering from multiple conducting and dielectric bodies ofarbitrary shape

A simple moment-method solution is presented for the problem of electromagnetic scattering from structures consisting of multiple perfectly conducting and dielectric bodies of arbitrary shape. The system is excited by a plane wave. The surface equivalence principle is used to replace the bodies by equivalent electric and magnetic surface currents, radiating into an unbounded medium. A set of coupled integral equations, involving the surface currents, is obtained by enforcing the boundary conditions on the tangential components of the total electric and magnetic fields. The method of moments is used to solve the integral equations. The surfaces of the bodies are approximated by planar triangular patches, and linearly varying vector functions are used for both expansion and testing functions. Some of the limitations of the method are briefly discussed. Results for the scattering cross sections are presented. The computed results are in very good agreement with the exact solutions and with published data     

High-frequency analysis of EM scattering from a conducting spherecoated with a composite material

The asymptotic high-frequency solution for the scattered field produced by a plane wave incident on a perfectly conducting sphere coated with a thin composite material is considered. In the shadow region, the high-frequency scattered field is entirely associated with the usual creeping-wave diffraction. In the lit region, the scattered field can be expressed as a sum of geometrical optics (GO) and creeping-wave diffraction terms. The field in the caustic regions, where the ray solution fails, is excluded. The appropriate formula for each term is derived, and the result is presented in a form suitable for computation. Numerical results for the bistatic scattering patterns of the coated sphere show excellent agreement with the rigorous eigenfunction solutions     

Shielding properties of an ensemble of thin, infinitely long,parallel wires over a lossy half space

A determination of the total electric field produced by an ensemble of thin, infinitely long, parallel wire scatterers over a flat lossy half space illuminated by a plane wave whose polarization is parallel to both the wires and the interface is presented. By invoking the thin wire idealization, a matrix equation is obtained for determining the currents on each wire, from which the total electric field is obtained. Several plots are given to show how the wires' radii, the earth's conductivity, the incident angle, and the total number of wires affect the shielding for a semicircular shell. In many cases, it is demonstrated that the shielding effectiveness can be as much as 70 dB; for other cases, when the structure is of resonant dimensions, the shielding can be degraded to 20 dB. It is also shown that a wire grid model can give shielding results similar to those of a continuous perfectly conducting structure of similar dimensions     

Analysis of a Transmission Cavity Wavemeter

A section of transmission line partially closed off at each end constitutes a cavity wavemeter. If fixed in length, it may be used as a reference cavity; or if tunable, it maybe used to determine frequency. Such a cavity is here treated systematically as a lossy transmission line, with the two end couplings either lossless or symmetrical. The analysis is by means of the transfer or wave matrix. Various expressions are derived which have previously not been obtained, or for which only approximate expressions have been derived from "equivalent circuits."     

Analytical aspects of plane wave illuminated thin wires and slits

The paper deals with simple elementary approximations for the current and charge response on different straight wire structures, dipoles and short slits in the receiving case. After proof that transmission line equations are also valid for single wires without discontinuities, these equations are formulated including the incoming wave. They turn out have simple particular solutions that could be expected for the case, when the electric field is parallel to the wire, but holds true for the general case too. Satisfying the boundary condition at discontinuities (wire ends, lumped elements) gives rise to additional waves appearing as solutions of the homogeneous wave equation. The formulation of currents along and voltages across a slit, including an illuminating magnetic field at one side of the screen, leads again to transmission-line type equations and, consequently, to the inhomogeneous wave equation. As slits in screens are usually small in terms of wavelength, an approximative solution for the short slit will do. For this case, even closed-form expressions are possible for the magnetic near field