In this paper, the unique features of periodic magneto-dielectric meta-materials in electromagnetics are addressed. These materials, which are arranged in periodic configurations, are applied for the design of novel EM structures with applications in the VHF-UHF bands. The utility of these materials is demonstrated by considering two challenging problems, namely, design of miniaturized electromagnetic band-gap (EBG) structures and antennas in the VHF-UHF bands. A woodpile EBG made up of magneto-dielectric material is proposed. It is shown that the magneto-dielectric woodpile not only exhibits band-gap rejection values much higher than the ordinary dielectric woodpile, but also for the same physical dimensions it shows a rejection band at a much lower frequency. The higher rejection is a result of higher effective impedance contrasts between consecutive layers of the magneto-dielectric woodpile structure. Composite magneto-dielectrics are also shown to provide certain advantages when used as substrates for planar antennas. These substrates are used to miniaturize antennas while maintaining a relatively high bandwidth and efficiency. An artificial anisotropic meta-substrate having /spl mu//sub r/>/spl epsiv//sub r/, made up of layered magneto-dielectric and dielectric materials is designed to maximize the bandwidth of a miniaturized patch antenna. Analytical and numerical approaches, based on the anisotropic effective medium theory (AEMT) and the finite-difference time-domain (FDTD) technique, are applied to carry out the analyzes and fully characterize the performance of finite and infinite periodic magneto-dielectric meta-materials integrated into the EBG and antenna designs.

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Magneto-dielectrics in electromagnetics: concept and applications

ed intelligence signals. 7.0 Russian Systems There has been extensive development of UWB systems and subsystems in the former Soviet Union and the present Russian Federation. The work arose out of programs to improve power systems in the 1950s. Even at this stage, the difference was noted between conventional continuous wave signal description methods and ultrashort pulse methods (e.g., Zernov, 1951). The simplicity of the methods of time domain analysis for short pulse UWB signals, as opposed to continuous, steady state signals, was described by Kharcevitch (1952). Initially, radio pulses of nanosecond duration were generated using traveling wave tube modulation (Astanin & Kardo-Sysoev, 2000). In 1957 Astanin at the A. Mozjaisky Military Air Force Academy developed an X-band 0.5 nanosecond duration transmitter for waveguide study. A receiver-correlator with a T-bridge waveguide and mechanically controlled delay was used (Astanin, 1964). At the same time, at the Radioelectronics Institute of the USSR Academy of Science, Kobzarev and collaborators conducted tests on indoor ranges of ultrashort pulse high resolution radars. These constituted the first stage of development of UWB systems in Russia (Astanin & KardoSysoev, 2000). The next stage of development utilized fast semiconductor switches, beginning with Shatz (1963), and continuing at the Ioffe Physico-Technical Institute (see below). As in the United States, progress was facilitated by the availability of fast sampling

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History of UltraWideBand (UWB) Radar & Communications: Pioneers and Innovators

An aspect independent radar target discrimination scheme based on the natural frequencies of the target is considered. An extinction-pulse waveform upon excitation of a particular conducting target results in the elimination of specified natural modal content of the scattered field. Excitation of a dissimilar target produces a noticeably different late-time response. Construction of appropriate extinction-pulse waveforms is discussed, as well as the effects of random noise on their application to thin cylinder targets. Also presented is experimental verification of this discrimination concept using simplified aircraft models.

Radar target discrimination using the extinction-pulse technique

A hybrid formulation which combines the method of moments (MM) with the finite element method (FEM) to solve electromagnetic scattering and/or absorption problems involving inhomogeneous media is discussed. The basic technique is to apply the equivalence principle and transform the original problem into interior and exterior problems, which are coupled on the exterior dielectric body surface through the continuities of the tangential electric field and magnetic field. The interior problem involving inhomogeneous medium is solved by the FEM, and the exterior problem is solved by the MM. The coupling of the interior and exterior problems on their common surface results in a matrix equation for the equivalent current sources for the interior and exterior problems. Combining advantages of both methods allows complicated inhomogeneous problems with arbitrary geometry to be treated in a straightforward manner. The validity and accuracy of the formulation are checked by two-dimensional numerical results, which are compared with the exact eigenfunction solution, the unimoment solution, and Richmond's pure moment solution. >

Coupling of finite element and moment methods for electromagnetic scattering from inhomogeneous objects

A classification technique focused on the identification of buried unexploded ordnance (UXO) using complex natural resonance (CNR) signature is considered. The total least square (TLS) Prony's (1795) method is used to extract CNRs from time-domain data, Full-scale UXO computational models and the body of revolution moment method (BORMM) code is used to obtain the backscattered fields, which are then used to give the theoretical free-space CNRs. Baum's (1993) transformation is used to relate a CNR in a lossy simple medium to the corresponding CNR in free space. A practical UXO classification yields an estimate of the UXO length from its CNR information. Successful UXO classification examples from actual measured data are presented.

Buried unexploded ordnance identification via complex natural resonances

An analytical formulation is presented for the computation of scattering and transmission by general anisotropic stratified material. This method employs a first-order state-vector differential equation representation of Maxwell's equations whose solution is given in terms of a 4 \times 4 transition matrix relating the tangential field components at the input and output planes of the anisotropic region. The complete diffraction problem is solved by combining impedance boundary conditions at these interfaces with the transition matrix relationship. A numerical algorithm is described which solves the state-vector equation using finite differences. The validation of the resultant computer program is discussed along with example calculations.

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Electromagnetic scattering by stratified inhomogeneous anisotropic media

A unified treatment of the natural mode representations for induced currents and scattered fields is obtained by use of fundamental concepts regarding causality and superposition. The transient scattered field response is shown to have the form of a constant coefficient complex exponential sum only in the "late-time," after the last driven reponse is received from the object. Prior to this, the "early.time" response is found to be due to direct physical optics fields as well as a sum of temporally modulated natural modes produced by the progressive illumination of the incident wavefront. Alternate representations and s -plane behaviors are considered. The implications of these results on natural resonance target identification schemes are discussed.

Singularity expansion representations of fields and currents in transient scattering

Eigenmode expansions are used for computing the currents on a dielectric loaded top-hat monopole radiating above an infinite conducting ground plane. The approach is facilitated by adding a parallel ground plane above the antenna, thus allowing use of cylindrical harmonic field expansions in each of three regions. Expansion coefficients are found by enforcing boundary and continuity conditions at conducting surfaces and regional interfaces. Convergence and accuracy are assessed using comparisons with three other methodologies, namely an integral equation solution, a quasi-static analysis, and multi-octave experimental measurements. >

Eigenmode analysis of dielectric loaded top-hat monopole antennas

A novel method for computing the input impedance and induced currents on cylindrical antenna structures is investigated. This technique considers the antenna when placed between two parallel ground planes, thus allowing the use of cylindrical harmonic Fourier expansions to represent the fields. Enforcement of boundary and continuity conditions on the tangential field components results in solutions for the discrete spectral coefficients of these expansions. Convergence is discussed and computational validations are presented for varied cases. >

Computation of monopole antenna currents using cylindrical harmonics

A new procedure is described for the solution of electromagnetic scattering problems in unbounded regions. Using this technique a spatial region enclosing the scatterer is initially decoupled from the exterior region and the fields therein are considered as the solution of an interior Dirichlet boundary value problem. The interior region solution is then recoupled to the unbounded exterior region by use of the equivalence principle. Such a process can be symbolically represented as a simple feedback system. The formulation is demonstrated for the case of plane wave scattering by finite metallic circular cylinders. A finite element solution of the interior problem is utilized in this example in conjunction with a field representation using a special case of coupled azimuthal potentials.

M.A. Morgan

Papers

Electromagnetic scattering by stratified inhomogeneous anisotropic media

Singularity expansion representations of fields and currents in transient scattering

Eigenmode analysis of dielectric loaded top-hat monopole antennas

Computation of monopole antenna currents using cylindrical harmonics

The field feedback formulation for electromagnetic scattering computations