Transient propagation in anisotropic laminated composites

A detailed analysis of the transient response of an electromagnetic pulse (EMP) and a Gaussian pulse in anisotropic laminated composites is presented. To this end, both the equivalent transmission line circuit (ETLC) model and the finite-difference-time-domain (FDTD) method are adopted in the time-domain analysis. Numerical results are presented for graphite/epoxy laminates, based on a model which treats each lamina as a homogeneous and anisotropic sheet. The factors that influence the transient response of anisotropic laminated composites, such as laminate thickness, fiber orientation, and the angle, frequency contents, and polarization of the incident wave, are also investigated     

Transient reflection properties of a dispersive and lossy bi-isotropic slab with an anisotropic laminated composite backing

This paper presents a detailed analysis of the transient reflection from a general bi-isotropic slab backed by a laminated composite. Either a nonreciprocally Tellegen medium or a reciprocally dispersive chiral medium with losses is considered as the bi-isotropic slab. The laminated composite is a graphite-fiber-reinforced-plastic composite (GFRPC), which has been widely used as a replacement for metals and may be treated as a multilayered anisotropic medium from the electromagnetic point of view. For the analysis, a theoretical model is developed based on a state-equation approach. On the basis of this model, both the time-domain and the frequency-domain plane-wave reflection results are obtained and studied. The factors affecting the results, such as the material parameters of the bi-isotropic slab and the fiber-orientation pattern of the GFRPC, are investigated in detail. Some special reflection properties, such as the twist-polarizer reflection and the antireflection, are observed based on this investigation. The properties may lead to a novel transient absorber technology for electromagnetic compatibility (EMC) applications.     

Full-wave high-order FEM model for lossy anisotropic waveguides

Anisotropic lossy waveguides are analyzed by applying the finite-element method with higher order interpolatory vector elements. The problem is formulated in terms of the electric field only. The transverse vector component of the electric field is numerically represented by higher order curl-conforming interpolatory vector functions, whereas the longitudinal component of the field is represented by higher order scalar basis functions. Due to the better interpolatory capabilities of the expansion functions, the metallic and material losses are modeled with a higher precision with respect to that provided by the other available numerical models. Furthermore, the use of higher order elements permits the correct modeling of the discontinuity of the normal field component at the interfaces between different materials     

EM propagation and backscattering discrete model for medium of sparsely distributed lossy random particles

Electromagnetic wave propagation and backscattering from a random medium are studied. The random medium is modeled by discrete lossy dielectric scatterers, for which the dyadic scattering amplitudes and orientation statistics are known. A method is developed to compute the propagation and backscattering coefficients. The technique is valid for scatterers having characteristic dimensions comparable to a wavelength. The procedure is valid when the albedo of individual scatterers is small, that is, when the scatterers are highly absorbing. Numerical calculations for the propagation and backscattering coefficients are presented, and compared with the literature.     

The propagation of bright and dark solitons in lossy optical fibers

An analytic perturbation solution to the nonlinear Schrodinger equation with loss Γ for both normal and anomalous dispersion is developed. Explicit results are obtained through second order in the perturbation Γ. The results show that the dark pulse spreads less rapidly than the bright one and that total spreading as well as the difference in spreading rate for the two types of pulses decreases with loss. Comparisons are made with a zeroth-order perturbation theory and with numerical simulations, which are found to bracket the second-order results     

Approximate solution for propagation modes on lossy multiconductor transmission lines in inhomogeneous media

The letter gives an approximate solution for wave propagation on a system of lossy multiconductor transmission lines with small losses. The solution leads to a general expression for the attenuation factor matrix. The expression gives a correct result for the cases of two and three conductors and can be used for any number of conductors.     

An anisotropic turbulence model for wave propagation near the surface of the Earth

A model of turbulent anisotropy of refractive index fluctuations near the surface of the earth is presented and used to calculate the mutual coherence function (MCF) of a propagating plane wave. it is found that there are measurable differences in the transverse (horizontal and vertical) MCF's thus making possible active remote sensing of turbulent anisotropy.     

A difference equation for transient signal propagation in cold homogeneous lossy isotropic plasmas

An extremely simple difference equation for the solution of transient signal propagation in a cold homogeneous lossy isotropic plasma has been found. This difference equation is derived from an integro-differential equation for the electric field by a finite difference method. The solution is generated in a recursive manner in time.     

Pulse propagation in lossy media using the low-frequency window for video pulse radar application

Pulse propagation in lossy media is considered. The frequency band is selected to lie in the low-frequency window (LFW) as defined by Gabbilard et al [5]. The antennas chosen are finite-length dipoles in electrical contact with the lossy media. The input impedance of these dipoles is observed to be approximately a constant resistance over the wide frequency band of interest thus making a broad-band match practical. It is shown that when propagation is in the LFW the input pulse propagates over distances of many kilometers with little distortion. Attenuation data ate presented as universal curves for the pulse amplitude generated at the terminals of a parallel receiving dipole immersed in the same-medium. The original application of these results is the evaluation of the limitations imposed by propagation losses and dispersion of broad-band video pulse-radar systems for deep probing in the earth. A potential video pulse-radar system for deep probing is discussed.     

The propagation of wave packets in inhomogeneous anisotropic media with moderate absorption

Hamilton's equations for geometric optics contain the quantity ∂ω/∂k which is complex in absorbing media. To investigate its physical meaning, the field of a wave packet (a pulsed beam) in a homogeneous medium with moderate absorption is calculated using the saddle-point method. The packet velocity v is approximately equal to the real part of ∂ωS/∂kSwhere kS, ωSare the saddle-point values of the propagation vector and the angular frequency. The imaginary part of ∂ωS/∂kScan physically be interpreted in terms of v and derivatives of Re kSċ σ, Re ωSτ in the direction of v, where σ^1/2and τ^1/2are beam-width and pulse duration, respectively.     

对"四论注入光敏器件的物理基础"一文的商榷

利用受光PN 结伏安特性方程,分别对受光PN 结的光电流输出、注入电流输出和零电流输出三种情况进行了讨论,进一步对注入光敏器件的物理基础进行探讨,并对一些实验问题进行讨论。     

A comment on "The critical transmitting range for connectivity in sparse wireless ad hoc networks"

In a previous paper P. Santi and D. Blough (see ibid., vol.2, no.1, p.25-39, Jan.-Mar. 2003) presented a number of results concerning the asymptotic connectivity of wireless ad hoc networks. This comment includes fixes to several of that paper's results.     

A FDTD scheme for the computation of VLF-LF propagation in the anisotropic earth-ionosphere waveguide

Due to the presence of the natural magnetic field, the ionosphere surrounding the earth is a gyrotropic medium. This paper presents a finite-difference time-domain scheme that can deal with such an anisotropic medium, allowing the propagation of VLF-LF radiowaves to be computed in the waveguiding structure composed of the earth surface and the ionosphere. The numerical scheme is described in detail, with a special emphasis on the problem of the numerical stability.     

Novel model for propagation loss prediction in tunnels

Radio signal propagation in a tunnel exhibits distinct near and far regions with quite different propagation characteristics. This paper proposes a model that can distinguish these propagation regions and predict their respective propagation losses in the tunnel. The model relies on a break point to separate the propagation regions and a hybrid technique to calculate the propagation losses. The location of the break point is determined with the solution of a novel tunnel-propagation equation for the first time. The solution shows that the location of the break point depends strongly upon frequency, antenna position, and tunnel transversal dimensions. The model is compared with data measured in various tunnels at different frequencies (900 MHz, 1.8 GHz, and 2.448 GHz). The results show reasonable agreement between predictions and measurements.     

A model for electromagnetic propagation in the lithosphere

The propagation properties of a crystal waveguide are derived using a model where dielectric constant and couductivity increase with depth according to a simple algebraic relation. The model is more realistic and consistent with currently available geological information and with reasonable extrapolations of crustal properties than are previously treated models. The mathematical analysis, moreover, is straightforward and rigorous, leading to an explicit analytical solution in the form of a mode expansion, the individual terms of which can be written in closed form. Numerical evaluation of the model leads to a total attenuation rate (for the dominant mode) of 0.256 dB km for a frequency of 1 kHz. A geologically critical attribute of the model is a positive temperature gradient with depth as the principal controlling influence on the electric profile. A temperature gradient lower than the one used in constructing the model would significantly reduce the attenuation rate.     

Comments on the hybrid model for propagation loss prediction in tunnels

The hybrid model for propagation-loss prediction in tunnels is discussed and its limitations are given. The values from the waveguide model with /spl gamma//sub 2/<2 are applicable for only short distances and for tunnels with low specific attenuation factor (/spl alpha/). In general, /spl gamma//sub 2/ can take any value higher than two.