The scattering behavior of a slightly rough surface moving parallel to its mean plane with uniform velocity

The scattering of plane electromagnetic (EM) waves from a perfectly conducting, slightly rough surface moving in vacuum with uniform and constant velocity is examined by combining the boundary perturbation method and the special, homogeneous Lorentz transform. The direction of motion is assumed to be parallel to both the incidence plane and the mean plane of the moving rough surface. The analysis is confined to first-order scatter. The end result is the determination of the velocity-dependent expressions for the bistatic and backscatter incoherent cross sections per unit area of the scattering surface. The expressions for bistatic geometry are only valid for scattering inside the plane of incidence. A numerical example demonstrates the effect of the motion on the scattering behavior of the rough surface.     

The reciprocity and power conservation principles for boundaryconditions on a semitransparent surface

Generalized boundary conditions of coupling on a semitransparent surface are valid only if the Lorentz reciprocity theorem and the electromagnetic power conservation principle are satisfied on the surface with these boundary conditions. These criteria are reformulated as relations between the tensors in the boundary conditions. Some known boundary conditions of coupling are considered     

Deterministic modeling of the (shielded) outdoor power line channel based on the multiconductor transmission line equations

In this paper, a complete methodology is proposed for the deterministic modeling of the channel transfer functions associated with underground power line access networks, in the light of the Multiconductor Transmission Line Theory. Some multiconductor power line cables are analyzed in detail, and their fundamental propagation characteristics (definition of eigenmodes, phase velocity, characteristic impedance, lineic attenuation) are related to the geometry of the conductors and the material properties. A multidimensional scattering matrix formalism is then introduced to perform an accurate analysis of the global power line network including multiconductor cable segments, derivation points, and termination loads. An illustrative example of a power line access network with three types of cables (with different numbers of conductors), 20 derivations, and five remote terminals, is finally proposed.     

Incremental length diffraction coefficients for arbitrarycylindrical scatterers

Convenient expressions are derived for incremental length diffraction coefficients (ILDCs) in terms of the far fields of arbitrary cylindrical canonical scatterers composed of linear electromagnetic material. The derivation of these general expressions for ILDCs is based on a surface-current equivalence theorem that states that the electromagnetic fields outside cylindrical sources can be generated to any degree of accuracy by localized electric and magnetic surface currents that lie in a single plane within the source region. This equivalence theorem is proven with the help of cylindrical wave expansions and the Kottler-Franz formulas. It combines with a general even and odd decomposition of cylindrical electromagnetic fields to allow the use of previous formulas for planar surface current ILDCs in the derivation of the general expressions for ILDCs in terms of cylindrical far fields     

The effect of material stresses on electromagnetic boundary conditions

The appearance of a layer of charge or current on the boundary of a material usually implies that a discontinuity in an electromagnetic field occurs at that boundary. The converse is not necessarily true. The reaction of the electromagnetic field to the charge or current layer is a stress communicated across the boundary. Determination of the charge or current on a boundary must take all the stresses and electromechanical interactions at the boundary into account. Calculating surface currents or charges derived from electromagnetic field quantities alone, ignoring mechanical stresses, may lead to wrong answers at material boundaries. In short, the occurrence of charge or current on material in the presence of an electromagnetic field is intimately related to momentum conservation. A postulate equivalent to identifying the Poynting vector with energy flow is introduced, linking surface currents and charges to momentum balance at the surface. A formally correct method for calculating surface currents and charges based on momentum balance is described.     

On Einsteinian velocity addition and the notion of phase velocity

It is shown that conventional notions of the electromagnetic phase velocity in moving media are in general, inconsistent with the Einsteinian velocity addition. A new vector quantity, normalized phase slowness, is introduced and shown to be consistent with both the Lorentz transformation and the Einsteinian law of velocity addition.     

材料磁性对电磁理论中菲涅尔公式的影响

本文考虑材料的磁性,利用电磁场的边值关系研究磁导率对菲涅尔公式的修正。文中利用所定义的介质界面矢量将电磁场量正交分解,统一处理垂直和平行于入射面的场分量关系,提供推导菲涅尔公式的另外方法,并对结果进行深入讨论。结果表明,材料磁性对菲涅尔公式的修正可以由两种材料的相对磁导率之比描述。     

Dispersion equations in moving media

The derivation of dispersion equations for electromagnetic waves in moving media is considered. Absorption is included. It becomes clear that conduction current or polarization current models lead to identical dispersion equations in moving media. The invariance of dispersion equations is defined and discussed. The question of velocity induced wave modes in moving media is considered, and conditions for appearance of these new modes are stated.     

Unusual boundary conditions at an interface [EM theory]

The derivation of electromagnetic boundary conditions at an interface by applying Gauss' divergence theorem to a flat pillbox or Stokes' curl theorem to a narrow contour is considered for two types of unusual boundary condition. One is an electric field discontinuity and the other is a double layer of current. It is suggested that the usual continuity proofs, based on pillboxes and contours of vanishing dimension, must be handled with care. Specifically, a `δ function' behavior of certain components along the normal is possible     

Waves in a moving medium with finite conductivity

Solutions for the steady-state behavior of TEM, TE, and TM waves in an infinite conducting medium moving at a uniform velocity are presented, along with relations between the time average power transmitted through the medium, power dissipated in the medium, and power delivered to an external mechanical load that maintains the medium at a constant velocity. Exact solutions valid at any velocity are obtained using the Lorentz transformation. It is shown that the TEM and TE modes describe relativistic linear induction motors or ac MHD generators in which the usual J × B force is the dominant force mechanism. The TM mode describes a class of electroconvection devices in which the electric field acting on the induced surface charge on the interface between the medium and the air gap is the dominant force mechanism. Both the TE and the TM modes are shown to have efficiencies and force generation capabilities which are independent of the absolute value of velocity.     

Tapping of Love Waves in an Isotropic Surface Waveguide by Surface-to-Bulk Wave Transduction

A theoretical study of tapping a Love wave in an isotropic microacoustic surface waveguide is given. The surface Love wave is tapped by partial transduction into a bulk wave at a discontinuity. It is shown here that, by careful design of the discontinuity, the converted bulk wave power and the radiation pattern may be controlled. General formulas are derived for the calculation of these important characteristics from a relatively general surface contour deformation.     

Photon/magnon conversion near a material interface

Direct coupling between electromagnetic waves and spin waves caused by an abrupt discontinuity in material parameters is studied. It is shown that the simultaneous presence of an evanescent and propagating wave can lead to the conversion of electromagnetic power to exchange power. The partial conversion of an electromagnetic wave travelling in air, and incident normally upon a ferromagnet, is analysed in detail for the case when the material is uniformly magnetised to saturation along the direction of propagation.     

Uniform diffraction solution for a discontinuity in curvature

A uniform diffraction solution is given for an electromagnetic wave incident on perfectly conducting surface having a discontinuity in curvature. The solution is derived from an asymptotic evaluation of the physical-optics integral, and correction factors are deduced to retrieve the rigorous nonuniform result away from the reflection boundary.     

On Guided Waves in Moving Anisotropic Media

Based upon the Maxwell-Minkowski theory, the equations governing the propagation of electromagnetic waves in a cylindrical waveguide of an arbitrary cross section filled with a moving anisotropic medium are derived. The governing equations are reducible to a pair of coupled wave equations in the axial components of the electric and magnetic fields, which in turn can be solved through the solution of a single second order scalar homogeneous Hehnholtz equation. For a general anisotropic medium no pure TM or TE modes can exist in the waveguide. However, if the moving medium is uniaxially anisotropic, TM and TE modes are possible. It is interesting to note that the cutoff frequencies are always lowered by a factor which depends upon the velocity of the medium and is independent of the guide geometry. The formulas for the characteristic wave impedance and power flow in a waveguide for a moving uniaxial medium, if expressed in terms of the new cutoff frequency, have the same forms as those for a moving isotropic medium. The propagation characteristics of waveguides of rectangular and circular cross sections filled with a moving uniaxial gyroelectric medium are discussed.     

A study of the mechanism of sea surface scattering

The mechanism by which electromagnetic waves are scattered from the sea surface is studied herein. Principal applications for such studies occur in over-water communication channels and radar. Predictions based on rigorous formulations for diffraction from a (moving) time-space periodic surface are correlated with the experimental results of other authors. Such predictions explain the existence of strong coherent scattering from the sea surface in nonspecular directions (grating lobes) and also anticipate the appearance of well-defined, harmonically related, frequency shifts (time-space harmonics) in scattered signals observed in these experiments. On the basis of such manifestations of the periodic nature of the sea surface, a model is formulated for use in calculation of the angular power spectrum of scattered electromagnetic waves. This model, accounting for spatial coherence effects, assumes for the fluctuation of heights on the water surface a Gaussian distribution with a narrow band-pass (nearly sinusoidal) spatial spectrum. Calculations of the angular, scattered spectrum due to this model show strong predominantly coherent wave summation in directions corresponding precisely to the grating lobes of a purely periodic structure of period equal to the center period of the band-pass spectrum. Spectral (angular) spreading increases with increased bandwidth of the spatial spectrum of the surface. Calculations for the present study are made on the basis of a perfectly reflecting boundary and hence do not display the proper distinction of polarization expected for a completely realistic model. The results, however, display the spectral charactral characteristics necessary to explain strong scattering in nonspecular directions by a random surface.     

Experimental study on surface quality of silicon ingots prepared by electromagnetic continuous casting

The effects of technical parameters on the forming of surface defects of the silicon ingots prepared by electromagnetic continuous casting were investigated by the orthogonal experiment, and the corresponding defect formation mechanisms were discussed and established. The results indicate that the forming of the ripple and lap are mainly caused by the shrinkage of shell and controlled by the electromagnetic pressure, and the unmelted granules are those moving from the center to the edge on the liquid surface, which was directly cooled by the cold crucible wall and kept as unmelted state. The surface quality was shown to be significantly improved by increasing the input power, pulling velocity and the heat preservation time, and among them, the input power is the most important one.     

Waveguides Containing Moving Dispersive Media

Perturbation formulas are derived for the changes in the dispersion curves and phase velocity for the modes in an arbitrary composite waveguide structure containing dispersive media in relative motion. The formulas are also valid when the media are fluids with arbitrary velocity distributions. It is shown that the relativistic transformation laws for the frequency and wave vector of uniform plane waves are also valid for waveguide modes provided that all moving media that make up the guide move with the same velocity. There are also difficulties when the moving media are dispersive. In general, one most therefore obtain the dispersion relation directly from the field equations or from the perturbation formulas. An example involving a simple surface wave along the interface of a moving plasma and a dielectric is worked out by both methods. As an interesting side result, it is found that plane waves in an unbounded isotropic plasma have phase velocities independent of the motion of the plasma.     

The deformation of the moving magnet plate of a commutated magnetically levitated planar actuator

This paper presents a method to calculate the deformation of the magnet plate of a commutated magnetically levitated planar actuator using a linked electromagnetic–mechanical model. The force and torque distribution on the moving magnet array is obtained from an electromagnetic model based on the surface charge method and the Lorentz force and torque integral. The mechanical model is a state-space model derived from FEM. This mechanical model uses the force and torque distribution to determine the deformation of the magnet plate during movement due to the commutated coil set.     

Surface losses in transmission lines

Two derivations of the loss parameter R, entirely from wave considerations in the presence of a metallic interface, are presented. R, which represents surface losses, occurs in transmission line equations for propagation of the voltage across the plates and of the current in the plates. Explanation of the surface losses involves some radiation of the interior fields into the metallic surfaces that absorb electromagnetic energy, and that ultimately convert it into ohmic losses (heat). Although the approach may restrict the derivation to a planar interface, i.e., to a parallel-plate transmission line, that restriction is not so essential that it cannot be softened     

Time-derivative Lorentz material model-based absorbing boundarycondition

A physically realizable electromagnetic absorber that is constructed from a time-derivative Lorentz material (TD-LM) model for the polarization and magnetization fields is used to define an absorbing boundary condition (ABC) for the finite-difference time-domain method. This Maxwellian ABC is shown to have performance characteristics that are comparable to the non-Maxwellian-Berenger (see J. Comput. Phys., vol.114, p.185-200, Oct. 1994) perfectly matched layer (PML) ABC