Radiation from a uniformly moving distribution of electric charge in an anisotropic, compressible plasma

The radiation characteristics of a linear distribution of electric charge moving with a uniform velocity in a homogeneous electron plasma of infinite extent are investigated for the case in which a uniform static magnetic field is impressed externally throughout the medium. The linear distribution of charge and its direction of motion are assumed to be parallel and perpendicular, respectively, to the direction of the external magnetic field. Of the two possible modes of waves of small amplitude, namely, the modified electromagnetic mode and the modified electron plasma mode, the uniformly moving charge distribution excites the modified electron plasma mode. The emitted radiation has no frequencies less than the plasma frequency. For a particular value of the ratio of the gyrotropic to the plasma frequency of the electrons, the frequency and the angular spectrum of the emitted radiation are determined for two values of the velocity of the charge.     

Radiation From a Uniformly Moving Charge in an Anisotropic Plasma

The radiation from a point charge moving uniformly in a plasma is investigated when the charge is moving in the direction of an external magnetic field. In general there are two modes, for each of which all the components of the electric and magnetic field are present. The two parameters of interest in this problem are the ratio u/c/sub 0/ of the velocity of the charges to the free-space velocity of electromagnetic waves and the ratio R of the gyromagnetic frequency to the plasma frequency of the electrons. For two sets of values of these parameters the frequency and the angular spectrum of the emitted radiation are obtained. In certain cases, as many as three Cerenkov rays are found to propagate in the same direction; these multiple rays, however, correspond to different frequency components and to different modes of propagation. The motivation for this investigation is indicated briefly.     

Radiation resistance of an oscillating dipole in a moving compressible plasma

The radiation resistance of an electric dipole located in a moving compressible plasma is treated. First, the wave equations for a uniformly moving compressible plasma in the presence of a current source without any restriction upon the magnitude of the velocity are derived. Then, using these wave equations the radiation resistance of the present problem is obtained and its characteristics are investigated numerically. It is shown that if the moving velocity of plasmaupsilon_{0}exceeds the thermal velocity of the electronupsilon_{t}, a region appears in which the plasma wave component of the radiation resistance becomes negative. In this case, the numerical calculation shows that there is a range ofupsilon_{0}where the radiation efficiency of the electromagnetic wave is much improved. The cutoff frequency of the plasma wave in a moving compressible plasma becomes lower than that of the plasma wave in the rest frame of the plasma.     

The Green's Dyadic for Radiation in a Bounded Simple Moving Medium

The studies here show that the wave equation for electromagnetic wave propagation in an isotropic and uniformly moving medium is solvable by the separation method in four coordinate systems. Solutions in the form of complete sets of eigenfunctions are possible for problems where boundary surfaces are presented. A Green's dyadic for finite or semi-infinite domain problems involving sources in the moving medium has been formulated through vector operation on the eigenfunction solutions of the homogeneous wave equation. The case of electromagnetic waves excited by a current loop, immersed in a moving medium, and confined by a circular cylindrical waveguide, was examined. The electric and magnetic field intensities in such a waveguide were compared with those obtained through a different approach. The Green's dyadic for electromagnetic waves in an infinite domain moving medium was shown to be obtainable from the finite domain Green's dyadic through a limiting process.     

Radiation propagating transverse to the external magnetic field from an electromagnetic source in an unbounded plasma

The radiation characteristics of a line source of magnetic current embedded in an unbounded plasma are investigated for the case in which a uniform magnetic field is impressed externally throughout the medium in the direction of the source. The plasma is assumed to be a homogeneous and macroscopically neutral mixture of compressible gas of electrons and ions. A two-fluid continuum theory of plasma dynamics is employed. It is shown that it is possible to define three suitable wave functions which satisfy separately simple wave equations whose solutions are written down by inspection. These wave functions specify the three possible modes which are identified, respectively, to be the modified forms of the electromagnetic, the electron plasma and the ion plasma modes. The limiting behavior of these modes are discussed for the following two cases: 1) infinite source frequency and 2) vanishing external magnetic field. The dispersion relations for the three modes are examined in detail for the general case using a perturbation procedure. It is shown that the modified ion plasma (MIP) mode propagates for all frequencies whereas both the modified electron plasma (MEP) mode and the modified electromagnetic (MEM) mode have a low-frequency cutoff. Explicit expressions for the cutoff frequencies are obtained. The power radiated in each of the three modes is also evaluated. It is found that the power radiated in the MEM mode is always lower than that due to the line source in free space, whereas the power radiated in the two plasma modes is higher than that value for certain ranges of frequency.     

Radiation in a warm plasma from an electric dipole with a cylindrical column of insulation

The radiation in a warm plasma due to an axially oriented electric dipole on the axis of a cylindrical column of insulation of infinite length is studied using the linearized continuum plasma theory, and approximate boundary conditions. Two types of surface waves are excited along the cylindrical column of free space immersed in a plasma and their dispersion and power relations are examined. The uncoupled electromagnetic (EM) and the plasma (P) space waves are excited. The radiation pattern and the normalized radiation resistance of the EM and the P modes are studied both as a function of frequency and as the radius of the insulating column. The effect of the insulating column is found to be very significant only in the case of the P mode. It is found that the power transfer into the P mode becomes less at higher frequencies and larger thickness of the insulation. The results of this investigation may be used to predict in a crude manner the effect of the "ion sheath" that is formed around the antenna in the ionosphere.     

Wave propagation and dipole radiation in a suddenly created plasma

Propagation and radiation of electromagnetic waves from oscillating sources in a suddenly created plasma are studied in this investigation. Field expressions are derived through the use of Laplace transformations. The spatial distribution of sources is taken to be arbitrary but confined. Two cases are considered in detail: 1) plane wave propagation in a source-free region and 2) electric point dipole radiation. In the case of plane wave propagation, various aspects such as wave split, frequency shift, phase and group velocities, amplitude changes, power flows, and energy relations are discussed. In the case of electric dipole radiation, the electromagnetic fields and instantaneous radiated power are calculated and expressed in terms of Lommel functions of two variables. Asymptotic expressions and graphical results of numerical calculations of these quantities are presented. Many interesting properties of the spherical waves and power radiation are discussed.     

Radiation from a vertical dipole in a warm plasma--Part I

The radiation characteristics of a vertical electric dipole which is situated in a homogeneous, warm plasma half-space above a perfectly conducting plane, is considered. The problem is formulated in terms of two vector potential functions, one of which is used to represent the electromagnetic mode, while the other is used to represent the acoustical mode. The potentials are expressed in terms of their Fourier integral representations, and the formal solution is found by the use of signal-flow graphs.     

Using a chromatic-aberration correction system to achieve sub-1.6-nm resolutions of a focused-ion-beam microscope designed for characterization and processing

The resolution of focused-ion-beam (FIB) microscopes, whether analytical or processing ones, is known to be determined by three main factors: (i) the size of a Gaussian (ideal) image of the ion source, (ii) the ion-optical aberrations, and (iii) the scattering of primary ions and secondary particles (both ions and electrons) in the target. This paper shows that each of them can be significantly reduced owing to recent advances in the field. These are (i) innovative field-ionization ion sources with a sub-1-nm effective emission area and improved brightness and ion-energy spread, (ii) the concept of a combined electromagnetic mirror for correcting axial aberrations in ion optics, and (iii) fresh data on the collision of low-energy He⁺ ions with a pure-metal surface and on the deceleration of low-energy O ₂ ⁺ ions in diamond. On this basis, a new concept of FIB microscope is proposed and discussed that is capable of operating in both analytical and processing mode, and offers better resolution by reducing all of the above-mentioned factors. It is shown that the combined electromagnetic mirror proposed enables one to achieve a 1.6-and a 1.5-nm resolution in analytical and processing mode, respectively, when only chromatic aberration is corrected. With perfect axial-aberration correction, a 0.6-and a 1-nm resolution should be attainable in analytical and processing mode, respectively.     

Radiation and scattering from electrically small conducting bodies of arbitrary shape

A simple moment solution is given for low frequency electromagnetic scattering and radiation problems. The problem is reduced to the corresponding electrostatic and magnetostatic problems. Each static problem is solved using the Method of Moments. The surface of the perfectly conducting scatterer is modeled by a set of planar triangular patches. Pulse expansion functions and point matching testing are used to compute the charge density in the electrostatic problem. For the magnetostatic current a set of charge-free vector expansion functions is used. The problem is formulated assuming the scatterer to be in an unbounded homogeneous region. Scatterers of various shapes, such as the circular disc, the sphere, and the cube are studied. Special attention is paid to a conducting box with a narrow slot. The computed results are the scattered fields, the induced charge and current distributions, and the induced electric and magnetic dipole moments. These are in close agreement with whatever published data are available.     

Radiation from a Current Filament Located inside a Cylindrical Frequency Selective Surface

We consider electromagnetic field radiation properties of a current filament placed at the origin of a cylindrical frequency selective surface (CFSS). The CFSS consists of free standing metal strips with two‐dimensional periodicity. The analysis is based on a cylindrical Floquet mode wave expansion technique. We observed that near the half wavelength resonance frequencies, there exist some specific frequencies at which the surface becomes totally transparent.     

涡旋电磁波无线通信技术的研究进展

It is known from electromagnetic momentum that electromagnetic waves can carry Spin Angular Momentum (SAM) related to polarization and Orbital Angular Momentum (OAM) related to the trajectory of the Poynting vector. When OAM is not zero, the wave-front electric field distribution of the electromagnetic wave is vortex-like and has the characteristic of propagating along the axial direction. Therefore, this electromagnetic wave is aptly named vortex electromagnetic wave. Based on the mathematical model of the plane electromagnetic wave field, the researchers introduce a Fourier factor that uses the topological charge (also called mode) of the OAM as a parameter to describe the field of the vortex electromagnetic wave. Therefore, the wave-front of the vortex electromagnetic wave with a “polarization” pattern associated with topological charge, the use of polarization patterns of vortex electromagnetic waves in different modes can further increase the spectrum effect of the wireless communication system. Studies show that although it is feasible to generate "planar" vortex electromagnetic wave beams from Uniform Circular Array (UCA) arrays in an open environment, to obtain modal multiplexing gain, and it is necessary to explore vortex electromagnetic wave beams based on orthogonal phase sequences distributed on a unit circle in the complex plane. At the same time, the paper also investigates the current research status of compatibility between OAM and Multiple Input Multiple Output (MIMO) systems in the field of radio frequency.     

基于电路参数模型的辐射电磁干扰噪声预估方法

针对Hubing电流驱动模型中认为电流在辐射线缆中是均一分布的, 幅值和相位保持不变即未考虑辐射线缆共模电流频率效应给辐射电磁干扰噪声预估带来的误差的问题.文中利用电流传输波动特性建立了辐射线缆长度与共模电流波长为同一数量级时的辐射线缆共模电流分布模型, 并设计电路模型进行测试预估.实验结果表明:采用文中方法预估辐射电磁干扰噪声与Hubing电流驱动模型预估方法相比能提高20.12 dBμV/m, 更加接近标准暗室测试结果, 从而为辐射电磁干扰(Electro Magnetic Interference, EMI)测试与分析提供理论依据.     

Scattering of electromagnetic waves by an anisotropic plasma-coated conducting cylinder

This paper considers the scattering of electromagnetic waves by an infinitely long anisotropic plasma-coated conducting cylinder. The source is assumed to be a magnetic current filament which gives rise to an incident magnetic field with only an axial component. Complete expressions for the scattered electric and magnetic fields are obtained. Scattering by an anisotropic plasma column and that by an isotropic plasma-coated conducting cylinder are special cases of the present problem.     

On the transient radiation of energy from simple currentdistributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method     

Response of Granular La1− xCaxMnO3 Film to 10 GHz and 35 GHz Frequency Electromagnetic Radiation

The nonbolometric response of La_1???xCa_xMnO₃ film to 10 GHz and 35 GHz frequency electromagnetic radiation is investigated in the case when, in addition to the strong electric field of the wave, the film is subjected to a stationary electric bias field. Dependences of responses on the radiation power P at temperature T = 80 K are presented. In the low power region, a linear dependence of the response on P is observed at both frequencies whereas for high powers the dependence behaves as ~P ^1/2. The obtained results are explained taking into account that the nonbolometric response originates from the intergranular junctions that operate in the reverse current regime. There two effects take place: (i) at low powers the detection resistance decreases with increasing power P, and (ii) at higher powers in addition to that the film resistance decreases as P ^1/2 due to the avalanche of charge carriers in the electric field of the electromagnetic wave.     

Electromagnetic Potentials and Field Expansions for Plasma Radiation in Waveguides

In order to calculate the raditaion from plasmas placed in waveguides it is necessary to know the field produced by arbitrarily moving charged particles in a waveguide. In this paper modal expansions for the vector and scalar potentials due to arbitrarily moving charged particles in a waveguide are derived and provide the extension of the Lienard-Wiechert potentials to a waveguide environment. In addition, for a plasma filled waveguide, a modal expansion is given of the electric field directly in terms of mode coupling with the charge motion. Expressions for the spectral distribution of the radiation are given, both in general and for cyclotron radiation. Some specific results for the H/sub 10/ mode excited in a rectangular guide by cyclotron motion are also presented.     

The Annular Slot Antenna in a Lossy Biological Medium (Short Papers)

An integral equation is formulated for a coaxially fed annular aperture antenna. The integral equation in terms of the unknown tangential aperture electric field is solved numerically by the Method of Moments. The coaxial feed line is air filled while the exterior region consists of i) air, ii) fat or bone, and iii) muscle. Results are given for the aperture electric field, apparent input admittance, and contours of constant power absorption when the excitation frequency is 2.45 GHz.     

基于模态分析的散布式扬声器幅频特性研究

从薄板弯曲振动方程出发,论述了散布武平板扬声器薄板振动模态与声辐射模态和辐射声功率的内在关联。研究表明：振动模态简正频率密度和主导声辐射模态是影响辐射声功率的主要因素。试验证明,激励力所能激发的模态密度、模态阻尼对平板扬声器幅频特性有较大影响。     

Radiation from an aperture in a conducting cylinder coated with a moving plasma sheath

The electromagnetic radiation from an aperture on a conducting cylinder coated with a moving isotropic plasma sheath is considered. Numerical results are presented to illustrate the radiation patterns as functions of sheath velocity and plasma frequency for the circumferential slot and axial slot apertures. It is found for the circumferential slot aperture that the radiation is enhanced in the direction of sheath motion when the plasma is overdense and that relatively little change occurs when the sheath is underdense. For the axial slot, it is found that an electromagnetic field is radiated whose polarization is normal to that of the field radiated under stationary conditions, in addition to a field of the usual polarization. Significant alterations of radiation patterns from their form when the sheath is stationary can occur at relatively small velocities if the wave frequency is near the plasma frequency.