Plane wave reflection from a rectangular-mesh ground screen

The theory of scattering of electromagnetic plane waves of arbitrary incidence and polarization from an infinite rectangular-mesh ground screen is treated. The screen is composed of thin wires of circular cross section, and is parallel to the interface between two homogeneous media. The theoretical results for parallel-wire screens are obtained in the limit of large wire spacing for one dimension of the mesh screen. Results presented for incident parallel polarized plane waves indicate that both the parallel-wire and mesh screens may exhibit a change in reflecting properties as the plane of incidence is varied from the wire axis direction. In addition, it is shown that the parallel-wire screen can produce appreciably higher cross-polarized fields than a square-mesh screen of the same wire spacing.     

On the radiation from a vertical dipole with an inductive wire-grid ground system

The far field of a vertical electric dipole on a sectionally homogeneous ground plane is considered. The specific model used is a dielectric-like ground which is modified by using an inductive wire grid or mesh screen in a region surrounding the dipole. Attention is focused on the modification of the radiation pattern resulting from the presence of the inductive ground screen. It is demonstrated that the low-angle radiation may be greatly enhanced by a ground screen which extends out to 15 or more wavelengths.     

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     

EM pickup and scattering by a wire

This article treats pickup and scattering by a single wire in free space or over a ground plane. The wire may be uniform or nonuniform and infinite or finite. We only treat the case where wire radius is so small compared to a wavelength and the other problem dimensions that scattering by the wire is independent of azimuth. Solutions based directly on Maxwell's equations are compared with solutions based on the telegrapher's equations; for 1 mm radius Cu wire at 1 GHz, equilibrium CW currents as computed from the two models, for a uniform, infinite wire, differ by 6 dB. In general, the wire-current solutions are separated into a homogeneous part and a particular or driven part. The driven part couples and scatters fields, while,at least on an infinite wire in the far field, the homogeneous part does not     

Note on the impedance of a wire grid parallel to homogeneousinterface

Numerical data are provided for the correction factor that is used to calculate the impedance of a planar wire grid parallel to the interface between two dielectric half-spaces. Comparisons with earlier investigations clarify, extend, and supersede previous computations. The authors show more clearly the significant influence of the interface on the equivalent grid impedance     

The near field of a wire grid model

The extreme near-field behavior of the wire grid model of a conducting surface is examined. Using a wire grid model of an infinite transverse magnetic circular cylinder, it is verified that the best accuracy for the E-field is obtained when the wire satisfies the `same surface area' rule of thumb. Two excitations are considered: a uniform surface current and plane wave incidence. In the first case, although the boundary value match between the wires is poor the extreme near field is still quite accurate. In the second case, the near field is also accurate, however, the largest errors occur not between the wires, but in the interior of the scatterer. In both cases, the boundary match between the wires as a check on the accuracy of the solution is misleading     

Electromagnetic scattering by a thin resistive wire

A theoretical investigation concerning scattering of a plane electromagnetic wave by a straight resistive wire of finite length in an isotropic homogeneous medium is reported. The work is based on the travelling-wave solution obtained by repeated use of the Wiener-Hopf method. Some numerical data for backscattering cross-section at broadside incidence are given.     

Dispersion of homogeneous and inhomogeneous waves in the Yeefinite-difference time-domain grid

The numerical dispersion relation governing the propagation of homogeneous plane waves in a finite-difference time-domain (FDTD) grid is well known. However, homogeneous plane waves, by themselves, do not form a complete basis set capable of representing all valid field distributions. A complete basis set is obtained by including inhomogeneous waves, where, in the physical world, constant phase planes must be orthogonal to constant amplitude planes for lossless media. In this paper, me present a dispersion analysis for both homogeneous and inhomogeneous plane waves in the Yee FDTD grid. We show that, in general, the constant amplitude and constant phase planes of inhomogeneous plane waves are not orthogonal, but they approach orthogonality for fine discretization. The dispersion analysis also shows that, for very coarsely resolved fields, homogeneous waves will experience exponential decay as they propagate and they may propagate faster than the speed of light. Bounds are established for the speed of propagation within the grid, as well as the highest frequency and the shortest wavelength that can be coupled into the grid. Analysis is restricted to the classic Yee algorithm, but a similar approach can be used to analyze other time-domain finite-difference methods     

A broad-band twist reflector

Polarization twisting reflectors, or twist reflectors, consisting of a wire grid spaced approximately3 lambda/8from a ground plane [1], [2] have a bandwidth of at most 30 percent. It is shown that the inclusion of an additional wire grid in front of the first grid makes bandwidths of 100 percent possible. Design formulas, theoretical performance in terms of cross polarization attenuation, and some experimental results for a twist reflector with two wire grids are given.     

Analysis of Radio Frequency Transmission along a Trolley Wire in a Mine Tunnel

An idealized model of a trolley wire communication system in a tunnel is considered. To facilitate the analysis the tunnel cross section is taken to be semicircular and the surrounding rock medium is homogeneous with a finite conductivity. The metallic rails or other conductors on the flat floor of the tunnel are represented by a plane metallic reflecting surface of infinite extent. The trolley wire and associated feeder line are assumed to be equivalent to a single cylindrical conductor that is parallel to the axis of the tunnel but located anywhere within the cross section. The appropriate modal equation is then solved for the propagation constant of the dominant mode. It is shown that the attenuation rate increases significantly as the trolley wire approaches the tunnel wall. However, even when the trolley wire is within 20 cm (8 in) of the wall, the attenuation rate at 200 kHz is still less than 2 dB per kilometer.     

On the theory of wave propagation along a thin dielectric coated wire in a stratified medium†

We consider propagation along a thin dielectric coated wire that is parallel to the interface between two homogeneous half-spaces. The solution obtained correctly accounts for the presence of the interface provided the coating radius is small compared with the distance to the interface. The final results are simplified in form when dealing with long insulated antennas buried in the earth and operating at low frequencies.     

基于遗传算法的栅格天线阵旁瓣电平优化方法

在推导出栅格天线阵列方向图的基础上,提出一种基于遗传算法的栅格天线阵旁瓣电平优化的方法。此方法首先简化栅格天线阵物理模型,给出等效模型下的阵列方向图,然后以栅格天线阵的短边电流幅度为优化参量,以阵列天线方位面的副瓣电平为适应度函数,利用遗传算法的最优化搜索得到满足副瓣要求的电流幅度,再通过电流幅度计算辐射单元阻抗,最终设计出满足要求的低旁瓣栅格天线阵。为了验证该方法的有效性,对一种频扫微带栅格天线进行了优化,在电磁仿真软件中对优化后的天线进行了仿真,根据设计结果加工制作了原型天线并进行了测试,测试结果显示优化后天线阵的副瓣电平降低了5dB,优化效果明显。     

Image Theory for Dipole Excitation of Fields above and below a Wire Grid with Square Cells

The exact imnage theory, recently introduced by two of the present authors for the Sommerfeld problem, is applied to the problem of a vertical electric dipole above a plane metallic grid with rectangular cells. The mesh size is assumed small compared to the wavelength and the wire diameter small compared to the mesh size, whence the Kontorovich averaged boundary condition concept is applicable. The image current consists of two exponential-line-current waves located in complex space. The theory is seen to produce simple asymptotic expressions for the field close to the grid surface. Also, an integral expression for the calculation of the change in the vertical-electric-dipole impedance due to the grid is constructed.     

Boundary conditions for absorption and excitation of extraordinary waves in a planar <Emphasis Type="Italic">R</Emphasis>τ grid

A technique is proposed for formulation of a boundary condition for excitation and absorption of E-polarized plane waves in magnetized plasma. The technique is based on the method of Rτ grids. It is shown that grid solutions corresponding to plane waves converge to exact solutions obtained for a homogeneous filling. Analytic expressions are derived for phase shifts per discretization step for eigensolutions corresponding to an Rτ grid in the case when waves propagate along the edges and a diagonal of the grid. Phase errors occurring during determination of the wavelength in the grid are estimated numerically for various propagation angles.     

ZnO-Ni壳核丝状阴极场发射特性研究

采用水热法在Ni丝上制备锥状ZnO,形成ZnO-Ni壳核丝状阴极。采用X射线衍射分析仪进行物相分析,表明该ZnO具有沿002面生长的取向。用扫描电子显微镜(SEM)观察其表面形貌,表明Ni的表面生长有一层分布均匀的锥状ZnO。通过场发射性能测试,结果表明ZnO-Ni壳核丝状阴极具有比平面结构中的ZnO纳米锥阴极更优良的场发射性能,并对该结果进行讨论。     

Courant induit par une onde incidente sur un conducteur rectiligne et filiforme situé au voisinage du sol

An analytical result for the current induced by a plane incident wave on a thin straight insulated wire buried in the ground and parallel to the interface is presented. Two methods are presented to solve this problem in the frequency domain; the first one uses the current on an infinite wire; the current induced on a finite wire is obtained adding to the preceding result the reflected currents of different orders. The second uses the reflection coefficients of the transmission line theory. The results are compared with those obtained from another method in which the Sommerfeld integrals are numerically evaluated.     

The RCS of a metal plate with a resonant slot

A numerical method based on the thin-wire electric field integral equation is used to calculate the radar cross section (RCS) of a wire grid with a resonant slot. The numerical results are in good agreement with experimental data for a wire grid, where the antiresonance is found to occur at a slightly lower frequency than a similarly slotted plate.     

Effects of Evanescent Space-Harmonics on the Transmission Characteristics of a Gaussian Beam by Cylindrical Wire Grids

When electromagnetic waves propagate through a wire grid, there will be some evanescent space harmonics generated around the wire grid. In this paper, we mainly investigate their effects on the transmission characteristics of a Gaussian beam by cylindrical wire grids. The results are compared with those without taking account of evanescent space-harmonics.     

周期性电抗加载介质结构电磁特性的等效分析

提出了一种周期性电抗加载介质结构传输特性的等效分析方法。在5～15GHz频率范围内,通过对A夹层介质结构中周期性加载金属丝网栅和金属条带网栅两种结构的电磁特性仿真分析,建立了金属条带宽度与金属丝直径之间的等效关系,并详细分析了电磁波入射方向、网栅加载位置、金属丝直径大小、网栅周期以及介质结构对等效的影响。仿真结果表明:电磁波任意方向入射,金属丝直径较小时等效条件为金属条带宽度为金属丝直径的π/2倍。因几何参数变化而导致的等效误差可以通过等效系数的经验公式予以修正。