Characteristics of circular loop antennas above a lossless ground plane

The circuit and radiation characteristics of a circular loop antenna above a lossless ground plane are studied. It is shown that the gain of this antenna is higher than that of a half-wave dipole above the ground plane. A gain as high as 10 dB is obtainable with this simple geometry.     

Near fields of the constant current thin circular loop antenna ofarbitrary radius

Assuming a known (constant) current distribution on the thin circular loop antenna of arbitrary radius in free space, an exact integration of the vector potential is performed without recourse to approximations. The only restrictions on the solution variables are that the observation point distance must be greater than the loop radius and that the polar angle must run between 0 and π. The resulting vector potential infinite series solution possesses a real part composed of linear combinations of complete elliptic integrals of the first and second kind and an imaginary part composed of elementary functions. Thus, it is possible to obtain an exact solution which is valid everywhere that r>a and 0⩽&thetas;⩽π. The electromagnetic field components of the constant current circular loop antenna are then determined by direct series differentiation. These solutions are valid in the near and induction fields, converging rapidly there, and are also valid in the far field, although many terms of the series are needed for convergence     

Low-frequency impedance of a circular loop over a conducting ground

Using a quasistatic approach, the input impedance of a circular wire loop is calculated for the case where the Earth is represented as a homogeneous conducting halfspace. It is shown that the input resistance is only proportional to the ground conductivity when the loop radius is small compared with the electrical skin depth.     

Resistance of a circular current loop in a cylindrical waveguide

The dependence of the resistance of a circular current loop situated coaxially inside a cylindrical waveguide on the size of the loop relative to the radius of the waveguide is examined.     

The radiation efficiency of a dipole antenna located above an imperfectly conducting ground

The dyadic Green's function technique is used to develop integral expressions for the radiation efficiency of three types of dipole antennas located above an imperfectly conducting, infinite ground plane. The three antennas treated are: 1) vertical Hertzian dipole, 2) horizontal Hertzian dipole, and 3) a vertical half-wave dipole with sinusoidal current distribution. The results of numerical evaluation of the integral expressions for several values of ground constants are presented in graphical form. The radiation efficiency of a vertical Hertzian dipole is found to exhibit a distinct peak when located at a height of one-eighth wavelength.     

The transient electromagnetic field of a pulsed line source located above a dispersively reflecting surface

An exact representation for the transient field of a pulsed line source above a plane reflecting surface is obtained as a finite integral over the transient plane-wave solution for complex angles of incidence. When applied to the reflection from a conducting half-space, a solution for the transient field is obtained as a finite double integral, which permits accurate calculations in a minimum of computer time. Comparison with early-time and late-time approximations available in the literature shows that there is a wide range of times for which neither is accurate.     

New representation for the current in a horizontal wire above ground

A new approximate formula is given for the current in a horizontal wire above ground. This formula, which is more accurate than previous ones, involves contributions from both proper and improper modes.     

The current in bare circular loop antennas in a dissipative medium

The distribution of current in bare circular loop antennas has been evaluated from the theory of T. T. Wu. Computations have been made for loops in air and in an infinite homogeneous isotropic dissipative medium. Currents are shown graphically for loops with circumferences up to two wavelengths in media, withalpha/betaranging from zero to one, wherek=beta-jalphais the complex wave number.     

Resistance of a circular loop with dipolar current distribution in a cylindrical waveguide

The resistance of a circular loop having a dipolar current distribution around its circumference and situated coaxially inside a cylindrical waveguide is determined and the dependence of the resistance on the size of the loop relative to the radius of the waveguide is examined.     

复杂陆地粗糙面及其上方坦克目标复合散射研究

研究了复杂陆地粗糙面及其上方坦克目标的复合散射特性.基于Monte Carlo方法,利用高斯谱函数模拟不同统计特性的陆地表面,运用加权反正切函数滤波处理以构造边界和连接相邻区域,构建了复杂陆地分区域复合粗糙面的几何模型.提出了一种快速精确求解复杂陆地粗糙面及其上方坦克目标复合散射的混合方法,利用迭代物理光学法求解目标与环境间的多次耦合,运用等效电磁流法修正棱边绕射.分析了复杂陆地粗糙面的散射特性,研究了入射角、均方根高度、相关长度和粗糙面表面结构对复合散射特性的影响.本研究为探测复杂环境上方的坦克目标提供了理论基础,数值仿真结果对雷达目标的探测与识别具有借鉴意义.     

On the excitation of a circular loop antenna by travelling-and standing-wave current distributions

Presented here is an exact formulation of the electric and magnetic (EM) fields radiated by a circular loop antenna, assuming both travelling- and standing-wave current distributions. By using a differential current element positioned in an azimuthal direction as the starting point, the paper systematically develops the EM fields radiated by the circular loop via a vector potential theory. This approach leads to a general integral representation for the radiation characteristics of the loop antenna where we completely evaluate the resulting expressions, when the excitations of the loop assume travelling- and standing-wave distribution forms. In addition, this paper briefly examines a method to generate approximately such current distributions by coupling the loop to a two- or four-wire transmission line. Furthermore, the paper discusses a graphical representation of the current distribution plotted as a function of frequency or loop size. From the field expressions determined, we derive generalized closed-form results for some important design parameters for the loop antenna. These parameters include the radial component of the Poynting vector, the total power radiated by the loop, the directivity, the radiation resistance, and the effective area and height of the antenna. When we specialize to the important case of uniform current excitation, the exact values of the parameters deduced from the general expressions, are summarized and exhibited in a comprehensive table. This table facilitates the computations of these important physical parameters. Further analysis involving small argument and asymptotic approximations in the residts for the travelling-wave current excitation leads to closed-form expressions in terms of tabulated functions. Numerical results presented, include the fields radiated by the loop when the standing-wave current excitation admits a Fourier series representation. The present approach via potential theory reveals that the fields can be calculated in any arbitrary direction : this is consistent with a previous observation of Knudsen (1951, 1953) who employed a different approach.     

Symmetric electromagnetic field of a circular loop antenna over the conductive cone of finite length

The diffraction properties of the field of circular loop antenna over the conic surface of finite length have been investigated. The relationships of the radiated power as a function of the antenna position on the symmetry axis of the cone were established. The directional patterns and field distribution in the near-field zone were obtained for different geometrical parameters of the diffraction system. The results obtained were compared with similar relationships for semi-infinite cones.     

FDTDSIBC混合方法计算近地导线表面电流

通过在FDTD方法中引进时域表面阻抗界条件来研究瞬态电磁脉冲对有耗地面附近电缆线耦合问题。由频域表面阻抗出发通过拉氏变换得到时域表面阻抗边界条件（SIBC）,并将这种边界条件应用于FDTD方法中来模拟有耗地面的反射。计算结果表明了些方法的有效性。     

Analysis of lightning-radiated electromagnetic fields in the vicinity of lossy ground

An antenna theory (AT) approach in the frequency domain is presented to compute electromagnetic fields radiated by a lightning return stroke. The lightning channel is modeled as a lossy-wire monopole antenna (a wire antenna with distributed resistance) energized by a current source at its base, and the ground is modeled as a lossy half-space. The method of moments is used for solving the governing electric field integral equation (EFIE) in the frequency domain. The resultant current distribution along the channel is used to calculate electromagnetic fields at different distances from the channel. All field components are evaluated using a rapid but accurate procedure based on a new approximation of Sommerfeld integrals. In contrast with the previous models, the approach proposed here is characterized by a self-consistent treatment of different field components in air or on the surface of a lossy half-space. It is shown that the omission of surface wave terms in the general field equations, as done in the perfect-ground approximation, can strongly affect model-predicted field components.     

On the nonuniqueness of equivalent current sheets supportingdiscontinuous electromagnetic fields

It is shown that the familiar boundary conditions across an interface are not unique. They can be reformulated in a number of alternative ways without affecting the solution outside the interface. Surface electric and magnetic current distributions used to compensate for the discontinuities in electric and magnetic tangential fields are interchangeable, leading to a variety of boundary conditions. In particular, the introduction of a magnetic surface current is not necessary to account for a discontinuity in the transversal electric field because it can be exchanged for a highly singular electric current, causing no difference in the solution except within the source region. Examples for conducting electric and magnetic current sheets are given     

K-pulse for a thin circular loop

Based on the pole-elimination concept, a time-limited input waveform, the K-pulse, is obtained for a thin conducting circular loop. The resultant response waveforms are also found to be time-limited (TL). Therefore, by employing the K-pulse input waveform, the resonant ringing associated with the circular loop has been eliminated. The concept and procedure for deriving the K-pulse are discussed.     

Influence of finite ground conductivity on the fields of a verticaltraveling wave of current

Following Wait (1998), we deal mainly with the radiation fields of a vertically directed and attenuated line current (lightning) above a plane Earth. In the present case, the conductivity and permittivity of the ground modify the radiation pattern in the air compared with the usually assumed case of infinite conductivity. The effect is quite striking at low elevation angles where the radiation pattern now has a null for grazing angles