Optimization of a conical antenna for pulse radiation: an efficientdesign using resistive loading

The conical monopole antenna with a section of continuous resistive loading is considered as a radiator for temporally short, broad-bandwidth pulses. The geometrical details of the coaxial feed and the resistive loading are varied to optimize this structure for pulse radiation. Compared with the perfectly conducting cone, the optimized resistive cone radiates a better reproduction of the pulse excitation with no loss in amplitude, and has internal reflections that are much smaller in amplitude. Graphical displays of the field surrounding the antenna are used to give insight into the physical processes for transient radiation from this antenna. Experimental models were constructed to verify the optimization and demonstrate the practicality of the design. Measurements of both the reflected voltage in the feed line and the time-varying radiated field are in excellent agreement with the theoretical calculations     

Feed models for coax-driven monopole and dipole antennas

A method is presented for accurately modeling a monopole or dipole antenna fed by a coaxial line. The base of the monopole is attached to a conducting plane through which the coaxial feed line extends to the feed. The feed structures considered are easily adaptable to physically rugged forms and are simple to construct. Equivalent models for the three regions of the structure are devised and coupled integral equations for aperture fields and surface currents are formulated to enforce the boundary conditions. Three variations of the feed configuration are discussed and the reflection coefficient of the antenna feed is determined from the data obtained from the solutions of the coupled integral equations. Computed reflection coefficient values are shown to agree well with values measured on laboratory models.     

Radiation from a dielectric coated hemispherical conductor fed by acoaxial transmission line

A dielectric-coated hemispherical conductor mounted on an infinite perfectly conducting ground plane and fed by a coaxial transmission line is investigated. Green's functions for the region above the ground line are derived with separated homogeneous and particular solution parts so as to be compatible with numerical analysis techniques. A magnetic field integral equation is constructed in terms of the unknown annular aperture tangential electric field and is solved by the method of moments. A comparison of the characteristics of the dielectric-coated hemispherical conductor and a flush-mounted coaxial line to an infinite homogeneous region above the ground plane is presented with respected to the tangential aperture electric field, with respect to the tangential aperture electric field, the coaxial line apparent input impedance, and the far radiated field     

On the electrically thick monopole: Part I--Theoretical solution

A new integral equation is formulated for the current distribution of an electrically thick, cylindrical antenna which is driven by a voltage across a finite gap on an infinitely large ground plane. Compared to the conventional integral equation of Hallen, this equation has an additional source term which can be expressed in terms of both the radius of the antenna and the width of the gap. A numerical solution of the integral equation has also been obtained by a so-called "approximate product-integration technique" and the results have been compared with the experimental data of a monopole antenna driven by a coaxial transmission line over an infinitely large ground plane. When the circumference of the antenna is comparable to a free-space wavelength, the experiment verifies that the theoretical model with finite gap excitation describes the experimental situation more properly than either the conventional model of a delta-function excitation on both outside and inside surfaces of the antenna, or the King-Wu model of a delta-fanction excitation only on the outside surface.     

TM scattering by an impedance sheet extension of a paraboliccylinder

An integral equation and method of moments (MM) solution are presented for the two-dimensional (2-D) problem of transverse magnetic (TM) scattering by an impedance-sheet extension of a perfectly conducting parabolic cylinder. An integral equation is formulated for a dielectric cylinder of general cross section in the presence of a perfectly conducting parabolic cylinder. It is then shown that the solution for a general dielectric cylinder considerably simplifies for the special case of TM scattering by a thin multilayered dielectric strip that can be represented as an impedance sheet. The solution is termed an MM/Green's function solution, where the unknowns in the integral equation are the electric surface currents flowing in the impedance sheet; the presence of the parabolic cylinder is accounted for by including its Green's function in the kernel of the integral equation. The MM solution is briefly reviewed, and expressions for the elements in the matrix equation and the scattered fields are given. Sample numerical results are provided     

Surface currents on impedance bodies of revolution

The surface currents induced by a plane wave axially incident on a rotationally symmetric body are determined by solving numerically extended form of Maue's integral equation. The relative surface impedance is independent of the azimuthal angle but may vary along the profile of the scatterer in any plane containing the axis of symmetry. Numerical results are shown for a sphere and a cone-sphere that are either perfectly conducting or perfectly absorbing. Apart from internal resonances, the computer code is found to provide accurate results well into the high-frequency region. A simple line-integral representation of the far field is given, and internal resonances are discussed for the backscattering radar cross section of a perfectly conducting and a perfectly absorbing sphere     

Electromagnetic diffraction from a dielectric-filled slit-cylinderenclosing a cylinder of arbitrary cross section: TM case

A simple moment solution to the problem of the diffraction of a TM plane wave from an infinite, perfectly conducting slotted cylinder of an arbitrary cross section is summarized. The slit cylinder encloses a smaller perfectly conducting cylinder of an arbitrary cross section, and the space between the cylinders is filled with a dielectric material. The equivalence principle is used to obtain a set of coupled integral equations for the induced/equivalent surface currents on the cylinders, and the method of moments is used to solve numerically the integral equations. The electric field integral equation formulation is used. The advantages and the limitations of the method are discussed. Sample results for the induced current, aperture field, internal field, and scattering cross sections are given. These are in good agreement with some of the available published data     

The Analysis of Monopole Antennas Located on a Spherical Vehicle: Part 2, Numerical and Experimental Results

Presented are various numerical results illustrating the behavior of thin monopole antennas located on a perfectly conducting sphere. The method of analysis, described in a previous paper, uses an integral equation solution for the unknown wire currents, and a modified Green's function to limit the range of integration to over the wires only. Studies are made of the input quantities, radiated currents and induced sphere currents for various antenna geometries. A comparison of the computed input impedance of monopole on the sphere is made with experimental data and good agreement is noted.     

Radiation from apertures in conducting cylinders of arbitrary cross section

An analysis of two-dimensional radiation from apertures in perfectly conducting cylinders of arbitrary cross section is given. Solutions are expressed in terms of generalized network parameters, obtained by applying moment methods to the superposition integral equation. Formulas are given for current distributions, self- and mutual admittances, and radiation patterns. Representative computations are included to illustrate the theory.     

A Hallen-type integral equation for symmetric scattering from lossycircular disks

A rigorous technique is presented for calculating the current induced on a thin lossy disk by rotationally symmetric sources, and the resulting scattered field. A Hallen-type integral equation is developed for the current using the magnetic vector potential, and it is solved by the method of moments. It is shown that the diffraction lobes usually associated with radiation above a finite circular ground plane can be reduced dramatically by the addition of loss. Application to a quarter-wave monopole radiating above a finite circular perfectly conducting ground plane shows good agreement with experiment     

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.     

Equivalent antenna radius for narrow slot apertures having depth

The problem of electromagnetic penetration of a narrow slot aperture in a thick perfectly conducting plane is reduced to the solution of Hallen's integral equation with an equivalent antenna radius. The depth and width of the slot are assumed to be small compared to both the length of the slot and the wavelength. The equivalent radius is evaluated in terms of the solution to the transverse static problem. Simple approximations for the equivalent radius is also given. Hallen's integral equation is solved by the Galerkin method with piecewise sinusoidal basis functions. Large slot depth-to-width ratios give rise to vanishingly small equivalent radii and thus large antenna resonance quality factors. A simple correction to the static field distribution in the slot is also given. This correction allows the depth to become somewhat larger relative to the wavelength     

Radiation from quarter-wavelength monopoles on finite cylindrical, conical, and rocket-shaped conducting bodies

A theoretical-numerical technique, based on Maue's integral equation, is applied for the determination of radiation patterns of airborne antennas comprising one or more quarter-wavelength monopoles, straight or bent, on finite cylindrical, conical, and rocket-shaped conducting airframes. The induced surface currents on the conducting body are first determined whence the total radiation fields are obtained. In the cases of a single monopole on a conducting body, the surface currents and radiation patterns are studied in view of varying the geometrical parameters of the body as well as the position and relative orientation of the monopole. For the case of two diametrically opposite monopoles on a finite cylinder, the effect of varying the relative phasing is studied. A physical interpretation of the variation with various parameters of the surface currents and the radiation fields is endeavored wherever feasible, and important results are concluded. Experimental investigation of radiation from one of the theoretically treated models, namely that of a bent monopole on a finite frustum of a cone, showed excellent agreement with computed results. This establishes the validity of the theoretical-numerical technique as well as the computed radiation patterns for various cases.     

The dielectric-filled tubular monopole

The integral equation for the current in a dielectric-filled tubular monopole when driven by a coaxial line with an assumed pure TEM mode is formulated and solved by a numerical method. It is found that when the dielectric-filled tube can support a TM01circular waveguide mode, the characteristics of the antenna are drastically different. In this case a very sharp resonance behavior is observed. The resonant length depends primarily on the properties of the internal waveguide mode rather than on the free-space behavior of the antenna.     

Dual-surface integral equations in electromagnetic scattering

A brief review is given of the derivation and application of dual-surface integral equations, which eliminate the spurious resonances from the solution to the original electric-field and magnetic-field integral equations applied to perfectly electrically conducting scatterers. Emphasis is placed on numerical solutions of the dual-surface electric-field integral equation for three-dimensional perfectly electrically conducting scatterers.     

一种求解目标内谐振时散射截面的有效方法

众所周知,在内谐振频率点上,用矩量法求解电场或磁场表面积分方程将得到不正确的表面电流。文中应用奇异值分解和正交化方法对由电场积分方程计算出的表面电流进行修正,从而得到目标表面上产生散射场的真实电流分布。文中计算了一无限长理想导体圆柱内谐振时的散射截面,所得结果与解析解一致,并对一无限长理想导体正方柱的后向散射截面进行了计算,结果表明本文方法是有效和准确的。     

Input admittance of a multilayer insulated monopole antenna

An efficient numerical technique based on the modal-expansion method in conjunction with a recursive algorithm is developed for a multilayer insulated monopole antenna fed by a coaxial transmission line. The modal-expansion analysis is facilitated by introducing a perfectly matched boundary (PMB) at a variable height over the ground plane of the monopole. The current distribution and input admittance are computed by finding the expansion coefficients of the electromagnetic field expressions. Numerical results for the input admittance of a dielectric-coated monopole antenna and an air-insulated monopole are compared with experimental ones available in the literature. Good agreement is achieved. Calculated results for the effects of various parameters on the input admittance of an air-insulated monopole antenna are presented and discussed     

基于奇异值分解的方法求解目标内谐振时的散射截面

表面积分方程已被广泛地用来分析电磁散射问题,但在离散的内谐振频率点上,用矩量法求解积分方程将得到错误的结果.本文基于电场积分方程,应用奇异值分解找出谐振模电流,并采用正交化方法将其舍去,从而得到非谐振模电流的分布.文中计算了一无限长理想导体圆柱内谐振时的散射截面,所得结果与解析解一致,并对一无限长理想导体三角柱的前向散射截面进行了计算,结果表明本文方法是有效和准确的.     

Analysis and optimization of an electrically small receivingantenna

A theoretical and experimental study of an optimum receiving antenna configuration that fits within certain allocated space requirements is presented. This optimum monopole antenna configuration is applicable for Loran-C reception. The antenna analysis is based on a quasi-static numerical study of a conducting body of revolution above a perfectly conducting ground plane. A general numerical algorithm is developed to determine the input impedance and the effective height of the antenna. In addition, the amplifier noise and its role in the choice of the optimum antenna are examined. Results are presented for cylindrical and truncated conical structures both with and without a top load     

Tubular monopole of arbitrary dimensions: The radiation field

The complete radiation field of a tubular monopole of arbitrary length and radius, which is base driven by a low-impedance coaxial transmission line, is derived. The integral equation for the current distribution, which includes a term for the feed-point correction is solved by the inversion of a matrix under the assumption that the current is a continuous, piecewise linear function. The far-zone electromagnetic field established by this current is expressed in closed form. It is found that the effect of thickness is to round off the nulls predicted by previous theory for vanishingly small radius and to enhance the high-angle radiation.