Resonant modes of a concentric spherical cavity with conicallystratified medium

An analysis on electromagnetic fields of a cavity formed by two concentric conducting spheres with a conically stratified medium is presented in this paper. Angular transmission formulation and the radial eigenfunction expansion are used to formulate the field components. Boundary matching methods are applied to obtain the characteristic equations containing various infinite series of spherical Hankel functions and Legendre functions of complex order for resonant frequencies. The first two resonant frequencies and field expansion coefficients are determined numerically. The distribution pattern of angular field components and the forms of typical electric field lines and magnetic field lines for the first resonant fields are also indicated     

Coupled antennas in a dissipative medium

The Fourier-transform method used to solve for the current distribution and the driving-point admittance of a single antenna in free space and in a dissipative medium can be extended to cover the case of two coupled antennas.     

Coupled dipoles in a dissipative medium

In Part I the admittance and impedance of a two-element array when immersed in an arbitrary dissipative medium are determined from a generalization of the King-Sandier theory of arrays. Numerical results are given for half-wave and quarter-wave dipoles with two different thicknesses. The distance between the elements is the independent variable. In Part II, experiments carried out in a tank filled with a conducting solution to determine the self and mutual admittances of coupled dipoles are described. The agreement between the measured values and the theory reported in Part I is quite good.     

Electromagnetic fields due to dipole antennas embedded in stratified anisotropic media

A method is presented for calculating the electric and magnetic fields from dipoles embedded in anisotropic stratified media. By decomposing the fields into transverse electric (TE) and transverse magnetic (TM) modes, the results are obtained more directly and are more computationally efficient than methods using the Hertz potential. The electromagnetic fields are obtained for four types of dipole sources: horizontal electric, horizontal magnetic, vertical electric, and vertical magnetic. The source is embedded within one of several anistropic layers, which are further sandwiched between two semi-infinite media.     

Electromagnetic fields of elementary dipole antennas embedded instratified general gyrotropic media

Using matrix methods in conjunction with Fourier transformation techniques, the field excited by arbitrarily oriented elementary electric or magnetic dipole sources in the presence of a uniaxial or biaxial stratified gyrotropic medium is obtained in the form of a full dyadic Green's function. Both the permittivity and permeability tensors, being completely unrestricted, assume their most general forms in each layer separately. The singular behavior of the solution in the close vicinity of the source-point is properly taken into account by separate, suitably selected dyadic delta function terms. Using suitably selected upward or downward wave amplitude matrices, the field inside any of the layers is determined     

Magnetodynamical modes in a spherical ferrite cavity

The axial-symmetrical modes of a spherical cavity containing magnetised-ferrite material are calculated. It can be shown that the different modes of the resonator are coupled by the gyrotropic medium. The field of the gyrotropic resonator is represented by an infinite sum of the eigenfunctions of the spherical resonator containing an isotropic medium. Under consideration of the boundary conditions the dependence of the resonance frequency on the magnetic-bias field may be obtained.     

The current in a parasitic antenna in a dissipative medium

A relatively simple analytic formula is derived for the current in a bare conducting cylinder of length2hand radiusawhen embedded in an isotropic homogeneous dissipative medium and excited by a periodic electric field with uniform amplitude and phase along its axis. In the derivation the same approximations are made as in an earlier analysis of the center-driven antenna for which complete experimental verification has been obtained. Computations are reported specifically for an antenna immersed in the ocean and excited by a field of very low frequency. The principal parameter ish/lambdain the range from 0.03 to 0.23 at a frequency of 5 Hz, wherelambdais the wavelength in sea water. The effect of changes in the radiusaare also shown successively witha/lambdaandh/aas the variable parameter. When the results of this paper are combined with those for the center-driven element, they provide the means for determining the response of a center-loaded dipole receiving antenna when immersed in a dissipative medium. Such an antenna is of interest in problems that relate to communication with submerged submarines and off-shore facilities.     

Radiation of a Hertzian dipole immersed in a dissipative medium

The general radiation formula for a Hertzian dipole immersed in an isotropic dissipative medium of infinite extent has been derived. As a boundary condition of the source, it is assumed that the dipole moment is a given quantity. When the conductivity of the medium is finite, the total radiating power is found to be infinite. Thus, in order to define a finite physically meaningful quantity, the dipole must be “insulated.” The total radiating power is then a function of the thickness of the insulator and the constants of the media. When the radius of the spherical insulator is large compared to a wavelength, the reflection coefficient of the wave traveling from the dielectric to the dissipative medium with the dipole as a source reduces to that of a plane wave as derived from Fresnel's equations. The similarity between this and the problem by Weyl (1919) is discussed     

地下SLF/ELF水平电偶极子在球形地-电离层中产生的场

在非理想导电地面与电离层条件下,导出了地下SLF/ELF水平电偶极子在地上、地下及电离层中产生的电磁场的球谐级数表达式.并提出了一种加速收敛算法,算出了大气层及电离层中的电磁场分布.计算结果表明:地下几十公里的水平电偶极子产生的场除了增加了一个固定衰减外,与地面上的水平电偶极子产生的场分布完全相似,它产生的电磁场可理解为电波首先垂直地透过土壤,然后在地一电离层腔体中传播.在SLF频段,地一电离层空腔中的电磁场可理解为两个"行波"的叠加.在ELF频段,空腔中的电磁场是驻波,其频率变化规律能正确反映出"舒曼"谐振现象.     

Admittance of insulated loop antennas in a dissipative medium

A small circular loop antenna is located in an insulating layer within a dissipative medium. The admittance of the loop is calculated using a two-term trial function for the loop current in a variational formulation. Thin insulating layers tend to maintain a nearly uniform current around the small loop, in particular for highly lossy surrounding media. The assumption of a uniform loop current is shown to be justified for a range of antenna parameters.     

The circular loop antenna immersed in a dissipative medium

Investigations were made of the radiating and circuit properties of circular loop antennas when immersed in conducting media with various loss tangents. The following quantities were experimentally examined and compared with the available theoretical results. 1) Driving point admittanceY_{in}of circular loops as a function of the circumference per wavelength,2pib/lambda = betab. 2) Current amplitude and phase distributions. 3) Field patterns. In each case the loss tangentsigma/omegaepsilon_{r}epsilon_{0}of the medium (conductivitysigma, permittivityepsilon_{r}epsilon_{0}) is taken as a parameter. The experimental results are in good agreement with the theory. A comparison between the behavior of the loop in a conducting medium and those of a linear dipole was made wherever possible. It was discovered that there is a cut off size beyond which the driving point admittance of a linear dipole is the same whether or not its ends are connected to form a loop. This particular loss tangent is 1.06, and the cut off size islambda/2. A circular loop antenna withbetab= 1.0is excited predominantly in a dipole mode and its field pattern resembles that of a two element dipole array. A study was made to determine how this pattern becomes more like that of a monopole as the loss tangent of the solution is increased.     

The small bare loop antenna immersed in a dissipative medium

The input admittance of a small thin-wire circular loop antenna, driven by a slice generator, immersed in a dissipative medium, is considered. It is found that the solution given by Storer for the loop antenna in a lossless medium can be carded over readily by replacingzeta_{0}byzeta, andk_{0}byk. The numerical values of the normalized input conductance and input susceptance of a small loop antenna, namelybeta b leq 0.3,Omega = 10, are calculated. It is to be noted that the input susceptance is practically independent ofkwhile the input conductance changes as much as seventeen times in this special case.     

Electromagnetic fields due to a horizontal electric dipole antenna laid on the surface of a two-layer medium

With applications to geophysical subsurface probings, electromagnetic fields due to a horizontal electric dipole laid on the surface of a two-layer medium are solved by a combination of analytical and numerical methods. Interference patterns are calculated for various layer thickness. The results are interpreted in terms of normal modes and the accuracies of the methods are discussed.     

Electromagnetic analysis of an antenna embedded in a compositeenvironment

This paper addresses the problem of an antenna embedded in a hole dug in the ground. The composite medium configuration consists of a half-space dielectric (representing the Earth-air interface) containing a cylindrical hole filled with a different dielectric medium. The wire antenna resides within this hole, on the axis. The solution strategy is based on decomposing the problem into simpler subproblems, which are treated sequentially. First we calculate a numerical dyadic Green's function for the composite medium by solving an integral equation formulated over a background consisting of the unperturbed dielectric half space (for which the Green's functions are known in a spectral integral form). This integral equation is solved via the fictitious currents method, which is a special case of the method of moments. We then solve the integral equation for the antenna currents using this numerical Green's function and determine the input impedance and radiation pattern     

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.     

Electromagnetic transient response of a spherical conducting shell

A quasistatic solution is obtained for the fields of a time-varying magnetic current source located in the vicinity of a spherical conducting shell. It is shown that the transient response has a relatively simple physical interpretation.     

Observations on a concentric spherical cavity laser oscillator

Experimental results on a number of ruby rods in concentric spherical cavities are presented and discussed. The observations include intensity variations (spiking) as a function of input energy, longitudinal and transverse mode structure, and output energy distribution. The characteristics of flat and concentric cavities are compared using the data presented. The spiking frequencies and decay times are compared with simple rate equation predictions during the quasi-CW time of operation with reasonable agreement. Results indicate that the cavity effects will be similar regardless of the type of laser material used.     

Transient response of an infinite cylindrical antenna in a dissipative medium

The transient solution of an infinite cylindrical antenna in a dissipative medium caused by an impulse excitation at a delta gap is obtained via an exact solution in the form of definite integrals and a simple asymptotic formula. This formula is used to obtain an integral for calculating the antenna current caused by a double exponential input voltage at the gap; furthermore, the resulting integral reduces to Sunde's classic result under the diffusion limit. These results are applied to the transient response of a buried wire subject to an electromagnetic pulse (EMP) or nearby lightning incident wave.     

The admittance of bare circular loop antennas in a dissipative medium

The normalized input admittance of thin 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. A comparison is also made with Storer's theory of the loop. Numerical results are given in the form of graphs for several wire sizes and for loops up to two and one-half wavelengths in circumference. The properties of the medium are represented by the ratioalpha/betain the range from zero (perfect dielectric) to one (good conductor);alphaandbetaare the imaginary and real parts of the complex propagation constantk=beta-jalpha = omegasqrt{mu(epsilon-jsigma/omega)}wheremuis the permeability,epsilonthe dielectric constant, andsigmathe conductivity of the medium.