An impedance sheet approximation for thin dielectric shells

An approximate equivalence between an impedance loaded surface and a thin dielectric shell is given. This approximation is used to compute the backscattering from a thin circular dielectric tube and the results are compared to the exact solution. Computations for backscattering from a thin dielectric cone-sphere and resonant wire loop inside of a thin dielectric cylinder are also given as further illustrations of the method.     

A Metrology Application of Reverberation Chambers: The Current Probe Calibration

This paper presents how it is possible to characterize a current probe in terms of its transfer impedance inside a reverberation chamber. The probe catches a floating wire placed inside the working volume of the chamber. The current on the wire is calculated from the knowledge of the total measured average field along the wire in the working volume. The current is calculated by representing the field in terms of a summation of random plane waves, where a finite-difference time-domain (FDTD) code is used to compute the current induced by each plane wave along the wire caught by the probe under test. Results for the current distribution along the wire are reported for several frequencies. Finally, the transfer impedance of commercial probes is recovered from the knowledge of the current and compared to the values given by the manufacturers and by an external laboratory     

The Exact Analytical Solution for Large Circular Loops Radiating Around a Dielectric Coated Conducting Sphere

In this paper, the electromagnetic field of electrically large circular loop antennas, with different radii and different nonuniform current values, around a dielectric coated conducting sphere is considered. One or more loop antennas are located on the outer surface of a spherical dielectric shell covering a conducting sphere. Eigenfunction series solutions for the field are assumed in two regions. The current distribution on the wire loop, driven by a voltage source, is determined by Fourier series expansion and all necessary harmonics are taken into account. Exact analytical field expressions in closed forms are derived and field patterns are plotted. The antenna model and formulation presented in this paper offer exact analytical solutions to several loop antenna problems.      

Wire antennas in the presence of a dielectric/ferrite inhomogeneity

A moment method solution for treating thin-wire antennas in the presence of an arbitrary dielectric and/or ferrite inhomogeneity is presented. The wire is modeled by an equivalent surface current density, and the dielectric/ferrite inhomogeneity is modeled by equivalent volume polarization currents. The conduction currents on the wire and the polarization currents in the dielectric/ferrite inhomogeneity are treated as independent unknowns and determined in the moment method solution. The method is applied to the problem of a loop antenna loaded with dielectric or ferrite. Numerical results are presented, and are in good agreement with measurements and previous calculations.     

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.     

Electromagnetic Simulation of Missile Exhaust

A simple moment solution is given for the problem of electromagnetic transmission through dielectric-filled slots in a conducting cylindrical shell of arbitrary cross section. The exciting source is assumed to be either a TM plane wave (receive mode) or an electric line current placed inside the shell (transmit mode). The surface-equivalence principle is used to replace the surfaces of the shell and the dielectrics by equivalent surface currents radiating in an unbounded medium. The application of the appropriate boundary conditions yields a set of coupled integral equations for the surface currents. The moment method with pulse expansion and point-matching testing procedures is used to solve the integral equations. Shells of different cross-sectional shapes are considered. Special attention is paid to circular and rectangular shells. In the transmit mode, the total far field transmitted through the slot is computed. In the receive mode, the aperture field and the field at the center of the shell are computed. For the case of air dielectric filling the slots, the computed results are in very good agreement with available published data.     

Method of moments analysis of EM fields in a multilayered spheroid radiated by a thin circular loop antenna

This paper presents derivation and computation of electromagnetic (EM) fields inside a dielectric prolate spheroid radiated by a loop antenna. The dielectric spheroid is considered to be multilayered, and a thin circular loop antenna that is loaded by a voltage source radiates on the top of the prolate spheroid. The multiple interaction of transmitted and reflected waves with the spheroid is characterized by applying the method of moments (MoM) to both the circular loop antenna wire and the stratified spheroidal interfaces. The dyadic Green's function in the expansion form of eigenvector wave functions is used to derive the EM fields, so the formulation is quite compact. Different basis and weighting functions are used inside the method of moments procedure for obtaining in an efficient way the unknown current distributions along the antenna wire and the unknown expansion coefficients of their resulted EM fields. Current distributions and the transmitted fields inside the spheroidal model are computed numerically and the convergence issues are discussed.     

TM scattering by a dielectric cylinder in the presence of a half-plane

The problem considered is the transverse magnetic (TM) scattering by a dielectric cylinder in the presence of a perfectly conducting half-plane. An integral equation, involving the half-plane Green's function in its Kernel, is obtained for the equivalent volume currents representing the dielectric cylinder. This integral equation is solved by the method of moments. Numerical results are compared with measurements for the echo width of a dielectric slab on a half-plane. The dielectric slab surface impedance and the fields inside the dielectric are also shown.     

Rectangular and zonal slots on a sphere with a backing shell: theory and experiment

A rectangular slot antenna on a spherical cavity with a conducting backing shell is investigated rigorously. The Green's function approach is used to formulate an integral equation for the magnetic slot current, which is solved using the moment method. Two previously developed methods to evaluate the admittance integrals are used and compared with each other. Both methods use recurrence formulas and analytical integration to speed the computation. The slot and cavity resonances of the structure are studied. In particular, two simple formulas for the first two natural resonances of the cavity structure are given. The effects of the shell size, slot length, and cavity dielectric constant on the input impedance are discussed. Measurements were carried out to verify the theory, and very good agreement between theory and experiment is obtained. In addition, the zonal slot antenna on a spherical cavity with a conducting backing shell is studied. The effect of the shell radius on the input impedance is investigated and, again, very good agreement between theory and experiment is found.     

Radiation from a circular loop in the presence of sphericallysymmetric conducting or dielectric objects

The problem of electromagnetic radiation from a circular loop antenna of radius a, carrying a current I is considered. The loop may be radiating in the presence of one or more of the following objects: a centrally located dielectric or perfectly conducting sphere of radius ba and outer radius d, a perfectly conducting spherically symmetric cap at radius b, and another such cap at radius d. Typically geometric structures considered are shown. It is demonstrated how the presence of the sphere, shell, and caps can change and direct the radiation pattern of the loop     

Determination of reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution

A simple theory is presented to predict the lumped reactance loading for circularly polarized circular loop antennas with a uniform traveling-wave current distribution. The reactive load is located on a circular wire loop of one-wavelength circumference at a position of 45/spl deg/ away from the feed point. To achieve a uniform traveling-wave current distribution, the loading reactance and the input impedances of the loaded and unloaded loop antennas need to satisfy certain conditions. First, the input resistance and the input reactance of the unloaded loop antenna should have the same absolute value. Second, the input impedance of the loaded loop must be purely resistive and its value needs to be two times of the input resistance of the unloaded loop. Third, the loading reactance should be chosen to be two times in value and opposite in sense of the input reactance of the unloaded loop. These conditions can be approximately met when the circular loop is placed above a ground plane. The loading reactance is determined from the input impedance of the unloaded loop and is optimized for an optimal performance of circular polarization. It is found that the reactive load must be capacitive and its value depends on the height of the loop above the ground plane and the thickness of the wire. The characteristics of the circular polarization and the input impedance of the capacitance-loaded circular loop antennas are investigated. An experimental example is presented to verify the theoretical prediction.     

A multicomposite, multilayered cylindrical dielectric resonator forapplication in MMIC's

A method, based on the mode matching technique, to study various resonant modes of a multicomposite, multilayered cylindrical dielectric resonator is presented. Dielectric sphere and cone resonators placed in practical environments are investigated. The calculated resonant frequencies show very good agreement compared with the numerical results of the finite difference method for the spherical resonator and with the measured values for the conical resonator, respectively. Experimental investigations show that the conical resonator can be coupled to a circular microstrip line, which has small size at high frequencies and may be integrated together with an oscillator     

Scattering from thin dielectric disks

A solution for scattering from thin dielectric disks has been obtained by approximating the currents induced inside the disk with the currents which would exist inside a dielectric slab of the same thickness, orientation and dielectric properties. This procedure yields an electrostatic approximation when the disk thicknessTis small compared to the wavelength of the incident radiation and yields a conventional physical optics approximation when the dimensionA, characteristic of the geometrical cross section of the disk, is large compared to wavelength. When the ratioA/Tis sufficiently large one or the other of these approximations applies, regardless of the frequency of the incident radiation. Consequently, the solution provides a conventional approximation for the scattered fields at allkA. As a check on this conclusion, a comparison has been made between measurements of the radar cross section of thin dielectric disks and the cross sections predicted using this theory. Agreement was found for thin disks with both large and small values ofkA.     

Inductance of a coil in a slotted shield

The inductance of a coil in a slotted, conducting shell is investigated. At frequencies for which the coil is very small relative to wavelength, an integral equation is formulated and numerically solved for the electric field induced in the slots. From knowledge of the slot electric field, the inductance of the coil in the slotted shell can be calculated. The dependence of inductance on the presence of the slotted shell is assessed quantitatively and explained on the basis of physical principles. The inductance of a one-turn loop inside a slotted shell is measured, and the data are found to be in excellent agreement with computations. As an unslotted shield closes on a coil, inductance is reduced noticeably, but if the shield possesses axial slots, the reduction is much less significant     

Radar cross sections of dielectric or plasma coated conducting spheres and circular cylinders

Radar cross sections for a variety of spherical and cylindrical scatterers having homogeneous dielectric or plasma shells are obtained by using both the exact boundary value solutions and approximate, semi-empirical methods based on physical principles. The plasma is assumed to have the macroscopic properties of a lossless dielectric with a permittivity less than that of free space. A superposition approximation for the radar cross section of a dielectric coated conducting body is obtained by considering the scattered field to be the phasor sum of two principal components, the field scattered by the air-dielectric interface and the field scattered by an equivalent conducting body which differs from the actual body because of the lens action of the shell. This approximation yields very good agreement with the exact solutions for both spherical and cylindrical dielectric clad scatterers with radii in the Rayleigh region and in the resonant region, and for bistatic scattering as well as for backscatter. The echo area of a conducting sphere with nonconcentric spherical dielectric shell calculated by means of the superposition approximation is in excellent agreement with experimental measurements, thus demonstrating the validity of this method in a case for which the exact solution cannot be obtained.     

Folded loop antenna for mobile hand-held units

The vertical folded loop antenna, modeled as wire and printed radiating element mounted on a conducting box, simulating a cellular telephone with and without dielectric coating, is analyzed. The finite-difference time-domain (FDTD) method is used to calculate radiation patterns and input impedance. The results are compared with measurements and with NEC data. Very good agreement is obtained in all cases. Parasitic loading is used to enhance the bandwidth of the printed element. The antenna meets the design requirements for existing and future mobile communication systems     

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     

Exact circuit analysis of spherical waves

A circuit representation for the impedance of spherical waves radiating from a spherical boundary was derived using a partial fraction expansion by Chu in order to establish gain bandwidth limitations for antennas. These circuits are derived directly from the recurrence relations for spherical Bessel functions. They provide an exact analog of the field solution both inside and outside of a spherical surface for any values of permittivity (epsilon) and permeability (mu). Since the circuits have the form of high-pass filters, they provide significant physical insight into scattering and radiation problems, suggest suitable asymptotic or approximate forms and allow circuit concepts and theorems to be brought to bear in order to solve specific configurations or to set general performance bounds. As an example of their application, compact computer programs for the radar cross section of conducting and dielectric spheres, the minimumQ's for antennas, theQ's and resonant frequencies of dielectric spheres and the induced current at the specular and shadow points of a conducting sphere are given. Other illustrative results are the short pulse responses of a solid dielectric and a dielectric-coated sphere computed using the circuit surge impedance and multiple reflection terms; and the response of a hypothetical scatterer in which the TE and TM modes are coupled.     

Measurement of AC magnetic field distribution using magneticresonance imaging

Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single-tone broadband frequency modulated (FM) waveform with the AC magnetic field strength determining the modulation index. An algorithm is developed to calculate the AC magnetic field intensity at each voxel using the frequency spectrum of the voxel signal. Experimental results show that the proposed algorithm can be used to calculate AC magnetic field distribution within a conducting sample that is placed in an MRI system     

Response of an overhead wire near a NEMP simulator

The response of an overhead wire illuminated by a simulated nuclear electromagnetic pulse (NEMP) was experimentally investigated. The wire is 70 m long, 7 mm in diameter, and situated 5 m above the ground. It is located 20 m away from a hybrid-type EMP simulator. The simulator is a resistively loaded elliptical loop structure with its pulse generator located 20 m above the ground. The overhead wire is terminated with various combinations of short circuit, open circuit, and characteristic impedance at the two ends, and the current response is measured at one end and in the middle. The measured responses are compared with calculated values from available analytical models