Showing papers on "Near and far field published in 1994"

A new aperture admittance model for open-ended waveguides

Abstract: A new model for the aperture admittance of open-ended waveguide structures radiating into a homogeneous, lossy dielectric is presented. The model is based on the physical and mathematical properties of the driving point admittance of passive, stable one-port networks. The model parameters, which depend upon the geometry of the waveguide and aperture, are determined from a relatively small number of computed admittances. This computed data is obtained by a full-wave moment method solution and, hence, includes the effects of radiation and energy storage in the near field and the evanescent waveguide modes. The accuracy of the numerical method is demonstrated by comparison with measured values. As an example, the model parameters are determined for the coaxial-line geometry. The accuracy of the model, for both the direct and inverse problem, is verified and a rigorous sensitivity and uncertainty analysis is performed. The new model has important applications in the field of dielectric spectroscopy. >

Electromagnetic response of material media

Abstract: General Theory of Interaction of Electromagnetic Field with Matter. QuantumMechanical Theory of Linear and Nonlinear Susceptability. Plasma in Electromagnetic Field. Atoms and Molecules in Electromagnetic Field. Interaction of Electromagnetic Radiation with Crystals. Index.

The electromagnetic field of a vertical electric dipole over the Earth or sea

Abstract: The analysis of King (1990) of the electromagnetic field of a vertical electric dipole over an imperfectly conducting half-space is applied to obtain the far field when the dipole is at a specified height d The contributions by the space wave with its 1/r dependence and the lateral surface wave with its 1/r/sup 2/ dependence are separated and studied in detail when the dipole is over a wide range of media such as sea water, wet and dry Earth, lake water and dry sand Graphs of the far-field patterns are shown >

Authors' reply [to comments on "Theoretical model for an MRI radio frequency resonator"]

Abstract: A theoretical model of magneto-acoustic current imaging is derived, based on fundamental equations of continuum mechanics and electromagnetism. In electrically active tissue, the interaction between an applied magnetic field, B, and action currents, J, creates a pressure distribution. In the near field limit, this pressure obeys Poisson's equation, with a source term (∇ x J ) · B. The displacement and pressure fields are calculated for a dipole (q), oriented either parallel or perpendicular to the applied magnetic field (B), at the center of an elastic, conducting sphere (radius a, shear modulus G). Surface displacements are on the order of qB/(4πGa), which is about 1 nm for typical biological parameters. If the applied magnetic field is changing with time, eddy currents induced in the tissue may be larger than the action currents themselves. The frequency of the pressure and displacement arising from these eddy currents, however, is twice the frequency of the applied magnetic field, so it may be possible to eliminate this artifact by filtering or lock-in techniques. Magneto-acoustic and biomagnetic measurements both image ∇ x J in a similar way, although magneto-acoustic current imaging has the disadvantage that acoustic properties vary among tissues to a greater degree than do magnetic properties.

A fast integral-equation solver for electromagnetic scattering problems

Abstract: We describe a new fast integral-equation solver applicable to large-scale electromagnetic scattering problems. In our approach, the field generated by a given current distribution M decomposed into near and far field components. The near field is computed using the conventional method of moments technique with the Galerkin discretization. The far field is calculated by approximating the original current distribution by an equivalent current distribution on a regular Cartesian grid, such that the two currents have identical multipole moments up to a required order m. As the result of this discretization, the original full impedance matrix is decomposed into a sum of a sparse matrix (corresponding to the near field component) and a product of sparse and Toeplitz matrices (corresponding to the far field component). Because of the convolution nature of the Toeplitz kernel, the field generated by the equivalent current distribution can be then obtained by means of discrete fast Fourier transforms. The single computational domain solver based on our formulation requires both memory and computation time of order O(N log N) (in volume problems) or O(N/sup 3/2/) (in surface problems), where N is the number of unknown current elements. In the domain-decomposed parallelized version of the solver, with the number P of processors equal to the number of domains, the total memory required in surface problems is reduced to O(N/sup 3/2//P/sup 1/2/). The corresponding speedup factor is equal to the number of processors P. During the talk, applications of the solver to 2- and 3-dimensional electromagnetic volumetric and boundary-value problems will be presented. >

Current and SAR induced in a human head model by the electromagnetic fields irradiated from a cellular phone

Abstract: The near field finite-difference time-domain method was used to calculate the current and specific absorption rate (SAR) distributions in an inhomogeneous model of a human head exposed to the electromagnetic waves irradiated from a cellular phone. The human head was simulated by a model of 57,263 block cells with inhomogeneous dielectric constant and conductivity. The cellular phone was modeled by an equivalent dipole antenna with an equivalent resistor of 120 ohms located at the center gap between the two arms of this dipole antenna. The transmitted power of the cellular phone was assumed to be 0.6 watts at a frequency of 835 MHz. For the head near the dipole antenna in the range of 1.0/spl sim/2.5 cm, the maximum currents and SAR's induced in the head were found in the ranges of 356/spl sim/551 mA and 1.23/spl sim/2.63 W/kg, respectively. It was also found that the maximum SAR induced in the head was below the IEEE's upper safety limit of 1.6 W/kg for the head to keep a distance from the dipole antenna by longer than 2.0 cm. >

Planar near-field scanning in the time domain .1. Formulation

Abstract: Time-domain planar near-field measurement techniques are formulated for acoustic and electromagnetic fields. Probe correction is ignored in that it is assumed that the probe measures the exact values of the field on the scan plane. Two fundamentally different approaches are used in deriving three sets of formulas that give the fields in the source-free half space z>z/sub 0/ in terms of their values on the scan plane z=z/sub 0/. In the first approach the time-domain formulas are obtained by inverse Fourier transforming the corresponding frequency-domain formulas. In the second approach the time-domain formulas are derived directly in the time domain by working with time-domain Green's functions. >

Near field measurements of optical channel waveguides and directional couplers

Abstract: Near field microscopy is used to investigate the guided mode intensity distribution of optical channel waveguides and directional couplers with subwavelength spatial resolution. The directional coupler consisted of two single mode optical ridge channel waveguides formed with silicon nitride deposited on a lower cladding layer of SiO2 on a silicon substrate. A near field measurement of the guided mode intensity profile in the transverse direction parallel to the waveguide surface was performed across one of the optical channel waveguides. These variations are compared with model calculations. Similar transverse measurements of light propagating through a directional coupler were performed at many locations along the coupler, providing a view of the evolution of optical power transfer.

Scattering of a surface plasmon polariton by a surface defect.

Abstract: By means of a Green-function, volume-integral-equation approach we study numerically the scattering of a surface plasmon polariton at a planar vacuum-metal interface by dielectric and metallic defects that are either embedded in the metal substrate or are situated in the vacuum region on the substrate. We calculate the transmission and reflection coefficients for the surface plasmon polariton, as well as the efficiency of its conversion into volume electromagnetic waves in the vacuum propagating away from the surface. We also compute the near field in the vicinity of the surface defect.

Planar near-field to far-field transformation using an array of dipole probes

Abstract: A method is presented for computing far-field antenna patterns from measured near-field data measured by an array of planar dipole probes. The method utilizes the near-field data to determine some equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna. These currents are then used to find the far fields. The near-field measurement is carried out by terminating each dipole with 50 /spl Omega/ load impedances and measuring the complex voltages across the loads. An electric field integral equation (EFIE) is developed to relate the measured complex voltages to the equivalent magnetic currents. The mutual coupling between the array of probes and the test antenna modeled by magnetic dipoles is taken into account. The method of moments with Galerkin's type solution procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient-fast Fourier transformation (CG-FFT) method exploiting the block Toeplitz structure of the matrix. Numerical results are presented for several antenna configurations to show the validity of the method. >

Near field scattered by a dielectric rod below a metallic surface

Abstract: Scattering of a plane wave by a two-dimensional particle (i.e., a rod) below a metal surface is examined by means of a volume-integral approach based on Green’s dyadic. A detailed study of the structure of the near field when surface plasmons are excited is reported. The role of image enhancement is described for p polarization. A method of measuring the dielectric constant of a metal based on the structure of the near field is proposed.

Heating patterns in biological tissue phantoms caused by millimeter wave electromagnetic irradiation

Abstract: Distribution of millimeter wavelength electromagnetic energy absorption in surface layers of biological tissue models was studied using methods of Infrared Thermography. 0.1 mm thin-layer phantoms were irradiated in the near field using different types of horn antennas in the 37-78 GHz frequency range. Heating patterns were recorded during microwave irradiation, and surface SAR distributions mere calculated. The temperature resolution was better than 0.05 K. It was found that horn antennas produced nonuniform heating patterns in irradiated objects. These nonuniform patterns were due to a geometrical resonance resulting from a secondary wave-mode interaction between an irradiated object and the corresponding critical cross-section of the horn antenna. Local SAR values in hot spots exceeded the spatially averaged values by over 10 times, and the widths of these hot spots at 5 times the average SAR were often 1 mm or less. The location, quantity, number and size of the local field absorption maxima of irradiated objects strongly depended on the frequency of electromagnetic irradiation, with equivalent Q-factors of 500 or more. These findings provide an explanation for a number of frequency-dependent effects of millimeter wave electromagnetic irradiation. >

The bi-polar planar near-field measurement technique, part II: near-field to far-field transformation and holographic imaging methods

Abstract: A novel customized bi-polar planar near-field measurement technique is presented in a two-part paper. This bipolar technique offers a large scan plane size with minimal "real-estate" requirements and a simple mechanical implementation, requiring only rotational motions, resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper introduced the bi-polar planar near-field measurement concept, discussed the implementation of this technique at the University of California, Los Angeles (UCLA), and provided a comparative survey of measured results. This paper examines the data processing algorithms that have been developed and customized to exploit the unique features of the bi-polar planar near-field measurement technique. Near-field to far-field transformation algorithms investigated include both interpolatory and non-interpolatory algorithms due to the a typical arrangement of the bi-polar near-field samples. The algorithms which have been tailored for the bi-polar configuration include the optimal sampling interpolation (OSI)/fast Fourier transform (FFT), Jacobi-Bessel transform, and Fourier-Bessel transform. Additionally, holographic imaging for determination of antenna aperture fields has been incorporated to facilitate antenna diagnostics. Results for a simulated measurement of an array of infinitesimal dipoles and a measured waveguide-fed slot array antenna are included. Appropriate guidelines with respect to the advantages and disadvantages of the various processing algorithms are provided. >

Far field sheaths from waves in the ion cyclotron range of frequencies

Abstract: Experimental evidence suggests that unabsorbed wave energy in ion cyclotron range of frequency fast wave (FW) experiments can result in deleterious edge interactions. A model describing the formation of far field sheaths due to FW interaction with material surfaces is presented. Near conductors that do not conform to flux surfaces, an incoming FW causes the generation of a slow wave (SW) component. The E∥ of the SW drives an RF sheath, in a manner similar to what has been previously discussed for antenna (near field) sheaths. To assess the importance of the proposed mechanism, a heuristic scaling model of the resultant sheath voltage V is developed and compared with a numerical code. The model illustrates the important dependencies of V on the single pass absorption, edge density, FW frequency, FW cutoff location, and limiter/wall geometries and yields qualitative agreement with the experimental observations.

Importance of confined fields in near-field optical imaging of subwavelength objects

Abstract: The detailed imaging process of subwavelength objects deposited on a planar surface is studied within the framework of a three-dimensional model of scanning near-field optical microscope. The model consists of a truncated pointed fiber approaching a planar surface on which a three-dimensional protrusion is deposited. For this geometry, Maxwell's equations are solved exactly by applying the field-susceptibility method in the direct space. The technique provides precise evaluations of the physically relevant near and far fields. In order to refine the understanding of the imaging process of subwavelength objects, we present simulated images of low-symmetry protrusions for two different modes of polarization and as a function of the approach distance. These simulations show clearly that subwavelength surface defects induce confined optical near-field distributions that are directly related to the shapes of the objects. We conclude that the central problem of near-field optical microscopy is the optimal detection of the confined fields that are set up by the objects themselves.

Phase conjugation of an optical near field

Abstract: Using degenerate four-wave mixing in an Fe:LiNbO(3) 0.04-wt. % crystal and an external-reflection near-field optical microscope, we have achieved phase conjugation of light emitted by a fiber tip. We observe that the phase-conjugated light at a wavelength of 633 nm can reach a power of ~0.1 nW and produce a 180-nm-wide spot image in the near-field microscope. This is the first direct demonstration, to our knowledge, of the phase conjugation of near-field components of optical fields.

Frequency independence property of radiation spatial filters

Abstract: It is known that a time harmonic boundary normal velocity field of a radiating object immersed in an acoustic fluid can be decomposed into a (efficiently) radiating and a non (efficiently) radiating components, belonging to two orthogonal ‘‘radiating’’ and ‘‘evanescent’’ subspaces. Numerical results show the validity for a capped cylinder of the conjecture that the ‘‘radiating’’ subspace at a certain frequency f1 is a subspace of the radiating subspace at frequency f2≳f1. The implications are (a) from the output of a bank of spatial filters designed for the max frequency of operation the far field can be identified over the entire frequency band, and, perhaps more importantly, (b) if in an active control scheme the outputs of the filters are driven to zero, broadband cancellation of the radiating noise is achieved.

A modified dual space method for solving the electromagnetic inverse scattering problem for an infinite cylinder

Abstract: A modified dual space method is used to solve numerically the electromagnetic inverse scattering problem for an infinite cylinder. An eigenvalue problem for the far field operator is introduced and its usefulness in studying the inverse scattering problem is examined.

Electromagnetic scattering by indented screens

Abstract: The problem of three dimensional electromagnetic scattering from a perfectly conducting screen with a bounded indentation is formulated as a system of boundary integral equations for the electric current density on the cavity wall and the interface between the cavity and free space. It is shown how the fictitious current density on the interface may be eliminated resulting in an integral equation of the second kind for the current density on the cavity wall only, with no integration over the infinite screen. In addition, integral representations are derived that represent the field everywhere in space in terms of the current density on the cavity wall only. Furthermore, asymptotic expressions for the far field are also presented. The equations and representations simplify considerably in the two-dimensional scalar case and results are presented for both TE and TM polarization. >

Experimental evaluation of a new expression for the far field of a diode laser beam.

Abstract: The validity of a newly derived expression for the far field of a diode laser beam is examined by comparison of the measured intensity profiles with the calculated ones for typical double-heterostructure diodes. I also clarify which of the new and conventional expressions for the intensity profiles in the plane normal to the junction is more useful. The cause of the observed discrepancy between the measured and calculated profiles in the plane parallel to the junction is investigated by consideration of the higher-order Hermite-Gaussian modes. Further, I examine how the axial intensity varies with the propagation distance.

Calculation of radiated electromagnetic fields from cables using time-domain simulation

Abstract: Radiated electromagnetic fields are produced by currents in cables or transmission lines interconnecting various circuits. An elegant method of computing the resultant electromagnetic field, produced by several radiating current elements, is given. The current in each radiating cable is first found from a time-domain simulation algorithm and this may be a steady-state or transient current. The radiated field is then calculated by assuming that a radiating transmission line can be treated as a chain of short radiating dipoles. The problems associated with the calculation of the near-zone term at low frequencies and the overall response near the radiator are clarified. The proposed technique is fully evaluated and compared with other methods. >

Simulation of Multidimensional, Anisotropic Ground Motions

Abstract: A recently developed technique for the digital simulation of spatially incoherent seismic ground motions is extended to include two or three spatial coordinates. This extended approach allows for four situations: (1) Simulation in the far field; (2) simulation in the near field of a small source; (3) simulation in the near field of an extended source; and (4) simulation in the near field of a limited source. In each situation, both direction‐dependent (isotropic) and direction‐dependent (anisotropic) coherency structures are accounted for. The developments proposed may facilitate the seismic analysis of structures involving foundation coupling in two or three dimensions accounting for ground motion spatial incoherence, response nonstationarity, and system nonlinearities. They can also be useful in evaluating the response of structures to other spatially variable random fields such as those of wave forces and turbulent winds.

Motion induced remote field eddy current effect in a magnetostatic non-destructive testing tool: a finite element prediction

Abstract: A hitherto unobserved phenomenon motion induced remote field eddy current effect, is presented in this paper. A numerical study of the non-destructive inspection of tubing with conducting walls, using a DC electromagnetic probe led to the detection of this interesting effect. This paper describes a bi-directional transmission of the electromagnetic field energy through the tube walls, similar to the phenomenon responsible for the Remote Field Eddy Current (RFEC) effect in eddy current (an AC electromagnetic nondestructive testing tool) inspection of tubing. Thus far it was considered that the RFEC effect by the nature of its physics was possible only in the presence of AC excitation in tubular geometries. However, it is shown in this paper that currents induced by magnetic flux moving over conducting material produce a RFEC effect even when a DC probe is used. This phenomenon may enable extraction of valuable information regarding the entire thickness of the tube wall from measurements made on the same side as the excitation source. >

Optical fiber characterization by simultaneous measurement of the transmitted and the refracted near field

Abstract: An experimental setup is presented which permits, in a routine way for R&D purposes, simultaneous measurement of the transmitted near field at 1300 nm and 1550 nm, and the refracted near field at 820 nm. A new method for the calibration of the refractive index is proposed. The obtained accuracy for the refractive index is dn+or-0.0002. The reproducibility of measurements of geometrical parameters like the mode field diameter, the core and cladding diameters, and concentricity error, is +or-0.1 mu m. Measurements of the mode field as a function of polarization state for four different hi-bi fiber are presented. >

Near-field corrections to site attenuation

Abstract: The theoretical model used for calculating normalized site attenuation for broadband antennas in ANSI C63.4-1992 and for antenna calibration in ANSI C63.5-1988 includes only the radiation terms in the electric field. The omission of the near field terms leads to errors of as much as 2.0 dB at 30 MHz for horizontally polarized antennas separated by 3 m. Corrected values of normalized site attenuation and E/sub D//sup max/ are presented for the 30-300 MHz frequency range. >

Far-field predictions from near-field measurements using an exact integral equation solution

Abstract: This paper presents a new approach to derive far-field data needed in antenna and EMI/EMC testing from near-field measurements. An exact integral equation solution to the wave propagation problem is used to transform the near-field data to the far field. The method requires near-field measurements on two closed surfaces enclosing all sources and inhomogeneities. The approach is validated with numerical simulation of measurements of fields radiated from a known antenna. >

Theoretical and experimental near-field characterization of perforated shields

Abstract: The shielding performance of an infinitely large, periodically perforated plane shield is investigated, both theoretically and experimentally. Attention is focused upon the near-field characterization by using two loop antennas at both sides of the shield. The electromagnetic coupling between the two loops is analyzed numerically using a plane-wave spectral representation of the radiated field and the method of moments. Comparison of these numerical results with near-field measurements yields a very good agreement up to high frequencies. The predicted classical far-field shielding effectiveness drastically overestimates the near-field shielding performance of the screen. >