Showing papers on "Near and far field published in 1982"

A Novel Method to Analyze Electromagnetic Scattering of Complex Objects

Abstract: The finite-difference time-domain (FD-TD) method is proposed as a means of accurately computing electromagnetic scattering by arbitrary-shaped extremely complex metal or dielectric objects excited by an external plane wave. In the proposed method, one first uses the FD-TD method to compute the near total fields within a rectangular volume which fully encloses the object. Then, an electromagnetic-field equivalence principle is invoked at a virtual surface of this rectangular volume to transform the tangential near scattered fields to the far field. To verify the feasibility of this method, the surface currents, near scattered fields, far scattered fields, and radar cross section of two canonical two-dimensional objects are presented. For these cases, it is shown that the FD-TD method provides magnitude of current and field predictions which are within ± 2.5 percent and further phase values within ± 30 of values predicted by the method of moments ( MOM) at virtually every point including in shadow regions.

The signature of an air gun array: Computation from near‐field measurements including interactions

Abstract: We designed a system to enable the signature of an air gun array to be calculated at any point in the water from a number of simultaneous independent measurements of the near-held pressure held [subject of a patent application]. The number of these measurements must not be less than the number of guns in the array. The underlying assumption in our method is that the oscillating bubble produced by an air gun is small compared with the wavelengths of seismic interest. Each bubble thus behaves as a point source, both in the generation of seismic waves and in its response to incident seismic radiation produced by other nearby bubbles. It follows that the intcraction effects between the bubbles may be described in terms of spherical waves. The array of interacting guns is equivalent to a notional array of noninteracting guns whose combined seismic radiation is identical. The seismic signatures of the equivalent independent elements of this notional array can be determined from the near-held measurements. The seismic radiation pattern emitted by the whole array can be computed from these signatures by linear superposition, with a spherical correction applied. The method is tested by comparing far-field signatures computed in this way with field measurements made in deep water. The computed and measured signatures match each other very closely. By comparison, signatures computed neglecting this interaction are a poor match to the

Phased array antenna employing linear scan for wide-angle arc coverage with polarization matching

Abstract: The present invention relates to an antenna arrangement which includes a polarization diplexer capable of bidirectionally directing two orthogonally polarized signals along one path in the far field of the antenna arrangement and along two separate paths in the near field for interception along at least one of the paths by an array of feed elements. The array is arranged to provide a fixed linear phase taper along one axis across the array to produce or intercept a beam squinted at an angle 90 degrees-α to the face of the array and including a signal polarized in a first direction. Phase shifting means selectively produce a linear phase taper along a second axis across the face of the array orthogonal to the first axis to cause the beam to traverse a predetermined arc in the far field of view. Polarization mismatch at the array from the diplexer is overcome by providing a single properly inclined 90 degree polarization rotator or by two properly inclined 90 degree polarization rotators depending on the direction of polarization and whether the array is a linear or a two-dimensional array.

Optical near-field scanning microscope

Abstract: This optical near-field scanning microscope comprises an "objective" (aperture) attached to the conventional vertical adjustment appliance and consisting of an optically transparent crystal having a metal coating with an aperture at its tip with a diameter of less than one wavelength of the light used for illuminating the object. Connected to the aperture-far end of the "objective" is a photodetector via an optical filter and an optical fiber glass cable. Scanning the object is done by appropriately moving the support along x/y-coordinates. The resolution obtainable with this microscope is about 10 times that of state-of-the-art microscopes.

Non-ionizing radiation

Abstract: Publisher Summary This chapter discusses nonionizing radiation. Radiation may be defined as a flow or stream of atomic and subatomic particles and waves. The two main types of radiation are electromagnetic waves and moving atomic particles. In the case of electromagnetic waves, energy transfer occurs by means of oscillating electric and magnetic fields. Particulate radiation depends on the atom, which consists of a nucleus of neutrons and protons surrounded by a cloud of electrons. In terms of electromagnetic radiation, dielectrics are materials that are not perfect, and the amplitude of an electromagnetic wave is attenuated when it passes through such materials, with the resulting energy dissipated as heat in the material. Absorption in biological material results in heating. The degree of absorption is dependent on the electrical properties of the material—namely, the dielectric constant and electrical conductivity. Microwave and radiofrequency radiation at certain frequencies, power levels, and exposure durations produce biological effects.

Spatial discrimination in SQUID gradiometers and 3rd order gradiometer performance

Abstract: A detailed description of SQUID gradiometers is presented with the aim of improving their near field detection response while operating in unshielded, magnetically harsh urban environments. Spatial gradiometers of various orders are compared (0th, 1st, 2nd. and 3rd) and the effects on performance of constructional errors, near and far noise sources, and gradiometer baseline are examined. It is shown theoretically that the noise characteristics of a 3rd order spatial gradiometer are expected to be superior to that of lower order designs, particularly for noise sources located within intermediate distances (1.5–50 m) from the gradiometer. Following this analysis, a 3rd order SQUID gradiometer has been designed, constructed, and evaluated. Data are presented on the instrument noise characteristics for operation within an unshielded urban laboratory and examples of the 3rd order gradiometer response to various biomagnetic signals are shown. The results support the theoretical expectations and confirm that the...

Transition radiation caused by a chiral plate

Abstract: A simple calculation is made of the electromagnetic field radiated due to a charged particle traversing a plate of chiral material. The transition radiation from this chiral plate is round to differ from the usual dielectrie transition radiation. Discussion is presented placing in evidence the characteristics of the radiation and comments are made concerning the possible applicability of the transition radiation mechanism.

Electromagnetic fields coupled into a cavity with a slot-aperture under resonant conditions

Abstract: The electromagnetic coupling of an incident plane wave through a slot-aperture backed by a lossy rectangular cavity is analyzed by using a generalized network formulation based on an application of the equivalence principle. Two types of aperture-cavity resonances manifest themselves. The conditions for the existence and the characteristics of such double aperture-cavity resonances are studied. General expressions for field strength in the aperture, field distributions in the cavity, and maximum power penetration are derived. Appropriate expressions for an equivalent magnetic current to replace the aperture are discussed. A numerical example is given.

Light scattering from macroscopic spherical bodies. I. Integrated density of states of transverse electromagnetic fields

Abstract: The integrated density of states (IDS) of light is examined by introducing a pseudopotential for the scattering of transverse electromagnetic fields. The two formulas, electromagnetic versions of the well-known formulas of electrons, are given for the change of IDS due to spherical scatterers, the one for a single scatterer and the other for a periodic array of scatterers. It is shown that they are expressed in terms of the phase shifts of light and have the forms quite analogous to the electronic counterparts except that the number of the phase shifts is doubled due to the transversality of electromagnetic fields.

A simple technique for solving E -field integral equations for conducting bodies at internal resonances

Abstract: A simple but effective method is presented to analyze electromagnetic radiation and scattering from condueting bodies at frequencies corresponding to internal resonances of a cavity of the same shape. The advantage of this technique is that it requires only the E -field integral equation and hot both E -field and H -field as required by the combined fields formulation. It is shown theoretically that this method produces a solution with minimum norm and converges monotonically as the order of the approximation is increased. The minimum norm solution for the current density given by the E -field integral equation is not the correct current density as there is a portion of the resonant current that exists on the body. However, the minimum norm solution indeed provides the true scattering fields. This technique may also be utilized for obtaining a minimum norm solution for nearly singular and singular matrix equations. Examples are presented to illustrate the application of this technique.

Method of determining excitation of individual elements of a phase array antenna from near-field data

Abstract: A near-field data measurement technique which provides sufficient data to enable the resolution of array element characteristics which are localized within a circle having a radius less than 0.61λ is disclosed. This allows phase correction of individual array elements having spacings substantially less than 0.61λ during the alignment of a phase array in a near-field test system.

Near-field absorption in a circular cylinder from electric and magnetic line sources

Abstract: A homogeneous, lossy circular cylinder is used as a simple model of a biological object in which interior heating is produced by the absorption of electromagnetic waves. For this model, we determined the optimum frequency, polarization, orientation and shape of applicators. Analytical and numerical results are given for both electric and magnetic line sources, with three different polarizations relative to the cylinder. Coupling efficiencies and contour plots are presented for a range of parameters. One particularly interesting result is the production of maximum energy deposition at the center of a cylinder of muscle tissue when exposed in the 100-MHz frequency range by the use of four applicators surrounding the cylinder.

Scattering by a strip with two face impedances at edge‐on incidence

Abstract: A high-frequency solution for the diffraction from a strip with two arbitrary face impedances, illuminated at edge-on incidence, is obtained by a spectral extension of the geometrical theory of diffraction. An asymptotic approximation of the solution given by Maliuzhinets for the half plane is used. Uniform expressions for the scattered far field are given for cylindrical and plane wave illuminations. Incidence perpendicular to the edges of the strip is considered in both TE and TM cases. In the case of plane wave illumination the expressions for the field are greatly simplified. Numerical results are presented. In particular for resistive strips, the backscattered field calculated from this solution compares very well with that calculated by other techniques.

Free surface displacements in the near field of a tensile crack expanding in three dimensions

Abstract: The discrete wave number method is applied to the study of the elastic motions due to an arbitrary three-dimensional tensile source. The source elastic field is formulated as a superposition of plane waves propagating in discrete directions. The discretization stems from an assumption of two-dimensional periodicity in the description of the source. Examples of the free surface motion induced in the near field of circular and rectangular dislocation sources expanding in a homogeneous half space are discussed.

Far field radiated by rectangular patch microstrip antenna

Abstract: Using the volume equivalence theorem, we introduce electric and polarisation current distributions for a rectangular patch microstrip antenna. These distributions are known from transmission-line-model analysis. Literal formulas are given especially to express the E-plane radiation pattern and the worst-case crosspolarisation level. Theoretical results are in good agreement with experiments.

Absorption Characteristics of Prolate Spheroidal Models Exposed to the Near Fields of Electrically Small Apertures

Abstract: Irradiation of prolate spheroidal models of biological models by the near fields of electrically small apertures is analyzed. The solution procedure involves the replacement of the aperture source by an equivalent configuration of electric and magnetic dipoles. The specific absorption rate (SAR) induced in the irradiated object is then calculated using the extended boundary condition method (EBCM). Numerical results are presented for the exposure case where the incident electric field at the spheroid location is parallel to the major axis of the model. This polarization is associated with the maximum low-frequency absorption in biological models and, hence, is the most important polarization case in microwave dosimetry.

Irradiation of prolate spheroidal models of humans and animals in the near field of a small loop antenna

Abstract: Analysis of the near-field irradiation of prolate spheroidal models of humans and animals by a small coaxial loop antenna is described. The near fields of the antenna are known exactly and hence are used to identify the suitable field parameters involved in the near-field absorption in the spheroidal model. An integral equation is formulated in terms of the transverse dyadic Green's function, and the fields radiated by the current loop are expanded in terms of the vector spherical harmonics. The extended boundary condition method is then employed to solve the integral equation. The power distribution and the average specific absorption rate (SAR) are calculated and plotted, for different human and animal models, as a function of the separation distance from the loop. It is shown that for distances less than 5λ the average SAR values oscillate about the far-field value. In particular, for d/λ < 0.4 an increase in the average SAR values was generally observed. It is also shown that in spite of the complicated nature of the near fields the absorption characteristics can still be explained in terms of the incident radiation. Furthermore, from the calculated SAR distributions at different frequencies it is shown that at all frequencies, excessive heating occurs at the surface of the spheroid while a limited absorption occurs in the central region around the major axis. This result is of particular importance in hyperthermia, where extensive efforts are being directed toward achieving deep-tissue heating by a coaxial coil carrying RF power at about 27 MHz.

Method and apparatus for the electronic measurement of the thickness of very thin electrically conductive films on a nonconductive substrate

Abstract: The invention concerns a method for measuring the thickness of very thin, electrically conducting films of the order of 1 to 1000 nanometers and measuring apparatus for performing the method. The thickness of the electrically conducting films is measured by means of high-frequency electromagnetic waves in the microwave range by use of the surface resistance and consideration of the conductivity of the film as a measure of the film thickness. The thickness determination makes use of the reflection factor and/or the transmission factor of the microwave on the conductive film. This can be done with the aid of two antennas in free space in the far field of these antenna arrangements or, if the antennas are designed as waveguide flanges, it can also be done in the near field, in which case the waveguide flanges are arranged opposite each other at a very small distance.

Lens and mount for use with electromagnetic wave source

Abstract: A lens cap and mount for use with an electromagnetic radiation emitter or sensor in the cap has an integral lens for collimating or converging the emitted or received radiation.

Numerical and Experimental Results for Near-Field Electromagnetic Absorption in Man

Abstract: Experimental results are presented for the whole-body-average energy absorption and the internal E-fields in man exposed to leakage-type near fields. An empirical relationship, previously presented.

Electromagnetic Wave Theory and Remote Sensing

Abstract: Electromagnetic waves are studied with applications to microstrip antennas, 1'2 geophysical subsurface probing,3' 4 scattering from helical structures5 '6 and Smith-Purcell radiation from metallic gratings.7 Radiation and resonance characteristics of two coupled circular microstrip disk antennas are studied rigorously using numerical techniques, matched asymptotic analysis, and the newly developed Hankel transform analysis.1,2 The electromagnetic fields due to dipole antennas in a two--layer dissipative medium is solved using the quasistatic approximation. 3 The solutions in integral forms are calculated with brute force numerical integration methods, the multi-image approach with the steepest descent method, the normal mode approach with the residue method, and a hybrid approach with combinations of the above methods. Electromagnetic wave scattering from helical structures has been studied using the physical optics and modal approaches. The fields scattered by a thin wire helix of finite extent is investigated using physical optics and the geometrical theory of diffraction.5 The geometrical theory of diffraction result is obtained as a high frequency limit of the physical optics approximation by applying the saddle point technique to the integral representation of the physical optics field. The electromagnetic wave scattering by a tape helix of infinite extent is studied by using Floquet wave expansion for its guided modes and scattered fields.6 The Smith-Purcell radiation problem is solved taking into account the penetrable properties of metallic gratings.7 When an electron beam streams across the surface of a metallic grating, emission of electromagnetic radiation occurs. It is shown that maximum radiation occurs when the surface plasmon mode is excited.

On a free-electron-laser in a uniform magnetic field - A solution for arbitrarily strong electromagnetic radiation fields

Abstract: Exact expressions are derived for the gain of a free-electron laser based on a uniform longitudinal magnetic field configuration operating in the single-particle, low-gain regime. The gain is calculated for different parameters of the system. For strong enough a field, the gain decreases with an increase of the amplitude, becoming negative when passing a threshold value, which depends on the system parameters. Implications regarding the saturation of the lasing process are discussed.

Out-of-plane scattering from glass waveguides: comparison of theory and experiment.

Abstract: The angular distribution of the radiation emitted into the far field by TE0 and TM0 optical guided waves was measured and compared with a theory assuming surface roughness to be the source of the scattering. The comparison with theory was made in such a way that a detailed knowledge of the surface under investigation was not required.