Showing papers on "Near and far field published in 1974"

Electromagnetic Fields Induced Inside Arbitrarily Shaped Biological Bodies

Abstract: A theoretical method has been developed to determine the electromagnetic field induced inside heterogeneous biological bodies of irregular shapes. A tensor integral equation for the electric field inside the body was derived and solved numerically for various biological models.

Multipole expansions and plane wave representations of the electromagnetic field

Abstract: A new and conceptually simple derivation is presented of the multipole expansion of an electromagnetic field that is generated by a localized, monochromatic charge‐current distribution. The derivation is obtained with the help of a generalized plane wave representation (known also as the angular spectrum representation) of the field. This representation contains both ordinary plane waves, and plane waves that decay exponentially in amplitude as the wave is propagated. The analysis reveals an intimate relationship between the generalized plane wave representation and the multipole expansion of the field and leads to a number of new results. In particular, new expressions are obtained for the electric and magnetic multipole moments in terms of certain components of the spatial Fourier transform of the transverse part of the current distribution. It is shown further that the electromagnetic field at all points outside a sphere that contains the charge‐current distribution is completely specified by the radiation pattern (i.e., by the field in the far zone). Explicit formulas are obtained for all the multipole moments in terms of the radiation pattern.

Near field transient radiation patterns for circular pistons

Abstract: The exact impulse response of field parameters for any field point on or off axis for the case where a circular disc radiator face is subjected to a displacement step corresponding to a velocity impulse is reviewed. By convolution, the transient field pattern for any arbitrary motion of the disc can be obtained. The exact response for a half-sine monopulse is computed. An approximate representation of the transient pressure response to the velocity impulse input at the disc is derived, and it is shown to correspond to the replica pulses described previously. The regions of validity of the approximation are quite limited and the replica pulses are displaced in time from the positions formerly attributed to them. The displaced replica approximation is applied to an examination of the structure of the near field for continuous sinusoidal excitation and a plot of positions of extrema is produced. It is shown that this approximation gives good agreement with the exact values and is superior to the previous published approach in this regard. For short sinusoidal pulses the effect of pulse length on the field pattern, and of field point on the time history of a transient wave are shown. When the excitation is a short sinusoidal pulse the effect of the pulse length and field point position on the field pattern and wave shape are demonstrated.

Application of electromagnetic particle simulation to the generation of electromagnetic radiation

Abstract: A three velocity and one‐space dimensional nonrelativistic electromagnetic particle simulation code employing the fast Fourier transform algorithm is described and used to simulate the amplification of electromagnetic radiation by an electron beam passed over a rippled static magnetic field. In the beam frame the rippled magnetic field looks like an intense electromagnetic pump and thus a parametric instability can be produced. In one case, it was observed that 30% of the beam energy was converted to electromagnetic radiation.

Efficient Numerical Computation of the Frequency Response of Cables Illuminated by an Electromagnetic Field (Short Papers)

Abstract: Computationally efficient numerical methods for determining the frequency response of uniform transmission lines consisting of a large number of mutually coupled conductors in homogeneous and inhomogeneous media, and illuminated by an electromagnetic (EM) field are presented.

Unstable resonator system producing a high irradiance beam in the far field

Abstract: Laser systems comprising an unstable resonator and optics for increasing the magnification of the output beam from the resonator to concentrate the power distribution in the far field are disclosed. The resonator device produces a beam of laser energy which is an annulus in cross section and matched optics change the energy distribution in the annulus; both refractory and reflecting optics are disclosed. Also graphs describing the intensity distribution of the annular beam under various conditions in both the near field and far field locations in the optical system are provided.

A unified theory on radiation of a vertical electric dipole above a dissipative earth

Abstract: Analytical expressions are derived for the electromagnetic field associated with a vertical electric dipole (VED) above a dissipative earth. The scattered field is shown to consist of a direct contribution from a perfect image source, and a correction due to the finite conductivity of earth which is expressible in terms of an incomplete Hankel function. The resultant expression readily reduces to the well-known asymptotic solutions in both the space-wave and ground-wave regions. Validity of the space-wave expression in the far zone is found to be dependent upon the observation angle as well as the refractive index of earth. In the near field, alternative approximate formulas in closed form are derived from the incomplete Hankel function expression for different elevation angles. These formulas are shown to yield accurate results whenever the distance from the image source is greater than about 0.2λc where λc is the effective wavelength (or skin depth) of earth. For distance smaller than 0.2λc, however, an analytical expression based upon a quasistatic approximation is generally applicable and has been discussed elsewhere.

Anomalous absorption of intense electromagnetic waves in plasma at high magnetic fields

Abstract: The authors present conclusive evidence of anomalous absorption of an intense RHCP wave at high magnetic fields omega ce/ omega >1, where omega ce and omega are the electron cyclotron frequency and the frequency of the electromagnetic wave.

An Introduction to Electromagnetic Theory

Abstract: Preface Part I. Basic Concepts: 1. Charged particles, force and field 2. Charge and current density 3. Practical units and magnitudes 4. Development of the theory Part II. The Vacuum Field of Charge and Current: Mazwell's Equations: 1. The field of static charge 2. The field of steady current 3. Faraday's law of induction 4. Maxwell's 'displacement' current 5. Maxwell's equations Part III. Electrostatics: 1. Field and potential 2. Energy 3. Potential theory 4. Dielectrics Part IV. The Magnetic Field of Steady and Slowly Varying Currents: 1. Steady currents in a conductor 2. Currents and field 3. Energy 4. Circuits 5. Magnetic media Part V. Electromagnetic Waves: 1. Maxwell's equations 2. Monochromatic plane waves in vacuum 3. Monochromatic plane waves in media 4. Radiation Part VI. The Electromagnetic Properties of Media: 1. Polarization and magnetization 2. Free electron gas 3. Dielectrics 4. Magnetic media Appendix Index.

On the field distribution of an aperture

Abstract: A study of the field distribution in a square aperture on a plane conducting screen illuminated by a plane electromagnetic wave at normal incidence is presented. In solving the problem, the square aperture is first formulated into a complementary square plate scattering problem. The distribution of current on the plate is then found by solving the corresponding wire mesh model with the aid of a computer program for linear wire antennas. A simple averaging process is then introduced in a straightforward manner to convert the currents flowing along the wires into continuously distributed equivalent surface currents on the square plate. The surface current density is, in turn, equal to the value of magnetic field intensity by the boundary condition for a perfect conductor. Finally, a direct application of the electromagnetic form of Babinet's principle gives the electric field distribution of the aperture. The far zone field in terms of a radar cross section of a square plate compares well with an experiment. A discussion is also included comparing the electric field distribution of a square aperture and circular apertures.

Decay of a plasmon into two electromagnetic waves

Abstract: In a plasma with a magnetic field, a plasmon can decay into two electromagnetic waves: a left-hand-polarized electromagnetic wave and right-hand-polarized whistler wave, both propagating in the direction of the magnetic field. Use of this process for a plasma laser is discussed.

Enhanced radiation coupling from unstable laser resonators

Abstract: Method and apparatus for providing a large fraction of the energy coupling from an unstable resonator in the central lobe of the far field pattern of the output beam from the resonator are disclosed The resonator is designed to allow a low order mode pattern to resonate about the optical axis therethrough and radiation is coupled out from both the axial region and the outer region of the mode The geometry of the systems is formed with transmissive and reflective optics in various configurations of linear and ring lasers

Noise generated in the total internal reflection electro-optic diffraction deflector/modulator

Abstract: The total internal reflection electro-optic deflector/modulator has so far been limited by the subdiffraction patterns that appear in the far field when light is transmitted through the device with zero applied voltage. This paper isolates the causes of these subdiffraction patterns and describes a simple means of removing them from the far field without affecting the performance of the device. The results of measurements taken at zero applied voltage are included and give an indication of the order of this effect.

Slit detector for optical detector system

Abstract: An optical system for detecting a time-varying signal spatially recorded upon a disc as a light diffraction grating in the form of a spiral track. The disc is driven, relative to a reading spot to develop a time-varying diffraction pattern comprising at least a zero order, +1 order and a -1 order. An aperture, having a dimension in the direction of the track which is less than onehalf the distance across which the zero order is distributed, is positioned in the far field of the diffraction pattern near the boundary of the zero order. A photodetector positioned adjacent the aperture derives an electrical signal representative of the signal spatially recorded on the disc.

Tolerances in optical mixing

Abstract: Analytical expressions are derived for the power spectral density of the current in optical heterodyning when phase front curvature mismatch, tilt or axis displacement occurs between the beating light beams. The signal beam is the far field of an incoherent source of circular shape, and the local oscillator is the fundamental mode of a stabilized laser. The cases in which the signal beam either illuminates the detector coherently or has a coherence area small compared with the detector area are shown to have simple solutions. An analogous analysis is carried out for homodyning (self-beating). The dependence of the signal-to-noise ratio on the detector aperture and optimum and tolerance values for the latter are obtained for a given misalignment.

High-Resolution Acoustic Imaging by Contact Printing

Abstract: In this paper we will present a technique which allows us to record high-resolution acoustic images of biological specimens. The image is detected in the extremely near field of the object and the imaging, itself, is accomplished in a thin liquid layer containing 1 µm polystyrene spheres. The response of the spheres to the “radiation pressure” of the acoustic fields traversing the liquid layer permits the spheres to condense into a pattern which produces the acoustic image. The force on an individual sphere as used in our device resulting from the radiation pressure can be calculated and by means of this calculation it is easy to optimize the magnitude and show that it is linearly proportional to the acoustic intensity — an effect necessary for our system of imaging.

The depolarization of electromagnetic radiation in a random medium: evolution of the Stokes parameters

Abstract: (1974). The Depolarization of Electromagnetic Radiation in a Random Medium: Evolution of the Stokes Parameters. Optica Acta: International Journal of Optics: Vol. 21, No. 9, pp. 721-735.

Diffusion of electromagnetic pulses into the earth from a line source

Abstract: The subsurface electromagnetic fields of a line source with a delta function current are considered. All displacement currents are neglected. The time-dependent electric and magnetic fields at the buried observer are illustrated. The resulting waveforms all have a leading spike followed by a slowly decaying tail of opposite polarity. The results have possible application to downlink signaling in mine rescue work.

An analysis of the radiation from apertures in curved surfaces by the geometrical theory of diffraction. [ray technique for electromagnetic fields

Abstract: In this paper the geometrical theory of diffraction is extended to treat the radiation from apertures or slots in convex perfectly conducting surfaces. It is assumed that the tangential electric field in the aperture is known so that an equivalent infinitesimal source can be defined at each point in the aperture. Surface rays emanate from this source which is a caustic of the ray system. A launching coefficient is introduced to describe the excitation of the surface ray modes. If the field radiated from the surface is desired, the ordinary diffraction coefficients are used to determine the field of the rays shed tangentially from the surface rays. The field of the surface ray modes is not the field on the surface; hence if the mutual coupling between slots is of interest, a second coefficient related to the launching coefficient must be employed. In the region adjacent to the shadow boundary, the component of the field directly radiated from the source is represented by Fock-type functions. In the illuminated region the incident radiation from the source (this does not include the diffracted field components) is treated by geometrical optics. This extension of the geometrical theory of diffraction is applied to calculate the radiation from slots on elliptic cylinders, spheres, and spheroids.

Far field construction for optical fourier transforms

Abstract: An optical reflective device for reflecting electromagnetic energy such asaser energy into a two-way spatial transform apparatus and onto a target for reflecting the energy back to the transform apparatus and onto a detector in an off-set input transform plane to cause far field pattern of the target to be located on the detector. Transmissive optics as well as reflective optics can be used in the optical reflective device.