Thin Conductive Sheet Research Articles

FE calculation of transient eddy currents in thin conductive sheets using dynamic boundary conditions

FE calculation of transient eddy currents in thin conductive sheets using dynamic boundary conditions

Focusing of Electromagnetic Waves through a Conducting Sheet into a Lossy Dielectric Half-Space

The ability to focus electromagnetic fields into a lossy dielectric region is considered for the case where a thin highly conducting sheet is located between the source aperture and the observer. An exact formulation is presented for the aperture illumination which may be any function of the horizontal (x, y) coordinates. The resulting expressions for the fields in the homogeneous dielectric region, above or below the thin conductive sheet, are evaluated numerically for selected cases. In particular, it is shown that the presence of the sheet does not impair the quality of the focus to any major extent, but the amplitude of the field is diminished significantly. The double numerical integration routines are validated by developing relatively simple asymptotic expressions for the fields which also can be employed as dependable field predictors for regions not near the array.

Estimation of the Load-Bearing Functions of the Patellar-Tendon-Bearing Orthosis.

Load-bearing functions and fracture-suspending effects of the patellar-tendon-bearing orthosis were evaluated by means of a contact pressure measurement system. The measurement of the dynamical skin/orthosis contact pressure distribution during walking was carried out using pressure-sensitive conductive rubber sensors. The sensor consists of a thin conductive rubber sheet sandwiched between a pair of copper electrodes and enabled determination of the contact pressure distribution in each of thirty-one equally sized areas covering the entire surface of the lower leg. Measurements were made using the three types of PTB, KBM and PTES orthosis under three sole conditions each. The relative pressure distributions measured were almost identical among the three types of orthosis, while for PTES the area around the patellar tendon made slightly low pressure contact. An experi-ment using an indentation device with an amputated above-the-knee specimen was also carried out to evaluate the fracture-suspending effects of the PTB orthosis. Results including the curve of the applied load-displacement for the specimens in the PTB orthosis were given.

Transient Electromagnetic Fields for a Polarized Conductive Sheet

We formulate the problem of a transient magnetic dipole located over a thin conductive sheet which may exhibit frequency dispersion or IP. (induced polarization). A first order analysis is carried through for the case when the source current is a step function. It is shown that the I.P. slow tail can be of opposite polarity to the principal electromagnetic response.

Anomalous vertical magnetic field for electromagnetic induction in a laterally varying thin conductive sheet

We employ a simple model to show how the natural electromagnetic field on the surface of the earth, which has a strong horizontal magnetic field component, can be converted to a significant vertical magnetic field at the surface. Such a conversion mechanism will be caused by lateral variations of the subsurface conductivity structure. Our idealized model is a thin conducting sheet with a periodic variation of the conductivity‐thickness product in one horizontal direction only.

Electromagnetic fields of a vertical magnetic dipole above a laterally inhomogeneous thin layer of conductive overburden

An analytical formulation is developed for the resultant electromagnetic field of an oscillating vertical magnetic dipole located over a thin conductive sheet of infinite extent. The sheet is characterized by a conductivity-thickness product or conductance σd that may be a function of the horizontal coordinates. The system of integral equations arising in the general formulation is simplified greatly when azimuthal symmetry prevails. Numerical results for a Gaussian variation of σd in the radial direction are presented for the case of a symmetrically located source. These results are for the fields at the level of the source dipole over the conductive sheet. It is shown that the quadrature response of the sheet is enhanced when there is rapid variation of the conductance. The null in the resultant wave tilt is also found to be shifted toward the direction of increasing conductance.

A model of the geoelectric structure beneath Japan.

A quantitative evaluation of the geoelectric subterranean model beneath Japan is made on the basis of the analyses of short-period geomagnetic variations. For the model it has been surmised that the surface of a highly conducting mantle layer must steeply drop to a depth of 200km or so from a depth of 40-50km beneath the Pacific coast of central Japan. The depression seems likely to become shallower toward the Japan Sea underneath which the depth of the layer again reaches 40km.As the above “depression model” was derived on the basis of a perfect conductor approximation, it is important to see whether the model can account for the frequency characteristics of geomagnetic variation as brought out by intensive observations in recent years.The electromagnetic response of a conductive half-space having a triangular depression on its surface is estimated entirely numerically. Similar studies are also made for a case in which thin conductive sheets representing the sea are added. It turns out that the model seems likely to account for the characteristics of geomagnetic variations having a period range of a few tens of minutes. Although there is no guarantee for uniqueness, the depression model cannot be ruled out as a possible geoelectric structure beneath Japan.

TIME DOMAIN ELECTROMAGNETIC RESPONSE OF A SHIELDED CONDUCTOR*

AbstractVelekin and Bulgakov (1967) in an interesting model experiment while studying the transient electromagnetic response of a conductive sphere placed below a thin conductive sheet found that at the earlier stages of the transience, the composite system response corresponded to the response due to the overlying sheet alone and at the later stages, it corresponded to that of the sphere alone. To examine whether such a separation of responses due to individual components can be analytically studied and applied to other source configurations, we have analyzed an idealized model consisting of two spherical shells. We find that in corroboration with the above results, the general nature of the curve consists of two humps representing the responses dominated by the outer shell and the inner shell respectively. In addition, however, we find that the two humps gradually disappear to yield a smooth decay curve for increasing values of the ratio σd1b/σ2d2a (where σ1, σ2 are the conductivities, d1, d2 are the thicknesses of the outer and inner shells respectively, and b and a are their respective distances from the centre) and the effect of inner shell on the composite system response is considerably reduced.

Low-frequency reflection in the ionosphere—I

A theory of low-frequency reflection in the ionosphere is developed. At wavelengths long compared to the layer thickness, the ionospheric layer is considered as a thin conductive sheet, that leads to a discontinuity of the electromagnetic field. A thicker layer is subdivided into many thin (or differential) sublayers. The field is then thought of as a superposition of many partial waves, each of which is reflected by an individual sublayer. An additional penetrating wave must be assumed. At the lowest frequencies—that is, in the case of thin-sheet reflection—the currents in the layer are horizontal and the reflection is of the metallic type, always showing reversal of the horizontal E-component. Dielectric-type reflection with sign or phase transition at the Brewster angle is obtained at somewhat higher frequencies, if vertical and horizontal current density components are of comparable magnitude. The present first part of the paper deals with the thin-sheet reflection theory and the fundamentals of the general theory. Specialized cases of the general theory and some consequences will be discussed in the second part. The frequencies under consideration are roughly 1–100 kc/s.