Ring-shaped Aperture Research Articles

A computational model for heat generation in a radially layered tissue inside a ‘coaxial TEM’ applicator

The electromagnetic heat dissipation in a radially layered biological tissue inside a circular cylinder has been investigated theoretically. The theory is based on a three-dimensional model and the electromagnetic field is assumed to be generated by a prescribed electric field along a ring-shaped aperture. The method of computation employs the spatial Fourier transform of all field quantities with respect to the axial coordinate, after which the field equations are solved in the spectral domain. Subsequently, an inverse Fourier transform is carried out to compute the quantities that are of interest to the clinical deep-body hyperthermia system at hand. For a number of representative configurations numerical results at 70 MHz are given.

Bistatic Synthetic-Aperture Radar Imaging of Rotating Objects

The imaging properties of a focused radar aperture synthesized by object rotation are considered. The ring-shaped aperture synthesized by the object rotation can provide high-resolution two-dimensional imaging using only monochromatic irradiation. The case of monochromatic irradiation is extended to the polychromatic case and to equivalent bistatic geometries. These techniques offer practical methods for radar imaging of rotating objects with scattering centers visible over large angular extents.

Magnetic polarizability of some small apertures

Curves are given for the dimensionless magnetic polarizabilities <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\nu_{mx}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\nu_{my}</tex> of a few characteristic apertures. The relevant integral equations are solved by the moment method. The subareas are triangular, and the basis functions for the triangles touching an edge take the edge singularity into account. Some data are included for a few typical ring-shaped apertures.

On the Theory of Diffraction by Concentric Arrays of Ring-Shaped Apertures

Lommel’s problem, the diffraction of spherical waves by a circular aperture and opaque disk, is extended to the case of a concentric array of ring-shaped apertures with arbitrary distribution of circle radii. The evaluation of the diffracted amplitude resulting from the entire array by the obvious process of numerical summation, applied to amplitude expressions pertaining to the sequence of circles involved, is found unsuitable and, instead, series expansions are investigated in which the location of the off-axis observation point and the interference effects of the ring openings are formally separated. The already available solutions of Lommel’s problem are examined from this viewpoint, and two new expansions in complex power series are derived. By means of two novel multiplication theorems for Lommel’s functions of two variables, convenient expansions in series of these functions are obtained for the amplitude diffracted by the annular array, thus affording the basis of a general theory of diffraction by ring systems, including the case of phase-reversal construction. Furthermore, equivalent general expressions are directly obtained from the power series formulas mentioned previously. A first application is made to the special case of the Soret zone plate. Finally Hopkins’ assumption of radially non-uniform amplitude is briefly dealt with in connection with the circular aperture and the annular systems.