Valdis V. Liepa

A general polarimetric radar calibration technique

A polarimetric radar calibration procedure is introduced and verified with experimental results. The procedure requires measurements of three known targets in order to determine the distortion matrices that characterize the effect of the measurement system on the transmitted and received waves. The scattering matrices for the known targets can be of any form, provided that a limited set of constraints is satisfied. A special case, wherein the transmit and receive distortion matrices are the transpose of each other, is considered. This case is useful for some single antenna systems and has the advantage that only two known targets are required. >

Coupling studies and shielding techniques for electromagnetic penetration through apertures on complex cavities and vehicular platforms

The multilevel fast multipole moment method is employed to compute the electromagnetic coupling and shielding of various aperture-coupled metallic enclosures. A phenomenological study of electromagnetic coupling, due to various slot shapes and sizes, with or without the presence of wire penetration is conducted. These slots are situated on over-moded cavities and two methods are proposed to mitigate slot coupling into the cavitys interior. As part of this work, we also investigate the coupling through similar slots and apertures within a complex platform such as an automobile. The proposed methods have shown to increase shielding by as much as 5-35 dB within the frequency range of interest.

Synthesis of tapered resistive strips

A technique for synthesizing a resistive taper that generates desired bistatic scattering and backscattering patterns from a strip is outlined. Antenna synthesis techniques relate the scattered field to the induced surface current density. Physical optics approximations then relate the induced current to the resistivity. The taper is checked by computing the surface current density and scattered fields of the tapered resistive strip using the integral equation formulation and comparing with the physical optics results.

The minimization of the back scattering of a cylinder by central loading

A theoretical and experimental study of the minimization of the back scattering by a thin cylinder by central loading is presented. The induced current on a centrally loaded cylinder illuminated by a plane wave at normal incidence was determined theoretically and experimentally. The magnitude and phase of the induced current can be greatly changed by a central impedance. The optimum loading to achieve zero back scatter in the broadside direction from a thin cylinder shorter than two wavelengths and the optimum central impedance for the purpose of minimizing the broadside back scattering from a thin cylinder over a wide range of frequencies has been determined.

A note on hybrid finite element method for solving scattering problems

The hybrid finite-element formulation (HFEM) originated by P. Silvester and M.S. Hsieh (1971) is modified in such a way that it results in a sparse or uniformly banded matrix, rather than a partly full and partly sparse nonuniform matrix. The modification is accomplished by changing the sequence of matrix substitutions and substantially improves the computational efficiency and enhances the capability of the method, which is demonstrated by numerical examples. A comparison with other numerical techniques is presented. >

Backscattering from tapered resistive strips

Resistive strips have lower backscattering cross sections than perfectly conducting strips, and this is true in particular when the illumination is edge-on with the electric vector parallel to the edge. The scattering then consists of edge contributions and attention is confined to this case. Using the available expressions for the edge contributions of uniform resistive strips it is shown that the front edge scattering decreases with increasing resistivity, whereas the rear edge scattering increases. This suggests that the resistivity should be tapered from a maximum at the front to zero at the rear, and numerical results are presented for the particular case of quadratic (parabolic) tapers. Values for the front and rear edge contributions are extracted from the scattered field data, and for strips more than about a half-wavelength in width it is found that the front edge contribution is almost identical to that for a uniform resistive half-plane. An empirical expression for the rear edge contribution is also derived, and the implications of the results are examined.

A Volume-Surface Integral Equation for Electromagnetic Scattering by Inhomogeneous Cylinders

This paper presents an integral equation formulation for electromagnetic scattering by inhomogeneous infinite cylinders having arbitrary scalar permittivities and permeabilities. The formulation involves both volume and surface integrals with only one unknown field component, and is applicable to both transverse electric and transverse magnetic cases. This volume-surface integral equation is well-suited for numerical implementation. In this paper, the integral equation is first derived by integrating the wave equation with the aid of the free-space Greens function, and then analyzed from the physical point of view, resulting in a new interpretation for the scattering mechanism. Finally, the equation is programmed using the method of moments with pulse expansion functions and point-matching. Numerical results are shown to demonstrate the validity of the formulation and the new interpretation of the scattering mechanism.

Crack determination from boundary measurements-Reconstruction using experimental data

In this work we assess the effectiveness of Electrical Impedance Tomography for determining the presence and the location of an interior crack from boundary measurements. Electrical Impedance Tomography uses boundary voltages and currents to image the interior of a region. We collect the data needed for this nondestructive evaluation technique by laboratory experiments and apply two numerical inversion algorithms to the data. Our experiments show that the data collected are sufficient to give good estimates of crack locations and crack sizes.

Modification to the scattering behavior of a sphere by reactive loading

The scattering of a plane electromagnetic wave by a metallic sphere loaded with a circumferential slot in the plane normal to the direction of incidence is investigated. The slot is assumed to be of small but nonzero width with a constant electric field across it, and under these assumptions the analysis is exact. The field scattered in any direction is then obtained by the superposition of the field diffracted by an unloaded sphere, and that radiated by an excited slot at the position of the load, with the radiation strength of the slot related to the loading characteristics in the combined problem. Numerical results are presented, primarily for the case of back scattering, and these are compared with experimental measurements made using a sphere with an equatorial slot backed by a radial cavity of adjustable depth. The agreement is excellent.

A moment method solution of a volume-surface integral equation using isoparametric elements and point matching (TE scattering)

Three integral equations for TE (transverse electric) scattering by dielectric cylinders having large values of permittivity are examined. A moment-method solution of the volume-surface integral equation is developed employing isoparametric elements and point matching. The solution is shown to be accurate and stable than the traditional solutions using pulse basis, particularly in the case of scatterers having large refractive indices. >