Radar cross section studies

The ultimate goal is to generate experimental techniques and computer codes of rather general capability that would enable the aerospace industry to evaluate the scattering properties of aerodynamic shapes. Another goal involves developing an understanding of scattering mechanisms so that modification of the vehicular structure could be introduced within constraints set by aerodynamics. The development of indoor scattering measurement systems with special attention given to the compact range is another goal. There has been considerable progress in advancing state-of-the-art scattering measurements and control and analysis of the electromagnetic scattering from general targets

Electromagnetic scattering from a class of open-ended waveguide discontinuities

A relatively simple high frequency analysis of electromagnetic scattering from a class of open-ended waveguide discontinuites was developed. The waveguides are composed of perfectly-conducting sections in which the electromagnetic field can be written as the sum of waveguide modes. Junctions are formed at the open end and also within interior regions where different sections are joined. The reflection and transmission properties of each junction are described in terms of a scattering matrix which is determined by combining the modal ray picture with high frequency techniques such as the Geometrical Theory of Diffraction (GTD), the Equivalent Current Method (ECM), and modifications of the Physical Theory of Diffraction (PTD). A new set of equivalent circuits are employed in this ECM analysis which leads to a simple treatment of many types of junction discontinuities. Also, a new procedure is presented to improve the efficiency of the aperture integration at the open end which is required in the PTD procedure for finding the fields radiated from (or coupled to) the open end. Once the scattering matrices are determined, they are then combined using a self-consistent multiple scattering method to obtain the total scattered fields

Discriminating the Frequency Dispersive Scattering Centres of Complex Targets by Wideband Group Delay Analysis

Abstract-Waveguide geometry is one of the most critical frequency Dispersive Scattering Centres (DSCs) in actual complex radar targets. Because of the occurrence of nonlinear dispersive scattering phase or range extension phenomena the nearby scatterers may be hidden in such cases. So, degradation of spatial resolution occurs in corresponding range profiles. According to a relatively simple parametric scattering model, a computationally efficient technique is introduced to analyze the complex range profiles including both backscattering field intensity and phase. The group delay of each scatterer is used as a criterion for discriminating the dispersive and non-dispersive ones. The wideband measured data samples are used for evaluating the technique, and the comparison is performed relative to Fourierbased results

Analysis of the EM scattering from arbitrary open-ended waveguide cavities using axial Gaussian Beam tracking

The electromagnetic (EM) scattering from a planar termination located inside relatively arbitrarily shaped open-ended waveguide cavities with smoothly curved interior walls is analyzed using a Gaussian Beam (GB) expansion of the incident plane wave fields in the open end. The cavities under consideration may contain perfectly-conducting interior walls with or without a thin layer of material coating, or the walls may be characterized by an impedance boundary condition. In the present approach, the GB's are tracked only to the termination of the waveguide cavity via beam reflections from interior waveguide cavity walls. The Gaussian beams are tracked approximately only along their beam axes; this approximation which remains valid for relatively well focussed beams assumes that an incident GB gives rise to a reflected GB with parameters related to the incident beam and the radius of curvature of the wall. It is found that this approximation breaks down for GB's which come close to grazing a convex surface and when the width of the incident beam is comparable to the radius of curvature of the surface. The expansion of the fields at the open end depend on the incidence angle only through the expansion coefficients, so the GB's need to be tracked through the waveguide cavity only once for a wide range of incidence angles. At the termination, the sum of all the GB's are integrated using a result developed from a generalized reciprocity principle, to give the fields scattered from the interior of the cavity. The rim edge at the open end of the cavity is assumed to be sharp and the external scattering from the rim is added separately using Geometrical Theory of Diffraction. The results based on the present approach are compared with solutions based on the hybrid asymptotic modal method. The agreement is found to be very good for cavities made up of planar surfaces, and also for cavities with curved surfaces which are not too long with respect to their width

High-frequency asymptotic methods for analyzing the EM scattering by open-ended waveguide cavities

Four high-frequency methods are described for analyzing the electromagnetic (EM) scattering by electrically large open-ended cavities. They are: (1) a hybrid combination of waveguide modal analysis and high-frequency asymptotics, (2) geometrical optics (GO) ray shooting, (3) Gaussian beam (GB) shooting, and (4) the generalized ray expansion (GRE) method. The hybrid modal method gives very accurate results but is limited to cavities which are made up of sections of uniform waveguides for which the modal fields are known. The GO ray shooting method can be applied to much more arbitrary cavity geometries and can handle absorber treated interior walls, but it generally only predicts the major trends of the RCS pattern and not the details. Also, a very large number of rays need to be tracked for each new incidence angle. Like the GO ray shooting method, the GB shooting method can handle more arbitrary cavities, but it is much more efficient and generally more accurate than the GO method because it includes the fields diffracted by the rim at the open end which enter the cavity. However, due to beam divergence effects the GB method is limited to cavities which are not very long compared to their width. The GRE method overcomes the length-to-width limitation of the GB method by replacing the GB's with GO ray tubes which are launched in the same manner as the GB's to include the interior rim diffracted field. This method gives good accuracy and is generally more efficient than the GO method, but a large number of ray tubes needs to be tracked

Nondestructive Testing Using mm-Wave Sparse Imaging Verified for Singly Curved Composite Panels

Nondestructive testing of composite materials is important in aerospace applications, and mm-wave imaging has been increasingly used for this purpose. Imaging is traditionally performed using Fourier methods, with inverse methods being an alternative. This communication presents a mm-wave imaging method with an inverse approach intended for nondestructive testing of singly curved composite panels with sparsely distributed flaws. It builds on previous work which was limited to imaging on planar panels. The move from planar to singly curved panels increases the applicability of the method for aerospace applications. The imaging method is reference-free due to a numerical source separation algorithm and exploits sparsity in reconstruction of scatterers. It is demonstrated using near-field measurements at 60 GHz of an industrially manufactured composite panel with deliberate flaws. Compared to a more traditional Fourier imaging method, our method generates images with higher resolution and higher dynamic range. Flaw detection is also easier using our method as it generates images with less background clutter

Performances of conformal and planar arrays

Static and dynamic deformations can have a severe impact on the performance of conformal antennas on aircrafts and other vehicles. Therefore it is essential to study the different deformation and vibration mechanisms and their influence on the antenna's radiation pattern. This presentation gives an overview of different approaches concerning electromagnetic modelling of array antennas and investigations on antenna deformations presented in the scope of TG20

Multi-Mode Analysis of Dual Ridged Waveguide Systems for Material Characterization

In this dissertation, two nondestructive dual ridged waveguide (DRWG) material characterization systems are investigated. The single and clamped DRWG probe geometries were analyzed in previous work; however, that research only incorporated the dominant DRWG mode. Here, that restriction is removed and the existence of evanescent higher- order modes is considered. Theoretical analysis of the single and clamped DRWG probes is presented and discussed. The approach taken here is similar to that presented in previous research: Love\u27s equivalence theorem is used to remove the DRWG apertures which are replaced with equivalent magnetic currents radiating in the presence of the background parallel-plate waveguide structure. Enforcing the continuity of the tangential magnetic fields in the DRWG and parallel- plate regions yields a system of coupled magnetic field integral equations (MFIEs). This coupled MFIE system is solved using the Method of Moments (MoM) where the tangential electric and magnetic fields in the DRWG are used as expansion and testing functions, respectively. Inversion of the resulting MoM impedance matrix produces theoretical expressions for the reflection and transmission coefficients which are numerically inverted (via nonlinear least squares) to yield estimates of the permittivity and permeability of the material under test. While the steps taken in the theoretical development are similar to previous work, the addition of higher-order modes into the analysis substantially complicates the derivation and is a significant extension of the existing dominant-mode-only literature. Lastly, simulation results of the two structures are presented. A comparison of the dominant-mode only reflection and transmission coefficients with the higher-order mode coefficients is provided