1,199 research outputs found
A High-Order Ultra-Weak Variational Formulation for Electromagnetic Waves Utilizing Curved Elements
The Ultra Weak Variational Formulation (UWVF) is a special Trefftz
discontinuous Galerkin method, here applied to the time-harmonic Maxwell's
equations. The method uses superpositions of plane waves to represent solutions
element by element on a finite element mesh. We discuss the use of our parallel
UWVF implementation called ParMax, and concentrate on methods for obtaining
high order solutions in the presence of scatterers with piecewise smooth
boundaries. In particular, we show how curved surface triangles can be
incorporated in the UWVF. This requires quadrature to assemble the system
matrices. We also show how to implement a total field and scattered field
approach, together with the transmission conditions across an interface to
handle resistive sheets. We note also that a wide variety of element shapes can
be used, that the elements can be large compared to the wavelength of the
radiation, and that a matrix free version is easy to implement (although
computationally costly). Our contributions are illustrated by several numerical
examples showing that curved elements can improve the efficiency of the UWVF,
and that the method accurately handles resistive screens as well as PEC and
penetrable scatterers. Using large curved elements and the matrix free
approach, we are able to simulate scattering from an aircraft at X-band
frequencies. The innovations here demonstrate the applicability of the UWVF for
industrial examples
UTD analysis of electromagnetic scattering by flat structures
The different scattering mechanisms that contribute to the radar cross of finite flat plates were identified and analyzed. The geometrical theory of diffraction, the equivalent current and the corner diffraction are used for this study. A study of the cross polarized field for a monopole mounted on a plate is presented, using novel edge wave mechanism in the analysis. The results are compared with moment method solutions as well as measured data
Accelerated stationary iterative methods for the numerical solution of electromagnetic wave scattering problems
The main focus of this work is to contribute to the development of iterative
solvers applied to the method of moments solution of electromagnetic wave
scattering problems.
In recent years there has been much focus on current marching iterative
methods, such as Gauss-Seidel and others. These methods attempt to march
a solution for the unknown basis function amplitudes in a manner that mimics
the physical processes which create the current. In particular the forward
backward method has been shown to produce solutions that, for some twodimensional
scattering problems, converge more rapidly than non-current marching
Krylov methods. The buffered block forward backward method extends
these techniques in order to solve three-dimensional scattering problems. The
convergence properties of the forward backward and buffered block forward
backward methods are analysed extensively in this thesis. In conjunction, several
means of accelerating these current marching methods are investigated
and implemented.
The main contributions of this thesis can be summarised as follows:
² An explicit convergence criterion for the buffered block forward backward
method is specified. A rigorous numerical comparison of the convergence
rate of the buffered block forward backward method, against
that of a range of Krylov solvers, is performed for a range of scattering
problems.
² The acceleration of the buffered block forward backward method is investigated
using relaxation.
² The efficient application of the buffered block forward backward method
to problems involving multiple source locations is examined.
² An optimally sized correction step is introduced designed to accelerate
the convergence of current marching methods. This step is applied to the
forward backward and buffered block forward backward methods, and
applied to two and three-dimensional problems respectively. Numerical
results demonstrate the significantly improved convergence of the forward
backward and buffered block forward backward methods using
this step
Electromagnetic on-aircraft antenna radiation in the presence of composite plates
The UTD-based NEWAIR3 code is modified such that it can model modern aircraft by composite plates. One good model of conductor-backed composites is the impedance boundary condition where the composites are replaced by surfaces with complex impedances. This impedance-plate model is then used to model the composite plates in the NEWAIR3 code. In most applications, the aircraft distorts the desired radiation pattern of the antenna. However, test examples conducted in this report have shown that the undesired scattered fields are minimized if the right impedance values are chosen for the surface impedance plates
Near Zone: Basic scattering code user's manual with space station applications
The Electromagnetic Code - Basic Scattering Code, Version 3, is a user oriented computer code to analyze near and far zone patterns of antennas in the presence of scattering structures, to provide coupling between antennas in a complex environment, and to determine radiation hazard calculations at UHF and above. The analysis is based on uniform asymptotic techniques formulated in terms of the Uniform Geometrical Theory of Diffraction (UTD). Complicated structures can be simulated by arbitrarily oriented flat plates and an infinite ground plane that can be perfectly conducting or dielectric. Also, perfectly conducting finite elliptic cylinder, elliptic cone frustum sections, and finite composite ellipsoids can be used to model the superstructure of a ship, the body of a truck, and airplane, a satellite, etc. This manual gives special consideration to space station modeling applications. This is a user manual designed to give an overall view of the operation of the computer code, to instruct a user in how to model structures, and to show the validity of the code by comparing various computed results against measured and alternative calculations such as method of moments whenever available
Quantum properties of atomic-sized conductors
Using remarkably simple experimental techniques it is possible to gently
break a metallic contact and thus form conducting nanowires. During the last
stages of the pulling a neck-shaped wire connects the two electrodes, the
diameter of which is reduced to single atom upon further stretching. For some
metals it is even possible to form a chain of individual atoms in this fashion.
Although the atomic structure of contacts can be quite complicated, as soon as
the weakest point is reduced to just a single atom the complexity is removed.
The properties of the contact are then dominantly determined by the nature of
this atom. This has allowed for quantitative comparison of theory and
experiment for many properties, and atomic contacts have proven to form a rich
test-bed for concepts from mesoscopic physics. Properties investigated include
multiple Andreev reflection, shot noise, conductance quantization, conductance
fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum
effects show up in the mechanical properties of the contacts, as seen in the
force and cohesion energy of the nanowires. We review this reseach, which has
been performed mainly during the past decade, and we discuss the results in the
context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have
been compressed. A higher-resolution version can be found at:
http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB
zip PostScript
Wideband mobile propagation channels: Modelling measurements and characterisation for microcellular environments
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Characterization and Measurement of Passive and Active Metamaterial Devices
This document addresses two major obstacles facing metamaterial development: uncertainty in the characterization of electromagnetic field behavior in metamaterial structures and the relatively small operational bandwidth of metamaterial structures. To address the first obstacle, a new method to characterize electromagnetic field behavior in a metamaterial is presented. This new method is a bistatic radar cross section (RCS) measurement technique. RCS measurements are well-suited to measuring bulk metamaterial samples because they show frequency dependence of scattering angles and offer common postprocessing techniques that can be useful for visualizing results. To address the second obstacle, this document characterizes the effectiveness of an adaptive metamaterial design that incorporates a microelectromechanical systems (MEMS) variable capacitor. Applying voltages to the MEMS device changes the resonant frequency of the metamaterial. In this research, computational models show that the size of the adaptive metamaterial unit cell should be increased to improve the responsiveness of the resonant frequency to changes in the MEMS capacitor
Electromagnetic Scattering from a Cavity in a Ground Plane: Theory and Experiment
The electromagnetic scattering from an arbitrarily shaped open cavity embedded in a perfectly conducting, infinite ground plane is examined. The cavity is filled with a linear, isotropic, homogeneous material. The fields in the cavity interior and above the ground plane are expressed in terms of the tangential fields on the cavity surface and aperture. A coupled set of three integral equations is developed governing the tangential fields on the aperture and cavity surface. The support of the unknown tangential fields is finite. A moment-method based algorithm to approximate the solution to the integral equations for axisymmetric geometries is developed. The unknown tangential fields are expanded using piecewise-linear functions in the elevation plane and complex exponentials in the azimuth plane. Orthogonality is exploited to reduce the size of the matrix. The algorithm yields a well-conditioned numerical solution. The solution obeys the edge condition at the aperture rim. The integral equations are uniquely solvable at frequencies where other integral equation-based techniques admit spurious solutions. Radar cross section calculations are compared to experimental measurements of full-scale physical models. Results show that an open cavity can serve as an effective radar cross section enhancement device
- …