1,574 research outputs found
A new technique for simulating composite material
This project dealt with the development on new methodologies and algorithms for the multi-spectrum electromagnetic characterization of large scale nonmetallic airborne vehicles and structures. A robust, low memory, and accurate methodology was developed which is particularly suited for modern machine architectures. This is a hybrid finite element method that combines two well known numerical solution approaches. That of the finite element method for modeling volumes and the boundary integral method which yields exact boundary conditions for terminating the finite element mesh. In addition, a variety of high frequency results were generated (such as diffraction coefficients for impedance surfaces and material layers) and a class of boundary conditions were developed which hold promise for more efficient simulations. During the course of this project, nearly 25 detailed research reports were generated along with an equal number of journal papers. The reports, papers, and journal articles are listed in the appendices along with their abstracts
Microwave antenna system for passive discrimination
A novel passive antenna system, capable of discriminating
specific electromagnetic signals is addressed. This
antenna system will be able to detect signals of certain
bandwidths, amplitudes and propagation directions. The
philosophy behind this design was to maximise the signal
discrimination at a stage prior to reception. The
development of such systems could relieve the work
involved in post detection discrimination, which may be
time consuming and expensive. A major motivation of these
studies lies in the difficulties inherent in signal
detection for mobile radio communication systems
operating at microwave frequencies. Such an antenna
system consists of two components. They are the filter
section and the detector array. The filter is designed in
such a way that only the near normal signal to the
locally flat area will be admitted and the rest
reflected. The detector array will be at an appropriate
position below the filter.
Two types of filter structures have been studied for this
angular filtering property. They are the Dielectric
Multilayers (DML) and periodic arrays of slots as
Frequency Selective Surfaces (FSS).
DML are constructed by stacking layers of dielectric
material whose permittivities vary in a near sinusoidal
manner. Such a structure is known to have the ability to
admit certain frequency bands of signals. The
conventional transmission/reflection matrix method is
used for its analysis. Also an optimisation procedure is
carried out to minimise the loss of the signal in the
DML. The characteristics of the DML as a beam-director and
Beam-shaper have also been investigated.
FSS exhibit the characteristics of band pass and band
stop filters, depending upon the nature of the surface
(periodic arrays of elements or slots) . Here the band
pass nature is utilised by using arrays of slotted
elements. These surfaces are tuned to admit narrow band
signals. The well-known modal analysis method has been
employed to study the FSS characteristics. The FSS have
been studied in the context of frequency scanning, beam
shaping, beam directing as well as angular scanning.
A prototype has been constructed to simulate a multi
signal environment in which the above structures have
been experimentally assessed
Conformal Microstrip Printed Antenna
© ASEE 2011In this paper, the comprehensive study of the conformal microstrip printed antenna is presented. The main advantages and drawbacks of a microstrip conformal antenna are introduced. The earlier researches in cylindricalrectangular patch and conformal microstrip array are summarized. The effect of curvature on the conformal Microstrip antenna patch on conical and spherical surfaces is studied. Some new flexible antenna is given for different frequencies. Finally, simulation software is used to study the effect of the curvature on the input impedance, return loss, voltage standing wave ratio, and resonance frequency
Ultra-Wideband FSS-Based Antennas
As antennas are indispensable elements in wireless systems, it is necessary to provide UWB antennas suitable for UWB systems. The most proposed UWB antennas have omnidirectional radiation, which provides the wide coverage area that is highly demanded by many conventional UWB applications. However, directional radiation is more beneficial for other UWB applications and it may even be beneficial for the conventional UWB omnidirectional applications in some environments that contain many sources of interference and distorting objects, where the omnidirectional radiation leads to high interference and loss of power in undesirable directions. Consequently, an immense research has addressed the issue of realizing UWB planar antennas with unidirectional radiation characteristics. Basically, the main technique used to create unidirectional radiation patterns is employing cavity-baking reflectors to redirect the back radiation, hence increasing the gain of the radiators. In addition, these reflectors can decouple the mounted radiator from the surroundings that can damage its characteristics. Therefore, we suggest the employment of UWB reflectors to achieve UWB planar antennas with directional radiation. Our research for designing optimal UWB reflectors has led to the investigation in the field of frequency selective surfaces (FSSs), which are valuable structures and can be of great interest to a wide range of applications especially UWB applications. Subsequently, the main aim of this chapter is to give a review of the fundamental uses of FSSs in antenna engineering and the basic physical concepts that have been employed to serve the purpose of enhancing antennas’ performances using FSSs with a variety of features and characteristics. Furthermore, it is geared toward the presentation of our proposed UWB FSS-based antennas. First, we use basic FSSs such as the capacitive and its complementary inductive FSSs to design UWB reflectors that can serve improving and stabilizing the gain of UWB antennas. Thereafter, a proposed UWB single-layer FSS is used to serve the same purpose. Then, the FSS is integrated and designed together with UWB radiator, which resulted in lower profile along with good performance
Gradient metasurfaces: a review of fundamentals and applications
In the wake of intense research on metamaterials the two-dimensional
analogue, known as metasurfaces, has attracted progressively increasing
attention in recent years due to the ease of fabrication and smaller insertion
losses, while enabling an unprecedented control over spatial distributions of
transmitted and reflected optical fields. Metasurfaces represent optically thin
planar arrays of resonant subwavelength elements that can be arranged in a
strictly or quasi periodic fashion, or even in an aperiodic manner, depending
on targeted optical wavefronts to be molded with their help. This paper reviews
a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised
to exhibit spatially varying optical responses resulting in spatially varying
amplitudes, phases and polarizations of scattered fields. Starting with
introducing the concept of gradient metasurfaces, we present classification of
different metasurfaces from the viewpoint of their responses, differentiating
electrical-dipole, geometric, reflective and Huygens' metasurfaces. The
fundamental building blocks essential for the realization of metasurfaces are
then discussed in order to elucidate the underlying physics of various physical
realizations of both plasmonic and purely dielectric metasurfaces. We then
overview the main applications of gradient metasurfaces, including waveplates,
flat lenses, spiral phase plates, broadband absorbers, color printing,
holograms, polarimeters and surface wave couplers. The review is terminated
with a short section on recently developed nonlinear metasurfaces, followed by
the outlook presenting our view on possible future developments and
perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic
Design, Concepts and Applications of Electromagnetic Metasurfaces
The paper overviews our recent work on the synthesis of metasurfaces and
related concepts and applications. The synthesis is based on generalized sheet
transition conditions (GSTCs) with a bianisotropic surface susceptibility
tensor model of the metasurface structure. We first place metasurfaces in a
proper historical context and describe the GSTC technique with some fundamental
susceptibility tensor considerations. Upon this basis, we next provide an
in-depth development of our susceptibility-GSTC synthesis technique. Finally,
we present five recent metasurface concepts and applications, which cover the
topics of birefringent transformations, bianisotropic refraction, light
emission enhancement, remote spatial processing and nonlinear second-harmonic
generation
Three-Dimensional Electromagnetic Scattering from Layered Media with Rough Interfaces for Subsurface Radar Remote Sensing
The objective of this dissertation is to develop forward scattering models for active microwave remote sensing of natural features represented by layered media with rough interfaces. In particular, soil profiles are considered, for which a model of electromagnetic scattering from multilayer rough surfaces with/without buried random media is constructed.
Starting from a single rough surface, radar scattering is modeled using the stabilized extended boundary condition method (SEBCM). This method solves the long-standing instability issue of the classical EBCM, and gives three-dimensional full wave solutions over large ranges of surface roughnesses with higher computational e±ciency than pure numerical solutions, e.g., method of moments (MoM). Based on this single surface solution, multilayer rough surface scattering is modeled using the scattering matrix approach and the model is used for a comprehensive sensitivity analysis of the total ground scattering as a function of layer separation, subsurface statistics, and sublayer dielectric properties.
The buried inhomogeneities such as rocks and vegetation roots are considered for the first time in the forward scattering model. Radar scattering from buried random media is modeled by the aggregate transition matrix using either the recursive transition matrix approach for spherical or short-length cylindrical scatterers, or the generalized iterative extended boundary condition method we developed for long cylinders or root-like cylindrical clusters. These approaches take the field interactions among scatterers into account with high computational efficiency. The aggregate transition matrix is transformed to a scattering matrix for the full solution to the layered-medium problem. This step is based on the near-to-far field transformation of the numerical plane wave expansion of the spherical harmonics and the multipole expansion of plane waves. This transformation consolidates volume scattering from the buried random medium with the scattering from layered structure in general. Combined with scattering from multilayer rough surfaces, scattering contributions from subsurfaces and vegetation roots can be then simulated. Solutions of both the rough surface scattering and random media scattering are validated numerically, experimentally, or both. The experimental validations have been carried out using a laboratory-based transmit-receive system for scattering from random media and a new bistatic tower-mounted radar system for field-based surface scattering measurements.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91459/1/xduan_1.pd
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