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F. Akleman
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ABSTRACT: In this study, an integral equation technique is presented in order to reconstruct the complex permittivity variation of a material loaded in a rectangular cross section waveguide. The problem is first reduced to the solution of a two-coupled integral equations in terms of S-parameters of the system, which contains the Green's function of the empty waveguide. The resulting inverse scattering problem for the unknown complex permittivity of the loading is solved by contrast source inversion (CSI) technique. The validity and reliability of the proposed method has been demonstrated with numerical applications.
IEEE Microwave and Wireless Components Letters 04/2008; · 1.72 Impact Factor
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ABSTRACT: Knowledge of the local groundwave-propagation characteristics is essential in wireless systems. Although Maxwell's equations establish the theoretical background, only a limited number of highly idealized groundwave-propagation problems have mathematically exact and/or approximate solutions. Therefore, semi-analytical/numerical and pure numerical simulation methods are almost the only way to handle realistic groundwave-propagation problems. To a certain extent, numerical simulators should be capable of taking non-flat, penetrable terrain and inhomogeneous atmospheric effects into account. Unfortunately, a generally applicable simulator has not yet appeared; there are many methods that have been developed under different assumptions and approximations, valid in different parameter regimes. It is therefore a challenge to apply these methods to the same physical problems, to do comparisons, and to evaluate numerical results. With all these factors in mind, a new MATLAB-based package GrMoMPE is introduced. It is first validated and calibrated, and then applied to some characteristic groundwave-propagation problems. The introduction of GrMoMPE has made it possible to do direct and accurate comparisons and reliable physical interpretations.
IEEE Antennas and Propagation Magazine 11/2007; 49(5):69-82. · 0.97 Impact Factor
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ABSTRACT: Rectangular vs. cylindrical finite-difference time-domain (FDTD) electromagnetic modeling is discussed, and characteristic tests and comparisons are presented depending on the analytical and numerical analysis of ring type circular resonators. A ring resonator is modeled with both rectangular-and cylindrical-FDTD packages which are also calibrated against analytical exact solution derived in terms of cylindrical Bessel functions. Applications of the periodic boundary condition in modeling circular (rectangular) structure via rectangular (circular) FDTD is also given.
Electromagnetics in Advanced Applications, 2007. ICEAA 2007. International Conference on; 10/2007
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ABSTRACT: Many natural or man-made guiding environments are characterized by physical parameters that render the wave equation non-separable in any of the standard coordinate systems. In particular, in the absence of transverse-longitudinal separability, it is not possible to define discrete or continuous normal modes (NM) that individually satisfy the transverse boundary conditions and that propagate longitudinally without coupling to other modes. When transverse-longitudinal separability is only weakly perturbed, one may define local (adiabatic) modes that adapt smoothly, without inter-mode coupling, to the slowly changing conditions. Adiabatic modes (AM) fail in cutoff regions, and can be made uniform there by intrinsic modes (IM), which are synthesized by a spectral continuum of adiabatic modes. These concepts have been elucidated and validated previously by investigating the wave dynamics in a simple test environment: a wedge waveguide with non-penetrable boundaries, viewed either in coordinate-separable cylindrical coordinates that yield exact field solutions in terms of normal mode, or in non-separable rectangular coordinates that yield approximate field solutions in terms of adiabatic modes and intrinsic modes. The present article is intended as a tutorial to enhance the utility and understanding of these analytical formulations through visualizations of the dynamic interaction between the various wave species, implemented through an educational MATLABtrade package. The visualizations for the full range of ray, mode, and hybrid options, parameterized in the spectral wavenumber domain, has been explored by us previously for the inherently separable canonical environment of a line-source-excited parallel-plate waveguide. In our present investigation of the wedge waveguide, we shall not attempt to mimic the variety of options because of the substantial complications and subtleties inherent in their rectilinearly weakly-non-separable implementation. For our purposes here,-
a single specific option suffices to address the normal-mode, adiabatic-mode, and intrinsic-mode phenomenologies. Throughout this article, the intended audience is expected to be familiar with asymptotic methods for the evaluation of integrals.
IEEE Antennas and Propagation Magazine 07/2007; · 0.97 Impact Factor
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ABSTRACT: This presentation is intended to tutorially enhance the utility and understanding of the analytical and numerical representations for the wave propagation problem inside a wedge-waveguide with both non-penetrable and penetrable boundaries; through visualizations of the dynamical interaction between the various wave species, implemented through educational Matlab packages. The first package has been prepared for the exploration of line-source-excited wave fields in terms of normal, adiabatic and intrinsic mode solutions. The second package belongs to one-way split-step parabolic equation modeling for the visualizations of modal leakage
Antennas and Propagation Society International Symposium 2006, IEEE; 08/2006
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12/2005: pages 55-63;
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IEEE Antennas and Propagation Magazine 09/2005; · 0.97 Impact Factor
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ABSTRACT: This work is intended as an educational aid, dealing with high-frequency (HF) electromagnetic wave propagation in guiding environments. It is aimed at advanced senior and first-year graduate students who are familiar with the usual engineering mathematics for wave equations, especially analytic functions, contour integrations in the complex plane, etc., and also with rudimentary saddle-point (HF) asymptotics. After an introductory overview of issues and physical interpretations pertaining to this broad subject area, detailed attention is given to the simplest canonical, thoroughly familiar, test environment: a (time harmonic) line-source-excited two-dimensional infinite waveguide with perfectly conducting (PEC) plane-parallel boundaries. After formulating the Green's function problem within the framework of Maxwell's equations, alternative field representations are presented and interpreted in physical terms, highlighting two complementary phenomenologies: progressing (ray-type) and oscillatory (mode-type) phenomena, culminating in the self-consistent hybrid ray-mode scheme, which usually is not included in conventional treatments at this level. This provides the analytical background for two educational MATLAB packages, which explore the dynamics of ray fields, mode fields, and the ray-mode interplay. The first package, RAY-GUI, serves as a tool to compute and display eigenray trajectories between specified source/observer locations, and to analyze their individual contributions to wave fields. The second package, HYBRID-GUI, may be used to comparatively display range and/or height variations of the wave fields, calculated via ray summation, mode-field summation, and hybrid ray-mode synthesis.
IEEE Antennas and Propagation Magazine 01/2005; · 0.97 Impact Factor
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ABSTRACT: The binary decision tree classification and feature extraction method based on texture features is applied to SAR data. In order to achieve more complex analysis it is advantageous to use binary decision trees, in which the decision between only two classes must be assigned at each node . Pixel based feature extraction methods reduce classification performance because of the speckle and also conventional texture analysis is not applicable to every part of an image. Therefore, a decision-making process, which can be applied to every pixel of an image, is required. The results show that computation time and accuracy of classification process are improved.
Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International; 08/2003
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ABSTRACT: A new time domain wave propagator (TDWP) based on the two-dimensional finite-difference time-domain (FDTD) technique (for original paper see ibid., vol. V-14, p. 302-307 (1966)) that was introduced in May 2000 issue) has been augmented in this paper so that it deals with impedance boundary condition or varying terrain heights and an example of each, with comparisons to other methods is presented.
IEEE Transactions on Antennas and Propagation 08/2003; · 2.15 Impact Factor
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ABSTRACT: In this letter, a novel time-domain wave propagator, based on the transmission line matrix (TLM) technique, is introduced. A two-dimensional (2-D) TLM algorithm is modified and the sliding window technique is applied to analyze ground wave propagation characteristics. The longitudinal propagation region over the Earth's surface is covered by a finite-size TLM computation space, as if the space slides from source to observation point. A short pulse is injected into the TLM computation space as a vertical initial source distribution near the left end and is traced within an adjustable window while propagating towards the right. Perfectly matched layer (PML) blocks on the left, top and right terminate the TLM computation space to simulate the semi-open propagation region. The ground at the bottom is a perfect electrical conductor (PEC). The PML blocks absorb field components that scatter back and top. The ground wave components (i.e., the direct, ground-reflected and surface waves) are traced longitudinally towards the right. Transient propagation can be observed at any range/altitude by accumulating the time history of the desired field components and any steady-state vertical and/or horizontal field profile at a desired frequency can be extracted by applying the off-line discrete Fourier transformation (DFT).
IEEE Transactions on Antennas and Propagation 08/2003; · 2.15 Impact Factor
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ABSTRACT: In this overview of groundwave propagation, we address a
particular class of propagation scenarios in the presence of surface
terrain and atmospheric refractivity. Beginning with idealized
analytically solvable models over a smooth spherical Earth, we trace the
progression toward more "reality" through physics-based numerical
algorithms, operating in the frequency and short-pulse time domain,
which take advantage of computational resources. An extensive sequence
of simulations for various terrains and atmospheric refractivities, as
well as different source-receiver arrangements and operating
frequencies, serves to calibrate these algorithms one against the other,
and establishes the range of problem parameters for which each is more
effective
IEEE Antennas and Propagation Magazine 03/2002; · 0.97 Impact Factor
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ABSTRACT: A novel time-domain wave propagator is introduced. A
two-dimensional (2-D) finite-difference time-domain (FDTD) algorithm is
used to analyze ground wave propagation characteristics. Assuming an
azimuthal symmetry, surface, and/or elevated ducts are represented via
transverse and/or longitudinal refractivity and boundary perturbations
in 2-D space. The 2-D FDTD space extends from x=0 (bottom) to
x→∞ (top), vertically and from z→-∞ (left) to
z→∞ (right), horizontally. Perfectly matched layer (PML)
blocks on the left, right, and top terminate the FDTD computation space
to simulate a semi-open propagation region. The ground at the bottom is
simulated either as a perfectly electrical conductor (PEC) or as a lossy
second medium. A desired, initial vertical field profile, which has a
pulse character in time, is injected into the FDTD computation space.
The PML blocks absorb field components that propagate towards left and
top. The ground wave components (i.e., the direct, ground-reflected and
surface waves) are traced longitudinally toward the right. The
longitudinal propagation region is covered by a finite-sized FDTD
computation space as if the space slides from left to right until the
pulse propagates to a desired range. Transverse or longitudinal field
profiles are obtained by accumulating the time-domain response at each
altitude of range and by applying the discrete Fourier transformation
(DFT) at various frequencies
IEEE Transactions on Antennas and Propagation 06/2000; · 2.15 Impact Factor
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ABSTRACT: In this study, the finite-difference time-domain (FDTD) method is
used to model mutual effects of a mobile phone and a human head in terms
of both biological effects and antenna design. A discrete human head
model and a hand-held receiver with various antennas mounted on top of
it, are located within a three dimensional (3D) FDTD algorithm is built
in Cartesian coordinates. Near fields are simulated directly in the time
domain for both sinusoidal and pulse type antenna excitations. The
antenna power, input impedance, absorbed power and the specific
absorption rate (SAR) distribution inside the head near the hand-held
receiver are calculated for various antenna types and human
head/hand-held receiver locations. The simulations are carried out for
both European GSM and DECT systems (at 900 and 1800 MHz, respectively).
The SAR distributions for various vertical and horizontal slices of the
head are calculated and are shown to agree with the available
calculation and measurement results. Besides, the effects of the human
head on the antenna radiation pattern are calculated where far field
simulations are obtained via a time-domain near-to-far-field (NTFF)
transformation based on Huygen's principle. Various hand-held receiver
antennas, including a quarter-wavelength wire, IFA, PIFA, symmetrical
and asymmetrical square or rectangular printed loops, are investigated
in terms of their radiation pattern, gain and efficiency. Different
antenna mountings are used and their effects to both antenna performance
and SAR distribution in the head are shown
Antennas and Propagation for Wireless Communications, 1998. 1998 IEEE-APS Conference on; 12/1998
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ABSTRACT: In this letter, a new implementation of the three-dimensional
(3-D) perfectly matched layer (PML) in finite-difference time-domain
(FDTD) applications is introduced. This technique is based on doubling
the cell dimensions in PML region where extra averaging of electrical
field components are necessary at the edges and faces along the PML-FDTD
interfaces. The presented numerical examples are for 3-D structures
which exhibit complex wave phenomena. Significant improvement obtained
after this implementation, especially at lower frequencies, is
demonstrated
IEEE Microwave and Guided Wave Letters 11/1998;
