-
[show abstract]
[hide abstract]
ABSTRACT: The booming DBS business climate creates a strong demand for contoured beam antennas. Prior designs need a large complex feed array with an offset paraboloidal reflector. The shaped reflector concept requires only a single feed to achieve the same contoured beam. Consequently, due to its simplicity, shaped reflector contoured beam antennas are the most “wanted” antennas in the aerospace industry today. Due to limited available space on a satellite, it is desirable (often required) to generate multiple beams from one reflector. The multiple beam shaped reflector antennas have been studied in the past. However, the issue of the interbeam isolation was not addressed. The objective of this study is to investigate the feasibility of implementing realistic multiple beam shaped reflector antennas on a spacecraft. The coverage which we selected in this study is a typical INTELSAT hemi-coverages
Antennas and Propagation Society International Symposium, 1996. AP-S. Digest; 08/1996
-
[show abstract]
[hide abstract]
ABSTRACT: The N-Star satellite Ku/S antenna system consists of the main
shaped reflector, frequency selective subreflector FSS, the S-band
helices, and the Ku-band corrugated feed horn. The Ku/S subreflector
consists of a planar frequency selective surface which is transmissive
at the S-band, and reflective at the Ku-band. The analysis and design
procedure, as well as the performance of the frequency selective
subreflector, are presented
Antennas and Propagation Society International Symposium, 1995. AP-S. Digest; 07/1995
-
[show abstract]
[hide abstract]
ABSTRACT: A new reflection and transmission analysis of a finite size
frequency selective subreflector (FSS) due to a finite source located in
the proximity of the FSS is presented. The field due to the finite
source (in this case a corrugated feed horn) is first expanded in terms
of the spectrum of the plane waves, and then, by employing the
reflection and transmission matrices of the FSS the physical optics
currents on a surface enclosing the FSS are constructed. From the
currents and employing the field equivalence principle, the transmitted
and reflected fields (scattered field) are found. The analysis presented
is of importance to the design of advanced communication satellite
antennas, where the FSS provides the multifrequency capabilities for the
shaped reflector
Antennas and Propagation Society International Symposium, 1994. AP-S. Digest; 07/1994
-
[show abstract]
[hide abstract]
ABSTRACT: A time-domain boundary-integral/finite-element algorithm for
transient electromagnetic field coupling into an enclosure (cavity) is
developed. The model is based on a finite-element technique, which is
coupled to the exterior region through the H -field integral
equation directly in the time domain. The global electric field,
throughout the interior region is driven by the tangential magnetic
field over the outer surface of the enclosure. The tangential magnetic
field, in turn, is related to the time-dependent incident pulsed field,
and the tangential electric field over the surface of the enclosure.
Hence, the electric and magnetic fields are coupled at the surface of
the enclosure; the coupled equations are solved by a leap-frogging
technique. Numerical based on the time-dependent
finite-element/boundary-integral implicit scheme are compared with
measurements. Some novel features of the newly developed algorithm are
also presented
IEEE Transactions on Antennas and Propagation 11/1992; · 2.15 Impact Factor
-
S. Barkeshli
[show abstract]
[hide abstract]
ABSTRACT: A less-singular representation for the complete electromagnetic,
electric and magnetic, dyadic Green's function for electric as well as
magnetic point currents for a planar anisotropic uniaxial multilayered
medium is derived. The electromagnetic dyadic Green's function is
developed via the scalarization of the vector-wave form of the
electromagnetic dyadic Green's function of the anisotropic uniaxial
multilayered medium with the Levine-Schwinger free-space dyadic Green's
function in mind. Also, through the newly developed form of the Green's
dyadic, the nature of the singularity of the electromagnetic dyadic
Green's functions is easily revealed. The new formulation is useful in
electromagnetic problems dealing with surface as well as volumetric
current distributions radiating within multilayered anisotropic uniaxial
material media which are of importance to high-frequency
electromagnetics and optics
Antennas and Propagation Society International Symposium, 1992. AP-S. 1992 Digest. Held in Conjuction with: URSI Radio Science Meeting and Nuclear EMP Meeting., IEEE; 08/1992
-
S. Barkeshli
[show abstract]
[hide abstract]
ABSTRACT: Explicit and relatively simple expressions for eigenvalues and
guided (propagating) eigenvectors of a general gyroelectric medium,
where the preferred guided wave direction, z ˆ, is parallel
to the gyrotropic axis and anisotropy is confined to a plane transverse
to z , are given. Some special cases of interest, namely,
Hermitian, symmetric (biaxial), and uniaxial permittivity tensors, are
also considered. The natural, or optic, coordinate basis is used to
derive the source-free eigenvectors and to explicitly reveal the
polarization states of those eigenvectors. Also under this basis, the
evolution of eigenvalues and eigenvectors as off-diagonal terms of the
permittivity tensor uniformly vanish, a transition from the biaxial to
the uniaxial case, is discussed
IEEE Transactions on Antennas and Propagation 04/1992; · 2.15 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A complete plane wave spectral eigenfunction expansion of the
electric dyadic Green's function for a planar multilayered
dielectric/magnetic media is given in terms of a pair of the
( z ˆ)-propagating solenoidal eigenfunctions, where ( z
ˆ) is normal to the interface, and it is developed via a
utilization of the Lorentz reciprocity theorem. This expansion also
contains an explicit dyadic delta function term which is required for
completeness at the source point. Some useful concepts such as the
effective plane wave reflection and transmission coefficients are
employed in the present spectral domain eigenfunction expansion. The
salient features of this Green's function are also described along with
a physical interpretation
IEEE Transactions on Microwave Theory and Techniques 02/1992; · 1.85 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A three-dimensional linearized model to reconstruct the flaw in
advanced anisotropic composite materials is developed. The inversion
scheme takes the form of an integral equation, which is discretized by
means of the method of moments. The measured data are then inverted by
means of the conjugate gradient (CG) fast Fourier transform algorithm,
which incorporates Levenberg-Marquardt regularization and adaptive
preconditioning schemes. The reconstruction of the flaw based on
simulated as well as measured data, along with some of the salient
features of the newly developed CG algorithm are presented
IEEE Transactions on Geoscience and Remote Sensing 02/1992; · 2.89 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A relatively simple closed-form asymptotic representation for the
single-layer microstrip dyadic surface Green's function is developed.
The large parameter in this asymptotic development is proportional to
the lateral separation between the source and field points along the
air-dielectric interface. This asymptotic solution remains surprisingly
accurate even for very small (a few tenths of a free-space wavelength)
lateral separation of the source and field points. Thus, using the
present asymptotic approximation of the Green's function can lead to a
very efficient moment method (MM) solution for the currents on an array
of microstrip antenna patches and feed lines. Numerical results based on
the efficient MM analysis using the present closed-form asymptotic
approximation to the microstrip surface Green's function are given for
the mutual coupling between a pair of printed dipoles on a single-layer
grounded dielectric slab. The accuracy of the latter calculation is
confirmed by comparison with numerical results based on a MM analysis
which employs an exact integral representation for the microstrip
Green's function
IEEE Transactions on Antennas and Propagation 10/1990; · 2.15 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A novel boundary integral/finite-element algorithm for transient
electromagnetic field coupling into an enclosure (cavity) is developed.
The model is based on a finite-element technique, which is coupled to
the exterior region through the H-field integral equation. The model is
fully three dimensional and accounts for the presence of a thin,
perfectly conducting wire that spans opposite walls of the enclosure.
Some numerical results based on the newly developed model are presented
Antennas and Propagation Society International Symposium, 1990. AP-S. Merging Technologies for the 90's. Digest.; 06/1990
-
S. Barkeshli
[show abstract]
[hide abstract]
ABSTRACT: A procedure for deriving a closed form asymptotic representation of the dyadic Green's function single- and double-layered microstrip configurations is outlined. Numerical results indicate that the proposed representation is quite accurate. This asymptotic solution remains valid even for field points very close to the source
Antennas and Propagation Society International Symposium, 1990. AP-S. Merging Technologies for the 90's. Digest.; 06/1990
-
[show abstract]
[hide abstract]
ABSTRACT: A recently obtained closed-form asymptotic representation due to
S. Barkeshli (1988) of the microstrip surface Green's function is known
to provide an efficient moment method analysis of microstrip problems.
However, in order to make this closed-form expression accurate even for
very small separation of source and field points (to within a few tenths
of a wavelength), both the proper and improper surface wave transition
effects must be included in the solution. This requires finding the
location of the corresponding proper and improper poles. A good initial
estimate that allows the poles to be located rapidly by using an
iterative procedure is presented. The number of proper and improper
surface wave poles that have to be considered and their values are
tabulated
IEEE Transactions on Antennas and Propagation 05/1990; · 2.15 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Inverse scattering models of the type that is often used to invert
eddy-current data are inherently nonlinear, because they involve the
product of two unknowns, the flaw conductivity and the true electric
field within the flaw. Computational inverse models, therefore, often
linearize the problem by assuming that the electric field within the
flaw is known a priori. In the present work, the authors describe such a
linearized model; it is fully three-dimensional and applies to metals,
such as stainless-steel, or to advanced composites, such as
graphite-epoxy. The model is based on an integral equation that is then
discretized by means of the method of moments. The measured data are
inverted by means of the conjugate gradient algorithm. an example is
shown in which a linear classifier algorithm is used to improve
convergence of the conjugate gradient algorithm
IEEE Transactions on Magnetics 04/1990; · 1.36 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A newly developed closed-form asymptotic representation of the
grounded dielectric slab Green's function is used in a moment-method
formulation to calculate the propagation constant of an infinite
microstrip transmission line and the input impedance of a finite-length,
center-fed printed dipole. In these problems, source and field points
are laterally rather than vertically separated with respect to the
substrate. The conventional Sommerfeld integral and the plane wave
spectral integral (PWS) representations of the microstrip Green's
function converge very slowly in this case. However, the asymptotic
closed-form representation of the Green's function does not have this
limitation, and it remains accurate even for very small lateral
separation between source and observation points. A modified form of the
Sommerfeld integral representation is used only for observation points
in the immediate vicinity of the source, while the asymptotic form is
used elsewhere. Some numerical results based on this approach are
presented and are shown to compare very well with previous results based
on the corresponding exact-integral or PWS forms of the Green's function
IEEE Transactions on Microwave Theory and Techniques 05/1989; · 1.85 Impact Factor
