الفهرس 
Objective and Outline of the ThesisOnly 14 pages are availabe for public view
 |  | from | 173 |  |  |
| | | from | 173 | | |
Abstract
This thesis is concerned with radar cross section reduction using
cloaking. In ideal cloaking the body to be hidden is coated with
anisotropic, radially nonhomogeneous material, in which the incident
wave is deflected around the cloaked body, and after traversing the
cloaked body returns to its original path without causing scattering.
Cloaking is studied using the coordinate transformation method in
which the cloaked object is transformed to a point in three dimensions
(as for a sphere) or to a line in two dimensions (as for a cylinder). The
values of some components of the permittivity and permeability
touching the cloaked body become infinite or zero, and both require
the use of metamaterials. One method to avoid this is to transform the
cloaked body to a small body rather than to a point or a line; however,
this causes some scattering depending on the size of the transformed
reduced body. The objective of the thesis is to study the scattering
properties with frequency and with scattering angle using such
approximate cloaking for cylindrical and spherical bodies. Approximate cloaking for conducting and dielectric cylindrical bodies
is studied for both TE
z
and TM
polarizations. For cloaking using a
cloak of anisotropic nonhomogeneous profile, the low frequency
asymptotic expressions for back scattering are obtained showing the
dependence on the reduced radius, the frequency and the dielectric
constant for dielectric cylinders. The implementation of the radial
variation of the cloaking material parameters requires the
discretization of the nonhomogeneous anisotropic material parameters
into many homogenous layers of small thickness w.r.t. the wavelength.
Each anisotropic layer can be further replaced by two equivalent
isotropic sub-layers with different values of the parameters in the two
layers based on the effective medium theory. For cloaking cylindrical
bodies using multi isotropic homogenous layers the anisotropic
traverses components of ( ) of a layer for TE
z
z
(TM
) case are
replaced by two isotropic layers, together with the single component
of ( ). The effect of using approximate cloaking on removing the
singular values of , components at the inner cloak radius is given.
The results show a reduction of scattering as the reduced radius
decreases, except for a range of low frequencies when cloaking a
dielectric body. The back scattering versus frequency shows
resonances when cloaking a dielectric cylinder. The behavior of the
scattering pattern and the back scattering versus frequency for
cloaking using isotropic multilayer cloak shows more variations than
when using nonhomogeneous anisotropic cloaking profile.
Approximate cloaking for conducting and dielectric spherical bodies is
studied. The solution for scattering is obtained in terms of the angular
harmonics modes. Cloaking is first studied with nonhomogeneous
variation of the anisotropic material parameters in the cloaking material. At low frequencies the total scattering cross section depends
on the reduced radius c as
both for dielectric and conducting
cloaked spheres, which is much better than approximately cloaking a
cylinder. At higher frequencies the scattering decreases on the average
as c decreases. The values of the radial permittivity and permeability
at the cloaked body radius are proportional to
ix
and to the cloaking
shell thicknesses.
For cloaking using multi pairs of isotropic homogenous layers,
different combinations of the values for the permittivity and
permeability in the two isotropic homogenous layers lead to different
scattering characteristics. Scattering decreases as the layer thickness
w.r.t. the wavelength decreases, and as the reduced radius decreases.
As the layer thickness increases the interaction between the layers
leads to more scattering for smaller reduces radius, instead of
decreasing the scattering. For the ideal case (c=0), the scattering is the
same for both conducting and dielectric spheres, and becomes
different as c increases.
Nonlinear coordinate transformations are also considered using
layered cloaking. Two transformation profiles are considered; in one
of which the permittivity and permeability change sharply near the
inner radius of the cloak as the nonlinearity degree increases. In the
other profile they change sharply near the outer radius of the cloak. It
is found that scattering is lower using the former profile for
nonlinearity degree less than unity.
z