| dc.description.abstract | To meet the demand of continuously growing RF and microwave technologies, scientists
and engineers have developed innovative materials consisting of conducting and dielectric
materials that overcome limitations in the properties of natural materials. Over the last
decades, the development of these artificial materials has evolved to be a new field. An
artificial material is a synthesized material that gains its electromagnetic properties from its
structure rather than inheriting them directly from those it is composed of. Planar versions
of these artificial materials are often characterized by small periodic conducting patches
over a dielectric substrate. In addition to the periodicity, the interaction of these structures
with the electromagnetic waves depends on the shape and size of the small patches.
Planar artificial materials pursued in electromagnetics include frequency selective surfaces
(FSS), high impedance surfaces (HIS), artificial magnetic conductors (AMC), and
electromagnetic bandgap (EBG) materials, which are characterized by periodic unit cells
whose lattice constant is comparable with the operational wavelength. One of the very
commonly seen geometry is the mushroom structure, which has a patch array with each
element connected to the metal on the other side of the dielectric with a via. These metalbacked
configurations are used in various antenna applications. Some narrowband
absorbers have also been suggested using these configurations. This thesis investigates new
artificial materials consisting of one- and two- layer of metal patch arrays that overcome
the requirement for vias, and examines their design and analysis for four different
applications (i) Circularly polarized antennas (ii) Surface wave suppression of microstrip
antennas (iii) In-phase reflection surface and (iv) Thin radar absorbing material.
Artificial materials with square patch arrays with or without vias can be designed as
artificial magnetic conductors. Arrays of rectangular patches with vias or square patches
with two vias or slots have been proposed for polarization sensitive reflectivity
characteristics. In this thesis we propose a simple geometry for polarization sensitive
reflection characteristics. This consists of a modification to square patches with variants of
fractal Minkowski curves as boundaries on two of its sides, printed over a metal backed
dielectric substrate without vias. The structure is compact, and due to its planar nature, it
can be fabricated easily using planar technology. Properties and performance of the
structure is analyzed numerically through simulations by varying fractal properties of the
sides. The asymmetry in the patch causes the reflection phase of the proposed structure to
depend on the polarization state of the incident wave and frequency. A phase difference
above 200 degrees between the x- and y-polarized reflected waves has been achieved with
small unit cells. Application of the proposed via-less structure to generate circular
polarization using simple dipole antenna is also demonstrated in this thesis.
Square patches arrays with vias have been proposed as high impedance surfaces (HIS) with
in-phase reflection and as electromagnetic bandgap (EBG) structures for suppressing
surface waves in microstrip antenna applications. The second structure proposed in this
thesis is an alternative for these mushroom structures, and consists of a periodic array of
square metal patches (on the top surface) and square metal rings (embedded within the
dielectric substrate). This structure does not require any vias for effective operation and is
analyzed extensively by numerical simulations. In-phase reflections due to high surface
impedance and surface-wave suppression characteristics similar to the mushroom structure
proposed by Sievenpiper have been validated. Application of the structure to reduce
mutual coupling between microstrip antennas and to improve the radiation pattern are
demonstrated through simulations. The structure is fabricated and experimental
measurements have been made to confirm surface wave suppression characteristics. A
waveguide-based experiment was done to demonstrate the in-phase reflection
characteristics. One of the main advantages of the proposed structure is that it is planar in
nature and easily fabricated using planar technology, without the need for any via
connections across dielectric layers. Another feature is that it exhibits in-phase reflection
and surface wave suppression bands at the same frequency band as in mushroom structure.
The scalability of this structure to operate in different frequency ranges is also
demonstrated in this thesis.
Modifications to EBG using either resistive patches or surface mounted resistors have been
used as absorber. In this thesis, we propose the use of the above ring-patch structure
printed on a moderately lossy substrate such as FR4, as a near-perfect electromagnetic
absorber. It is demonstrated that input impedance of the structure can be configured to
match the free space impedance by varying the width of the ring to result in near-perfect
absorption. This configuration causes a concentration of electric fields in the dielectric
region between the ring and patch, thereby enhancing the dissipation of the energy.
Monostatic and Bistatic radar cross section measurements have been used to ensure that
there are no scattered fields in other directions. The structure is thin, easy to fabricate, and
is scalable to operate at different frequencies. This does not use any resistive materials for
absorption. It is shown that 99% of the incident power is dissipated by either dielectric or
metal losses. The performance of this structure is analyzed using an equivalent circuit
approach. A method for improving the bandwidth of this absorber by combining four unit
cells and optimizing the dimensions of this sub-array is also proposed here. The
performance of this EBG based absorber configuration is similar to the metamaterial based
absorbers proposed recently with much smaller unit cells.
To summarize, this thesis investigates electromagnetic behavior of single and stacked twolayer
periodic metal patches without any interconnects, which are simple and easy to
fabricate using planar approaches. It has been established that the configurations proposed
in this thesis are equally effective for various electromagnetic applications as previously
reported geometries, often characterized by vias or surface mount components. | en_US |
