The absorbers usually employed in everyday applications and the ultimate layouts recently proposed in the literature and
based on unconventional material loading, are usually backed by a metallic plate. The metallic backing plays two main
roles. On one hand, it is used to avoid power transmission on the other side of the absorber. On the other hand, it enables
a boundary condition useful to create a reflected component that, combined with the impinging wave, cancels the
reflection from the screen. In order to avoid a metallic backing, proper resonant structures may be employed. In this
frame, it is possible to make use of metamaterials, that are artificial materials exhibiting properties not readily found in
natural materials.
The aim of the paper is to present a theoretical investigation relating to the design of compact microwave absorbers,
made by a proper combination of two metamaterial slabs. The operation principle of that layout is based on the
anomalous surface resonance, arising at the interface between two slabs, characterized by opposite signed values of the
real parts of permittivities and/or permeabilities. We show that these bi-layers, when excited by an impinging
electromagnetic wave, may support a localized interface resonance, whatever the total thickness of the entire setup is,
for any angle of incidence, and no matter what the nature of the backing on the other side of the structure is. We also
propose a new class of miniaturized inclusions, in order to be employed in practical layouts of innovative microwave
absorbers.
In this paper, we show how metamaterials can be used either to enhance the coupling values or to reduce the crosstalk
between the strips of coupled microstriplines. Coupling between regular coplanar microstriplines, in fact, is limited, due
to the small ratios between the characteristic impedances of even and odd TEM modes supported by the structure. The
broadside configuration or the employment of an overlay are often utilized to overcome this limitation, leading,
however, to more bulky components. On the other hand the coupling/crosstalk can be undesiderable in printed circuits.
The employment of metamaterials with a negative real part of the permittivity is able to increase or decrease the
coupling values, while keeping the profile of the structure very low. A quasi-static model of the structure is developed
and physical insights on the operation of the proposed components and the role of the metamaterial loading are also
given. Finally numerical results are shown for two proposed layouts.
The main radiation and scattering properties of cavity backed microstrip patch antennas loaded by homogeneous and inhomogeneous chiral, ferromagnetic and bianisotropic materials are investigated in this paper. The theoretical approach used is based on the variational formulation applied to a cavity filled by an arbitrary number of layers and in presence of an arbitrary number of metallizations (i.e. patches and ground planes of finite dimensions). In order to numerically solve the stationary equation for the electromagnetic field, a Finite Element Method (FEM) in conjunction with a Boundary Element Method (BEM) is applied and the main scattering and radiation features of the cavity antenna are straightforwardly carried out. Some numerical results showing the effects of different complex media on the radiation pattern, on the input impedance and on the radar cross section of multi-layered cavity antennas are, finally, presented. Such results show that chiral materials allow to reduce the antenna size for a fixed working frequency increasing, besides, the cross-polarization levels. Some bianisotropic materials, instead, allow to obtain the same size reduction as chiral media without degrading the cross-polarization purity.
The main applications of inhomogeneous materials with a continuous spatial variation of the electromagnetic properties as substrates for radiation components in the microwave and millimeter wave ranges are presented in this contribution. Firstly, we describe the way to obtain this kind of materials in the considered frequency ranges; secondly, we present some applications of such materials as substrates for integrated planar antennas showing the improvements respect to homogeneous ones. Particularly, we can observe improvements both in the radiation and in the scattering characteristics of such antennas.
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