| dc.description.abstract | During this PhD study, metamaterials incorporating active devices such as varactors and
non-Foster circuits, were researched and designed. Starting from the research on tuneable
metamaterials, an electronically controlled leaky-wave (LW) antenna based on composite
right/left handed (CRLH) transmission line (TL) structure was proposed which could
perform a broadband beam-fixing function with the frequency range from 1 to 4 GHz.
In addition, scanning from forward to backward at a fixed frequency can be achieved by
manipulating the biasing voltage applied to the varactors.
Most of this study has been devoted to the non-Foster active metamaterials. First, the
characterization of active magnetic metamaterials with non-Foster loads was presented.
Based on the equivalent circuit model, stability of an actively-loaded loop array was
examined through different analysis techniques, further to give the design specifications
to achieve the broadband non-dispersive negative-Re(μ) (MNG) or μ-near-zero (MNZ)
magnetic properties. Moreover, the wave propagation in the actively-loaded medium
was investigated. By relating the dispersion characteristics and the effective medium
properties, we henceforth proposed the design of zero-loss and broadband metamaterials.
This thesis also has covered the study of active high impedance surfaces (HIS) with
non-Foster loads. As a two-dimensional metamaterial structure, HIS have been widely
used in the microwave and antenna engineering. However it can be easily seen that the
performance of a general passive HIS is always limited by the narrow bandwidth, thus
making a broadband HIS desirable. In this work, an analytical solution to achieving a
stable broadband HIS structure is given by incorporating appropriate negative impedance
converter (NIC) circuits. Simulation results have verified the design approach.
Finally, the design of NIC circuits was presented as the key part of the realization of active metamaterials. Two schemes have been adopted to realize the design of NICs,
one is the operational amplifier (op-amp) based NIC, and another is based on discrete
transistors. Both types of NICs were introduced and studied in this thesis. | en_US |
