The research presented in this thesis concerns antenna designs based on metamaterial-inspired structures. Based on a review of the existing literature and understanding of the background theories, different metamaterial-inspired structures are applied to designs of resonant antennas (RAs) and leaky wave antennas (LWAs) for improved antenna characteristics. Extended composite right/left-handed (ECRLH) unit cell structures enable the RA designs with multiband or wideband properties; the novel metamaterial-inspired supercell structures enable the LWA designs with the dual-passband property and the backward-to-forward leaky-wave radiation characteristics in each passband. In addition, two tunable antennas are presented to mainly achieve the frequency reconfigurability and possibly the pattern reconfigurability by electronically controlling surface-mounted semiconductor varactors or discrete ferroelectric barium strontium titanium (BST) thin-film varactors. Furthermore, the uncertainty analysis in determination of permittivity of BST film materials from the characterization process is discussed in this thesis, in order to provide the design clues when the antenna with BST materials is designed. The conclusions are drawn and the possible future research directions are explained as well
