Wireless communication systems have grown dramatically during the last few years. Moreover, these systems have achieved a great popularity in society. Several examples can be mentioned: cellular communications (GSM, DCS, UMTS), personal area networks (Bluetooth), local area wireless networks (WiFi), radionavigation systems (GPS), etc. The current trend consists of using only one user terminal for several standards (e. g. GSM and UMTS terminals) and for more than one service (e. g. cellular communications, radionavigation systems and personal area networks). In addition, it is also important to note that current user terminals are more and more compact. For these reasons, it would be desirable to use only one antenna for all the standards and/or services covered by the terminal. However, it is important to note that each standard or service requires different antenna characteristics in terms of operating frequency and optimal radiation performance (radiation pattern, polarization, etc.). Hence, compact antennas with multifrequency (simultaneous operation over two or more bands) and multifunction performance (radiation pattern or polarization diversity, frequency reconfigurability, etc.) are a good solution as the radiating element of hanheld terminals. Furthermore, similar arguments can be made to justify the huge demand on multifrequency and multifunction compact antennas for the network elements such as base stations, hot-spots and other access points. Additionally, novel proposals, such as Cognitive Radio, and emerging radio applications like RFID are challenging from antenna engineering point of view. It is important to take into account that the antennas with the optimal characteristics stated above are very difficult to achieve by using conventional techniques. Thus, novel approaches are being developed to obtain radiating elements with the desired characteristics. One of these techniques is the use of metamaterial structures. Metamaterials can be broadly defined as electromagnetic structures engineered to achieve exotic or unusual properties. These features have been used in microwave engineering to develop devices with extraordinary properties such as miniaturization or operation over multiple frequency bands. On the other hand, the effort in the antenna field has been put on the use of metamaterials for travelling-wave antennas and as substrates and superstrates for antennas. Recently, there has been a great effort on miniaturized antennas based on metamaterial concepts. Nevertheless, from the author's point of view, the possibility of achieving multifrequency and/or multifunction antennas based on metamaterials has not been fully explored. The main goal of the proposed Thesis is the development of a novel design approach called metamaterial-loaded printed antennas. This solution consists of loading a conventional printed antenna with a set of metamaterial particles. Hence, the bene ts of printed antennas (low cost, compactness, low pro le, light weight, simplicity to integrate with circuitry and usefulness as elements for antenna arrays) are kept. Furthermore, the desired additional characteristics such as multifrequency and multifunction performance are obtained thanks to the proper design of the metamaterial loading elements. Several metamaterial-loaded printed antennas are proposed to provide solutions for a broad range of applications. In particular, two types of printed antennas are considered: printed wire antennas and microstrip patch radiators. The methodology used throughout the Thesis is the following: firstly, approximate models based on transmission line theory and equivalent circuits are developed to analyse and design the proposed antennas with low computational cost. Then, a full-wave study is carried out by making use of commercial and home-made solvers. Finally, the designed antennas are manufactured and measured to check their performance. Two different classes of wire antennas are proposed: printed dipole antennas loaded with metamaterial particles and printed wire antennas over ground plane with Left-Handed (LH) metamaterial loading. Regarding the dipole antennas, a multifrequency performance is achieved because these antennas have additional working bands close to the self-resonance frequencies of the metamaterial loading particles. Moreover, miniaturization is achieved when the additional modes are placed below the resonance frequency of the unloaded dipole. On the other hand, the use of LH loading allows developing antennas over ground plane (the monopole and half-loop antenna over ground plane) with additional features and small dimensions. The second type of antennas is microstrip patch antennas filled with metamaterial structures. Multifrequency and multifunction microstrip patch antennas are developed using this approach. In addition, this technique is extended to achieve multifunction patch antennas with polarization diversity and multifrequency performance. In particular, two applications are proposed: quad-frequency patch antennas with polarization diversity and dualfrequency circularly polarized patch antennas. Finally, it is proposed the application of the metamaterial-loaded antennas not as isolated radiating elements, but integrated into systems or antenna arrays. Specifically, the proposed dipole antennas are used to enhance the performance of log-periodic antenna arrays. Moreover, it is shown that metamaterial-loaded antennas are a good solution to fulfil the requirements of future communications systems (Cognitive Radio) and emerging applications such us RFID.---------------------------------------------------------------------------------------------------
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