Multifrequency and Multifunction Metamaterial-Loaded Printed Antennas

Abstract

Metamaterials can be broadly defined as electromagnetic structures engineered to achieve exotic or unusual properties (Caloz & Itoh, 2004; Eleftheriades & Balmain, 2005; Engheta & Ziolkowski, 2006; Marques et al., 2007). Recently these features have been used in microwave and antenna engineering to develop devices with extraordinary properties. For example, microwave devices with extraordinary characteristics such as miniaturization or operation over multiple frequency bands have been developed (Caloz & Itoh, 2004; Eleftheriades & Balmain, 2005; Marques et al., 2007). The effort in the antenna field has been put on the use of metamaterials for travelling-wave antennas and as substrates and superstrates to enhance the performance of the original antennas (Caloz & Itoh, 2004; Eleftheriades & Balmain, 2005; Engheta & Ziolkowski, 2006). One of the main applications of metamaterial structures in microwave engineering is the development of artificial LeftHanded (LH) Transmission Lines (TLs) (Caloz & Itoh, 2004; Eleftheriades & Balmain, 2005). These TLs are termed as LH because their behaviour is the dual of the conventional or Right-Handed (RH) ones. In the LH TLs, the electric field, magnetic field and propagation vectors form a LH triplet, allowing the propagation of backward-waves, contrary to the conventional case. If we consider a TL as the concatenation of infinite unit cells, the equivalent circuit model of a LH unit cell is a series capacitance and a shunt inductance which is the dual of a RH unit cell (a series inductance and a shunt capacitance) (Caloz & Itoh, 2004). During the last years, wireless systems have achieved a great popularity and penetration in society. Cellular systems, positioning systems (GPS, Galileo), personal area networks (Bluetooth) and wireless local area networks (WiFi) are good examples. This fact has made that user terminals designed for two or more of these services are very common nowadays. From the antenna engineering point of view, the radiating elements for these terminals require challenging features. The first one is multifrequency, which means that the antennas must work at two or more arbitrary bands simultaneously. Another challenging feature is multifunctionality because in some cases different characteristics, such as polarization or radiation pattern, are required at each working band. Moreover, all these antennas must be small to integrate them into compact handheld devices attractive for the users. Furthermore, 22