Band-Reconfigurable Unidirectional Antenna : A simple, efficient magneto-electric antenna for cognitive radio applications

With the tremendous growth of wireless communications, the electromagnetic frequency spectrum has become very congested. This attracts increasing attention to band-reconfigurable or frequency-reconfigurable communication systems for their ability to use the spectrum efficiently. Cognitive radio (CR) [1], [2] is one candidate for future wireless communication systems because it utilizes bandreconfigurable antennas to sense the spectrum and communicate at some unoccupied frequencies. In recent years, a great deal of research has been published on the design of band-reconfigurable antennas [3]-[14]. In general, a wideband antenna for sensing and a reconfigurable narrowband antenna for communication are needed [3]-[5]. In these designs, a relatively small antenna (reconfigurable narrowband antenna) was placed on the ground [3], [4] or free position [5] of a relatively large antenna (wideband antenna). The frequency reconfiguration was realized by using a varactor diode [3], tuning circuit [4], or even rotational motion [5]. Although these designs [3]-[5] can combine wideband and narrowband antennas, it is difficult to provide sufficient isolation between sensing and communication antennas within a limited space. Therefore, the approach that uses one wide-narrowband reconfigurable antenna for both sensing and communication [6]-[8] is a good choice for CR applications. An L-shaped slot antenna was demonstrated in [6] by using microelectromechanical switches. Two designs with band-switchable functionality [7], [8] were designed based on Vivaldi antennas. A wideband mode or several narrowband modes can be achieved with the help of the inserted reconfigurable filters [7], [8]. Furthermore, many designs have been proposed recently with frequency-reconfigurable narrowband characteristics [9]-[13], which may be used as communication antennas for CR applications.