Reconfigurable antennas using frequency selective surfaces.

Nowadays, because of the requirements of miniaturization and multifunction in the modern communication systems, more and more electronic devices are integrated into a single platform. By using this method, the communication quality can be improved significantly. However, another serious problem of interference has been introduced. As a solution for resolving this issue to enhance the performance of communication systems, radiation pattern reconfigurable antennas have received much attention. The conventional methods for designing radiation pattern reconfigurable antennas based on the phased antenna arrays incur large loss, and are complicated and expensive to be applied in practice. For this issues, frequency selective surfaces (FSSs) work as space filters to electromagnetic (EM) waves, which can be either transmitted or reflected in the operating frequency band. Therefore, radiation pattern reconfigurable antennas are realized by using frequency selective surface (FSS) in this thesis. This approach offers more antenna functionality, less cost and a significant save in terms of size and space. Thus, research in this field is very important and is one of the most popular fields nowadays. In this dissertation, first and foremost, a novel dual-band beam-sweeping antenna based on two independent cylindrical active frequency selective surfaces (AFSS) have been proposed. As the AFSS unit cell characteristic is the primary influential parameter that affects the sweeping radiation pattern functionality. Hence, two AFSS unit-cells with integrated pin-diodes have been proposed at two different frequency bands, 2.45 GHz and 5.2 GHz. When a dual-band omnidirectional monopole antenna is surrounded by two cylindrical AFSS screens, the proposed design has shown that it can effectively realize beam-sweeping which covers all azimuth angles at 2.45 GHz and 5.2 GHz simultaneously. The size of antenna system can be reduced greatly in comparison with the case where the two cylindrical FSS screens work independently from each other when they are loaded in the same antenna system. Secondly, a beam-switching antenna with high gain and flexible control of beam numbers has been proposed based on FSS. The proposed antenna is composed of an omnidirectional monopole antenna as a radiating source surrounded by a hexagon FSS screen and six metallic sheets. By changing the states of the pin-diodes in the hexagon FSS screen, the proposed antenna can not only sweep the beam six directions with gain enhancement in the azimuth plane, but also it can flexibly operates at multiple beam modes at 5.2 GHz. Then, a beam-tilting antenna with negative refractive index metamaterial (NRIM) loading has also been designed. The proposed antenna is composed of a double-feed dielectric resonator antenna (DRA) and 1×4 NRIM array, which is fixed over and in the middle of the DRA. This beam-tilting antenna can steer the main beam by ±38o in the xoz-plane over 5 to 5.5 GHz band. The reflection coefficient of the antenna is better than -10 dB in the band from 5 to 5.5 GHz. In the final design, a three layers quasi-yagi antenna has been designed with multi- beam directions in both elevation and azimuth plane at 5.2 GHz. There are four elements of quasi-Yagi antenna and eight pin-diodes as switches inserted them in the middle layer. The top and bottom layers include the parasitic elements each of which are inserted into pin-diodes. By controlling the pin-diodes in the different layers, the antenna can realize beam switching in the azimuth plane in four directions and beam tilting in the elevation plane.