Design Of Dual Circularly Polarized Antennas For The Polarization Diversity Systems

As the rapid development of wireless communication technologies and the growing demand of spectrum resources, there are more and more challenges to the communication systems, and meanwhile more and more requirements are imposed upon the antennas. In order to increase the system capacity and the spectrum utility efficiency, the polarization diversity technology is proposed. As a typical application of the polarization diversity technology, RFID system has been paid more and more attention. However, Comparing with the linear polarization, the circular polarization has the advantages of anti-fading, anti-interference and resisting multipath effects. Thus, the circularly polarized antennas are more suitable for the polarization diversity systems, which will improve the system capacity and the spectrum utility efficiency. Therefore, the dual circularly polarized antennas used for the polarization diversity systems become the new hotspot in the field of antenna research world widely. In this thesis, the dual circularly polarized antennas used for the polarization diversity system are investigated. Two types of the dual circularly polarized antennas used for the RFID system are designed and fabricated.Firstly, the principles and advantages of the antennas fed by the L-shaped probe are presented. The recent researches of the L-shaped probe are also outlined. According to the requirements of the RFID system, the dual circularly polarized antenna is designed and simulated. Effects of several critical parameters in the antenna on its performance are analyzed. Based on the simulated results, an antenna prototype is fabricated and measured. The measured results indicate that the impedance bandwidth of the antenna is up to 8%, while the isolation is better than 15dB.Secondly, the principle of the CPW is introduced and the characteristic impedance of the CPW is calculated. Also, some recent researches of the dual circularly polarized antennas fed by CPW are presented. Moreover, a dual circularly polarized antenna fed by the CPW is simulated. Based on the simulated results, the antenna prototype is fabricated and measured. The measured results show that the impedance bandwidth is 16.3%, the axial ratio bandwidth is 9.7%, and meanwhile, the isolation is better than 20dB. This antenna has some advantages such as low profile and easy integration.Finally, the performance of the proposed two antennas, including the circular polarization and isolation, is compared based on the aforementioned results. Also some drawbacks of these two types of antennas and the problems remain to be studied further are discussed.