Applications of Frequency Selective Surfaces in Polarization Control of Antennas

  • Author / Creator
    Khosravi, Farhad
  • The purpose of this dissertation is to address the problems associated with the current antennas deployed for wireless and navigation purposes. The focus in this work is on the circular polarization radiation of the antennas and the goal is to improve their radiation properties. The methods presented are based on Frequency Selective Surfaces (FSS); a group of periodic structures that behave as a spatial filter in principal but can be adjusted for other alterations to the incident electromagnetic wave. The FSS can be designed adequately so that the transmitted wave undergos a change in amplitude or phase. Moreover, adding the feature of selectivity over the incident polarization (TE or TM), can make the FSS shape the radiation pattern of the antenna in terms of amplitude or polarization; depending on whether the change is in the amplitude or phase of the wave transmitted through the FSS. The introduction chapter discusses the contemporary problems with these antennas and solution already proposed for these issues. Besides, it discusses drawbacks in current solutions and their limitations in those methods. The next chapter talks about bidirectional same sense circularly polarized antennas. The effort in this chapter is to present a method, besides those currently presented in literature, to achieve bidirectional antennas which radiate same sense of polarization on the two beams. A multilayer FSS is proposed to change the sense of the polarization by changing the phase of the wave passing through the structure. Then the FSS is combined with the antenna. Axial ratio of better than 3dB is achieved on both sides at the GPS L1 frequency band (f=1.575GHz). The next chapter is on improving one of the major issues in GPS antennas. Contemporary GPS antennas suffer from high axial ratio at angles close to 90o from the broadside direction. While there are some antennas available currently, they are either bulky or large in profile. It will be shown that, besides the presumed source of problem in previous works, another intrinsic factor in the radiation equations is also predominantly playing role in the corruption of axial ratio at those angles. Again, the approach presented for the aforementioned problem is based on FSS structures. The FSS will be appropriately designed for different angles to change the amplitude of the transmitted wave through the FSS and then is placed on the antenna. As will be shown, axial ratio is improved to the desired value with the low profile and low cost structure. As the results of this chapter shows, axial ratio of better than 2dB is achieved at angles up to 100o in all of the planes of the radiation pattern. Although the approach is to address one of the problems in the GPS antennas, the methodology seems pretty promising in many other applications where the radiation pattern of the antenna needs to be altered for different angles.

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  • Degree
    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.