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Résumé:
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In this thesis holographic antennas are investigated as an alternate technology to reflectors and microstrip phased arrays for use in Ka-band applications. These antennas combine the advantages of low profile printed technology with an unconstrained feed to avoid excessive losses associated with conventional phased array feeds. The main purpose of this work is to apply optical holography theory at microwave frequencies in the design of Ka-band holographic antennas. As with optical frequencies, the interference pattern between two waves is calculated in order to generate the hologram. The simplest pattern is a zone plate representing the destructive pattern which can be easily reproduced using microstrip technology. To fabricate the antennas, conductor strips are etched on a dielectric material where the two interfering waves are 180° out of phase. The hologram is then illuminated by the reference wave to reconstruct the desired antenna pattern. An interesting feature of these antennas is that they can be designed so that the feed is in the same plane as the holographic plate, making the structures almost flat and preventing feed aperture blockage. Measurements of various prototypes validated the holographic concept and have shown that the efficiency of the proposed holographic antennas is comparable to that of Ka-band passive phased arrays with conventional microstrip feeds. Also, based on the experimental results, some design guidelines are established along with a methodology for enhancing the efficiency. The efficiency-improvement methods include multi-layer structures, feed horns, dipole-holograms, diamond-shaped holograms, and many others. |
