Analysis and Design of Wideband Matched Feeds for Reflector Antennas

Research output: ResearchPh.D. thesis – Annual report year: 2018

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The offset single reflector antenna enables simple, high efficiency, high gain systems, but suffers from high cross polarization. The cross polarization can be reduced by matched feeds, but these suffer from being narrowband. In this thesis,
insights on the reasons for the inherent bandwidth limitation of matched feeds are presented and methods to overcome it are proposed and demonstrated. A thorough account of the working principles of conventional matched feeds is first presented, including an alternative, effective method for determining the required waveguide modes and their excitations. In this process it is discovered that TE01 and TM01 modes can be used in matched feeds instead of the TE21 modes reported in the literature thus far. A matched feed design using TM01 is presented as a demonstration. A design procedure for conventional matched feeds is proposed. The procedure involves use of optimization in several stages of the design including direct far-field optimization. With an axially corrugated horn, a 7 dB improvement of cross polarization is achieved across a 13 % bandwidth for dual polarizations. By surrounding a circular waveguide with a coaxial one, the modal phase dispersion which arises in conventional matched feeds, can be eliminated. This property is used to present a design method for a new type of mode launcher combining techniques from directional coupler theory and analytical mode coupling models. Designs featuring excellent performance over a 30 % bandwidth for both polarizations are presented. A dual-polarized matched feed using this kind of mode launcher is demonstrated in a subset of the band. It is shown that elimination of phase dispersion between modes can also be achieved using impedance surface waveguides. Horns made of a specific range of anisotropic impedance surfaces provide far superior conditions for wideband matched feed design than smooth or corrugated metal horns. With the analysis framework and the innovations presented in this thesis, matched feeds are several steps closer to becoming the standard for offset reflector antennas.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages145
StatePublished - 2017
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