In this thesis, we have investigated antenna elements that allow broadband operation and are relatively compact, while determining methods by which their geometry may be adapted, or various points in their structure loaded, in order to form reconfigurable antennas that may allow their polarisation or matching characteristics to be electronically altered. Much of the work undertaken has involved evanescent open-ended waveguide antennas, whereby the waveguide is operated below its cutoff frequency. Various novel matching methods have been developed, involving: (i) Using the Imaginary Smith Chart to design a match to the aperture admittance of an open-ended wave guide, and thereby to free space, with the match made reconfigurable through loading of the structure using a varactor diode. (ii) Use of capacitive coupling between an open-ended waveguide antenna's coaxial input feed and capacitive obstacles placed across the aperture of the antenna to obtain a broadband match, with polarisation-agile performance possible through varying the ex citations supplied at the input ports of the antenna. (iii) Application of band-pass filter techniques to design a match to the aperture admittance of an open-ended waveguide, through the use of alternating propagating/evanescent sections or by loading the antenna with capacitive obstacles. Radar cross-section reconfigurability is demonstrated, through a printed obstacle with switch implemented at the aperture of the waveguide. The Vivaldi antenna has also been the subject of investigation in the project, with the most notable result obtained being the design of an antipodal Vivaldi antenna loaded by shorted slotline stubs that have a variable capacitor placed across them; this allows wideband operation but with a tuneable rejection band, which enables rejection of cellular and ISM bands.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:557663 |
| Date | January 2012 |
| Creators | Ludlow, P. |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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