Browsing by Author "Klopper, Brandt"

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    Antenna elements for sparse-regular aperture arrays
    (Stellenbosch : Stellenbosch University, 2019-04) Klopper, Brandt; De Villiers, D. I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: The central theme of this dissertation regards the design and analysis of antenna elements in sparse-regular aperture arrays (AAs) for radio astronomy applications. Throughout this work, a set of modelling techniques are presented to efficiently analyse the impedance and radiation responses of sparse-regular AA elements, which are required to obtain key AA radiometric figures-of-merit. These modelling techniques are applied to a range of narrowband and broadband AA elements, including a novel sparse-regular candidate AA element for the Square Kilometre Array's Mid-Frequency Aperture Array (SKA MFAA). A thorough study of response models for sparse-regular AA elements is presented, considering several options for the radiometric and full-wave electromagnetic modelling of the antenna elements, as well as global surrogate models for multivariate AA element responses. A design study is presented in which global surrogate modelling techniques are applied for the first time to the design of broadband AA elements, with results that improve upon the per-element receiving sensitivity performance of prior work across a 4.5:1 bandwidth and multiple scan angles. To improve upon the limited scan and frequency coverage occurring in contemporary AA element design, a global modelling framework is proposed to efficiently estimate sparse-regular AA element impedance responses over a continuous and broad range of frequencies and scan angles. Special attention is paid to the incursion of grating lobes into visible space, which causes rapid response variation and can significantly degrade the elements' active impedance matching. A pre-sampling method is proposed to support the construction of adaptively sampled impedance response models, based on standard array theory and requiring no a priori information of the full-wave electromagnetic behaviour of the AA element under analysis. Global models built with the proposed method are shown to obtain significantly more accurate estimates of the global worst-case active reflection coeficient than models built with standard space-filling sampling and pure adaptive sampling techniques. The global impedance response modelling framework is extended to include the simultaneous modelling of the AA unit cell far-fields, thereby adding radiation responses to the modelling framework and allowing the subsequent determination of figures-of-merit such as receiving sensitivity and intrinsic cross-polarisation ratio. For efficient far-field modelling, two contemporary or-thogonal basis function decomposition techniques are considered, namely the Spherical Wave Expansion (SWE) and Characteristic Basis Function Pattern (CBFP) method. The two methods are tested for a variety of isolated antenna elements as well as elements in regular AAs, in the rst formal comparison of SWE versus CBFP for parametric modelling of antenna far-fields. Following consistent and clear evidence of higher modelling accuracy and computational efficiency, the CBFP method is chosen over SWE to be incorporated into the global modelling framework. A sparse-regular candidate AA element is proposed for use in SKA MFAA, in the form of a pyramidal sinuous AA element. The element geometry exhibits stable impedance behaviour over frequency and scan angle relative to other possible candidate elements, and is presented in dual-polarised form for MFAA. The element design is well-parametrised to allow further optimisation towards meeting all MFAA requirements. Finally, as part of realising an optimisation framework for antenna elements in sparse-regular AAs, an expedited performance modelling technique is proposed to rapidly estimate the sensitivity performance of sparse-regular AA elements. Instead of focusing on accurate global response model accuracy, this technique efficiently quantifies the multivariate response performance in a single scalar figure-of-merit incorporating response features such as the sensitivity minimum and overall response smoothness over its operating parameters. Narrowband and broadband examples yield accurate model results with few high-fidelity response samples, with reasonably accurate values provided for the MFAA pyramidal sinuous element within 350 samples.
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    Fast design and optimisation of one-dimensional microstrip patch antenna arrays
    (Stellenbosch : Stellenbosch University, 2016-03) Klopper, Brandt; De Villiers, D. I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This thesis proposes a method for the design of one-dimensional, uniformly spaced, nonuniform amplitude microstrip patch antenna arrays using computationally inexpensive transmission-line models and space mapping surrogate-based optimisation. The method provides consistent and effective results for both impedance specifications and radiation specifications, and is significantly faster than conventional full-wave optimisation procedures. The method is developed for microstrip-fed, probe-fed and aperture-fed rectangular microstrip antennas. Transmission-line models of each geometry are developed and run through a set of validation experiments to test their effectiveness over a range of substrate choices. The models are shown to perform with reasonable accuracy for a practical set of substrates, to the point where they are useful coarse models in space mapping optimisation procedures. An optimisation framework is set out to apply the space mapping paradigm to the design of the three aforementioned patch element geometries, as well as the design of a one-dimensional array. Several types of space mapping techniques are tested on each design problem, and the most efficient is selected to incorporate into the main design method of the thesis. The method incorporates the design of the individual patch antenna elements, the array layout and the finite antenna/feed substrate dimensions. The method is developed to accommodate microstrip feeds, probe feeds and aperture-coupled feeds to the patch antenna elements. A design procedure is developed to design a one-dimensional patch antenna array to a desired impedance and radiation specification. The procedure is tested with 2 experimental S-band designs; a narrowband probe-fed 1x8 patch array and a wideband aperture-fed 1x8 patch array. The aperture-coupled patch array achieves 10% fractional bandwidth over a -10 dB |𝑆11| passband, and exhibits 13.9° 3dB-beamwidth and -20.6 dB sidelobe-level in its gain pattern, across its passband. The total solver runtime of the entire aperture-coupled patch antenna array optimisation process amounts to 1708.14s (28.47 minutes). The design method is modular, efficient and integrated with a general space mapping optimisation software framework. It is highly expandable and is one of few, if any works to apply a transmission-line model to the space mapping optimisation of an aperture-coupled patch antenna.