Doctoral Degrees (Electrical and Electronic Engineering)

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by browse.metadata.advisor "De Villiers, D. I. L."

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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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    Expanding the field of view: station design for the AAMID SKA radio telescope
    (Stellenbosch : Stellenbosch University, 2019-12) Bij de Vaate, Jan Geralt; De Villiers, D. I. L.; Davidson, D. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: The discovery of radio astronomy dates back to 1928, when Karl Jansky made the first detection of cosmic noise static. Radio astronomy is therefore a relatively young science and its development is completely in parallel with the radio technology enhancements, including the modern information technology. Radio astronomy has been often at the forefront; the first adopters of new technologies and capabilities. Very often radio astronomy triggered inventions that have been of direct use for other sciences and society. With the advent of digital signal processing, faster and smaller computers, radio astronomy has progressed at a very high pace. Arrays are being designed and built that contain thousands of individual antennas, connected and processed with digital signal processing, enabling very high performance. The conception of the Square Kilometre Array can be directly linked to the potential of Aperture Arrays (AA). As early as 1994 first sketches of the Square Kilometre Array consisted of flat electronically steerable panels. This AA telescope, briefly called the HI telescope, after the HI resonant line at 1421 MHz, would run from 150 to 1450 MHz. Although AAs are technically feasible, SKA1 will use dishes for the frequencies above 350MHz. This dissertation addresses the requirements, system design and possible implementation of an AA system for the SKA2, the second phase of the square kilometre array project. Aperture Arrays have the potential of instantaneous all-sky observations, creating a very powerful telescope. However, the realization of this telescope is not without challenges in cost, power consumption and operational performance. The research in this dissertation explores the potential of the sparse-regular array concept. Sparse-regular arrays are in use for lower frequency telescopes but implementation of this concept has not been considered for higher frequencies; up to the HI line at 1421 MHz. It will be argued with a new proposed figure of merit, average sensitivity divided by system cost, that a sparse-regular design can be superior to a dense or sparse-random system. This dissertation therefore provides an alternative solution for aperture arrays for the SKA and in particular focusses on a possible realization. A realization which takes signal processing into account that goes a step further than ‘just’ build a telescope that can do the same as dishes, but a bit better. For this it is proposed to use Fast Fourier Transform signal processing, enabled by a regular antenna placement structure. FFT signal processing will reduce the processing load significantly and fully utilizes the potential of AA’s. Therefore, regular arrays, sparse or dense, will be lower in cost and power consumption and superior in performance when compared to random arrays. This dissertation concludes that the sparse-regular concept, although technically feasible, has a number of significant drawbacks and therefore might not be the design of choice for SKA2.
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    Partially filled radial power combiner with port isolation
    (Stellenbosch : Stellenbosch University, 2020-03) Du Toit, Hendrik J.; De Villiers, D. I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This dissertation documents the design and testing of a partially filled radial power combiner with a passive peripheral isolation network, at 4 GH'. A review of available literature on power combiners highlights the advantages of radial combiners. The theory of radial transmission lines is presented, and such a power combiner without an isolation network is designed first. A printed circuit board is utilised at the bottom of the radial line cavity to aid in transitions at the central and peripheral ports. The centre transition into a coaxial connector consists of a shunt capacitance realised by an annular ring on the PCB. This also acts as a base for the coaxial connector. At the periphery, inductive posts couple energy from the radial cavity into microstrip lines. This approach is taken for compatibility with the ensuing planar passive isolation network at the peripheral ports. Broadside hybrid quadrature couplers are arranged around two almost identical radial combiners to implement the isolation.