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# Doctoral Degrees (Electrical and Electronic Engineering)

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### Browsing Doctoral Degrees (Electrical and Electronic Engineering) by browse.metadata.advisor "De Villiers, Dirk"

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- ItemA figure of merit for the x-band all-sky survey(Stellenbosch : Stellenbosch University, 2023-03) Kriel, Scott; De Villiers, Dirk; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
Show more ENGLISH ABSTRACT: The X-Band All-Sky Survey (X-BASS), is an experiment aimed at mapping the sky, both in intensity and polarisation, in the frequency range 7—15 GHz. This is an extension of the C-Band All-Sky Survey (C-BASS), taking the form of a single-dish polarimeter located in Klerefontein, South Africa. Using Cassegrain-like reflector surfaces already in place as part of C-BASS, the work will focus on the design of a feed antenna forthe new frequency band of interest. In traditional design algorithms, generic Figures of Merit (FoMs) describing the radiation pattern variations are used as design goals. These multiple FoMs are heuristically determined, and it is normally difficult to ascertain the quantitative importance of each FoM when combining them to describe the actual system performance. This is especially true for X-BASS, where the specific role of the instrument poses a unique set of requirements, the trade-off of which is not immediately apparent. As such, this work establishes new FoMs which directly relate antenna properties to their effect on distinct science goals. In order to accomplish this, attention must be paid toward the specific purpose of X-BASS, which is to aid experiments observing the cosmic microwave Background (CMB). Radiation from the CMB is seen to fluctuate across the sky in both temperature and polarisation. It is in the measurement of these anisotropies that lies a wealth of information on the origins of the universe and the physical mechanisms governing its evolution. The next generation of experiments seek to observe the CMB polarisation with an extraordinary degree of accuracy, placing ever-increasing demands on the instruments used to perform them. As such, more tailored design approaches than those that currently exist are required to optimise antenna structures in a way which maximises the quality of these science outcomes. This work sets out to accomplish this, providing a FoM for the X-BASS antenna which is readily calculable and relates directly to the uncertainty seen in the determination of cosmological parameters from CMB observations. Furthermore, this description is not specific to X-BASS, but could be applied more generally to other instruments aimed at measuring the CMB. With the FoM developed here at hand, future work can focus on rigorous analysis and optimisation of the antenna structure, allowing the effect of specific design choices to be studied in detail. The prescription could also be extended to consider more non-ideal effects, adding to the complexity of the analysis and providing a tool by which the next generation of CMB instruments can be designed.Show more - ItemImpact and cause of sensitivity ripple in radio astronomy reflector antennas(Stellenbosch : Stellenbosch University, 2023-12) Cerfonteyn, William; De Villiers, Dirk; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
Show more ENGLISH ABSTRACT: This dissertation presents a study on the impact and cause of the frequency ripple in receiving sensitivity of electrically small reflector a ntennas. Knowledge of the shape and spectral content of the ripple is important in some radio astronomy applications. Although no alternative to high fidelity s ampling of the antenna response, using appropriate computational electromagnetic simulations, was found to accurately characterize the ripple response, the different physical causes of the ripple, and their relative impact on the final response, is comprehensively considered. For next-generation telescopes using wide-band room temperature low-noise amplifiers ( LNA), as opposed to extremely cold cryogenic systems, it is shown that the ripple may, in many cases, be reliably ignored during the initial design phase of the system - even for electrically very small systems. It is further illustrated how the ripple characteristics vary as a function of antenna pointing angle, and how, in some cases, the spillover energy onto the hot ground may dominate the effect. To date, such characterizations have been ignored in the literature, and focus has mainly been on the behaviour of the antenna main beam - which normally points at a relatively cold sky. The dissertation describes that the cause of the frequency ripple in receiving sensitivity is due to non-ideal effects. The sensitivity ripple is influenced only by the ripple in the antenna noise temperature (ANT), and the ripple in the aperture efficiency (AE), while the antenna and LNA ar e we ll ma tched. Furthermore, the ripple of the ANT and AE is determined only by the radiation intensity ripple, which is caused by stray radiation, due to non-ideal effects, interfering with the radiation pattern of the full reflector system. Non-ideal radiation or effects occur, when the reflector does not operate ideally, which occurs when the reflector is not infinitely large. The extent of the non-ideal radiation is correlated to the electrical size of the reflector, and thus electrically small reflectors start to diverge more from the ideal radiation proposed by geometric optics. Furthermore, it is highlighted that the ANT is a function of all directions, and thus the sensitivity is also. This results in certain directions or regions being significantly more important than others for the ANT calculation, in a specific pointing angle. In these regions, the ripple in the radiation pattern is observed in the ripple of the ANT, as expected. Heatmaps are constructed to illuminate these important angles which can be used to gain insight into which non-ideal effects dominate the ripple contribution, and prove the strong dependence of the ANT ripple on the pointing angle. Besides being a function of all directions, the ANT is also a function of many physical parameters. Some of these parameters and their effect on the ANT is investigated. During the design of radio telescope projects, such as the ngVLA, state-of-the art estimations for ANT and AE are used. The accuracy of these approximations for ANT and AE are investigated, and characterised. These strategies used for rapid approximation are fast, however, often neglect modeling the ripple. This is because precise calculation of the ripple is often expensive in terms of computation and storage, and usually not necessary during the optimisation phase. The modeling efficiency of these techniques is interrogated, which is a key component in the effective designing of reflectors for radio astronomy. Physical Optics (PO) simulation strategies are often used in larger radio telescope designs, compared to Method of Moments (MoM). For smaller designs, the accuracy between these techniques becomes important to consider. MoM accounts for more non-ideal effects, compared to PO, and as such models the ripple more accurately. In small designs, the Physical Theory of Diffraction (PTD) can be used in conjunction with PO, to more accurately model the influence of non-ideal effects. There is a breakpoint in frequency, where the ripple modeled with MoM and PO (with PTD) will converge, as the electric size of the reflector increases. These techniques are compared and analysed, to characterise their impact for use in modeling the ripple of the ANT in smaller designs. Finally in the conclusion, future work is considered, where possible ripple prediction methods are discussed. One of these methods uses a combination of techniques (including Validated Exponential Analysis or VEXPA) to recover a unique signal composition, from a sampling rate under the Nyquist rate. Besides this method, the viability of geometric arguments, or applying a preprocessed ripple, is considered for ripple prediction. The exclusive use of far-fields in the sensitivity calculation, without considering the near-field, is also discussed, with suggestions to aid the investigation of its effect.Show more - ItemInductance in superconductor integrated circuits(Stellenbosch : Stellenbosch University, 2023-03) Fourie, Coenrad; De Villiers, Dirk; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
Show more ENGLISH ABSTRACT: This dissertation presents an overview of the research and publications of the candidate and his research group on the design of superconductor integrated circuits around inductance as a key circuit parameter, on the development and verification of inductance extraction tools for complex, three-dimensional integrated circuit models, and on the application of self- and mutual inductance extraction and magnetic _eld analysis to the improvement of superconductor circuit and system design. The research spans more than two decades, and culminates in the extraction of compact simulation models for the analysis of superconductor integrated circuits in the presence of trapped ux and external magnetic _elds, which was not previously possible. The golden thread that ties all of his work together is inductance in superconductor integrated circuits.Show more - ItemSub-sampled exponential analysis applied to sparse planar antenna array configurations(Stellenbosch : Stellenbosch University, 2023-03) Raal, Ridalise; De Villiers, Dirk; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
Show more ENGLISH ABSTRACT: Considerable research efforts are currently ongoing for the planned Mid-Frequency perture Array of the Square Kilometre Array project, which is set to operate from 450- 1450 MHz. An important design criterion is the aperture array configuration, for which a choice has not yet been made. Until recently, the focus was mainly on dense regular or sparse random array configurations, since the strong grating lobes that are present in the beam pattern of a sparse regular array have made them historically unattractive. Random placement of antenna elements in a sparse array reduces the impact of grating lobes by spreading out their energy into sidelobes. A sparse configuration is attractive since fewer elements are required for the same effective area, which leads to an improved angular resolution and lower cost when compared to dense regular arrays. The increased spacing between elements additionally leads to reduced mutual coupling effects. A recently developed algorithm known as VEXPA (Validated EXPonential Analysis) allows recovery from aliasing after sub-Nyquist sampling of data. VEXPA was previously applied to the direction-of-arrival estimation problem when using a sparse regular linear array in a coprime configuration which consists of two sub-arrays that are designed to have the grating lobes of each add destructively. Whereas traditional coprime arrays require a computationally expensive pairing and matching step between solutions of the sub-arrays, VEXPA makes use of a shared linear structure of equations between them, which automatically pairs their output. It also provides additional features compared to traditional DOA algorithms, such as validation of the output and the ability to estimate the number of incoming signals of the problem without any prior knowledge. In this dissertation, VEXPA is adapted and used for two different applications concerning sparse array configurations. For the first application, the sparse regular linear antenna array case (1-D scanning) is extended to the sparse regular planar antenna array case (full 2-D scanning). Additionally, a non-collinear configuration is presented which allows the algorithm to be used with an existing sparse regular planar array that was not originally designed with the coprime configuration in mind. A small engineering test system was constructed and measured in an anechoic chamber. Due to limitations of the available measuring equipment, it was necessary to construct the planar array demonstrator to be sparse along one dimension and dense in the second dimension. Comparisons to full-wave simulations showed that the main source of error was due to mutual coupling between the densely spaced antenna elements, emphasising the importance of the use of a sparse antenna array configuration. The second application considers antenna position estimation in a sparse random array. The construction cost of an aperture array consisting of a large amount of antenna elements can be significantly reduced by relaxing the required placement accuracy, and allowing for errors when connecting the elements to the system back-end. The idea is to find the antenna element positions after the installation phase through low-cost in-situ measurement campaigns where odd harmonic signals are transmitted from an Unmanned Aerial Vehicle (UAV) flying above the array with a known position in the sky. The problem is formulated as an inverse direction-of-arrival method and a simplified version of VEXPA which does not require validation is used to solve the resulting sub-sampled exponential analysis problem. The method was first developed conceptually by assuming that the UAV is in the farfield of the array, which allows its zenith angle to be used to model the coprime scaling parameters that are required for recovery from aliasing. For large arrays, the UAV is in the near-field with a spherical phase front incident on elements in the array, which leads to the time delay between elements being described by a non-linear equation. Thus, an iterative approach was followed to estimate the antenna positions from a linearised approximation of the near-field equation. For each iteration, coprime scale parameters were extracted to allow for de-aliasing and subsequent updating of the approximation with a new estimation of the antenna position, until the solution converges. An optimal flight path for the UAV was considered and the method was shown to work well for toy examples simulated in Matlab.Show more