Browsing by Author "Cerfonteyn, William"

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    A 22.2 GHz Antenna for water vapour radiometry
    (Stellenbosch : Stellenbosch University, 2019-04) Cerfonteyn, William; De Villiers, D. I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: For this thesis a low-cost antenna solution for a low-cost water vapour radiometry system is designed. The most important specifications for the antenna is that it has to have a -20 dB sidelobe level, a 3° -3 dB beamwidth and have to be electronically steerable. It also has to be low-cost. The first design that is considered is the phased array solution as it can be electronically steered. Since components can be etched onto the design the manufacturing cost is also reduced. The number of elements required for this design to reach the specifications is determined to be more than 16x16 elements, and thus proves to be cost impractical. The analysis is tested by designing a 1x4 and 2x2 multilayered aperture coupled patch antenna array with a corporate feed network, to both test if a better antenna element design can improve the results, and if the analysis is accurate in determining the impracticality of an array design. This also proves that the array design is infeasible. A reflector solution is then considered. The reflector system is designed to statically observe in a few specified directions, removing the need for electronic steering. Two parabolic reflectors and a conical horn is designed. The reflectors are combined through interpolation into one larger reflector. After the horn design proves to be ineffective, a new horn is considered that almost allows the reflector system to meet the specifications. Finally the system is measured, but unfortunately the measured result proves that the physical system does not meet all the specifications. The sidelobe level is around -16 dB, however, the -3 dB beamwidth is well below 3°.
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    Impact 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.
    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.