Impact and cause of sensitivity ripple in radio astronomy reflector antennas

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dc.contributor.advisorDe Villiers, Dirken_ZA
dc.contributor.authorCerfonteyn, Williamen_ZA
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.en_ZA
dc.date.accessioned2023-11-20T08:52:07Zen_ZA
dc.date.accessioned2024-01-08T16:06:30Zen_ZA
dc.date.available2023-11-20T08:52:07Zen_ZA
dc.date.available2024-01-08T16:06:30Zen_ZA
dc.date.issued2023-12en_ZA
dc.descriptionThesis (PhD)--Stellenbosch University, 2023.en_ZA
dc.description.abstractENGLISH 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.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Hierdie proefskrif bied ’n studie aan oor die impak en oorsaak van die frekwensierimpeling in die ontvangsensitiwiteit van elektriese klein weerkaatsantennas. Kennis van die vorm en spektrale inhoud van die rimpeling is belangrik in sommige radiosterrekunde toepassings. Alhoewel daar geen alternatief vir hoëtrou simulasie van die antenna-weergawe, wat gebruik maak van toepaslike berekende elektromagnetiese simulasies, gevind was om die rimpelweergawe akkuraat te karakteriseer nie, word die verskillende fisiese o orsake v an die rimpeling, en hul relatiewe impak op die finale g edrag, omvattend oorweeg. Vir die volgende generasie teleskope wat breëband kamertemperatuur laeruis versterkers (LNA) gebruik, in teenstelling met uiters koue kryogeniese stelsels, word getoon dat die rimpeling in baie gevalle betroubaar geïgnoreer kan word tydens die aanvanklike ontwerpfase van die stelsel - selfs vir elektries baie klein stelsels. Dit word verder geïllustreer hoe die rimpeleienskappe verskil as ’n funksie van die antenna se wyshoek, en hoe, in sommige gevalle, die oorspoelenergie na die warm grond die effek kan o orheers. Tot o p h ede i s sulke karakteriserings in die literatuur geïgnoreer, en fokus was hoofsaaklik op die gedrag van die antenna hoofbundel - wat normaalweg na ’n relatief koue lug wys. Die proefskrif beskryf dat die oorsaak van die frekwensie-rimpeling in ontvangsensitiwiteit te wyte is aan nie-ideale effekte. Die sensitiwiteitsrimpeling word slegs beïnvloed deur die rimpeling in die antenna ruistemperatuur (ANT), en die rimpeling in die stralingsvlak benuttingsgraad (AE), terwyl die antenna en LNA goed aangepas is. Verder word die rimpeling van die ANT en AE slegs bepaal deur die stralingsintensiteitrimpeling, wat veroorsaak word deur verstrooiing, as gevolg van nie-ideale effekte, wat inmeng met die stralingspatroon van die volle weerkaatser stelsel. Nie-ideale stralingseffekte vind plaas wanneer die weerkaatser nie ideaal werk nie, wat plaasvind wanneer die weerkaatser nie oneindig groot is nie. Die omvang van die nie-ideale straling is gekorreleer met die elektriese grootte van die reflektor, en dus begin elektries klein weerkaatsers meer afwyk van die ideale straling wat deur geometriese optika voorspel word. Verder word dit uitgelig dat die ANT ’n funksie van alle rigtings is, en dus is die sensitiwiteit ook. Dit lei daartoe dat sekere rigtings, of gebiede, aansienlik belangriker is as ander vir die ANT-berekening, vir ’n spesifieke wyshoek. In hierdie gebiede word die rimpeling in die stralingspatroon waargeneem in die rimpeling van die ANT, soos verwag. Hittekaarte word gekonstrueer om hierdie belangrike hoeke uit te wys wat gebruik kan word om insig te verkryin watter nie-ideale effekte die rimpelbydrae oorheers, en om die sterk afhanklikheid van die ANT-rimpeling op die wyshoek toe te lig. Behalwe dat dit ’n funksie van alle rigtings is, is die ANT ook ’n funksie van heelwat fisiese parameters. Sommige van hierdie parameters en hul effek op die ANT word ondersoek. Tydens die ontwerp van radioteleskope, soos die ngVLA, word innoverende afskattingstegnieke van ANT en AE gebruik. Die akkuraatheid van hierdie benaderings vir ANT en AE word ondersoek en gekarakteriseer. Hierdie strategieë wat vir benadering gebruik word is vinnig, maar ignoreer tipies die modellering van die rimpeling. Dit is omdat presiese berekening van die rimpeling normaalweg duur is in terme van berekeningstyd, en gewoonlik nie nodig is tydens die optimeringsfase nie. Die modelleringsdoeltreffendheid van hierdie tegnieke word ondersoek, wat ’n sleutelkomponent is in die effektiewe ontwerp van weerkaatsers vir radiosterrekunde. Fisiese Optika (PO) simulasiestrategieë word dikwels vir die analise van groter weerkaatsantennas gebruik, in teenstelling met die moment metode (MoM) wat vir kleiner stelsels verkies word. Vir kleiner ontwerpe word die akkuraatheid van hierdie tegnieke belangrik om te oorweeg. MoM neem meer nie-ideale effekte in ag, in vergelyking met PO, en modelleer die rimpel dus meer akkuraat. In klein ontwerpe kan die Fisiese Teorie van Diffraksie (PTD) saam met PO gebruik word om die invloed van nie-ideale effekte meer akkuraat te modelleer. Daar is ’n breekpunt in frekwensie, waar die rimpeling gemodelleer met MoM en PO (met PTD) sal konvergeer, soos die elektriese grootte van die reflektor toeneem. Hierdie tegnieke word vergelyk en ontleed om hul impak te karakteriseer vir gebruik in die modellering van die rimpeling van die ANT in kleiner ontwerpe. Laastens in die slothoofstuk, word toekomstige werk oorweeg, waar moontlike rimpelvoorspellingsmetodes bespreek word. Een van hierdie metodes gebruik ’n kombinasie van tegnieke (insluitend Gevalideerde Eksponensiële Analise of VEXPA) om ’n unieke seinsamestelling te voorspel vanaf ’n stel monsters wat geneem word stadiger as die Nyquist-tempo. Benewens hierdie metode, word die lewensvatbaarheid van meetkundige argumente, of die toepassing van ’n voorafverwerkte rimpeling, oorweeg vir rimpelvoorspelling. Die eksklusiewe gebruik van ver-velde in die sensitiwiteitsberekening, sonder om die naby-veld in ag te neem, word ook bespreek, met voorstelle om die ondersoek na die effek daarvan moontlik te maak.af_ZA
dc.description.versionDoctorateen_ZA
dc.format.extentxxiv, 197 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/128927en_ZA
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch University,en_ZA
dc.subject.lcshAntennas, Reflectoren_ZA
dc.subject.lcshRadio -- Receivers and receptionen_ZA
dc.subject.lcshRadio -- Transmitters and transmissionen_ZA
dc.subject.lcshRadio astronomyen_ZA
dc.subject.lcshLow noise amplifiersen_ZA
dc.titleImpact and cause of sensitivity ripple in radio astronomy reflector antennasen_ZA
dc.typeThesisen_ZA
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