In this thesis, the concept of multi-phase centre virtual antenna is thoroughly investigated when a dual-mode primary feed is placed at the focal point of an offset reflector antenna. The virtual antenna requires that the antenna have multiple phase centre locations with identical radiation patterns. It is shown that different polarizations and mode content factors of a dual-mode circular waveguide can displace the phase centre location of an offset reflector antenna without changing its radiation patterns in any direction. This novel idea has promising applications in modern satellite, remote sensing, global positioning systems, and radars with moving platform. The concept is well matched for the modern Displaced Phase Centre Antenna technique, in which a simple signal processing technique is employed to electronically displace the phase centre locations. To avoid mechanically rotating the primary feed, a novel dual-mode feed is also proposed and implemented. It is capable of generating two perpendicular polarized TE11 modes with a fixed aligned TE21 mode. The antenna such developed has a potential application as a transceiver in ground moving target indicator radars. All numerical results are successfully validated by fabricating and testing a prototype antenna in practice. As known, an offset reflector fed by a conventional linear polarized feed suffers from high cross polarization level. This property is also investigated for single-phase centre antenna applications along with sidelobe level and aperture efficiencies. A simplified feed model is proposed utilizing the TE11 and TE21 type modes to reduce the cross polarization at both asymmetry and inter-cardinal planes. It is shown that an asymmetric dominant TE11 mode with the presence of the TE21 mode is sufficient in order to reduce the unwanted cross polarization. The results of this investigation are used to design practical dual-mode feeds using simple circular waveguide geometries. To complete the study, the effects of linear and quadratic phase errors associated with the optimized primary feed are also investigated on the cross polarization of the offset reflector antennas. In particular, the phase errors resulting in separate phase centre locations of each mode produce broadside-shape cross polarized patterns instead of boresight-null ones.

In this thesis, the concept of multi-phase centre virtual antenna is thoroughly investigated when a dual-mode primary feed is placed at the focal point of an offset reflector antenna. The virtual antenna requires that the antenna have multiple phase centre locations with identical radiation patterns. It is shown that different polarizations and mode content factors of a dual-mode circular waveguide can displace the phase centre location of an offset reflector antenna without changing its radiation patterns in any direction. This novel idea has promising applications in modern satellite, remote sensing, global positioning systems, and radars with moving platform. The concept is well matched for the modern Displaced Phase Centre Antenna technique, in which a simple signal processing technique is employed to electronically displace the phase centre locations. To avoid mechanically rotating the primary feed, a novel dual-mode feed is also proposed and implemented. It is capable of generating two perpendicular polarized TE11 modes with a fixed aligned TE21 mode. The antenna such developed has a potential application as a transceiver in ground moving target indicator radars. All numerical results are successfully validated by fabricating and testing a prototype antenna in practice. As known, an offset reflector fed by a conventional linear polarized feed suffers from high cross polarization level. This property is also investigated for single-phase centre antenna applications along with sidelobe level and aperture efficiencies. A simplified feed model is proposed utilizing the TE11 and TE21 type modes to reduce the cross polarization at both asymmetry and inter-cardinal planes. It is shown that an asymmetric dominant TE11 mode with the presence of the TE21 mode is sufficient in order to reduce the unwanted cross polarization. The results of this investigation are used to design practical dual-mode feeds using simple circular waveguide geometries. To complete the study, the effects of linear and quadratic phase errors associated with the optimized primary feed are also investigated on the cross polarization of the offset reflector antennas. In particular, the phase errors resulting in separate phase centre locations of each mode produce broadside-shape cross polarized patterns instead of boresight-null ones.

Thesis (M.S.)--Ohio University, August, 1998. / Title from PDF t.p.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 226-230). Also available via the Internet.

[pt] O modelamento de superfícies refletoras embarcadas em satélites tem se apresentado como uma alternativa eficiente à síntese de conjuntos de alimentadores. Existem várias técnicas disponíveis para a determinação do formato adequado para esta superfície, baseados em aproximações da ótica geométrica ou da ótica física. Neste trabalho, estudamos a aplicação da técnica das Projeções Sucessivas, um método interativo genérico, à síntese de superfície refletora e à síntese da distribuição de fase sobre a abertura. Este método é baseado nas aproximações da Ótica Física, o que garante uma boa precisão. Entre suas principais vantagens, estão a sua simplicidade e a sua eficiência computacional. / [en] The shaping of reflector surfaces for satellites hás been considered na efficient alternative to the available for the determination of the optimum shape for this surface, base don Geometrical Optics or Physical Optics approximations. This work investigates the application of the Technique of Sucessive Projections, a generic interative method, to the synthesis of a single reflector antenna and to the phase distribution on the aperture. This method is based on Physical Optics Techniques, which guarantees a good precision. Among its main advantages are its simplicity and its computational.

[pt] Neste trabalho, a técnica de interpolação de superfícies definidas numericamente por pseudo-splines é aplicada a antenas refletoras offset e com alto grau de modelagem. É feito um estudo comparativo com técnica de interpolação alternativa levando-se em conta os diversos métodos de análise de antenas refletoras. / [en] In this work, the pseudo-spline technique for numerically defined surfaces is applied to the approximation of single and dual-shaped offset reflector antennas. A comparative study with an usual alternative technique is undertaken, emphasizing the implications observed when different methods are employed for the evaluation of the antenna radiation field.

[pt] A maximização do ganho de antenas cassegrainianas ou gregorianas normalmente é obtida com a modelagem do sub-refletor e do refletor principal. Tendo em vista facilidade de construção e flexibilidade no uso do tipo de alimentador, apresenta-se a alternativa de modelar-se apenas o sub-refletor. No presente trabalho, além de se modelar o sub-refletor, ajusta-se os parâmetros do parabolóide usado como refletor principal, a fim de minimizar os erros de fase da abertura até um valor que justifique a utilização dessa alternativa. Inicialmente são consideradas antenas cassegrain simétricas (capítulo 2). A seguir o estudo é entendido á antenas gregorianas do tipo offset. (Capítulo 3). / [en] The mazimization of the gain of cassegrainian or gregorian antennas is generally obtained with the shaping of su-refletor and main reflector. Having in mind easiness of construction and flexibility in the use of feed, an alternative of shaping only the sub-reflector, is presented. In the present work, besides shaping the sub-reflector, the parameters of the paraboloid used as the main reflector, are adjusted to minimize phase erros in the aperture uo to a value that justifies the use of such alternative. Symetrical cassegrain antenas are first considered (chapter 2). In chapter 3, the study is extended to gregorian offset type antennas.

Modifications of large reflector antennas, such that their observing capabilities are enhanced in the range of about 10-500~MHz without affecting operation of the pre-existing higher-frequency systems, are addressed in this dissertation. The major contributions of this dissertation can be divided into two parts: 1) designing new low frequency feeds, and 2) developing new analysis methodologies which, as opposed to traditional techniques, are suitable for analyzing low frequency systems. First, we consider the performance of existing schemes that provide low frequency capability. Then, a new class of dipole-based low frequency feeds - namely, the ``distributed feed array'' - is designed to cover the frequency range of interest without affecting operation at higher frequencies. As an example, distributed feed arrays are designed for the Expanded Very Large Array (EVLA) to cover the range of 50-250~MHz. A method of moments (MoM) model of an EVLA antenna is developed for this purpose. The new design shows performance comparable to the existing 4 m system on the EVLA in the range of 50-88~MHz, and introduces observing capabilities in the range of 110-250~MHz (currently not covered by the EVLA). Moreover, the blockage presented to the existing EVLA L-band system is reduced significantly when the existing 4 m system is replaced by the proposed system. At low frequencies, external noise can be a significant or dominant contribution to the total noise of the system. This, combined with mutual coupling between the array elements of the distributed feed array, makes it difficult to predict the sensitivity of these systems. This dissertation describes a system model and procedure for estimating the system equivalent flux density (SEFD) - a useful and meaningful metric of the sensitivity of a radio telescope - that accounts for these issues. We consider the efficiency of methods other than MoM - in particular, Physical Optics (PO), Uniform Geometrical Theory of Diffraction (UTD), and hybrid methods - for accelerated computation at low frequencies. A method for estimating the blockage presented by low frequency systems to the pre-existing higher-frequency systems is also described. / Ph. D.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The o set Gregorian dual re ector antenna is eminently well suited to a radio telescope antenna application as it o ers a narrow beam width pattern (i.e high gain) and good e ciency. The focus of this work is on the analysis of characteristics of such a Gregorian antenna. The design of the class of re ector antennas is normally based on the use of ray-optics, with this simpli ed approach being able to predict antenna performance based on approximate formulas for example the beam width against aperture size. However for compound antennas such as the Gregorian re ector there are several interdependent parameters that can be varied and this reduces the applicability of the simple ray-optic approach. It was decided that, if a fast enough analysis of a con guration can be found, the technique of design through interactive analysis would be viable. To implement a fast analysis of the main beam performance of such a Gregorian antenna, a solution algorithm has been implemented using a plane wave spectrum approach combined with a custom aperture integration formulation. As this is able to predict the beam performance within about a second on a PC, it is suitable for iterative design. To implement the iterative design in a practical manner a user interface has been generated that allows the user to interactively modify the geometry, see the physical layout, and then nd the antenna pattern. A complete working system has been realised with results comparing well to a reference solution. The limitations of the technique, such as its inaccuracy in predicting the side lobe structure, are also discussed. / AFRIKAANSE OPSOMMING: Die afset Gregoriaanse dubbelweerkaatser antenna is uiters gepas vir radioteleskoop toepassings aangesien dit 'n nou bundelwydte (ho e aanwins) en 'n goeie benuttingsgraad bied. Die fokus van hierdie werk is op die analise van die eienskappe van so 'n Gregoriaanse antenna. Die ontwerp van die klas van weerkaatsantennas is normaalweg gebaseer op straal-optika, waar hierdie vereenvoudigde tegniek, deur benaderde formules, gebruik kan word om antennawerkverrigting af te skat soos bv. die bundelwydte teen stralingsvlakgrootte. Vir saamgestelde antennas soos die Gregoriaanse weerkaatser is daar egter verskeie onafhanklike parameters wat verstel kan word en die toepaslikheid van die eenvoudige straal-optiese benadering verminder. Dit was besluit dat, indien die analise van die kon gurasie vinnig genoeg uitgevoer kon word, die tegniek van ontwerp deur interaktiewe analise werkbaar kan wees. Om 'n vinnige analise van die hoofbundelwerkverrigting van so 'n Gregoriaanse antenna te bewerkstellig, is 'n oplossingsalgoritme gemplementeer wat gebruik maak van 'n platvlakgolfspektrum benadering in kombinasie met 'n doelgemaakte stralingsvlakintegrasieformulering. Aangesien hierdie strategie die hoofbundel binne ongeveer 'n sekonde op 'n persoonlike rekenaar kan voorspel, is dit gepas vir iteratiewe ontwerp. Om die iteratiewe ontwerp op 'n praktiese wyse te implementeer is 'n gebruikerskoppelvlak geskep wat die gebruiker toelaat om, op 'n interaktiewe wyse, die geometrie aan te pas, die siese uitleg te sien en dan die stralingspatroon te bereken. 'n Volledige werkende stelsel is gerealiseer met resultate wat goed ooreenstem met 'n verwysingsoplossing. Die tekortkominge van die tegniek, soos die onakkuraatheid in die voorspelling van die sylobstruktuur, word ook bespreek.

[pt] Este trabalho apresenta uma nova técnica para aproximação de uma superfície refletora definida numericamente, i.e., por pontos fornecidos pelo processo de síntese da antena. As limitações inerentes às técnicas usuais são aqui eliminadas pela utilização de Pseudo-Splines Quínticas que interpolam uma distribuição arbitrária de pontos por uma superfície suave, com derivadas primeiras e segundas contínuas, assegurando uma representação única para o domínio de interesse. O procedimento é, então, aplicado ao subrefletor modelado de uma antena Cassegrain, com subseqüente cálculo do campo eletromagnético espalhado, permitindo uma análise detalhada de sua aplicabilidade. Uma teoria assintótica uniforme de difração é, também, aqui desenvolvida de modo a acomodar o espalhamento de feixes Gaussianos, descritivos, em freqüências altas, do diagrama de irradiação de alimentadores comumente empregados em sistemas refletores, por superfícies condutoras, através do rastreamento do campo eletromagnético ao longo de raios no espaço complexo. A análise do problema canônico (difração por semi-plano) estabelece as particularidades do método e a comparação com a solução rigorosa existente comprova sua acurácia, permitindo a extensão a problemas tridimensionais vetorais. A teoria Complexa da Difração, assim formulada, é, então aplicada ao cálculo do campo espalhado por diferentes geometrias de antenas refletoras, ilustrando a versatilidade do método bem como suas limitações. / [en] In order to evaluate the electromagnetic field scattered by shaped reflector antennas, one has to fit a surface to a set of points furnished by a synthesis technique. A new method, capable of interpolating arbitrarily located data points by a smooth surface is here presented. The interpolating function, called Quintic Pseudo-Spline, has continuous first and seconde order derivatives and yields a unique representation for the entire domain. The method is tested on the shaped subreflector of a Cassegrain antenna providing a thorough investigation of its applicability. Also, an uniform asymptotic theory of diffraction is derived in order to analyse the scattering of Gaussin beams, descriptive of the high-frequency radiation pattern of feed horns commonly employed in reflector systems, by conducting surfaces with edges. The constraints inherent to usual methods of analysis are hereby avoided by tracking these beam-type fields along straight rays in a complex coordinate space. Investigation of the canonical problem of scattering of a Gaussian beam by a conducting half-plane establishes the characteristics of the complex ray diffraction process. Comparison of the results thus obtained with the rigorous solution reveals the accuracy of the proposed theory and permits its extension to the three-dimensional vector problem. The resulting Complex Theory of Diffraction is then applied to the evaluation of the scattered field for several reflector antenna geometries, illustrating the versatility of the method as well as its limitation.