Thesis (Ph.D.) -- University of Western Sydney, 2001. / "Submitted in fulfilment of requirements for the degree of Doctor of Philosophy, School of Engineering and Industrial Design, University of Western Sydney" Includes bibliography.

Surface errors on parabolic reflector antennas degrade the overall performance of the antenna. They cause amplitude and phase errors in the aperture field which lower the gain, raise the side lobes, and fill in the nulls. These are major problems in large ->space reflector antenna systems. F or example, future multiple beam antenna systems requiring spatial isolation to allow frequency reuse could be rendered useless if high side lobes are present. Space antenna structures are difficult to build. They must maintain a nearly perfect parabolic shape in a harsh environment while remaining lightweight. The restrictions on the structure become more severe as science and technology requirements demand electrically large antennas. Mechanically, there are technologies [4)r building antennas with adaptive surfaces that can compensate for many of the larger distortions caused by thermal and gravitational forces. However, as the frequency and size of the reflectors increase, the subtle surface errors become significant and degrade the overall radiation pattern. It is for this reason that another method must be used to further improve the radiation pattern. Electromagnetic compensation for surface errors in large apace reflector antennas has been the topic of several research studies. Most of these studies try to correct the focal plane fields of the reflector near the radiation pattern. The compensation is implemented by weighting the elements of an array feed. In most of the studies, a precise knowledge of the reflector surface is required. An alternative approach to electromagnetic compensation is presented in this study. The proposed technique uses pattern synthesis to compensate for the surface errors. It differs from previous methods in two major respects. The previous studies used global algorithms that try to correct the entire focal plane field near the focal point or the aperture plane field and, hence, modify the entire radiation pattern. The pattern synthesis approach uses a localized algorithm in which pattern corrections are directed specifically towards portions of the pattern requiring improvement. The second major difference is that the pattern synthesis technique does not require knowledge of the reflector surface, but instead uses radiation pattern data to perform the compensation. / Ph. D.

Thesis (MScEng)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: This thesis presents a thorough study of the printed sinuous antenna and its characterisation as a feed for re ector antenna applications. Two di erent techniques are used in this study, namely a parametric study and an e cient surrogate based optimisation strategy. A planar sinuous antenna over a re ecting ground plane, with no absorber lining, is designed following a parameter study from which e ective design guidelines are derived. The designed prototype displays a bandwidth ratio of more than 3 : 1 from 1:9􀀀6:2 GHz, at a measured return loss of 10 dB, representing a signi cant improvement over the octave band previously achieved with a similar antenna. An optimisation based approach is followed in formally investigating a conical sinuous antenna over a re ecting ground plane. An e cient surrogate based optimisation strategy, in which the antenna's response is approximated by a Kriging model, is used. The search for optimal design parameters as well as improvements in the accuracy of the Kriging model is accomplished by using expected improvement as the in ll sampling criterion. The antenna is optimised for return loss, aperture e ciency for a prime-focus paraboloid re ector as well as cross-polarisation and results from the optimisation are used to derive e ective design guidelines and performance limitations. The investigations are conducted for the 2 􀀀 6 GHz band and the obtained results can be easily applied for designs with wider bandwidths. Simulation results reveal improved return loss, aperture e ciency and cross-polarisation performances compared to what has previously been reported for this antenna. / AFRIKAANSE OPSOMMING: Hierdie tesis bied 'n deeglike studie van die gedrukte stroombaanbord sinuous antenna, sowel as die karakterisering daarvan as voer vir re ektor antenna toepassings. Twee verskillende tegnieke word gebruik, naamlik 'n parametriese studie en 'n surrogaat-gebaseerde optimering strategie. E ektiewe doeleindes vir die ontwerp is van 'n parameter studie afgelei, waarvolgends 'n planêre sinuous antenna met 'n weerkaatsingsgrondvlak ontwerp is sonder enige absorberende materiale. Die prototipe vertoon beter as 'n 3 : 1 bandwydte van 1:9 GHz tot 6:2 GHz teen 'n gemete weerkaatsingskoë siënt van beter as 􀀀10 dB, wat dui op 'n aansienlike verbetering teenoor die oktaaf bandwydte wat voorheen met 'n soorgelyke antenna bereik is. 'n Optimering-gebaseerde benadering is gebruik om ondersoek in te stel in die gebruik van 'n koniese sinuous antenna met 'n weerkaatsingsgrondvlak. 'n Doeltre ende surrogaat-gebaseerde optimeering strategie is gebruik, waar die antenna se weergawe deur 'n Kriging model benader word. Die verwagte verbetering is gebruik as maatstaf in die soektog vir optimale ontwerpsparameters, sowel as om die akkuraatheid van die Kriging model te verbeter. Die antenna is geoptimeer vir sy weerkaatsingskoë siënt, stralingsvlak e ektiwiteit for 'n paraboloïed antenna sowel as kruispolarisasie. Resultate van die optimering is gebruik om e ektiewe riglyne vir die ontwerp en grense vir die werkverrigting op te stel. Die antenna is ondersoek vir die 2 tot 6 GHz frekwensieband en die resultate wat verkry is kan maklik op ontwerpe met selfs wyer bandwydtes toegepas word. Simulasie resultate dui op 'n verbetering in weerkaatsingskoë siënt, stralingsvlak e ektiwiteit en kruispolarisasie in vergelyking met wat berig is vir hierdie antenna.