[pt] Neste estudo faz-se uma análise do desempenho de antenas cônicas compactas para operar em largas bandas de freqüência. Esta análise será feita observando o comportamento da impedância da antena. Serão, primeiramente, consideradas as estruturas convencionais formadas por cones e cones sobrepostos próximos a um plano condutor. Posteriormente, será considerada uma estrutura cônica onde o arredondamento das bordas resulta em um comportamento mais uniforme para a impedância de entrada com variações da freqüência, conferindo um aumento na banda para esta configuração. / [en] The discone antenna is well known as an easy to build multioctave broadband antenna with a omnidirectional radiation pattern in the H-plane, It comprises a plane conductive disc element spaced close to and axially aligned with a conductive cone element. For applications in the UHF and microwave ranges, high performance antennas have been designed to operate from 0.5 to 5 GHz with a VSWR of about 3.5:1 or less. If the length is finite, the impedance still strongly dependent on apex angle as long as the cone is longer than about quarter of wavelength and the apex angle is relatively larger. A problem that is experienced with these antenna designs is the relatively large size required to operate at the low frequencies. In applications where utilization of this antenna. In this work, the analysis and design of these antenna is obtained by employing a rigorous formulation of the electromagnetic scattering problem. As a design tool, we employ Method of Moments for the analysis of rotationally symmetric structures excited by TEM mode. To properly account the variations in driven-point impedance with frequency, the coaxial waveguide used to feed the antenna is also included in the analysis. The excitation is simulated by a distribution of equivalent electric and magnetic currents placed inside the coaxial cable, over a cross-section plane. These currents are defined such that only excite the mode TEM towards the cable-antenna junction. These numerical tool is employed in the shaping the metallic surfaces involved in the discone-type antennas in order to obtain more compact structures. Simple solutions can be easily obtained making the center fed cone and the disc element with radially outer edge portions rolled backwardly and away from each other to form donut-shaped configurations. The ruled edges not only reduce the diameter of the antenna but also permit the current to flow around them smoothly and without reflections that degrade the VSWR at low end of the frequency band.

[pt] O projeto de antenas refletoras para a geração de feixes modelados envolve a construção de algoritmos baseados em métodos de análise eletromagnética associados a técnica de otimização. Em geral, estes algoritmos demandam considerável tempo de processamento computacional, impondo limites para sua utilização intensiva na exploração de diferentes conjuntos de especificações e na busca de soluções mais compactas. Neste trabalho, será considerada a síntese de antenas refletoras circularmente simétricas para a geração de feixes modelados circularmente simétricos. Em função das características de simetria da antena, simplificações serão introduzidas na formulação do problema, reduzindo substancialmente o tempo de processamento e viabilizando sua utilização intensiva. Para a análise eletromagnética da antena refletora serão consideradas as aproximações da Ótica Física (PO) e das correntes de franja (CF). Considerando alimentadores radiando um campo com dependência azimutal n=1, o algoritmo foi adaptado e aplicado no modelamento de antena refletora para atender as especificações do CBERS (China-Brazil Earth Resources Satellite). Para alimentadores com dependência azimutal n=0, foram considerados sistemas de antenas omnidirecionais de um e dois refletores para a geração de diagramas modelados no plano vertical. Para estes dois tipo de sistema, o algoritmo de modelamento foi utilizado para maximizar o diagrama em direção e para produzir diagramas com dependência do tipo cossecante ao quadrado no plano vertical. A validação da técnica de análise eletromagnética desenvolvida foi obtida comparando os resultados com os gerados via Métodos dos Momentos. / [en] The usual algorithms employed in the design of shaped reflector antennas for satellite applications embed an electromagnetic analysis method in an optimization technique. In general, these algorithms demand considerable computation time, limiting its intensive use in the study to explore different sets of specifications and to search for more compact geometries. In this work, it is considered the particular case of the shaping of circularly symmetric reflectors antennas for the generation of circularly symmetric radiation patterns. The use of the reflector surface properties simplifies the formulation of the electromagnetic scattering and substantially reduces the computer time involved in the optimization iteration. For the electromagnetic scattering, the algorithm employed here considers the approximations given by the Physical Optics and Fringe Currents methods. Firstly, the shaping algorithm was applied in the design of a single reflector antenna to comply with the specifications of the China-Brazil Earth Resources Satellite (CBERS). For this case, the feed radiation pattern was represented by model with azimuthal dependence n=1. As a second case, the shaping algorithm was applied in the design of single and dual reflector omnidirectional antennas. Theses antennas were feed by a TEM coaxial horn with azimuthal dependence n=1. For these two type of antenna configurations, the modelling algorithm was used to maximize the diagram in one direction or to produce radiation patterns with dependence of the type cosecant to the square in the vertical plan. A Methods of Moments electromagnetic analysis was used to validate the scattering fields yielded by the PO+Fringe currents approximations.