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- Adapted raised cosine window function for array factor control with dynamic range ratio limitationPublication . Santos, Filipe E. S.; Azevedo, Joaquim A. R.The use of window functions to improve the side lobe level of antenna arrays is hindered by high value of excitation currents dynamic range ratio. This paper proposes a fast and iterative window function generation strategy aimed at achieving improved side lobe level starting from a preset current dynamic range ratio. Based on this strategy a new window function is developed for standard set of conditions.
- Synthesis of planar arrays with elements in concentric ringsPublication . Azevedo, Joaquim A. R.The circular symmetry of concentric ring arrays becomes an advantage for situations where the radiation patterns are symmetric in the azimuthal direction. For planar arrays with elements in an equispaced grid, the fast Fourier transform can be applied in the calculations involved between the array factor and the source distribution. However, for circular arrays the elements are in a non-equispaced grid, making the application of fast computation algorithms difficult. Therefore, two techniques are proposed to synthesize circular arrays. The first one provides the control of the pattern by sampling the array factor. In this case, the fast Fourier transform can be applied. The windows technique is also utilized to control the sidelobe levels and the ripple structure of the shaped beam. The second technique considers the symmetric nature of concentric circular arrays and the control of the array factor is performed imposing some specific points of the pattern.
- On the use of Fourier transform with acoustical fieldsPublication . Casimiro, António Manuel E. S.; Azevedo, Joaquim A. R.The Fourier Transform is a powerful tool to deal with the radiated far field pattern of source distributions [l], PI. In this paper it is made a theoretical introduction to the possibility of using the Fourier Transform to study the fields produced by distributions, either continuous or discrete, of acoustical source distributions. With this new approach we can have a deep insight in the field and it is possible to develop and simplify methods of analysis and synthesis of fields produced by acoustical sources. All the procedure is based on the Theorem of the Small Complex Translation presented. In this case, and with a convenient choice of variables, the far field pattern is related with an window on the Fourier Transform of source distribution. The analysis and synthesis are made using the Fourier Transform firoprieties and the tools provided by the signal processing theory using the Fourier Transform
- Dealing with the useful window related with the far field patternPublication . Casimiro, António Manuel E. S.; Azevedo, Joaquim A. R.; Grilo, Alberto J. V. V.With a convenient representation, the radiation pattern of a spatial distribution of radiating sources is always related with that distribution by a window in the inverse Spatial Fourier Transform of the distribution. This fact is used to change the distribution of sources without disturbing the radiation pattern. On the other hand, the error, in the mean square sense, made by source elimination is also related with the Fourier Transform of the distribution. So it spreads all over the domain and not only inside the useful window. This must be taken in account and, with some signal processing, it is possible to find the error only in the far field pattern using the spatial distributions of sources.