Design and performance improvement of shaped-beam parabolic reflector antenna for small region coverage by non-symmetrical array feed technique

Design and analysis of a shaped-beam 12.2 GHz array-fed reflector antenna for broadcasting satellite application and its significant performance improvement are presented in this paper. In the beginning, an elementary design employing a cluster of feed horns illuminating a single reflector has been proposed for multi beam antenna (MBA) system to produce contoured beam for Peninsular Malaysia. In this configuration, the horn positions were determined precisely through a newly developed caustic locus graph, which is the simplified technique for calculating the exact feed locations in both elevation and azimuth plane. The accuracy of the two-dimensional caustic locus graph, which was developed based on a ray tracing approach, has been verified through EM simulations. A universal feed position equation, derived from the technique has been very useful in designing the multiple feed structures employed in this research to ensure optimum radiation to the exact locations on the earth. Based on the initial design (a cluster of feed horns) due to the small size of the coverage area, an issue with regards to physical constructability of the feed horns is raised. To solve this problem, the MBA is modified by utilizing 18-element microstrip array as the feed, where each element positions are calculated by using the same caustic model. In this approach, calculations of accurate feed positions and optimum excitation coefficients in widely distributed feed areas are considered as among the most critical aspects in designing the shaped-beam antenna system. By using the proposed solution, the preceding issue is solved and higher gain shaped-beam coverage with optimized radiation pattern is generated. This paper shows the results of the contoured beam antenna that have been achieved for beam scanned over a small coverage size of approximately 0.9° long and 0.5°wide. Small variation of radiation level, which is less than 3 dB within the EOC, is also demonstrated in the performance analysis.