Enhanced Two Level EBG Antenna for a High F/D Multibeam Reflector Antenna in Ka Band: Design and Characterization

This paper deals with the design of an EBG antenna dedicated to feed a side front offset Cassegrain antenna (SFOCA) for a multibeam coverage of Europe in Ka band. Indeed, the EBG structure which permit to interlace equivalent radiating apertures, represents a new passive technology to realize multi beam coverage with only one reflector at an attractive cost compared to usual systems with beam forming networks or with several reflector antennas. The telecommunication mission studied requires on the one hand a multibeam reflector antenna with large F/D ratio to decrease the parasitic defocusing effects for high scanned beams and on the other hand, a circular polarization concept which reduces the price of the subscriber terminals assembly. These characteristics induce some focal structure specifications which cannot be reached with a classical EBG antenna. Also, the first step has been to improve the EBG antenna performances thanks to a more efficient feed which is a conical horn. The replacement of the patch by the horn permits to double the radiation bandwidth and to decrease the side lobes (-15 dB) of a 24 dB EBG antenna. The second job has been to reduce the parasitic interferences which occur in a multifeed EBG antenna especially that no polarization decoupling is achievable. Consequently, a multihorn two level EBG antenna with corrugations which permit to emplace the feeds outside the EBG cavity has been designed. As the performances of this structure are as well as those obtained with an EBG antenna fed by only one feed, we have study the whole system composed of the SFOCA and the multihorn two level EBG structure. The results obtained agree with the specifications because a EOC gain higher than 43 dB and sideslobes lower than -20 dB have been obtained for 90% of the beams.