Reflector focal array based on multi-feed EBG antenna for Ka-band space applications
The demand for multimedia communications via satellite systems have increased considerably in the last years. Normally, the satellite coverage is composed of a hexagonal lattice of many small beams. An example of an experimental satellite coverage named “Agora” is composed of 40 0.65°-diameter spots, using 4 frequency channels in a 4x reuse scheme with the same circular polarization. A first solution which meets the requirements of such a mission (C/I, efficiency, roll-off) is the use of 4 Rx/Tx parabolic reflectors on the same satellite, providing each a family of spots at the same frequency. This solution has the drawback of heavy weight and the challenging accommodation of 4 reflectors on the platform. These 4 reflectors could be replaced by only one, when using a focal array which is capable of generating a specific illumination law (overlapped beams). This focal array could be composed of an active BFN which is, unfortunately, expensive and has moderate RF efficiency. Several studies have shown that overlapped beams could be obtained by using multi-feed EBG antennas [1, 2]. The major problem of these structures is the mutual coupling between different feeds. It was demonstrated recently that suitable filter designs could be connected to each feed [3, 4] in order to ensure the isolation between different channels and also to reconstruct properly the radiation patterns which are otherwise disturbed by the coupling. In [4] and [5] it is shown that the use of a matched multi-feed EBG antenna with 2 linear polarizations and 2 frequency channels provides a 4x reuse scheme that simplifies the system architecture. In fact, the use of a matched antenna relaxes the need for the filters to provide matching inside the operating band; they just need to act like a short circuit with the appropriate phase outside their operating band, in order to reconstruct properly the radiation patterns. On the other hand the use of 2 frequency channels reduces the number of required filters (2 filters instead of 4). Finally, linear polarizations are considered in order to prevent the design of bulky filters for circular polarization (6 poles filters with 6 cavities are needed to ensure the transmission of circularly polarized wave and isolation between channels). In this paper, a metallic multi-feed EBG antenna operating in Ka band [29.5–30] GHz is presented, and its performances before and after connecting the filters will be shown. Finally, this antenna is used to illuminate a reflector and the coverage characteristics (C/I) will be given.