Main candidate concepts to provide overlapped beams from a single aperture have been studied:
a) the number of feeds may be reduced with the FAFR concept, studied and prototyped at EQM level by TAS-F. It is one of the candidate concept for the specified mission; compliant performances have been shown, with around 120 feeds for 19 spots. The BFN becomes feasible in an industrial process by gathering always 16 feeds and tuning only the phase-law. However, it is still a "top-level" solution with significant cost due to the 120 or 240 redounded LNA's, and not feasible for transmit. The same number of SSPA's would present much too high volume, DC consumption and cost.
b) A single oversized reflector associated to classical feeds (within the SFB concept) ? It is also a candidate solution. It has been recently assessed within ESTEC contract 19075/05/NL/NH; But we can yet tell that the over-sizing is very significant: the final reflector (after shaping optimization) has the same surface than 4 classical ones. An even more serious drawback is the very long focal length (8m for a TX antenna; 5.4m for a RX one), which does not allow accommodation on the satellite Earth panel.
c) Finally, the "EBG structure over a focal array" concept has been recently proven able to provide overlapped feeds fitted to the main mission requirements (without significant reflector oversizing), by at least 2 laboratories in Europe (XLIM in Limoges, and TNO in The Hague).
However associated developments are still at small scale, not at EM-standard for material and processes and have not yet solved some critical issues: how to manage the unavoidable coupling between overlapped apertures (in a phase-coherent way over the whole band) is one of them.
So we will propose that the present study will be focused on a significant improvement of the EBG concept, to reach better performances than the "oversized reflector solution" and similar performances than the FAFR, while avoiding any BFN.
As proposed in ESA requirements, we will develop this concept for a receive antenna, as it appears as a 1st simpler step:
- the relative bandwidth is smaller than in transmit (500 MHz related to 30 GHz instead of 20 GHz, for users link);
- the power-handling issues are not present in receive.
However, once the EBG concept will have proven its efficiency, how to apply it to a single transmit antenna could be assessed in further studies with a good background.
For developing the "EBG focal array concept", we have associated in the tendering team:
- Thales Alenia Space France, as one of the world leader in Satellite antennas, whose Research Section pointed out for several years the interest of the EBG technique for multi-beam antennas.
- XLIM (Limoges laboratory, part of the National French Research Centre 'CNRS'), a pioneer in the EBG developments from more 10 years.