Operational and next generation satellite telecommunication systems often require multiple beam capability.
In particular Ka band multi-beam down links are being used to achieve efficient coverage of the continents. The two fundamental ways to generate a large number of high gain beams over a large coverage area with reflector antennas are overlapping feed arrays with sophisticated feed networks in front of a single reflector and interleaved beams with multiple apertures (typically 4 reflectors) and a single feed per beam design.
The reason why this multi aperture antenna concept is needed and a single aperture with single feed per beam with a nominal sized reflector is not considered is the fact that the latter suffers from reduced performance.
The trade-off between inter-feed spacing (and the related feed diameter) and beam separation leads to inefficient illumination of the reflector and insufficient performance in terms of Edge Of Coverage Gain and isolation levels. Conversely, by using 4 antennas and interleaved beams, efficient illumination of the reflectors can be obtained because the aperture diameter of the feeds can be increased by a factor 2.
For future telecommunication satellites, it is particularly important to study alternative antenna architectures based on a single aperture antenna. A single aperture antenna would safeguard advantages especially in terms of costs, mass and spacecraft accommodation. Possible solutions to generate a multi-beam coverage starting from a single aperture is the Focal Array Fed Reflector or using an oversized shaped reflector in front of a single feed per beam focal array.
An alternative approach is made possible with the advent of Electromagnetic Band-gap materials, novel concepts have emerged and a few research groups have now demonstrated that based on this technology a single-feed-per-beam option in front of a nominal size reflector is viable and would significantly reduce the costs of multi-beam systems.
This study has covered the architectural trade-off at focal plane array level, performance analysis at antenna level, and focal plane array detailed design. An EM at focal plane array level permitted to demonstrate the feasibility and performances of this innovative concept.
Many challenges were proposed in this ARTES:
- No solution was "on shelf" at the beginning of the study. Very challenging to develop and manufacture a competitive solution in a short delay
- Antenna design selected relatively sensitive to planarity-parallelism default: We demonstrate that we have very good performances (better that state of the art in Ka)
- Very stringent requirement for filters part (phase response)
- Not classical spacing between feeds which leads to develop all transitions, loads,… with drastic manufacturing constraints
- Demonstration of the overlapped effect
For regular multi-beam lattice:
Need of overlapped feeds to achieve simultaneously:
- efficient illumination of the reflector
- low ‘roll-off’ between adjacent beams
=> an EBG ‘radome’ can provide a good alternative solution: widens & guides the electromagnetic field issued from each focal array feed
High interest to replace (for a regular Ka spots-arrangement) :
- Either 4 multifeed antennas(TX/RX) à Critical accommodation & mass, especially for a multi-bands satellite
- Or an active FAFR à rather complex & expensive
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.
The study can be considered in 4 main phases:
1. Antenna architecture trade-off, state of the art and requirements concerning overlapping focal plane array
In this first step, we compare performances available with a FAFR, an oversized reflector and a solution using an EBG focal array. a trade-off matrix with different antenna architectures leads to the solution with EBG feed (complexity, performances on agora mission, …). Retained solution comes from the knowhow from XLIM which makes a review of different EBG concepts. Outcomes of this trade-off permits to establish requirements of the overlapping focal plane array.
2. Antenna performances and EM definition
We optimise in this task design to reach optimal performances of the overlapping focal plane array and of the global antenna architecture (reflector illuminating by focal array (filters assembled to antenna part)). Many eligible material were studied but Invar design presents some guaranties mainly for thermal management and is coherent to filter’s manufacturing process.
3. Design and manufacturing of the EM
In accordance with the EM definition, a design of the complete EM (EBG screen, Feeds and Filters) was proposed. As planarity and parallelism requirement were very stringent, we propose a tuning capability to match the antenna performances with expected one. Manufacturing of the EM presents some difficulties dues to Ka constraints associated to EBG concept tolerances. The final EM was composed of 14 EBG feeds.
4. Electrical testing of the EM and antenna performances update
This last part is dedicated to tuning the antenna and filters parts and characterised the full focal plane array in term of [S]-parameters, directivity and gain. Directivity diagrams are used to determine performances on Agora coverage of the full focal plane array placed in front of the reflector.
Study concludes positively with many fulfilment :
- Good measure performances of the filters and very close to the filter’s simulated response
- Good agreements between simulation and measurements for the full focal array ([S]-parameters, Radiation diagrams)
- Good performances of the associated reflector and full focal array on the 40 spot coverage Agora
- Lot of retro-simulations leads to consider that slight differences between measures and simulation are due to manufacturing issues
- Demonstration of the overlap effect with near to +6dB in directivity when exciting a single EBG feed compared to a classical horn
To conclude, a market study shows that an EBG solution is competitive for moderate to medium capacity systems. If we can follow development on this topic, the next step consists in an EQM manufacturing.