Space multi-beam antenna with very high figure of merit, for Ka-band multimedia via satellite transmission Yann CAILLOCE, Gerard CAILLE: Alcatel Space Industries, B.P. 87, 3037 Toulouse Cedex, France. e-mail: yann.cailloce@space.alcatel.fr Summary : A typical multimedia via satellite transmission needs to cover, from a geostationary orbit, a large terrestrial zone with 0 to 50 semi-contiguous beams, for very high gain & re-using 0 to times the same frequency resource, by dividing the available bandwidth in sub-bands, spot-beam over. Such multi-beam antenna much increases the System capacity, for providing broadband services directly to users at home or office, wherever in the world. We point out the Focal Array Fed Reflector as the most suited receive-antenna architecture, for various kinds of coverages, to provide high figure of merit (G/T) and inter-beams isolation. A demonstrator has being built under the IST/ MultiKaRa contract, co-funded by the European Union. We present the design and performances of the whole antenna, especially the highly integrated active receiving feeds, and explain how a cold thermal control of the active focal array improves the antenna G/T by db, allowing cheaper User terminals (0% less power to be transmitted). I. Multimedia via satellite systems Numerous projects are emerging, from International Public institutions (Intelsat, Eutelsat,...) or private Operators, for providing "multimedia via satellite, direct to home" services to subscribers: very fast INTERNET, videoconferencing, visiophone, tele-teaching, telemedicine, access to large data bank, etc. To provide such services, mainly to users in the most populous regions, the simplest approach is to use a small number of geostationary satellites facing these regions. The C- and Ku-bands traditionally used for satellite communication are nearly saturated; the only band that can offer sufficient capacity ( Gbit/s needs around GHz in each transmission direction) is the Ka-band (8/3 GHz for the uplink; 8/ GHz for the downlink). So, many demands (filings) for using the Ka-band have been issued for multimedia satellite projects: Spaceway, Cyberstar, Ge*star, Euroskyway, Intelsat BSS, Pioneer, Ka-Sat.: - some concentrate their services on regional coverages (Europe, USA/Canada ) with very narrow spotbeams (0.5 to 0.7 diameter): see an example on the left figure here-under; - others aim at covering North to South with the same satellite, with a little larger spots, to. (right figure). F F F F F3 F F F Regional multi-beam coverage with a regular lattice and : frequency reuse scheme Very large coverage with 50 regular / dispersed spot-beams In all cases, to increase the system capacity, in terms of transmitted data rate, a frequency reuse pattern is associated with the multi-beam coverage (F,F,F3,F on the left figure). The equivalent total bandwidth is equal to: N spot x overall bandwidth / nb of different sub-bands. As example, using 8 spots and sub-bands, provides a times higher System capacity than a single beam coverage, provided a good spatial isolation is
ensured between antenna patterns corresponding to the same sub-band. Besides, covering the same footprint with 8 beams instead of induce an increase in antenna directivity roughly equal to 0 log8 = 7 db. Especially, a very high G/T (figure of merit) is compulsory for the receive satellite antennas, to counteract high rain attenuation around 30 Ghz, and allow the use of low-cost user terminals, which are a key to the success of such systems: - small size antennas, with shape and surface accuracy well suited to mass production; - low-power Solid State Power Amplifiers: at these frequencies, they represent a significant part of the terminal cost. To fit these challenging demands an European Consortium was built by Alcatel Space in 999, and issued a proposal for a detailed study, including an important hardware Demonstrator, within the IST Programme: the MultiKaRa (Multi-beam Ka-band Receive antenna for Multimedia systems) project was accepted by the European Commission. The main antenna specifications have been derived from the projected coverage, presented on the last right figure: - frequency band : 8350-30000 MHz ; frequency reuse : ; Isolation: (C/ΣI) >6.5 db - 50 spot-beams,.5 wide; linear polarisation - G/T > db/k (which requires G>0 dbi with a classical receiver noise temperature) on the whole coverage. II. Focal Array Fed Reflector receive antenna design From several trade-off analyses performed by Alcatel Space, the FAFR provides a high antenna gain using a light reflector, low side lobes and a much lower number of RF paths, individually matched in amplitude/phase, than a Direct Radiating Array, even for numerous spots as required here. Each beam is generated by combining signals coming from a cluster of 7 or active feeds, composed of small hexagonal horns (3 mm wide) and LNA's. Besides, each feed contributes to several beams ( to 5, depending on its position in the focal array), thanks to matched dividers, then combiners, which losses are masked by the LNA gain. To provide the 50 spots, over a very large field of view, shown on the right coverage I, a -FAFRs solution (one per hemisphere) was selected, as it required a lower overall number of feeds. A complete optimisation process was led to maximise both the minimum directivity over all the spots and isolation between those using the same frequency sub-band: the optimal offset geometry was a φ=.m reflector, with m focal length; 7 radiating elements are necessary to generate the 3 beams of the North hemisphere, 96 for the 6 South beams, while complying with all requirements. BFN spot S : : a 0 a 0 RFP : RFP S- S N redunded LNA Divider q 3 : a 0 : a 0 BFN spot n : a 0 : a 0 : a 0 a 0 RFP BFN spot 3 RFP spot s+ BFN spot : a 0 spot BFNspot s : a 0 : a 0 a Ø : a 0 RFP : a 0 RFP ps spot 3 spot spot S- : a 0 : a 0 a 0 p FAFR Antenna example Electrical diagram of the multi-beam focal array.
III. Active feeds design & development A partial focal array was developed by Saab Ericsson Space, comprising 7 active feeds (each with parallel LNA chains in a cold redundancy arrangement), embedded among terminated horns. They succeeded to comply with stringent RF requirements within a very small volume (the centre-to-centre spacing is.8 mm, within an hexagonal array lattice). Antenna Horn Array Terminations Transformers WG switches LNA units Cold Plate, linked to OSR by CPL - Active gain > 30 db (two 3-stages MMIC- LNAs on each of the 7 x chains). - DC consumption < 00 mw per chain. - Overall Noise Figure <.7 db at ambient; 0.dB /0 C decrease until 60 C. - Radiating efficiency of the hexagonal horns > 90 % vs πs/λ.. - Mass : 7g per active feed in the assembly. IV. Cold thermal control, for decreasing Noise Temperature The objective of the cold thermal control, was to maintain the LNAs at low temperature, for lowering their Noise Figure. The basic concept for cold thermal control is to link each focal array to one or several dedicated baffled thermal radiators by using a Capillary Pump Loop (CPL) : Focal array Heat Transfer Capillary Pump Loop (CPL) Cold plate / evaporator interface box Cold Space OSR Cold thermal control principle & CPL demonstrator (OSR=Optical Solar Reflector). The CPL is a very reliable equipment (without any motor, using thermal properties of evaporation / natural convection / condensation), allowing an efficient heat transfer and hence a remote location of the radiators, fitted to the best accommodation on the spacecraft: 3
Assembling FAFR antennas and 3 thermal radiators on ALCATEL Spacebus platform (left: deployed / right: stowed under the Ariane 5 fairing) The CPL built by SABCA (Belgium) was extensively tested; the results were injected in an overall spacecraft thermal model by ALCATEL-Cannes, which confirmed an efficient cooling of the focal array: the LNA s temperature is kept < -30 C as worst case (end of life, hottest orbit), compared to 50 C inside the spacecraft with a classical thermal control. This induces a db increase for G/T (antenna figure of merit), V. RF tests of the active focal array and the reflector antenna ± 30 useful cone Embedded pattern of an active feed (horn + LNAs)
The radiating patterns of the 7 active feeds, embedded in the partial focal array, have been measured as very identical, with good axi-symmetry and low cross-polarisation (see the last figures). 3 feed-clusters for adjacent beams Active focal array Alignment support Test range interface Carbon-fibre reflector The FAFR antenna demonstrator consists of a very light carbon-fibre parabolic reflector and a partial Rx active focal array, linked to a test Beam Forming Network; all units are assembled on a specific structure, which interfaces with the far-field antenna test bench in Alcatel-Space Toulouse. The radiating patterns and antenna directivity, for 3 adjacent beams, are under measurement by moving the 7- active-feeds cluster at the 3 central positions, while keeping terminated the external crown of the partial focal array. Here-below, the iso-dbi contours for the first of the 3 beams, show a good agreement between measurements and simulations. More extensive results will be presented at the ICSTI conference. : VI. Conclusions Simulated dd Measured d The MultiKaRa project has proven the advantages of FAFR antennas, w.r.t. classical feed per beam ones: - only antennas instead of 6 are needed to cover with numerous high-gain spot-beams very large regions, from North to South ( antenna instead of in case of a restricted zone, e.g. Europe or North America) - the reduced antennas volume lets place on the spacecraft Earth panel, to large thermal radiators, linked to the focal arrays by a bi-phase Capillary Pump Loop; the LNA's operating temperature is kept between -50 and -30 C during the 5-years satellite life: this increases by db the antenna figure of merit (G/T).. - an improved isolation between beams reusing the same frequency band (by more than 3 db) offers a much larger transmission capacity for multimedia systems. 5