Antenna design for Space Applications M. Sabbadini European Space Agency, Noordwijk, The Netherlands

Transcription

1 Antenna design for Space Applications M. Sabbadini European Space Agency, Noordwijk, The Netherlands

2 Day 2 overview Satellite communications Communication satellite antenna design Example of reflector antenna sizing Antenna technology for communication satellites 2

3 Satellite Communications

4 System architectures Fixed satellite communications Telephony TV distribution Data transmission C band Ka band Direct satellite broadcasting TV (analog and digital) Digital radio Ku band Mobile satellite communications Telephony Multimedia Broadband L, S + Ku band* Ku-Ka band * Link to ground station 4

5 Payload function Receive signals from ground Amplify signals Convert carrier frequency from uplink to downlink bands Transmit signals to ground 5

6 Fundamental parameters Capacity: amount of information that can be transmitted in unit time, strictly related to the density of the radio wave flux that the satellite can generate on ground. Availability: percentage of time in which the system operates properly, needs to be very close to 1 for communication systems (e.g %). 6

7 Payload sizing G: ~ 120dB BW: GHz 10W-1000W H D pw Geostationary orbit case: H=35786km D=~42000km 7

8 Satellite orbits Elliptic orbits Geostationary orbit LEO orbit 8

9 Earth viewing angle r θ h r = 6378km h = 35860km θ = sin 1 r r + h 8.7 Minimum satellite elevation angle for good visibility over the Earth horizon γ = 5-10 r θ = sin 1 cosγ r + h 9

10 Communication Satellite Antenna Design

11 Typical European coverages 11

12 Flux density and beam width Flux density (dbm -2 ) edge level for fixed beam width peak level for fixed beam width peak level of global beam edge level of global beam beam edge beam centre deg 60deg 50deg 40deg 30deg 20deg 10deg satellite altitude (km) 12

13 Gain and spot size d c a e b Coverage a b c d e Diameter (deg) Antenna size (λ) Minimum gain (dbi)

14 Multiple beams To increase capacity with finite power beams become smaller. Several of them are needed to cover the same area. Spatial diversity 14

15 How to generate multiple beams? Use several antennas (but they take a lot of space on the satellite, which is small) But thenhow is it possible to separate the beams from one another? There is need of some form of orthogonality. 15

16 Multibeam antennas optical mirror reflector antenna feeds 16

17 Reflector antenna parameters parabola D h C V θ θ f ϕ F Aperture plane Focal plane d Key quantities Diameter Focal length Offset Feed spacing View angle Beam spacing Beam deviation factor D f h d φ θ κ=θ/θ κ<1, 1 17

18 Feed layout The layout of feeds in the reflector focal plane is the mirror image of the desired coverage. 18

19 Orthogonality Having identified a way to separate the input ports, there is need of a way to separate the fields radiated from of them, i.e. some form of diversity. There are 4 possibilities: Polarisation Frequency Time Code very useful, but there are only 2 distinct ones heavily used, but bandwidth is limited and filters do not have infinitely sharp edges of limited use since the information flow must be continuous in most communication applications used in some cases, it makes the receiver more complex 19

20 Beam crossover The flux level across the coverage area varies since gain changes across each beam footprint. Minimum ripple is best from the system point of view. G min G max G min ΔG G max The maximum of G min for a given aperture is obtained with ΔG 4.3dB. ΔG 3dB is often preferred to improve the power budget. 20

21 Isolation Signals falling in the frequency band allocated to one beam and coming from others are an interference. Beam orthogonality (spatial and polarisation diversity) requires some level of decoupling (isolation) among beams. G min Copolar Isolation Crosspolar Isolation 21

22 Sidelobe level The sidelobe level is dictated by the higher spatial frequencies of source currents in the source region. Linear aperture x=[0,1] with illumination changing from uniform to sin(x). 22

23 Centred reflector systems Centred reflector systems have high sidelobe levels due to the blockage effect of the feed(s) or subreflector and to the scattering of its supporting structure. A 23

24 Offset reflector systems Offset reflector systems have better performances, however the feed does not illuminate equally the rim due to the difference in path attenuation if pointed along the axis of cone intersecting the rim. The feed is instead pointed at the projection on the reflector surface of the aperture centre, so that the illumination of the rim becomes approximately balanced. θ θ D 2 D 2 24

25 Surface distortions Surface distortions also affect the sidelobe level. They are due to manufacturing and to thermal loads causing deformations. Systematic (e.g. due to segmentation) and periodic (e.g. due to the supporting structure) deformations give rise to specific sidelobe patterns linked to their spatial frequencies. Random surface errors of relatively small entity can be assumed to generally reduce the peak gain and increase sidelobe. The gain reduction can be considered as a reduction in efficiency given by Ruze s formula η = e 4πσ 2 λ 25

26 Cross polarisation Even if the feed has very pure polarisation, e.g. a corrugated horn, the reflector curvature causes some cross polarisation to appear. In a centred system the revolution symmetry ensure a relatively low level, in an offset one the level is much higher. Currents on reflector Cross-polar radiation 26

27 Beam scanning Some other departures from an ideal behaviour of reflector antennas are linked to the use of feeds out of focus. Issues: C V H θ F d Beam deformation Loss of gain Higher sidelobes Higher crosspolar Irregular beam grid 27

28 Improving scan performances Scan losses, pattern distortion, sidelobe and cross polar level increase with the distance of the feed from the focus. Effects are marked when the distance is larger than a few λ. The larger the f/d ratio of the reflector the lesser the effect (the reflector surface is flatter). Part of the effect could be removed by re-pointing the feeds toward the centre of the reflector but this complicates manufacturing as feeds are not parallel any more. 28

29 Contoured beams In many cases it is important to concentrate the power flux only where it is really useful and circles or ellipses do not abound in geography. Elliptic coverage Contoured coverage 29

30 Two alternatives for contouring Contoured beams can be obtained by: Using an array with suitable complex excitation Using a reflector antenna fed by an array Using a reflector antenna with a non-parabolic reflector Since the shape is usually fixed the use of an array with a rather complex beam forming network and a large number of elements is not justified. Using a feed array it is easy to generate multiple shaped beams, using a shaped reflector this is much more difficult, but the antenna is much simpler. 30

31 Contoured-beam antennas Feeding multiple feeds with the same signal, possibly with different amplitude and phase weights, the reflector antenna generates a shaped beam by superposition. 31

32 Reflector shaping Changing the shape of the reflector will alter the phase and, to a lesser extent, the amplitude of induced current (or of the equivalent aperture distribution) thus modifying the beam. Σ Σ A A Φ Φ Parabolic reflector F Shaped reflector F 32

33 Shaped surface The surface profile is modified mainly looking at the phase of currents. Clearly the variations could be limited to 2π, but the reflector surface needs to be continuous. The resulting surface may differ from the initial parabolic profile by several wavelengths. 33

34 Double shaped reflector The non-uniform aperture phase distribution reduces the antenna efficiency. A double reflector system can be used to minimise the phase differences while increasing amplitude variations and still produce a contoured beam. At the cost of adding a (small) reflector. F Σ Φ A 34

35 Area-gain product The efficiency of countered beam antennas is rather low, i.e. their gain is much lower that what could be obtained with the same aperture, therefore a different measure of efficiency is required. The area-gain product is usually applied in these cases to have a measure of how well an antenna matches the requirements. Area is the measure in steradians of the coverage extent Gain is the minimum gain achieved over the area 35

36 Double reflector antennas Double reflector antennas offer additional flexibility. The sub-reflector viewing angle may be differ from the main reflector viewing angle and the equivalent f/d ratio may differ from that of the main reflector. However they have higher losses and a higher mass. Also their scan capability is limited. Sub-reflector viewing angle Main reflector viewing angle 36

37 Compensated reflector systems An interesting option offered by multiple reflector systems is that they can be arranged to behave as a centred system, e.g. have the minimum of optical aberrations, like cross polar. If constructing the image with respect to the prime focus of the main reflector, i.e. the ideal surface obtained reflecting the main reflector surface into the chain of sub-reflectors, the feed axis coincides with the axis of the main reflector image then the system is equivalent to a centred one. Central ray Image of main reflector 37

38 Dragone configuration An interesting solution to reduce the level of crosspolar radiation is to use two parabolic-cylinder reflector one providing focusing in one plane and the other in the orthogonal plane. A cylindrical surface does not change the polarisation of the field upon reflection. Ku-band Dragone double reflector antenna (courtesy of EADS-CASA) 38

39 Example of reflector antenna sizing

40 Design parameters Requirements Geostationary satellite at 16 E European coverage Frequency 20/30 GHz Minimum gain 40dBi Unknowns Reflector diameter Focal length Number of beams Feed size 40

41 Design procedure All dimensions expressed in λ = 10,15 mm G dbi,peak = 3+G dbi,min (η=0.5), G peak =0.5(πD) 2 D = ( /π) = 113 θ -3dB = 70/D = 0.62 Assume beam spacing of 0.5 (to be adjusted later) and h=d/4, f/d=1 φ = Assume feed gain at reflector edge G feed,peak -6dB φ feed,-3db = φ/ 2 ϕ = G A πd Gmax = η 4π = η 2 λ λ λ θ 3dB = k D 4 f ( h + D) 4 fh arctan( ) arctan( ) f ( h + D) 4 f h ( θ ) = G( 0) + ( G( θ ) G( )) 0 0 θ θ

42 42 Design procedure cont d Derive feed diameter d = 60/φ feed,-3db Compute scan angle θ, assuming κ =1 Finally check consistency with assumption of 0.5 D k db λ θ = 3 θ C H V F d = f D h f D h d arctan 2 2 θ

43 Antenna technology for communication satellites

44 Rigid Reflectors The reflector (shell) mainly use composite materials, i.e. a sandwich consisting of: Two surface skins in CFRP (Carbon Fibre Reinforced Plastic) (fibre + resin) An honeycomb supporting structure in: Aluminum Carbon Kevlar Nomex

45 Thick-shell Reflectors 1.4*1.8 m shaped reflector developed by Thales Alenia Space (France) in the frame of EXPRESS AM2 program (Ku-band Tx/Rx) 45

46 Stiffened Thin-shell Reflectors Manufacturing process Final product Reflector Mould Reflector draping and co-curing Assembly of stiffeners

47 Ultra-light Reflectors 3.8m shaped reflector using triaxial skin thin sandwich of 1.5Kg/m 2. Developed by Astrium and Thales Alenia Space. 47

48 Ultra-light Reflectors 2.5m, 1Kg/m 2 Ku-band dual-shaped Gregorian reflector using triaxial carbon fibre membrane with stiffening web. Developed by EADS- CASA 48

49 Ultra-light Reflectors 2.2m, 1.2Kg/m 2 CFRP ultra-light shaped reflector built by Thales Alenia Space 49

50 Dual-Gridded reflectors Shaped dual-gridded reflector of 2.3 m using a Kevlar front reflector and CFRP back one. EADS-CASA distribution forbidden without written Antennas consent of for the Space authorapplications Antennas University for of Pisa,, Space March Applications 8 th

51 Earth deck module Ka band earth deck antenna module incorporating 4 Rx and 1 Tx antennas together with LNA box and RF sensing developed by Thales Alenia Space in the frame of Hotbird VI program. 51

52 Reflector antennas with large F/D Courtesy of Alcatel-Alenia Space 52

53 Foldable antennas Courtesy of Alcatel-Alenia Space 53

54 Dual-reflector multi-beam antenna Courtesy of Alcatel-Alenia Space 54

55 Waveguide feed array Courtesy of Thales Alenia Space 55

56 Thuraya satellite for mobile communications (more than 200 beams ) 12.5m mesh reflector Courtesy of RUAG Aerospace 56

57 3 m reflector for L-band ARTEMIS Telecom technology satellite (ESA) ARTEMIS in the ESTEC CPTR for testing Courtesy of RUAG Aerospace 57

58 Courtesy of MDA 58

59 L-band feed array 59

60 C-band beam forming network Courtesy of EADS-Astrium 60

61 Multi-beam feed system Courtesy of Alcatel-Alenia Space 61

62 Active antenna for LEO Support Structure Power unit Rx Radiating Panel Tx Radiating Panel Tx BFN Rx LNA / BFN Assembly Pointing mechanism 0.57m x 0.33m 14 kg Courtesy of Thales Alenia Space 62

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