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Antenna designfor
Space Applications
M. SabbadiniEuropean Space Agency, Noordwijk, The Netherlands
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Day 2 overview• Satellite communications• Communication satellite antenna design• Example of reflector antenna sizing• Antenna technology for communication satellites
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Satellite Communications
Antennas for Space Applications
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System architecturesFixed satellite communications
TelephonyTV distributionData transmission
Direct satellite broadcastingTV (analog and digital)Digital radio
Mobile satellite communicationsTelephonyMultimediaBroadband
C bandKa band
Ku band
L, S + Ku band*Ku-Ka band* Link to ground station
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Payload function• Receive signals from ground• Amplify signals• Convert carrier frequency
from uplink to downlinkbands
• Transmit signals to ground
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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. 99.95%).
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Payload sizing
10-100 pW
10W-1000WG: ~ 120dB
BW: 0.2-1.5GHz
Geostationary orbit case:H=35786kmD=~42000km
H D
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Satellite orbits
Geostationary orbit
Ellipticorbits
LEO orbit
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Earth viewing angle
h
θ
r = 6378km
h = 35860km°≅⎟
⎠⎞
⎜⎝⎛
+= − 7.8sin 1
hrrθ
r
⎟⎠⎞
⎜⎝⎛
+= − γθ cossin 1
hrrMinimum satellite elevation angle for
good visibility over the Earth horizon γ = 5°-10°
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Communication Satellite Antenna Design
Antennas for Space Applications
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Typical European coverages
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Flux density and beam width
250 5000 10000 20000 30000 35768-140
-130
-120
-110
-100
-90
-80
satellite altitude (km)
Flux
den
sity
(dB
m-2
)
beam edgebeam centre
5deg
20deg 10deg30deg40deg
60deg 50deg
edge level of global beam
peak level of global beampeak level for fixed beam width
edge level for fixed beam width
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Gain and spot size
a
b
d
c
e
Coverage a b c d eDiameter (deg) 3 2 1.5 1 0.75Antenna size (λ) 20 28.3 40 56.6 80Minimum gain (dBi) 30 33 36 39 42
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Multiple beams
To increase capacity with finite power beams become smaller.Several of them are needed to cover the same area.
Spatial diversity
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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.
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Multibeam antennas
reflector antennaoptical mirrorfeeds
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Reflector antenna parameters
θ’C
h
V d
Dθ
f
Focal plane
Key quantities
Diameter DFocal length fOffset h Feed spacing dView angle φBeam spacing θBeam deviation κ=θ/θ’factor κ<1, ≈1
ϕF
Aperture plane
parabola
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Feed layout
The layout of feeds in the reflector focal plane is the mirror image of the desired coverage.
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OrthogonalityHaving 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:
used in some cases, it makes the receiver more complexCode
of limited use since the information flow must be continuous in most communication applications
Time
heavily used, but bandwidth is limited and filters do not have infinitely sharp edges
Frequency
very useful, but there are only 2 distinct onesPolarisation
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Beam crossoverThe flux level across the coverage area varies since gain changes across each beam footprint.Minimum ripple is best from the system point of view.
Gmin
Gmax
Gmin
GmaxΔG
The maximum of Gmin for a given aperture is obtained with ΔG ≈ 4.3dB.ΔG ≈ 3dB is often preferred to improve the power budget.
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IsolationSignals 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.
Gmin
CopolarIsolation
CrosspolarIsolation
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Sidelobe levelThe 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).
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Centred reflector systemsCentred 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
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Offset reflector systemsOffset 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 coneintersecting the rim.
θθ
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.
2D
2D
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Surface distortionsSurface 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 24
⎟⎠⎞
⎜⎝⎛−
= λπσ
η eRuze’s formula
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Cross polarisationEven 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
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Beam scanning
θ
C
HV F
d
Issues:
Beam deformationLoss of gain
Higher sidelobes
Higher crosspolar
Irregular beam grid
Some other departures from an ideal behaviour of reflector antennas are linked to the use of feeds out of focus .
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Improving scan performancesScan 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.
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Contoured beams
Elliptic coverage Contoured coverage
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.
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Two alternatives for contouringContoured 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, butthe antenna is much simpler.
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Contoured-beam antennasFeeding multiple feeds with the same signal, possibly with different amplitude and phase weights, the reflector antenna generates a shaped beam by superposition.
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Reflector shaping
Parabolic reflector Shaped reflectorF F
Σ Σ
AA
ΦΦ
Changing the shape of the reflector will alter the phase and, toa lesser extent, the amplitude of induced current (or of the equivalent aperture distribution) thus modifying the beam.
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Shaped surfaceThe 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.
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Double shaped reflector
F
Σ
A
Φ
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.
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Area-gain productThe 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 extentGain is the minimum gain achieved over the area
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Double reflector antennasDouble 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
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Compensated reflector systemsAn 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
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Dragone configuration
An interesting solution to reduce the level of cross-polar 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)
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Example of reflector antenna sizing
Antennas for Space Applications
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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
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Design procedureAll dimensions expressed in λ = 10,15 mm
GdBi,peak = 3+GdBi,min → (η=0.5), Gpeak=0.5(πD)2
D = √(4·104/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 Gfeed,peak -6dB
φfeed,-3dB = φ/√2
2
2max 4 ⎟⎠⎞
⎜⎝⎛==λπη
λπη DAG
DkdBλθ =−3
( ) ( ) ( ) ( )( )2
00 00 ⎟⎟
⎠
⎞⎜⎜⎝
⎛−+=
θθθθ GGGG
)4
4arctan())(4
)(4arctan( 222 hffh
DhfDhf
−−
+−+
=ϕ
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Design procedure cont’dDerive feed diameterd = 60/φfeed,-3dB
Compute scan angle θ, assuming κ =1
Finally check consistencywith assumption of 0.5°
DkdBλθ =−3
θ
C
HV F
d
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠⎞
⎜⎝⎛ +
−+⎟⎠⎞
⎜⎝⎛ +
=
f
DhfDh
d
42
2
arctan2
2
θ
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Antenna technology for communication
satellites
Antennas for Space Applications
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The reflector (shell) mainly use composite materials, i.e. a sandwich consisting of:
Rigid Reflectors
Two surface skins in CFRP (Carbon Fibre Reinforced Plastic)(fibre + resin)
An “honeycomb” supporting structure in:
• Aluminum• Carbon• Kevlar• Nomex
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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)
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Antennas for Space Applications© distribution forbidden without written consent of the author
Stiffened Thin-shell Reflectors
Reflector
Mould
Reflector draping and co-curing
Assembly of stiffeners
Manufacturing process Final product
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Ultra-light Reflectors3.8m shaped reflector using triaxial skin thin sandwich of 1.5Kg/m2. Developed by Astrium and Thales AleniaSpace.
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2.5m, 1Kg/m2 Ku-band dual-shaped Gregorian reflector using triaxialcarbon fibre membrane with stiffening web.
Developed by EADS-CASA
Ultra-light Reflectors
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2.2m, 1.2Kg/m2
CFRP ultra-light shaped reflector built by Thales AleniaSpace
Ultra-light Reflectors
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50Antennas for Space Applications© distribution forbidden without written consent of the authorAntennas for Space applications University of Pisa,, March 8th 2005
Dual-Gridded reflectorsShaped dual-griddedreflector of 2.3 m using a Kevlar front reflector and CFRP back one. EADS-CASA
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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.
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Reflector antennas with large F/D
Courtesy of Alcatel-Alenia Space
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Foldable antennas
Courtesy of Alcatel-Alenia Space
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Courtesy of Alcatel-Alenia Space
Dual-reflector multi-beam antenna
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Waveguide feed array
Courtesy of Thales Alenia Space
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Thuraya satellite for mobile communications
(more than 200 beams )
12.5m mesh reflector
Courtesy of RUAG Aerospace
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ARTEMIS in the ESTEC CPTR for testing
3 m reflector for L-band
ARTEMISTelecom
technology satellite(ESA)
Courtesy of RUAG Aerospace
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Courtesy of MDA
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L-band feed array
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C-band beam forming network
Courtesy of EADS-Astrium
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Multi-beam feed system
Courtesy of Alcatel-Alenia Space
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Courtesy of Thales Alenia Space
0.57m x 0.33m14 kg
SupportStructure
Power unit
Rx LNA / BFNAssembly
Tx BFN
Tx Radiating Panel
Pointingmechanism
Rx RadiatingPanel
Active antenna for LEO
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