Antenna design for Space Applications

Transcription

Antenna design
for
Space Applications
M. Sabbadini
European Space Agency, Noordwijk, The Netherlands
[email protected]
© distribution forbidden without written consent of the author
Day 2 overview
•
•
•
•
Satellite communications
Communication satellite antenna design
Example of reflector antenna sizing
Antenna technology for communication satellites
Antennas for Space Applications
2
Satellite Communications
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
Payload function
• Receive signals from ground
• Amplify signals
• Convert carrier frequency
from uplink to downlink
bands
• Transmit signals to ground
5
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%).
6
Payload sizing
G: ~ 120dB
BW: 0.2-1.5GHz
H
D
10W-1000W
10-100 pW
Geostationary orbit case:
H=35786km
D=~42000km
7
Satellite orbits
Elliptic
orbits
LEO orbit
Geostationary
orbit
8
Earth viewing angle
r
θ
h
r = 6378km
h = 35860km
Minimum satellite elevation angle for
good visibility over the Earth horizon
γ = 5°-10°
⎛ r ⎞
⎟ ≅ 8.7°
⎝r+h⎠
θ = sin −1 ⎜
⎛ r
⎞
θ = sin ⎜
cosγ ⎟
⎝r +h
⎠
−1
9
Communication Satellite
Antenna Design
Typical European coverages
11
Flux density and beam width
-80
-90
Flux density (dBm-2)
-100
edge level for fixed beam width
peak level for fixed beam width
peak level of global beam
-110
edge level of global beam
-120
beam edge
beam centre
-130
5deg
40deg
30deg
20deg
10deg
60deg 50deg
-140
250
5000
10000
20000
satellite altitude (km)
30000
35768
12
Gain and spot size
d
a
c
e
b
Coverage
a
b
c
d
e
Diameter (deg)
Antenna size (λ)
3
2
1.5
1
0.75
20
30
28.3
33
40
36
56.6
39
80
42
Minimum gain (dBi)
13
Multiple beams
To increase capacity with finite
power beams become smaller.
Several of them are needed to
cover the same area.
Spatial diversity
14
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
Multibeam antennas
optical mirror
reflector antenna
feeds
16
Reflector antenna parameters
parabola
Key quantities
Aperture
plane
D
C
θ
θ’
ϕ
h
V
F
Focal
plane
d
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
f
17
Feed layout
The layout of feeds in the reflector focal plane is the mirror image of the desired
coverage.
18
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 very useful, but there are only 2 distinct ones
Frequency
heavily used, but bandwidth is limited and filters do not
have infinitely sharp edges
Time
of limited use since the information flow must be
continuous in most communication applications
Code
used in some cases, it makes the receiver more complex
19
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.
Gmax
Gmin
Gmin
ΔG
Gmax
The maximum of Gmin for a given aperture is obtained with ΔG ≈ 4.3dB.
ΔG ≈ 3dB is often preferred to improve the power budget.
20
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.
Gmin
Copolar
Isolation
Crosspolar
Isolation
21
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
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
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.
D
2
θ
θ
D
2
24
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
2
Ruze’s formula
η =e
⎛ 4πσ ⎞
−⎜
⎟
⎝ λ ⎠
25
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
Beam scanning
Some other departures from an ideal behaviour of reflector
antennas are linked to the use of feeds out of focus .
Issues:
Beam deformation
C
Loss of gain
θ
Higher sidelobes
Higher crosspolar
Irregular beam grid
d
V
H
F
27
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
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
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
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
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
F
F
Parabolic reflector
Shaped reflector
32
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
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.
Σ
A
Φ
F
34
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
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
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
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
Example of reflector
antenna sizing
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
Design procedure
All dimensions expressed in λ = 10,15 mm
GdBi,peak = 3+GdBi,min → (η=0.5), Gpeak
=0.5(πD)2 Gmax = η 4π A2 = η ⎛⎜ πD ⎞⎟
λ
2
⎝ λ ⎠
D = √(4·104/π) = 113
θ −3dB = k
θ-3dB= 70/D = 0.62°
λ
D
Assume beam spacing of 0.5° (to be adjusted later) and
h=D/4, f/D=1
φ =…
ϕ = arctan(
4 f (h + D)
4 fh
) − arctan( 2
)
2
2
4 f − (h + D)
4f −h
Assume feed gain at reflector edge Gfeed,peak -6dB
φfeed,-3dB = φ/√2
⎛θ ⎞
G (θ ) = G (0 ) + (G (θ 0 ) − G (0 ))⎜⎜ ⎟⎟
⎝ θ0 ⎠
2
41
Design procedure cont’d
Derive feed diameter
d = 60/φfeed,-3dB
Compute scan angle θ, assuming κ =1
θ = arctan
θ −3dB = k
λ
D
d
2
⎛
⎞
D
⎞
⎛
⎜
⎜h + ⎟ ⎟
2
D⎞ ⎜
2⎠ ⎟
⎛
⎝
+
+
−
h
f
⎜
⎟ ⎜
⎟
2⎠
4f
⎝
⎜⎜
⎟⎟
⎝
⎠
C
θ
Finally check consistency
with assumption of 0.5°
d
V
H
F
42
Antenna technology
for communication
satellites
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
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
Stiffened Thin-shell Reflectors
Final product
Manufacturing process
Reflector
Mould
Reflector draping and co-curing
Assembly of stiffeners
Ultra-light Reflectors
3.8m shaped reflector using
triaxial skin thin sandwich of
1.5Kg/m2. Developed by
Astrium and Thales Alenia
Space.
47
2.5m, 1Kg/m2 Ku-band
dual-shaped Gregorian
reflector using triaxial
carbon fibre membrane
with stiffening web.
Developed by EADSCASA
48
2.2m, 1.2Kg/m2
CFRP ultra-light
shaped reflector built
by Thales Alenia
Space
49
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 writtenAntennas
consent offor
theSpace
authorapplications
Antennas
Space
Applications
University for
of Pisa,,
March
8th 2005
50
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
Reflector antennas with large F/D
Courtesy of Alcatel-Alenia Space
52
Foldable antennas
53
Dual-reflector multi-beam antenna
54
Waveguide feed array
Courtesy of Thales Alenia Space
55
Thuraya satellite for mobile
communications
(more than 200 beams )
12.5m mesh reflector
Courtesy of RUAG Aerospace
56
3 m reflector
for L-band
ARTEMIS
Telecom
technology
satellite
(ESA)
ARTEMIS in the ESTEC CPTR for testing
Courtesy of RUAG Aerospace
57
Courtesy of MDA
58
L-band feed array
59
C-band beam forming network
Courtesy of EADS-Astrium
60
Multi-beam feed system
61
Active antenna for LEO
Support
Structure
Power
unit
Rx Radiating
Panel
Tx Radiating
Panel
Rx LNA / BFN
Assembly
Tx BFN
Pointing
mechanism
0.57m x 0.33m
14 kg
Courtesy of Thales Alenia Space
62
63
64

Antenna design for Space Applications

Transcription

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