Antenna design for Space Applications - · PDF fileAntenna design for Space Applications ......

© distribution forbidden without written consent of the author

Antenna designfor

Space Applications

M. SabbadiniEuropean Space Agency, Noordwijk, The Netherlands

[email protected]

2Antennas for Space Applications© 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


Satellite Communications

Antennas for Space Applications


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


Payload function• Receive signals from ground• Amplify signals• Convert carrier frequency

from uplink to downlinkbands

• Transmit signals to ground


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%).


Payload sizing

10-100 pW

10W-1000WG: ~ 120dB

BW: 0.2-1.5GHz

Geostationary orbit case:H=35786kmD=~42000km

H D


Satellite orbits

Geostationary orbit

Ellipticorbits

LEO orbit


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°


Communication Satellite Antenna Design



Typical European coverages


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


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


Multiple beams

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

Spatial diversity


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.


Multibeam antennas

reflector antennaoptical mirrorfeeds


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


Feed layout

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


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


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.


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


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).


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


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


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


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


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 .


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.


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.


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.


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.


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.


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.


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.


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


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


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


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)


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


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


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


Antenna technology for communication

satellites


Antennas for Space Applications© distribution forbidden without written consent of the author

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


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)

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


Ultra-light Reflectors3.8m shaped reflector using triaxial skin thin sandwich of 1.5Kg/m2. Developed by Astrium and Thales AleniaSpace.


2.5m, 1Kg/m2 Ku-band dual-shaped Gregorian reflector using triaxialcarbon fibre membrane with stiffening web.

Developed by EADS-CASA

Ultra-light Reflectors


2.2m, 1.2Kg/m2

CFRP ultra-light shaped reflector built by Thales AleniaSpace

Ultra-light Reflectors

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


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.


Reflector antennas with large F/D

Courtesy of Alcatel-Alenia Space


Foldable antennas




Dual-reflector multi-beam antenna


Waveguide feed array

Courtesy of Thales Alenia Space


Thuraya satellite for mobile communications

(more than 200 beams )

12.5m mesh reflector

Courtesy of RUAG Aerospace


ARTEMIS in the ESTEC CPTR for testing

3 m reflector for L-band

ARTEMISTelecom

technology satellite(ESA)

Courtesy of RUAG Aerospace


Courtesy of MDA


L-band feed array


C-band beam forming network

Courtesy of EADS-Astrium


Multi-beam feed system



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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