a luneburg lens for the ska summary of the mnrf research project into the manufacture of a low-cost...
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A Luneburg Lens for the SKASummary of the MNRF research project into the manufacture of a low-cost microwave refracting spherical lens for radioastronomy
John Kot, CSIRO
René Magritte: “Voice of Space”
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The vision: a spherical lens seeing the whole radio sky
• The full bandwidth and collecting area of the telescope is available for multiple, simultaneous, independent users.
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Spherical microwave lenses• The classical Luneburg lens is a spherically-
symmetric, graded index lens which images a point on the celestial sphere to a point on the lens surface.
• A practical lens would comprise uniform shells, with focus away from the lens surface.
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Materials development
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Dielectric lens materials
• Simple calculation based on the dielectric properties of polymer foam and the price of raw materials (oil) shows that a foamed polymer lens would be far too expensive for the SKA
• Only artificial materials seem to offer low loss and low density.
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Artificial dielectrics
• Artificial dielectrics are made by distributing small polarizable particles in a uniform background material – a macroscopic analogue of a “natural” dielectric.– Controllable dielectric properties– Reduced loss and density
• Two main classes:– Metallic particles (traditional artificial dielectric)– Dielectric particles (composite dielectrics)
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Nike-Zeus acquisition radar
90 ft
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Measured results for Cu wire artificial crystal
Artificial Wire Crystal AD
1
1.1
1.2
1.3
1.4
1.5
1.6
0.105 0.11 0.115 0.12 0.125 0.13 0.135
density (g / cm^3)
die
lect
ric
con
sta
nt
Artificial Wire Crystal AD
1
1.1
1.2
1.3
1.4
1.5
1.6
0.105 0.11 0.115 0.12 0.125 0.13 0.135
density (g / cm^3)
die
lect
ric
con
sta
nt
Structure 1
Structure 2
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Composite dielectrics
• Candidate material is a composite of ceramic particles in low-density polymer foam.
• Ceramics: Titanium dioxide has high dielectric constant (100), is widely used in microwave components, and is available cheaply in large quantities (paint pigment)
• Standard industrial processes can mass-produce loaded polymer foams.
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Physics of dielectric mixtures• The dielectric
properties of a dielectric mixture depend strongly on the distribution of the different fractions of the composite.
• Two mixtures of identical amounts of the same materials can have radically different properties.
Opal Inverse opal
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0 0.2 0.4 0.6 0.8 1Volume Fraction
0
20
40
60
80
100
effe
Dielectric constant for different TiO2 mixtures: opal & inverse opal
diel
ectr
ic c
onst
ant
volume fraction
opalinverse opal
0 0.2 0.4 0.80.6 1
100
80
60
40
20
0
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Random mixture of Al2O3 disks in EPS
Dielectric (Al2O3 / EPS) mixture AD
1
1.05
1.1
1.15
1.2
1.25
1.3
0 0.01 0.02 0.03 0.04 0.05
volume fraction of inclusions
die
lect
ric
con
sta
nt
Disks
Measured
Spheres
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Development of a shaped ceramic particle composite
• Shaped particles can approach an ideal mixture.
• Production method for shaped TiO2 particles developed by CSIRO MIT, including extensive work on doping to reduce loss
• Polymer foam extrusion process compatible with TiO2 particles developed by CSIRO MS
• Production of simple shapes by moulding process• Material design and measurement done by CSIRO
TIP / ICT Centre
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Effective dielectric constant for uniform random mixture of TiO2 disks in air
fractional volume of inclusions
diel
ectr
ic c
onst
ant
2.2
2
1.6
1.4
1
1.2
1.8
0 1% 2%
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Measured results for TiO2 plates in EPS
Dielectric (TiO2 / EPS) mixture AD
1
1.1
1.2
1.3
1.4
1.5
1.6
0 0.01 0.02 0.03 0.04 0.05 0.06
volume fraction of inclusion
die
lect
ric
con
sta
nt
Measured
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Limitation of the current material
The ideal mixing rule is approached by high eccentricity particles. The present extrusion process limits the eccentricity to around 30:1, giving roughly 6x increase in density.
volume fraction0 10.5
diel
ectr
ic
cons
tant
Eccentricity:1:110:130:1100:1
100
60
40
0
20
80
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Manufacturing a prototype lens
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Manufacture & testing of a 1m prototype lens
Trial assembly of lens parts The lens arrives at Marsfield
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Manufacture & testing of a 1m prototype lens
Testing in the antenna range
Radiometric measurement of material loss
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Results for prototype lens• Dielectric loss: excellent; loss tangent < 10-4
• Uniformity: good; OK for < 10 GHz; some further development needed for >10 GHz
• Manufacturing by moulding process: successful proof of concept
• Density: Approximately 20% improvement over foamed polymer lens, but still 6x higher than theoretical limit, due to limitations of extrusion process
• Isotropy: Poor; limits efficiency above 4 GHz; caused by excessive compression during moulding
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Conclusions
• The project has studied the feasibility of refracting spherical lenses for the SKA, and found that both metal wire artificial crystals and TiO2-based composite dielectrics could in principle be used to manufacture lenses at low cost and low loss.
• A TiO2-based composite dielectric material has been developed, and a proof-of-concept prototype lens successfully produced with a process scalable to cheap mass production. The IP is protected by a provisional patent.
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Conclusions (II)• The performance of the current material is limited
by the manufacturing process we have available. Within the budget & time limits of the NTD project there is no realistic prospect of developing the new manufacturing process needed to advance the lens development.
• Under the current project, we plan to round off the current work to the stage where it can be easily picked-up again in the future, should the need or will arise.
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Conclusions (III)• CSIRO will extend the current patent for 3 years,
and actively seek partners for commercial and scientific application of the technology such as:– CSIRO’s wideband dielectric-loaded feed horn
technology, as used by project SETI– Use of small spherical lenses for Ka-band mobile satcom
applications, e.g. video surveillance by UAVs.
• The hybrid lens / aperture array proposal presented by Peter Hall at Capetown remains the most attractive option for the SKA that offers unconstrained multiple fields-of-view across the SKA frequency band