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”

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.

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.

Materials development

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.

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)

Nike-Zeus acquisition radar

90 ft

Measured results for Cu wire artificial crystal

Artificial Wire Crystal AD

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1.1

1.2

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1.6

0.105 0.11 0.115 0.12 0.125 0.13 0.135

density (g / cm^3)

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Artificial Wire Crystal AD

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0.105 0.11 0.115 0.12 0.125 0.13 0.135

density (g / cm^3)

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Structure 1

Structure 2

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.

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

0 0.2 0.4 0.6 0.8 1Volume Fraction

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effe

Dielectric constant for different TiO2 mixtures: opal & inverse opal

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onst

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volume fraction

opalinverse opal

0 0.2 0.4 0.80.6 1

100

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Random mixture of Al2O3 disks in EPS

Dielectric (Al2O3 / EPS) mixture AD

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1.05

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1.15

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0 0.01 0.02 0.03 0.04 0.05

volume fraction of inclusions

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Disks

Measured

Spheres

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

Effective dielectric constant for uniform random mixture of TiO2 disks in air

fractional volume of inclusions

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2.2

2

1.6

1.4

1

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0 1% 2%

Measured results for TiO2 plates in EPS

Dielectric (TiO2 / EPS) mixture AD

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0 0.01 0.02 0.03 0.04 0.05 0.06

volume fraction of inclusion

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Measured

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

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Eccentricity:1:110:130:1100:1

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Manufacturing a prototype lens

Manufacture & testing of a 1m prototype lens

Trial assembly of lens parts The lens arrives at Marsfield

Manufacture & testing of a 1m prototype lens

Testing in the antenna range

Radiometric measurement of material loss

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

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.

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.

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