A Luneburg Lens for the SKA Summary 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 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

Dielectric constant for different TiO 2 mixtures: opal & inverse opal dielectric constant volume fraction opal inverse opal

Random mixture of Al 2 O 3 disks in EPS

Development of a shaped ceramic particle composite Shaped particles can approach an ideal mixture. Production method for shaped TiO 2 particles developed by CSIRO MIT, including extensive work on doping to reduce loss Polymer foam extrusion process compatible with TiO 2 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 TiO 2 disks in air fractional volume of inclusions dielectric constant % 2%

Measured results for TiO 2 plates in EPS

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 fraction dielectric constant Eccentricity: 1:1 10:1 30:1 100:

Manufacturing a prototype lens

Manufacture & testing of a 1m prototype lens Trial assembly of lens partsThe 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 < Uniformity: good; OK 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 TiO 2 -based composite dielectrics could in principle be used to manufacture lenses at low cost and low loss. A TiO 2 -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