A Luneberg lens is a microwave lens consisting of a series of concentric shells of differing dielectric constants. The highest dielectric constant, or refractive index, resides at the core and the lowest, at the outer shell. Microwaves passing through this arrangement of shells are focused in the same manner as light passing through a glass lens.

Known for decades, Luneberg lenses have been fabricated from materials such as polystyrene foam, foamed glass, and other cellular materials. The dielectric constant is controlled by simply controlling the bulk density of the foam. Materials, such as foamed polystyrene, are useful for ground-based applications, but space deployment imposes severe restrictions on their use. The closed-cell materials used in the past are frequently difficult to fabricate and are high in mass and tend to exhibit electrical and dimensional instabilities. They may also be expensive and vulnerable to fracture by ground handling or launch vibrations.

This concept proposes Luneberg lenses suitable for space applications and fabricated from open-celled elastomeric foams. An ideal candidate is the class of foamed urethane polymers. The dielectric constant of these materials is controlled by varying the density of the foam, but may also be varied by addition of polar groups (e.g., ethoxy, sulfonyl, carboxylate, trifluoropropoxy, and the like) to the polymer backbone. Unlike increasing the dielectric constant by the addition of conductive compounds, this approach maintains the electrons in the valence band and consequently keeps the loss tangent low.

Urethane is the most widely used chemistry in the fabrication of open- celled foam materials, with densities ranging from 1.2 to 10 lb/ft3 (19.2 to 160 kg/m3). Dielectric constants typically vary from 1.05 to 4.0 at high densities, and loss tangents vary from immeasurably small to 5 ¥ 10-3. Both mechanical and dielectric properties are identical in all axes (isotropic), solving a significant problem with previous materials choices. These open-celled foams are inexpensive, easy to machine and low in density. Their open cell structure results in rapid and complete air release in the vacuum of space. The inherent resilience of these materials gives them high mechanical damping, resistance to fracture during handling and launch, and additionally provides a positive restoring force to their dimensions. This "self-correcting" property overcomes the "airgap" problem of previous designs, in which a slight separation between the shells results in an internal reflection and loss of RF (radio-frequency) efficiency. Finally, these materials have extremely low thermal conductivity (typically 0.05 W/m-K). This last characteristic prevents the lens structure from undergoing dramatic temperature swings in response to a changing thermal environment, and further improves their electrical stability. Luneberg lenses fabricated from these inexpensive open-celled urethane foams may additionally be supported for use by a thin-walled lightweight shell of some other material, such as fiber glass.

This work was done by Paul B. Willis of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

This Brief includes a Technical Support Package (TSP).
Luneberg Lenses Made of Open-Cell Polyurethane Foams

(reference NPO20339) is currently available for download from the TSP library.

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This article first appeared in the January, 1999 issue of NASA Tech Briefs Magazine.

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