A proposed lightweight, off-axis reflector structure for a microwave communication antenna would be made of a carbon/carbon composite material. The structure was conceived for use aboard the Solar Probe spacecraft, where it would also serve as a shield to protect the spacecraft against solar radiation at perihelion. The basic concept of the carbon/carbon reflector structure could be also adapted to design lightweight, strong, off-axis reflector structures for antennas to be used on Earth.

Carbon/carbon was chosen as the class of structural materials because such materials offer a combination of light weight, high strength, good radio-frequency (RF) reflectance properties, and low mass loss at high temperatures. Results of tests of candidate materials suggest that the proposed shield/antenna structure would function well at a temperature greater than 2,000 K. The major drawback of materials in this class is that they are expensive.

In the original Solar Probe application, the dual use of the structure as a solar shield and antenna reflector was made possible by a fortuitous combination of optimum shield and antenna shapes that was effected by designing the spacecraft trajectory to obtain Sun/spacecraft/Earth quadrature at spacecraft perihelion. The combined shield/antenna would also enable a reduction of overall spacecraft diameter: According to an older design concept, the solar shield would be a separate, conical structure and the antenna reflector would lie within the shadow of the shield. The overall spacecraft diameter according to that concept would be 4 meters. The overall diameter according to the proposed simplification would be reduced to 1 meter, and the overall mass and cost of the spacecraft would be concomitantly smaller. Of course, whether or not such simplification and reduction in size could be effected in other applications would depend on the geometries and design and operational requirements specific to those applications.

This work was done by James Randolph of Caltech for NASA's Jet Propulsion Laboratory. NPO-20318

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Carbon/carbon shield/antenna structure

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This article first appeared in the June, 1998 issue of NASA Tech Briefs Magazine.

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