Radio-frequency reflective fabric
This is a division of application Ser. No. 123,843, filed Nov. 23, 1987. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to radio-frequency reflectors, and more particularly to a fabric for use as a radio-frequency reflective surface in extraterrestrial applications. 
     BACKGROUND OF THE INVENTION 
     Knitted wire meshes have been used in the prior art for radio-frequency reflective surfaces in earth satellite applications. For example, the reflector of the Tracking Data Relay Satellite System, and also the Galileo reflector marketed by Harris Corporation of Melborne, Fla., use a wire mesh reflective surface made of gold-plated molybdenum. TRW Corporation has marketed a so-called tulip reflector having a radio-frequency reflective surface made of a welded-wire fabric comprising filaments made of stainless steel and silver. 
     Knitted wire meshes of the prior art have exhibited a phenomenon known as &#34;pillowing&#34; when subjected to biaxial loads applied over curved frameworks. &#34;Pillowing&#34; of a mesh is a distortion characterized by bulges (or &#34;pillows&#34;) that occur in the mesh due to mechanical strain. &#34;Pillowing&#34; in a knitted wire mesh used as a radio-frequency reflective surface generally degrades performance, and causes side lobes of radio-frequency energy reflected from the mesh. 
     Knitted wire meshes used as radio-frequency reflective surfaces in the prior art have also exhibited significant intermodulation products at knit junctions of the meshes. Intermodulation products generally cause excessively high noise levels in radio-frequency receiving channels. Many presently operating satellite systems (e.g., INMARSAT, INTELSAT and COMMSAT) have such high noise levels attributable to intermodulation products that operational capabilities of receiving channels are seriously impaired. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a yarn having a coefficient of thermal expansion of effectively zero over a useful temperature range, which yarn can be woven into a fabric for use as a radio-frequency reflective surface. 
     It is a particular object of the present invention to provide a fabric for use as a radio-frequency reflective surface for an antenna to be deployed in extraterrestrial space, where the fabric has an effective coefficient of thermal expansion of zero over a useful operating temperature range for the reflective surface, and where intermodulation products produced on the reflective surface are practically insignificant. 
     It is a further object of the present invention to provide a fabric drapery woven from a yarn according to the present invention for enclosing a region in which an emitter of radio-frequency electromagnetic radiation is located, where the drapery functions as a shield to prevent leakage of radio-frequency energy from the enclosed region. 
     In accordance with the present invention, a yarn having an effectively zero coefficient of thermal expansion is formed by combining a metallic filament having a positive coefficient of thermal expansion with rovings made of a non-metallic material having a negative coefficient of thermal expansion, where the coefficient of thermal expansion and the tensile modulus of the metallic filament are related to the coefficient of thermal expansion and the tensile modulus of the non-metallic rovings so that mechanical strain thermally induced in the metallic filament is substantially counteracted by mechanical strain thermally induced in the non-metallic rovings at temperatures in a range at which a fabric woven from the yarn is designed to operate as a radio-frequency reflective surface. 
     In exemplary embodiments of yarns according to the present invention, the metallic filament is made of: (a) copper, (b) silver, (c) molybdenum, (d) an alloy of copper and beryllium, or (e) gold-plated molybdenum; and the non-metallic rovings are made of: (a) graphite, or (b) Kevlar aramid. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of an antenna reflector having a radio-frequency reflective surface that is made of a fabric woven from a yarn according to the present invention. 
     FIG. 2 is an enlarged view of a portion of the fabric from which the radio-frequency reflective surface shown in FIG. 1 is made. 
     FIG. 3 illustrates a segment of a yarn from which the fabric shown in FIG. 2 is woven. 
     FIG. 4 schematically illustrates counteracting strains thermally induced in the metallic filament and the non-metallic rovings of a portion of the yarn segment shown within the line 4--4 in FIG. 3. 
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     An antenna reflector intended for deployment in extraterrestrial space should ideally have certain features that might not be particularly important in an antenna reflector used for terestrial applications. Thus, for example, an antenna reflector designed for deployment in extraterrestrial space should be light-weight, and should be packageable in a relatively small volume prior to deployment. Furthermore, an antenna reflector designed for deployment in extraterrestrial space should be made of materials that are chemically stable under exposure to ultraviolet radiation, and should be dimensionally stable under thermal stresses induced by extreme temperature fluctuations. 
     FIG. 1 is a generalized illustration of an antenna reflector, which has a radio-frequency reflective surface comprising a plurality of sectors made of a flexible woven fabric stretched over a light-weight framework. The fabric is chemically stable under exposure to ultraviolet radiation, and is dimensionally stable under thermal stresses induced by extreme temperature fluctuations when the antenna reflector is deployed. Chemical stability of the fabric under ultraviolet radiation is provided by covering the fabric with an elastomeric paint such as Dow Corning DC6-1104 formulation. The framework, which forms ribs for supporting the fabric, can be made of a graphite-epoxy composite material. The fabric is foldable, thereby enabling the framework with the fabric attached thereto to be collapsed into a storage configuration that occupies a much smaller volume than is required when the framework is opened to a deployment configuration in which the fabric is stretched taut to form the reflective surface. 
     The fabric from which the radio-frequency reflective surface of the antenna reflector shown in FIG. 1 is made is woven from a yarn 10 according to a pattern that may be conventional. A typical weave for the fabric, viz., a conventional Marquisette Leno weave, is illustrated in enlarged view in FIG. 2. The yarn 10 comprises a metallic filament around which rovings made of a non-metallic dielectric material are wound. The metallic filament provides the fabric with the required radio-frequency reflective property to enable the fabric to function as a reflective surface. The non-metallic rovings working in concert with the metallic filament provide dimensional stability for the fabric under thermal stresses. 
     In principle, a single non-metallic roving could be used for the yarn 10. However, in the preferred embodiment as illustrated in FIG. 3, the yarn 10 comprises a longitudinally extending metallic filament 11 around which two rovings (e.g., a roving 12 would in a clockwise direction, and a roving 12&#39; wound in a counterclockwise direction) are wrapped. The metallic filament 11 typically has a diameter on the order of 0.001 inch, and the rovings 12 and 12&#39; are of about 55 denier. The filament 11 and the rovings 12 and 12&#39; have coefficients of thermal expansion that substantially counterbalance each other, thereby resulting in an effective coefficient of thermal expansion of zero over a useful temperature range for the fabric woven from the yarn 10 when the yarn 10 is subjected to thermal loading forces that occur when the antenna reflector is deployed. The fabric woven from the yarn 10 is intended primarily for use as the reflective surface of an antenna reflector for extraterrestrial applications in which large temperature fluctuations are to be expected, generally in the range from about -150° C. to about 100° C. 
     If a fabric for use as a radio-frequency reflective surface were to be woven from a metallic filament that is not covered by an electrically insulating covering, intermodulation products would be produced at yarn intersections of the fabric, i.e., where crossing elements of the metallic filament come into contact with each other. Such intermodulation products would produce unacceptable noise levels in the receiver bands of the antenna, which could make it necessary for separate antennas to be used for the transmitter and receiver bands. Such intermodulation products could in principle be eliminated by winding or otherwise covering the metallic filament with any sort of dielectric that would prevent contact between crossing elements of the metallic filament. However, it is not sufficient simply to eliminate intermodulation products. Unless the roving material is also capable of accepting approximately half of the loading forces imposed upon the yarn by stresses thermally induced in the fabric, the metallic filament would bear most of the loading forces and the net coefficient of thermal expansion of the fabric would have substantially the value of the coefficient of thermal expansion of the metallic filament. 
     The filament 11 illustrated in FIG. 3 is made from a metal whose coefficient of thermal expansion over a useful temperature range is substantially counteracted by the coefficient of thermal expansion of the rovings 12 and 12&#39; over the same temperature range, so that the effective coefficient of thermal expansion for the yarn 10 over that temperature range is substantially zero. The useful temperature range for a radio-frequency reflective surface designed to operate in extraterrestrial space is from about -150° C. to about 100° C. Therefore, an investigation was made of changes in the coefficients of thermal expansion with respect to temperature over the range from -150° C. to 100° C. for a variety of materials that were considered as possible candidates to use for the metal filament 11 and the non-metal rovings 12 and 12&#39; of the yarn 10 of the present invention. As a result of the investigation, it was determined that: 
     1. The filament 11 could be made from: (a) copper, (b) silver, (c) molybdenum, (d) an alloy of copper and beryllium, or (e) gold-plated molybdenum; and 
     2. The rovings 12 and 12&#39; could be made from: (a) graphite, or (b) a Kevlar aramid, where &#34;Kevlar&#34; is a trademark of Du Pont Corporation. 
     The coefficient of thermal expansion and the tensile modulus for each of the five above-listed candidate materials for the metallic filament 11 are listed in tabular format as follows: 
     
                       TABLE I                                                     
______________________________________                                    
METAL FILAMENTS                                                           
         Coefficient of                                                   
         Thermal Expansion                                                
                     Tensile Modulus                                      
Material   10.sup.-6 /deg C.                                              
                         psi        mPa                                   
______________________________________                                    
Copper     17.64         17.0 × 10.sup.6                            
                                    117,500                               
Silver     19.62         11.0 × 10.sup.6                            
                                     76,000                               
Molybdenum 5.04          50.0 × 10.sup.6                            
                                    354,700                               
Cu and Be Alloy                                                           
           17.64         18.5 × 10.sup.6                            
                                    127,900                               
Au-plated Mb                                                              
           5.04          50.0 × 10.sup.6                            
                                    345,700                               
______________________________________                                    
 
    
     where the tensile modulus for each material is given in pounds per square inch (psi) and also in magapascals (mPa). 
     The cofficient of thermal expansion and the tensile modulus for each of the candidate materials for the non-metallic rovings 12 and 12&#39; are listed in tabular format as follows: 
     
                       TABLE II                                                    
______________________________________                                    
NON-METAL ROVINGS                                                         
       Coefficient of                                                     
       Thermal Expansion                                                  
                     Tensile Modulus                                      
Material 10.sup.-6 / deg C.                                               
                         psi       mPa                                    
______________________________________                                    
Graphite *               *                                                
Kevlar   -4.21           17.5 × 10.sup.6                            
                                   121,000                                
______________________________________                                    
 *No particular values for the coefficient of thermal expansion and the   
 tensile modulus are given for graphite, because such values are dependent
 upon the process used to form the graphite from a hydrocarbon material   
 (typically, rayon fibers). The values for the coefficient of thermal     
 expansion and the tensile modulus of graphite can generally, within      
 limits, be tailored to meet a specific requirement.                      
 
    
     The condition that must be met in order for mechanical strain thermally induced in the filament 11 to be counteracted by mechanical strain thermally induced in the rovings 12 and 12&#39; of the yarn 10 so that the fabric woven from the yarn 10 retains dimensional stability while undergoing large temperature fluctuations is that 
     
         |α.sub.F A.sub.F E.sub.F |=|α.sub.R A.sub.R E.sub.R | 
    
     where α F  and α R  are the coefficients of thermal expansion of the filament 11 and of the rovings 12 and 12&#39;, respectively; where A F  and A R  are the cross-sectional areas of the filament 11 and of the rovings 12 and 12&#39;, respectively; and where E F  and E R  are the tensile moduli of the filament 11 and of the rovings 12 and 12&#39;, respectively. For any given roving material, the product A R  E R  is effectively determined by the manufacturer. In general, for a fabric to be used in extraterrestrial applications, it is desirable that the rovings 12 and 12&#39; have a weight that is as light as possible and a diameter that is as fine as possible. For graphite and Kevlar fibers, a density of 55 denier provides about as light a weight and as fine a diameter as can be obtained commercially at the present time. 
     Combining the data from Tables I and II enables an &#34;effective&#34; coefficient of thermal expansion to be determined for a yarn comprising a filament selected from Table I and a roving material selected from Table II. Thus, for a yarn comprising a Be/Cu filament that is 0.001 inch in diameter and has two rovings made of 55 denier Kevlar material, the effective coefficient of thermal expansion is seen to be substantially zero over the temperature range from -150° C. to 100° C. Similarly, an effective zero coefficient of thermal expansion over the same temperature range can be achieved using a yarn comprising a 0.0012-inch diameter filament made of silver and two rovings made of 55 denier Kevlar. 
     When a fabric woven from the yarn shown in FIG. 3 is placed under a tensile loading (as occurs when a radio-frequency reflective surface made of such a fabric is deployed), a rise in temperature causes the metallic filament 11 to experience a mechanical stress that expands the filament 11 longitudinally. The same rise in temperature, however, causes the rovings 12 and 12&#39; to experience a contraction due to an oppositely directed mechanical stress. A small portion of the yarn 10 enclosed within line 4--4 of FIG. 3 is shown in magnified view in FIG. 4 along with arrows representing stress forces tending to elongate the metallic filament 11 longitudinally and to contract the rovings 12 and 12&#39; in the opposite direction. As indicated in FIG. 4, the net result of thermally induced stresses on the yarn 10 is that the yarn 10 retains dimensional stability, i.e., undergoes no significant expansion or contraction. 
     The particular embodiments described herein of a yarn having a substantially zero coefficient of thermal expansion over a useful termperature range, and of a fabric woven from such a yarn, are illustrative of the invention, which is defined more generally by the following claims and their equivalents.