Patent Publication Number: US-6342677-B1

Title: High frequency cable having a dual-layer structure

Description:
This application claims the benefit of U.S. Provisional Application No. 60/135,764 filing date May 25, 1999, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to high-frequency cables, and more particularly to a multi-layer high-frequency or coaxial cable and a method for manufacturing the same. 
     BACKGROUND ART 
     Coaxial cables and other high frequency cables are known in the art for transmitting, for example, television signals and other communication signals. As shown in FIG. 1, a conventional coaxial cable  100  is formed out of an inner tube  102 , a dielectric material  103 , and an outer tube  104 . The two tubes  102 ,  104  are made of metal or another electrically conductive material and are disposed concentrically with the dielectric material  103  sandwiched in between the two tubes. The conductivity of the material used to form the tubes  102 ,  104  and the relative permittivity and dissipation factor of the dielectric material  103  will determine the RF attenuation of the resulting coaxial cable. As is known in the art, at radio frequencies the current flowing through the tubes  102 ,  104  in the cable  100  tends to flow only in and directly beneath the surfaces of the conducting tubes. This is commonly known as a “skin effect”. More particularly, current flows through and directly beneath an inside surface of the outer tube  102  and an outside surface of the inner tube  104 . 
     Each tube  102 ,  104  is manufactured by bending a flat strip of conductive tape into a round tube and welding the longitudinal edges of the tape together to form a seam. To minimize manufacturing costs, the material selected for forming the tubes  102 ,  104  is preferably one that is easy to form and weld. However, the materials that provide the best manufacturing characteristics do not necessarily offer the conductivity required for minimizing RF attenuation. 
     There is a need for a coaxial cable that has high conductivity, to minimize RF attenuation, and yet preserves the ease of manufacture and welding provided by less conductive materials. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a coaxial cable apparatus and method having a dual-layer structure for both its inner and outer tubes. Each tube is formed out of a flat strip having a base layer and a high conductivity layer disposed on the base layer. In a preferred embodiment, the high conductivity layer is disposed on less than the entire surface of the base layer, leaving the margins on the longitudinal edges of the base layer free of high conductivity material to form edge clearances. The flat strip is then bent to form a tube, with the edges of the tube being welded together. The edge clearances allow the edges of the base layer to be welded together without the weld joint touching the high conductivity layer, thereby avoiding potential problems associated with welding materials having different metallurgical properties. The preferred coaxial cable structure is arranged so that the high conductivity layer is on the outer surface of the inner tube and on the inner surface of the outer tube, following the normal current flow pattern in coaxial cables. By providing a separate high conductivity layer for each tube, the inventive dual-layer structure improves the RF attenuation characteristics of the coaxial cable while preserving the ease of manufacture provided by the material used in the base layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art coaxial cable structure; 
     FIG. 2 is a cross-sectional view of a coaxial cable according to the present invention; and 
     FIG. 3 is a perspective view of a section of a layered strip before it is formed into the inventive coaxial cable. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2 and 3, a preferred coaxial cable  200  according to the present invention includes an inner tube  202  and an outer tube  204  with a dielectric material  203  disposed in between the inner and outer tubes  202 ,  204 . Both the inner tube  202  and the outer tube  204  are formed from a strip  205 , preferably metal, that include a base layer  206 ,  208  and a high conductivity layer  210 ,  212  (i.e. low ohmic resistance). The material for the base layer  206 ,  208  can be selected based on its ease of forming and welding to ensure that the inner and outer tubes  202 ,  204  can be manufactured efficiently. Possible materials for the base layer  206 ,  208  include, but are not limited to, steel, aluminum, and copper. The material for the high conductivity layer  210 ,  212  can be selected based on its conductive characteristics; the forming and welding characteristics of the high conductivity layer  210 ,  212  are not as important because the edges of the high conductivity layer  210 ,  212  preferably will not be welded together. Alternatively, or in addition, the selection of the material combination to be used for the high conductivity layer  210 ,  212  and the base layer  206 ,  208  is based on the differential thermal expansion between the two materials. Possible materials for the high conductivity layer  210 ,  212  include, but are not limited to, gold, silver, and copper. The high conductivity layer  210 ,  212  can be disposed on the base layer  206 ,  208  using any known method, including cladding, electrodeposition, sputtering, and plating. Both smooth and corrugated cables can be used to form the inventive structure. 
     As can be seen in FIGS. 2 and 3, the high conductivity layer  210 ,  212  does not cover the entire base layer  206 ,  208 . Instead, each base layer  206 ,  208  has edge clearances  214 ,  216  that are free from the high conductivity material. The edge clearances  214 ,  215 ,  216 ,  217  can be formed by any known means, including selective deposition of the high conductivity material on the base layer  206 ,  208 , machining, etching, sputter deposition, plating, or electrochemical methods. Once the high conductivity layer  210 ,  212  is deposited on the base layer  206 ,  208 , the corresponding edge clearances  214 ,  215 ,  216 ,  217  for each strip  205  are brought together and welded to form the inner and outer tubes  202 ,  204 . Weld joints  218 ,  220 , which connect the edge clearances  214 ,  215 ,  216 ,  217  together, touch only the base layer  206 ,  208  because of the margins between the edges of the base layer  206 ,  208  and the edges of the high conductivity layer  210 ,  212 , which are provided by the edge clearances  214 ,  215 ,  216 ,  217 . As a result, the welding material and process can be selected based on the characteristics of only the base layer  206 ,  208  material, without having to consider the characteristics of the high conductivity layer  210 ,  212  material or deal with the formation of brittle intermetallics and other problems normally associated with welding materials having different metallurgical properties. 
     As can be seen in FIG. 2, the inner and outer tubes  202 ,  204  are arranged concentrically with respect to each other. The resulting coaxial cable  200  has high conductivity layers  210 ,  212  disposed on the outer surface of the inner tube  202  and the inner surface of the outer tube  204 , which are the areas in the coaxial cable  200  through which current flows, as explained above with respect to FIG.  1 . The inventive coaxial cable  200  therefore provides high conductivity areas in the regions where current normally flows, allowing the current to flow more freely and thereby reduce RF attenuation, while preserving the ease of welding and forming provided by the material used in the base layer. 
     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.