Abstract:
Thermal mass compensated foam support structures for coaxial cables such as inner conductors and or inner conductor support structures. The foam support structures provided with an adhesive solid or high density foam polymer or blend layer to increase the thermal mass of the support structure enough to allow the foam to surround the adhesive solid or high density foam polymer or blend layer without forming unacceptably large voids in the foam dielectric as the foam dielectric cures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/306,793 titled “ Coaxial Cable with Fine Wire Inner Conductor”, filed Jan. 11, 2006 now U.S. Pat. No. 7,446,257 by Mark Witthoft, currently pending and hereby incorporated by reference in the entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Prior attempts at coating support structures having a low thermal mass with dielectric foam, such as the fine wire inner conductor or plastic rod inner conductor support of a coaxial cable, have suffered from an unacceptably high number of longitudinal voids in the applied dielectric foam, proximate the support structure. 
     A prior art coaxial cable with void(s)  5  around the fine wire inner conductor  10 , for example as shown in  FIG. 1 , is difficult to prepare for interconnection because the exact inner conductor position is variable. Also, in contrast to a cable where the inner conductor  10  is fully supported by the foam dielectric  15 , any pressure upon the inner conductor  10  during interconnection may cause it to bend and collapse into the void(s)  5 , away from the cable end. 
     Commonly owned U.S. Pat. No. 6,800,809, titled “Coaxial Cable and Method of Making Same”, by Moe et al, issued Oct. 5, 2004, hereby incorporated by reference in the entirety, discloses a coaxial cable structure wherein the inner conductor is formed by applying a metallic strip around a cylindrical filler and support structure comprising a cylindrical plastic rod support structure with a foamed dielectric layer there around. The resulting inner conductor structure has significant materials cost and weight savings compared to coaxial cables utilizing solid metal inner conductors. 
     Competition within the coaxial cable industry has focused attention upon reducing materials and manufacturing costs, electrical characteristic uniformity, defect reduction and overall improved manufacturing quality control. 
     Therefore, it is an object of the invention to provide a coaxial cable and method of manufacture that overcomes deficiencies in such prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic end view representation of a prior art fine center conductor coaxial cable. 
         FIG. 2  is a schematic end view representation of a fine center conductor coaxial cable according to the invention. 
         FIG. 3  is a schematic manufacturing process diagram. 
         FIG. 4  is a close up of the quench area  50  of  FIG. 3 . 
         FIG. 5  is a schematic end view representation of a prior art support structure utilizing a plastic rod. 
         FIG. 6  is a schematic end view representation of a support structure according to the invention. 
         FIG. 7  is a schematic end view representation of an inner conductor structure incorporating the support structure of  FIG. 6 . 
         FIG. 8  is a schematic end view representation of an exemplary coaxial cable with a low thermal mass inner conductor structure according to the invention. 
         FIG. 9  is a schematic representation of an alternative exemplary coaxial cable with a low thermal mass inner conductor structure according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Continuous production manufacture of coaxial cables including dielectric foam applied about an inner conductor or other supporting structure having a low thermal mass has previously either included an unacceptably high number of longitudinal voids appearing in the dielectric foam, proximate the inner structure, or necessitated design changes such as increasing the size and thus thermal mass of the support element. The inventor(s) have recognized the reason these voids appear. 
     The foam dielectric area of a high impedance cable will be larger than in an otherwise similar low impedance cable. During the foam dielectric expansion step, the foam dielectric relies upon the thermal mass of the inner conductor to assist with the curing of the dielectric foam towards the center of the cable rather than just towards a cooling quench flowing around the exterior. Even if a traditional thin adhesive coating of an unexpanded plastic is present around the inner conductor, if insufficient inner conductor thermal mass is present to receive heat transfer from the dielectric foam, i.e. cool the core of the foam dielectric as it is expanded, the foam dielectric will pull away from the inner conductor, creating voids around the inner conductor. Similarly, the inner conductor support structure of U.S. Pat. No. 6,800,809 has an oversized diameter—to provide sufficient thermal mass to obtain a uniform foam dielectric layer without unacceptably large voids. 
     The inventor&#39;s research has verified that applying a thick outer layer of adhesive resin such as a solid or high density foam polymer or blend around the foam dielectric support structure increases the thermal mass and improves the combined support structure and dielectric foam combination mechanical characteristics during further manufacturing steps. The increased thermal mass and improved mechanical characteristics of the coated support structure results in a fine wire inner conductor coaxial cable with significant improvements in uniformity of characteristic impedance and ease of use. 
     As shown in  FIG. 2 , a first exemplary embodiment of the invention has a fine wire inner conductor  10  surrounded by a, for example, polyolefin adhesive resin coating, or other solid or high density foam polymer or blend layer  20  that has a thickness at least 30% of the inner conductor 10 diameter. The inner conductor  10  of the first exemplary embodiment shown in  FIG. 2  has an inner conductor 10 diameter of 0.02 inches. Therefore, the solid or high density foam polymer or blend layer  20  according to the invention should be at least 0.06 inches thick. In this embodiment, after the solid or high density foam polymer or blend layer  20  is applied to the inner conductor  10 , the resulting coated inner conductor  25  will have an overall exterior diameter of at least 0.32 inches. 
     The solid or high density foam polymer or blend layer  20  is surrounded by a foam dielectric  15  that is surrounded by the outer conductor  30 . In the exemplary embodiment, the foam dielectric  15  and solid or high density foam polymer or blend layer  20  are polyolefin resins selected to have compatible molecular properties. The solid or high density foam polymer or blend layer  20  may also be selected to provide suitable adhesion to the inner conductor  10  as well as acceptable signal loss characteristics. 
     The fine wire inner conductor  10  of the first embodiment may have a steel core for improved tensile strength. Copper or other high conductivity metal electroplating may be applied to the steel core to protect it from corrosion and improve conductivity. An outer layer of tin may also be applied to simplify soldered connections to the inner conductor. 
     The outer conductor  30  may be a solid aluminum or copper material with or without corrugations, as desired. Alternatively, foil and or braided outer conductor(s)  30  may also be applied. If desired, a plastic outer protective sheath may be added. 
     During a continuous manufacturing process according to the present embodiment, as shown in  FIG. 3 , the fine wire inner conductor  10  is delivered to a first extruder  35  that applies the solid or high density foam polymer or blend layer  20  around the inner conductor  10  to a thickness at least 30% of the inner conductor  10  diameter. Passage through a cooling tube  40  or other cooling mechanism cools the conductor  10  and surrounding hot solid or high density foam polymer or blend layer  20  (coated inner conductor  25 ). Where sufficient process space is available, the cooling mechanism may be formed as an extended transport path through open air. 
     A second extruder  45  applies a foam dielectric resin layer to the coated inner conductor  25  that expands to form foam dielectric  15  upon exiting the second extruder  45 . Expansion is controlled by passage through a quench area  50 , as shown in  FIG. 4 , until the foam dielectric  15  reaches its desired expansion. Because the inner conductor  10 , coated by the solid or high density foam polymer or blend layer  20 , has a significantly higher thermal mass than prior high impedance fine wire inner conductor coaxial cables, the inner conductor  10  and solid or high density foam polymer or blend layer  20  is able to draw heat from the hot foam dielectric  15  as it expands. Thereby, the formation of void(s)  5  between the coated inner conductor  25  and the foam dielectric  15  that are larger than a cell size of the dielectric foam are minimized and or essentially eliminated. 
     The foam dielectric  15  coated inner conductor  25  may be cured for a desired period or passed directly to the outer conductor  30  application process (not shown). The desired outer conductor  30  may be applied, for example by seam welding a solid metal outer conductor  30 , coaxial with the inner conductor  10 , around the foam dielectric  15 . Methods for applying outer conductor  30  to a foam dielectric  15  coated inner conductor  25  are well known in the art and as such are not described in further detail here. 
     To minimize material requirements, the solid or high density foam polymer or blend layer  20  thickness, and thereby the thermal mass of the plastic rod  55  and solid or high density foam polymer or blend layer  20  combination may be adjusted until an acceptable thermal mass is present to generate the desired foam dielectric  15  application parameters and thereby the finished coaxial cable characteristics. 
     With respect to an inner conductor support structure  52  according to U.S. Pat. No. 6,800,809, to avoid unacceptable voids and or position shift between the plastic rod  55  and the layer of foamed dielectric  15 , the plastic rod  55  has previously been applied with an increased diameter, for example as shown in  FIG. 5 . Because the materials cost of the plastic rod  55  per unit of cross sectional area is much higher than the materials cost for adhesive  60  and/or foam dielectric  15  polymer layers, as the diameter of the plastic rod  55  is increased, the material cost of the resulting inner conductor support structure also significantly increases. 
     Although the plastic rod  55  may have a larger diameter than a fine wire inner conductor  10  described herein above, plastic material generally has a lower thermal mass per cross sectional area than metal. Therefore, the inventors have also observed surrounding foam dielectric  15  void creation and or position shift problems with plastic rods  55  having significantly larger diameters. As with a fine wire inner conductor  10 , applying a solid or high density foam polymer or blend layer  20  to the plastic rod  55  increases the thermal mass of the plastic rod  55 , enabling application of a significantly smaller plastic rod  55  diameter, for example as shown in  FIGS. 6 and 7 , without encountering unacceptable low thermal mass foam dielectric  15  application void defects. 
     To improve adhesion between the plastic rod  55  and the solid or high density foam polymer or blend layer  20  an intermediary adhesive layer  60  may be applied. Similarly, an intermediary adhesive layer  60  may be applied between the solid or high density foam polymer or blend layer  20  and the foamed dielectric  15 . 
     In a plastic rod  55  support structure  52  embodiment, the inner conductor  10  is further formed by surrounding and or otherwise metalizing the outer diameter of the entire plastic rod support structure with metal  65 , applied for example by seam welding a metal strip applied around the outer diameter of the foam dielectric  15 , as is well known in the art. 
     The diameter of the inner conductor  10  for a coaxial cable is generally selected according to the desired coaxial cable structural and impedance characteristics. Within the largest diameter commonly manufactured coaxial cable including a conventional plastic rod inner conductor supporting structure such as disclosed by U.S. Pat. No. 6,800,809, the plastic rod may be required to be as large as 3.5 mm in diameter. According to the invention, the diameter of the plastic rod  55  may be dramatically reduced. For example, a 3.5 mm plastic rod  55  may be replaced with a plastic rod  55  with a diameter of 1.0 mm or less by applying a solid or high density foam polymer or blend layer  20  with a thickness of approximately 30 percent of the selected plastic rod  55  diameter. 
     As the diameter of the plastic rod  55  is reduced, tensile strength limitations of the plastic rod material may become significant. Examples of high tensile strength plastic rod(s)  55  include Kevlar fibers and or glass reinforced plastic. Where the plastic rod  55  is provided in a high strength polymer material with suitable tensile strength characteristics, the plastic rod  55  diameter may be further reduced and the solid or high density foam polymer or blend layer thickness increased, for example to 50% or more of the plastic rod  55  diameter. 
     A method for manufacturing the inner conductor support structure  52  is analogous to the procedure for preparing the fine wire inner conductor  10  coated with a solid or high density foam polymer or blend layer  20 , herein above, with the plastic rod  55  replacing the fine wire inner conductor  10  and adjusting the thickness of the layers accordingly to generate an inner conductor  10  structure that is then applied as an input to a traditional production process to produce a completed coaxial cable. Additional steps in the production of the inner conductor  10  structure may include the intermediate coating of the plastic rod  55  and/or the solid or high density foam polymer or blend layer  20  outer diameter(s) with an additional intermediary adhesive layer  60 , if desired. 
     The invention has been demonstrated with respect to a fine wire inner conductor  10  and plastic rod  55  support structure  52  for an inner conductor exemplary embodiment(s). One skilled in the art will appreciate that the cable design and manufacturing process herein is applicable to coaxial cables having a foam dielectric thickness corresponding to a desired characteristic impedance and solid inner conductors of up to 0.1 inch in conductor diameter. For coaxial cables having thicker solid metal inner conductors, the thermal mass of the inner conductor  10 , uncoated, should be sufficient to avoid the appearance of the void(s)  5  described herein, during curing of the foam dielectric  15  as long as the inner conductor  10  is not delivered to the second extruder  45  for foam dielectric  15  coating at an excessive temperature. 
     One skilled in the art will recognize that the invention is also applicable to other coaxial cable inner conductor  10  structures having a low thermal mass, such as a plastic rod  55  or tube  70  with a metal  65  outer diameter as shown for example in  FIGS. 8 and 9 . In this instance, the diameter of the inner conductor  10  is not a limitation of the solid or high density foam polymer or blend layer  20  thickness. Instead, the solid or high density foam polymer or blend layer  20  may be applied at thicknesses selected to achieve a desired thermal mass and thereby the void minimizing effect during dielectric foam  15  application, as described herein above. 
     The metal  65  outer diameter of the plastic rod  55  may be applied by metalizing the plastic rod  55 , for example, by seam welding a metal strip folded around the plastic rod  55 , coating, depositing and or plating operations. Alternatively, the metalizing may be via application of a metallic foil upon the outer diameter of the plastic rod  55  or tube  70 . 
     Although the manufacturing process is described as a continuous process, the process may be divided into several discrete sections with work in progress from each section stored before feeding the next section, without departing from the invention as claimed. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Table of Parts 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 5 
                 void 
               
               
                 10 
                 inner conductor 
               
               
                 15 
                 foam dielectric 
               
               
                 20 
                 solid or high density foam 
               
               
                   
                 polymer or blend layer 
               
               
                 25 
                 coated inner conductor 
               
               
                 30 
                 outer conductor 
               
               
                 35 
                 first extruder 
               
               
                 40 
                 cooling tube 
               
               
                 45 
                 second extruder 
               
               
                 50 
                 quench area 
               
               
                 52 
                 support structure 
               
               
                 55 
                 plastic rod 
               
               
                 60 
                 adhesive layer 
               
               
                 65 
                 metal 
               
               
                 70 
                 tube 
               
               
                   
               
             
          
         
       
     
     Where in the foregoing description reference has been made to ratios, integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth. 
     While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant&#39;s general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.