Abstract:
A high impedance coaxial cable with a fine wire inner conductor and method of manufacture. An inner conductor less than 0.1 inches in diameter is coated with an adhesive resin. The coating having a thickness at least 50 percent of the inner conductor diameter. A foam dielectric surrounds the adhesive resin and an outer conductor surrounds the foam dielectric. Adjusting the thickness of the adhesive resin coating varies the thermal mass of the inner conductor to reduce the appearance of voids between the adhesive resin and dielectric foam.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Coaxial cables for high frequency signal transmission may be designed for specific operating impedances by adjusting the spacing between the inner conductor and the surrounding outer conductor. To design a coaxial cable for high impedance characteristic, the distance between the inner conductor and the outer conductor is increased and or a dielectric with a higher specific gravity is used. However, application of dielectric materials with higher specific gravities increases the materials cost, weight and signal loss characteristics of the cable. To minimize the overall diameter of a high impedance cable, where high signal power capacity is not a design parameter, the diameter of the inner conductor may be minimized down to that of a fine wire.  
         [0002]     A coaxial cable with a fine wire inner conductor, surrounded by a foam dielectric that is covered by the outer conductor presents several manufacturing challenges. A fine wire inner conductor is very fragile. This makes it difficult to smoothly guide the inner conductor with the required precision through a traditional continuous coaxial cable manufacturing process.  
         [0003]     Prior high impedance fine wire inner conductor coaxial cables have been observed with an unacceptably high number of longitudinal voids in the dielectric foam, proximate the fine wire inner conductor. These voids introduce variances to the dielectric value of the area between the inner and outer conductor, create a moisture/corrosion path within the cable and also allow the position of the inner conductor within the foam dielectric to vary. Together, these factors introduce a significant error between the designed and the measured characteristic impedance of the finished cable that may vary length to length of the cable.  
         [0004]     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.  
         [0005]     Competition within the coaxial cable industry has focused attention upon electrical characteristic uniformity, defect reduction and overall improved manufacturing quality control.  
         [0006]     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  
       [0007]     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.  
         [0008]      FIG. 1  is a schematic representation of a prior art fine center conductor coaxial cable.  
         [0009]      FIG. 2  is a schematic representation of a fine center conductor coaxial cable according to the invention.  
         [0010]      FIG. 3  is a schematic manufacturing process diagram.  
         [0011]      FIG. 4  is a close up of the quench area  50  of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0012]     The inventor has recognized the reason voids appear in prior high impedance fine wire inner conductor coaxial cables.  
         [0013]     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.  
         [0014]     The inventor&#39;s research has verified that applying a thick outer layer of adhesive resin around the fine wire inner conductor increases the thermal mass and improves the inner conductor mechanical characteristics during further manufacturing steps. The increased thermal mass and improved mechanical characteristics of the coated fine wire inner conductor results in a fine wire inner conductor coaxial cable with significant improvements in impedance characteristic uniformity and ease of use.  
         [0015]     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  20  that has a thickness at least 50% 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 adhesive resin coating  20  according to the invention should be at least 0.01 inches thick. In this embodiment, after the adhesive resin coating  20  is applied to the inner conductor  10 , the resulting coated inner conductor  25  will have an overall exterior diameter of at least 0.04 inches.  
         [0016]     The adhesive resin coating  20  is surrounded by a foam dielectric  15  which is surrounded by the outer conductor  30 . In the exemplary embodiment, the foam dielectric  15  and adhesive resin coating  20  are polyolefin resins selected to have compatible molecular properties. The adhesive resin coating  20  also is selected to provide suitable adhesion to the inner conductor  10  as well as acceptable signal loss characteristics.  
         [0017]     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.  
         [0018]     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.  
         [0019]     During a continuous manufacturing process according to the invention, as shown in  FIG. 3 , the fine wire inner conductor  10  is delivered to a first extruder  35  that applies the adhesive resin coating  20  around the inner conductor  10  to a thickness at least 50% of the inner conductor  10  diameter. Passage through a cooling tube  40  or other cooling mechanism cools the conductor  10  and surrounding hot adhesive resin coating  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.  
         [0020]     A second extruder  45  applies a foam dielectric resin layer to the coated inner conductor  25  that expands into the foam dielectric  15  upon exiting the second extruder  45 . Expansion is aided 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 adhesive resin coating  20 , has a significantly higher thermal mass than prior high impedance fine wire inner conductor coaxial cables, the inner conductor  10  and adhesive resin coating  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 eliminated. Any void(s)  5  present before application of the outer conductor  30  may be removed by the compression of the foam dielectric  15  during outer conductor  30  application.  
         [0021]     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.  
         [0022]     To minimize material requirements, the adhesive resin coating  20  thickness may be adjusted until an acceptable level of void(s)  5  is obtained in the finished coaxial cable.  
         [0023]     The invention has been demonstrated with respect to a first exemplary embodiment. 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 characteristic impedance greater than 85 ohms and solid inner conductors of up to 0.1 inch in conductor diameter. For lower impedance and or thicker inner conductor cables, 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.  
         [0024]     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   adhesive resin coating       25   coated inner conductor       30   outer conductor       35   first extruder       40   cooling tube       45   second extruder       50   quench area                  
 
         [0025]     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.  
         [0026]     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.