Patent Publication Number: US-7583489-B2

Title: Tungsten shorting stub and method of manufacture

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No.: 60/747,920 filed May 22, 2006 and hereby incorporated by reference in the entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to improvements in the operating power level and or surge capacity of RF devices such as shorting stubs for coaxial cables. More particularly, the invention relates to improved materials and manufacturing processes for these devices. 
     2. Description of Related Art 
     A major limitation in the power handling of a helical and or spiral planar shorting stub is its resistance to deformation when surged by lightning. The positive benefits of the fields generated by the interaction of the “rings” of the spiral become a liability when the calculated geometry is deformed by a surge and the device is no longer electrically balanced for its target frequency range. 
     Prior shorting stubs have significant surge limitations and or size requirements because of the characteristics of the conventional materials previously applied (Brass, Phosphor Bronze, Aluminum). Where the shorting stub has a helical or spiral geometry the interactive effects of the fields generated during a surge event will damage and or destroy the shorting stub if the surge is of too high a level. 
     For example, limitations in the range of 25-30 KA are known to exist for shorting stub assemblies utilizing conventional materials unless the overall size of the shorting stub is extended to the point where the size and materials cost(s) become unacceptable. 
     Competition within the electrical cable and associated accessory industries has focused attention on increased manufacturing efficiencies, overall component size reduction and increased power handling capability. 
     Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the 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 isometric view of an exemplary multi-planar shorting stub, including a plurality of inner diameter supports. 
         FIG. 2  is a schematic top view of  FIG. 1 . 
         FIG. 3  is a schematic front side view of  FIG. 1 , 
         FIG. 4  is a schematic back side view of  FIG. 1 , with the plurality of supports removed. 
         FIG. 5  is a schematic top view of  FIG. 4 . 
         FIG. 6  is a schematic isometric top view of a planar Archimedes spiral shorting stub. 
         FIG. 7  is a schematic isometric top view of a planar circular spiral shorting stub. 
         FIG. 8  is a schematic isometric side view of a helical spiral shorting stub. 
         FIG. 9  is a schematic isometric side view of a multi-planar shorting stub including a plurality of outer diameter supports. 
         FIG. 10  is a schematic isometric side view of a multi-planar shorting stub including a unitary support band. 
         FIG. 11  is a schematic isometric side view of a multi-planar shorting stub including a plurality of post supports. 
     
    
    
     DETAILED DESCRIPTION 
     The electro-mechanical characteristics of Tungsten and other metals and or metal alloys are known: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Tensile 
                 Thermal 
               
               
                   
                 Conductivity 
                 Elasticity 
                 Yield 
                 Stability 
               
               
                 Material 
                 % IACS) 
                 (PSI) 
                 (PSI) 
                 (μin/in-° C.). 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 (CTE) Bronze 
                 28 
                   16 + 10e6 
                 63,100 
                 20.3 
               
               
                 Phosphor Bronze 
                 16 
                   16 + 10e6 
                 74,700 
                 16.0 
               
               
                 Aluminum (7075) 
                 33 
                 10.3 + 10e6 
                 73,000 
                 23.2 
               
               
                 Tungsten 
                 30 
                 59.5 + 10e6 
                 109,000 
                 4.6 
               
               
                   
               
            
           
         
       
     
     Although a shorting stub may survive a relatively high power surge event, deformation of the shorting stub resulting form the surge event may destroy the electrical characteristics of the shorting stub for ongoing operation. The inventor has recognized that, within a common assembly size constraint, a primary limitation of shorting stub design for higher surge capacities is the electro-mechanical characteristics of the materials applied to the shorting stub. 
     While almost as conductive as Aluminum (high conductivity is a desirable characteristic because higher conductivity lowers the resulting “let thru” of the shorting stub), The inventor&#39;s research has revealed that Tungsten will deform far less for vastly higher surge capability (Elasticity and Tensile Strengths) and is more thermally stable thus less prone to frequency response drift. However, the significantly higher material costs of Tungsten material have previously made application of Tungsten cost prohibitive. Although the actual amount of Tungsten required in a finished shorting stub is relatively low, materials waste due to extensive machining and or stamping procedures required to form complex shorting stub geometries increased the materials costs significantly. Further, Tungsten is brittle at ambient temperatures, requiring specialized procedures during machining, stamping, bending and or folding manufacturing operations which further increase manufacturing costs. 
     Metal Injection Molding (MIM), also known as Powder Injection Molding (PIM), is a net-shape process for producing solid metal parts that combines the design freedom of plastic injection molding with material properties near that of wrought metals. With its inherent design flexibility, MIM is capable of producing an almost limitless array of highly complex geometries in many different metals and metal alloys. Design and economic limitations of traditional metalworking technologies, such as machining and casting, can be overcome by MIM. 
     In a typical MIM process, finely granulated metal material is uniformly mixed with a wax or polymer binder and injection molded. A “green” molded part is then extracted from the mold. A de-binding step extracts the majority of binder from the green part via application of low temperature and or a solvent. The de-bound green part is then sintered at high temperature wherein the de-bound part is proportionally shrunk to the final target size, concentrating the metal density and strength characteristics to close to that of a casting made from the same material by conventional means. 
     The inventor has recognized that modified MIM manufacturing technologies may be applied to form the complex shapes of shorting stubs and other RF components using Tungsten and or Tungsten alloys to reduce both the increased materials and machining costs previously associated with Tungsten. Thereby, the invention enables the design and manufacture of shorting stubs and other RF structures that benefit from the improved electromechanical properties of Tungsten and or Tungsten alloys. 
     Because of the minimal waste inherent in the MIM manufacturing process, although the superior electromechanical properties of Tungsten are realized, the increased costs associated with the application of Tungsten are minimized. Via the present invention, a surge suppressor with improved electrical characteristics including improved multiple strike survivability and significantly increased maximum strike magnitude capacity is enabled. 
     Exemplary highly compact Multiple Planar Inductive Loop Surge Suppressor configurations and the shorting stubs thereof are disclosed in U.S. patent application Ser. No.: 11/306,872 filed Jan. 13, 2006 titled “Multiple Planar Inductive Loop Surge Suppressor” by Howard Davis and Kendrick Van Swearingen, co-owned with the present application by Andrew Corporation of Westchester, Ill. and hereby incorporated by reference in the entirety. 
     As shown for example in  FIGS. 1-5 , a shorting stub may be formed via MIM having a multi-planar configuration. The multi-planar configuration is useful to increase the inductive aspect(s) of the shorting stub, without undesirably increasing the overall size requirements of the finished assembly, and further to reduce the mechanical spring response to surge characteristics of a helical and or spiral configuration. The shorting stub  10  is formed extending outward from an inner conductor connection  12 , through a connecting portion  14  that may include one or more loop segment(s)  16  before reaching an outer conductor connection  18 . The loop segment(s)  16  may be arranged in parallel planes joined one to another by a transition segment  20 . 
     While the invention has been demonstrated in detail with respect to a specific embodiment of a multiple planar shorting stub, one skilled in the art will recognize that other shorting stub configurations such as single plane spiral and or helical may be similarly applied. As demonstrated in  FIGS. 6-8 , the loop segment(s)  16  may be formed in a wide range of configurations, or combinations of configurations such as linear, circular, arcurate, spiral, helical or the like. The loop segment(s)  16  may each extend from the inner conductor connection  12  to a common or multiple outer conductor connection(s)  18 . Alternatively, the loop segment(s)  16  may be joined end to end. The inner conductor connection  12  and or the outer conductor connection  18  may be formed, for example, as loops, pins, tabs, wedges, screw ends, sockets or the like. 
     To support multiple planar loop segments in the desired configuration during the mold retraction and or sintering step(s) of the MIM manufacturing process, one or more support(s)  22  may be included in the design that are later easily removed from the finished shorting stub. 
     Forming each of the support(s)  22 , for example, parallel to a longitudinal axis of the inner conductor and with a frangible connection to each of the multiple planar loop segment(s)  16  enables easy removal of the supports without requiring an additional machining step. Placement of the supports along an inner diameter of the loop segment(s)  16  minimizes the overall size requirement of the MIM mold. 
     Alternatively, as demonstrated by  FIGS. 9-11 , the support(s)  22  may be formed, for example, along the outer diameter of the loop segment(s)  16 , as a unitary support band  24  or as post(s)  26  positioned between the loop segment(s)  16 . Support ( 22 ) configurations of this type may introduce cavities and overhanging portions not easily obtained by single piece molding. In these configurations separate parts may be molded to obtain “green” molded pieces that are then stacked together for the sintering step. The sintering step then joins the mating surfaces of the stacked pieces together to form a single integral component. If the support(s)  22  are formed in a configuration not easily adapted for removal by breaking a frangible joint, they may be removed with a secondary machining operation. 
     One method of manufacture according to the invention includes the steps of forming a shorting stub  10  according to a desired configuration via MIM manufacturing process(s), the shorting stub  10  formed from Tungsten and or a Tungsten alloy. Any support(s)  22  included in the configuration are removed after at least the sintering steps of the MIM manufacturing process(s) have been completed. 
     Adaptations to standardized MIM procedures advantageous when Tungsten and or Tungsten alloy material is being applied include selection of a compatible polymer and solvent pair for the de-binding step. Polymer rather than wax may be applied and nitric acid used as the solvent for polymer removal during de-binding. Nitric acid would react with Copper and Copper alloy material, but provides desirable de-binding results when applied to Tungsten or Tungsten alloy material. 
     Tungsten and or Tungsten alloys may be applied to other RF devices with similar benefit. For example, previously RF filter elements have been manufactured from specialized alloys such as INVAR™ (FeNi36) a Nickel Iron alloy, known for having an extremely low thermal expansion property (2 μin/in-° C.). Application of 
     Tungsten in place of INVAR™ provides an acceptable thermal expansion characteristic at a significant cost reduction. 
     While a MIM manufacturing process has been identified the invention is not limited thereto, a shorting stub or other RF device such as a filter element may be formed according to the invention from Tungsten and or a Tungsten alloy by other manufacturing processes. 
     One skilled in the art will appreciate that the present invention represents a significant improvement in power capability, overall size requirements, manufacturing and cost efficiency. 
     Table of Parts 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 10 
                 shorting stub 
               
               
                 12 
                 inner conductor connection 
               
               
                 14 
                 connecting portion 
               
               
                 16 
                 loop segment 
               
               
                 18 
                 outer conductor connection 
               
               
                 20 
                 transition segment 
               
               
                 22 
                 support 
               
               
                 24 
                 unitary support band 
               
               
                 26 
                 post 
               
               
                   
               
            
           
         
       
     
     Where in the foregoing description reference has been made to ratios, integers, components or modules 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.