Abstract:
A shield terminator for sealing the splice of a ground lead wire and the shield of a coaxial cable. The shield terminator includes a solder band and a dam of thermoplastic material which has at least two lumens therein. One larger lumen receives the coaxial cable while the small lumen receives the ground lead wire. The presence of the thermoplastic material around both the cable and the lead wire insures that all interstices between the cable, wire and the sleeve of the shield terminator will be filled, preventing the escape of solder from the sleeve, and precluding entry of water and other contaminants into the splice.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to electrical connectors and cable splicing, and more particularly to a heat-shrinkable shield terminator having a multilumen sealing ring. 
     2. Description of the Prior Art 
     There are presently available a wide variety of electrical couplers and splicing devices for connecting wires and cables in electrical systems. The subject invention relates to such a device primarily used in conjunction with coaxial cables. 
     A typical coaxial cable (also known as a coax, coaxial transmission line, or concentric cable) has a central wire surrounded by an insulating layer, which is further surrounded by a conductive sheath or shield, and an insulating covering or jacket (see FIG. 1). The conductive shield is usually connected to the electrical ground, whether it be earth ground, a chassis, or simply a common tie. 
     It is sometimes necessary to connect the ground shields of two or more coaxial cables together, often at a point along the coax where it is undesirable to completely cut through the cable. In such cases, a section of the outer jacket may be removed without cutting the remaining layers of the cable, thereby exposing the ground shield. One end of a ground lead wire may then be soldered to the ground shield and the other end connected to the second coax or other ground. Throughout the specification, the term &#34;lead&#34; in the phrase &#34;ground lead wire&#34; is meant in the verb sense, not as the metal lead (Pb). Similarly, the word &#34;ground&#34; is not to be taken in a limiting sense; as noted above, the lead wire may simply be connected to a common tie. 
     A problem arises, however, with respect to the exposed ground shield. If left bare, the ground wire may accidentally become detached, or environmental influences, particularly water, could corrode the shield, as well as affect the insulative ability of the layer surrounding the central wire. Also, when cables are so exposed in the presence of water (such as underground cables after a rain), the water may seep in between the outer jacket and the inner insulating layer, where it is then conveyed down the cable, sometimes for hundreds of feet. If openings in the inner insulating layer exist, even as small as a pinhole, the water will create a resistive fault in the cable. Even if there are no flaws in the insulating layer, the presence of water will still create a capacitive fault, and alter the impedance characteristics of the coax. Such defects can become critical in certain situations, such as in airborne equipment or combat vehicles. 
     It is, therefore, necessary to provide an environmental seal around the exposed area of cable. Devices for this purpose are known in the art, and are generally a protective sleeve. They are cylindrical in shape, and are usually constructed of a thermoelastic, or heat-shrinkable material. In this manner, the sleeve may be loosely fitted around the exposed section of cable and heat applied, resulting in a tight seal about the shield ground. 
     Improved versions of the protective sleeve include a solder band placed midway along the sleeve, and two rings of sealing material, or dams, placed at each end of the sleeve (see FIG. 2). This particular design, known generically as a shield terminator, is disclosed in U.S. Pat. No. 3,312,772 issued to Raychem Corporation on Apr. 4, 1967, which is hereby incorporated by reference. Other related U.S. Pat. Nos. are: No. 3,305,625 issued to R. Ellis; No. 3,316,343 issued to H. Sherlock; No. 3,396,460 issued to J. Wetmore; No. 3,525,799 issued to R. Ellis; No. 4,206,786 issued to J. Wetmore; and No. 4,341,921 issued to D. Simpson. Each of these patents is hereby incorporated by reference. 
     The shield terminator is slipped around the exposed portion of the cable shield, an the ground lead wire is inserted between the sleeve and dam material (FIG. 3). When heat is applied, the solder melts, eliminating the separate step of soldering the ground lead wire to the ground sheath. At the same time, the sealing material becomes somewhat fluid and, as the thermoelastic material begins to shrink, the sealant should fill any gaps between the ground lead wire and the outer jacket of the cable. Notably, the abovereferenced patent does not disclose use of the sealing material to protect against external influences, but rather contemplates use of the sealant only to keep the solder from flowing out of the protective sleeve. A detailed discussion of desirable performance characteristics and use of these shield terminators may be found in U.S. Department of Defense military specifications MIL-S-83519A, MIL-S-83519/lC and MIL-S-83519/2A, which are hereby incorporated by reference. 
     Unfortunately, however, the sealing material does not always completely fill the space between the ground lead wire and the outer jacket. Failures sometimes occur at interstices between the sleeve and the ground lead wire, above the dam, as illustrated in FIG. 4A. Alternatively, if the ground lead wire is inserted between the dam material and the cable jacket, failures may occur adjacent to the jacket as shown in FIG. 4B. Such failures completely nullify the effectiveness of the shield terminator, and may lead to catastrophic results under certain conditions. It would, therefore, be desirable and advantageous to devise a shield terminator having improved sealing means which overcomes this deficiency. 
     Accordingly, the primary object of the present invention is to provide a shield terminator for electrical wires and cables. 
     Another object of the invention is to provide such a device which has improved sealing means for protection against environmental influences. 
     Still another object of the invention is to provide a dam for a shield terminator which may be used to interconnect several wires or cables. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in a shield terminator having at least one multilumen dam. In the simplest embodiment, one of the two dams present in the terminator is a bilumen dam, having one lumen or channel for the cable, and another lumen for the lead wire. In this manner, the user is assured of a quality seal encompassing the exposed portion of the cable, and protecting the shield against environmental influences. Where a chain of interconnected cables is required, the shield terminators may include two such bilumen dams, one at each end; alternatively, a trilumen dam may be provided. Dams having more than three lumens are also envisioned as being useful in specific applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of the invention are set forth in the appended claims. The invention itself, however, will best be understood by reference to the accompanying drawings, wherein: 
     FIG. 1 is an elevational view of a coaxial cable with a portion of the outer insulating jacket removed, exposing the shield; 
     FIG. 2 is a perspective view of the prior art shield terminator; 
     FIG. 3 is a side elevational view with a partial cutout illustrating the prior art shield terminator with the ground lead wire inserted therein; 
     FIG. 4A is a cross-sectional view taken along lines 4A-4A of FIG. 3 showing failure points along the dam after heat has been applied to the shield terminator; 
     FIG. 4B is similar to FIG. 4A except that the ground lead wire has been placed under, rather than over, the dam; 
     FIG. 5 is a perspective view of one embodiment of the present invention showing a bilumen dam in broken lines; 
     FIG. 6 is a front elevational view of the bilumen dam of the present invention; 
     FIG. 7 is a side elevational view with a partial cutout showing the ground lead wire inserted into the shield terminator and bilumen dam; 
     FIG. 8 is a cross-sectional view taken along lines 8--8 of FIG. 7; 
     FIG. 9 is a perspective view of the capped shield terminator of the present invention; 
     FIG. 10 is a front elevational view of the trilumen dam embodiment of the present invention; and 
     FIG. 11 is a perspective view depicting use of the trilumen dam in a chain arrangement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the drawings, and in particular with reference to FIG. 1, there is depicted a conventional coaxial cable 10 with a portion of the outer insulating jacket 12 removed, exposing shield section 14. An annular cut is normally made in the insulating jacket to expose shield section 14, but it is sufficient to cut a smaller &#34;window&#34; of jacket 12 to expose the cable shield. The conductive shield 14 should be woven conductive filaments, normally braided copper wire which is tinned or silvered for solderability. Some shields are wrap-around aluminum sheeting (aluminized Mylar foil), but it is difficult to make a solder bond with such material. Also shown is the terminal end 16 of coaxial cable 10, including inner insulating layer 18 and central wire 20. Of course, those skilled in the art will appreciate that the present invention is not limited to use on coaxial cables, but may be used in any application requiring electrical connection to an insulated conductor. 
     Although the conductive shield of coaxial cable 10 may be connected to the electrical ground proximate terminal end 16, this is often insufficient for sensitive equipment, especially where shorts or lightning strikes are likely to occur. Therefore, grounding of shield section 14 at an intermediate point 15 such as that shown in FIG. 1 is often desirable. In anticipation of this requirement, several shield terminators of the present invention may be prepositioned about coaxial cable 10, such that they may be conveniently maneuvered along cable 10 to the location of exposed sections 14. 
     Referring now to FIGS. 5 and 6, the shield terminator 22 of the present invention is explained. Shield terminator 22 generally comprises tubular sleeve 24, prior art dam 26, solder band 28, and bilumen dam 30. Sleeve 24 is constructed of a heat-shrinkable material usually formed from polymeric and elastomeric resins. Suitable thermoelastic materials include flexible polyolefins, polychloroprene, polyvinylidene fluoride, polytetrafluoroethylene and fluoroelastomers. Use of polychloroprene is eschewed, however, as it is not transparent (which is desirable for observation of a proper solder joint), and it does not have the heat stability of the other compounds. These substances typically have a shrink ratio of approximately 2:1, meaning that, in tubular form, they may shrink to 50% of their original diameter. Sleeve 24 may therefore easily slide over cable 10, and still provide a tight fit around cable -0 after heating. 
     Sleeve 24 may come in various sizes according to the diameter of the particular cable being grounded. The thermoelastic material comprising sleeve 24 is optionally color-coded according to diameter, length, thermoelastic material, solder type, etc., and may further contain a thermochromic temperature indicator which causes sleeve 24 to change color when an appropriate temperature is applied, namely, the minimum temperature necessary to melt solder band 28 and plasticize dams 26 and 30. 
     Dam 26 is a prior art dam, and is essentially a band or ring of thermally stabilized thermoplastic. A material should be chosen which melts when heat (sufficient to melt solder band 28) is applied, but the material should remain somewhat viscous to preclude escape of any solder from sleeve 24. The inventor prefers the use of a mixture of ethylene vinyl acetate copolymer and polyvinylidene fluoride. Solder band 28 is also known in the art, and may be any type of tinlead alloy, with the optional provision of flux. Solder band 28 may be replaced by other fusible bonding materials. 
     Bilumen dam 30 (shown in FIG. 6) is generally constructed of the same material as prior art dam 26; however, bilumen dam 30 has two separate lumens or holes 32 and 34. The size of bilumen dam 30 (as well as the size of prior art dam 26 and solder band 28) depends on the size of cable 10 and sleeve 24. 
     With further reference to FIGS. 7 and 8, lumen 32 provides for passage of coaxial cable 10, while lumen 34 accommodates ground lead wire 36. FIG. 7 depicts the placement of ground lead wire 36 through lumen 34, with the leads 38 of ground lead wire 36 placed adjacent solder band 28. FIG. 8 in turn illustrates how bilumen dam 30 surrounds both ground lead wire J6 and coaxial cable 10, insuring a complete seal once heat is applied and sleeve 24 shrinks (for clarity, shield 14, inner insulating layer 18 and lead wire 16 have been omitted from FIG. 8). The compression from the sleeve contraction causes bilumen dam 30 to seal about cable jacket 12 and lead wire 36. Heat may be applied by convection, infrared radiation, or any other convenient method. In addition to providing improved sealing ability, bilumen dam 30 also affords greater strain relief for the electrical connection. 
     It is contemplated that shield terminator 22 may be sold either with or without ground lead wire 36 in place. In other words, lead wire 36 may be inserted into lumen 34 before packaging, simplifying installation of the device. Also, if a ground connection is desired at the terminal end 16 of cable 10, rather than at some intermediate segment as heretofore discussed, shield terminator 22 can have a variable diameter, i.e., the diameter of sleeve 24 at prior art dam 26 will be smaller than at bilumen dam 30 in order to fit more tightly around the internal insulating layer 18 of cable 10. Additionally, if a connection needs to be made to the terminal end of a plain wire such as plastic insulated cable (PIC), rather than on a coaxial cable, then shield terminator 22 may take the form of a cap 39, such as that shown in FIG. 9, having only bilumen dam 30 and solder band 28 therein. 
     In certain applications, it is necessary to create a &#34;chain&#34; of connected lead wires to guarantee a common ground for several cables. In such a case, it is useful to have two ground lead wires connected to a single cable shield. This arrangement may be facilitated by the use of two bilumen dams 30 in shield terminator 22, one at each end. Alternatively, a trilumen dam 40, such as that shown in FIG. 10, may be employed. Trilumen dam 40 is simply substituted for bilumen dam 30 in shield terminator 22. Trilumen dam 40 has a first lumen 42 for passage of cable 10, and second and third lumens 44 and 46 which receive the two ground lead wires. FIG. 11 depicts a chain of cables connected by means of the trilumen dam shield terminators 48. As those skilled in the art will appreciate, dams having more than three lumens will also be useful where it is necessary to connect more than two ground wires to the cable. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover such modifications that fall within the true scope of the invention.