FLEXIBLE SHIELDED CABLE AND METHOD OF MANUFACTURE

Abstract: A flexible shielded cable (20) includes at least one elongate flexible metal conductor (24) and a layer (26) of a flexible di­ electric material disposed about the conductor. The cable has a flexible metallic shield (28) positioned about the dielectric with the shield including a copper foil (30) having overlapping edges (36) and a copper braid (32) about the foil. A layer (34) of metal bonds together the overlapping edges, bonds the braid and the foil and closes the openings of the braid.

Description: The present invention relates to electrical cables and, more specifically, to a flexible coaxial cable having excellent shield effectiveness over a broad frequency range. Shielded cables are typically classified as flexible, semirigid or rigid, with cables having greater rigidity typically having more predictable electrical properties. A flexible shielded cable usually has a shield formed of braided copper. While such a shield may perform satisfactorily at low frequencies, the openings in the braid permit high frequency energy transfer thus limiting the use of such cables. A common type of semirigid coaxial cable includes a copper tubing into which the core assembly (made up of the central conductor and its dielectric jacket) is inserted. This type of coaxial cable is relatively expensive because it is not manufactured in a continuous process. A length of the core assembly is inserted into a length of the tubing, and the tubing shrunk by swaging resulting in a tight fit. While the formed copper tubing does provide a smooth, continuous inner shield surface for effective shielding over a wide frequency range, it does have severe mechanical shortcomings. This type of coaxial cable is relatively heavy, it is not very flexible, and special tools are required for bending without kinking or breaking the shield. The use of the copper tubing, which has minimum elasticity, also limits the maximum operating temperature of the cable. A recently proposed coaxial cable includes a layer of conductive or semi-conductive matter surrounding the dielectric. A shield, which may be a braid, is embedded in the layer which is softened by heating. For further information regarding the structure and operation of this cable, reference may be made to US-A-4,486,252. GB-A-2130430 discloses a flexible shielded cable comprising at least one elongate, flexible metal conductor; a layer of a flexible dielectric material disposed about the conductor; and a flexible metallic shield disposed about the layer, the shield including a copper foil having overlapping edges, and a copper braid about the foil; and, according to the present invention, such a cable is characterised in that the shield also includes a layer of metal which closes any opening between the overlapping edge, bonds the braid and the foil and closes interstices of the braid whereby the shield is flexible and has no openings therein. The invention also includes a method of forming a metallic shield about at least one flexible metal conductor encompassed by a layer of dielectric material to form a flexible coaxial cable, the method comprising wrapping a copper foil about the layer of dielectric material so that the foil has overlapping edges; and applying a copper braid over the foil; characterised by passing the braided cable through a bath of a molten metal which bonds to the braid and the foil so that any opening between the edges of the foil is closed and interstices of the braid are closed. The new cable offers effective shielding over a wide frequency range and can undergo relatively sharp bending without the use of any special tools and without damage to the shield. The cable also is usable at higher operating temperatures than copper tubing coaxial cables. Additionally, the cable can be made in very long continuous lengths as opposed to semirigid cable with a solid copper tubing shield, which is limited in length because a length of dielectric core must be shoved into the copper tubing prior to swaging. The cable has long service life, is reliable in use and is easy and economical to manufacture. In the accompanying drawings: FIG. 1 is a cross-sectional view of a shielded cable embodying various features of the present invention; FIG. 2 is a perspective view of the cable of FIG. 1, with various components removed to illustrate underlying components, having a shield made up in part by a longitudinally wrapped foil; FIG. 3, similar to FIG. 2, illustrates an alternative embodiment of the shielded cable of the present invention wherein the foil is helically wound; FIG. 4 is a diagram illustrating application of the foil and application of a braid about the core assembly of the cable of FIG. 1; FIG. 5 is a diagram, partly block in nature, depicting application of solder or tin which bonds the braid to the foil and closes the openings of the braid; and FIG. 6, similar to FIG. 1, illustrates another alternative embodiment of a cable embodying various features of the present invention wherein a plurality of insulated conductors are present in the core assembly. FIGS. 1 and 2 show a cable 20 having a core assembly 22 made up of an elongate, flexible central metallic conductor 24 which is preferably copper and could be either solid or made up of a number of strands. While only a single conductor 24 is illustrated in the core assembly in FIGS. 1-3, it will be appreciated that a number of conductors insulated from each other, could be included. Encompassing the conductor 24 is a flexible layer 26 of dielectric material in intimate contact with the conductor. Disposed about the dielectric layer 26 is a flexible metallic shield 28 made up of a copper foil 30, a copper braid 32 about the foil 30 and a layer 34 of metal such as solder or tin which bonds the braid 32 to the foil 30 and closes the openings or interstices of the braid. As best shown in FIG. 2, the foil 30 has overlapping, longitudinally extending edges 36. The layer 34 of metal also bonds the overlapping edges 36 together to provide the shield 28 with an inner surface 37 which is substantially smooth and has no openings through which energy could be radiated. It will be appreciated that this approximates the smooth inner surface of the copper tube of a semirigid coaxial cable. Thus the shield 28 greatly reduces undesirable energy or signal transfer through the shield due to electrical, magnetic or electromagnetic fields. The cable 20 can be used over a broad frequency range, from dc to 20 gigahertz. Grounding of the shield 28 results in predictable cable impedance and signal attenuation. More specifically, the copper foil (which preferably has a thickness in the range of .003 to .0003 inch (.076 to .0076 mm) functions to limit high frequency signal penetration. It will be appreciated that the only discontinuity in the foil, where the edges 36 overlap, extends in the axial direction of the cable. Current tends to flow in the direction of the discontinuity. Because the discontinuity does not take an arcuate path, there is no substantial increase in inductive signal coupling through the shield 28 due to the presence of the discontinuity. The braid 32 functions to limit penetration of low frequency signals. The use of the braid 32 over the foil 30 results in low radio frequency leakage and low susceptibility to electrical noise. The braid 32 being bonded to the foil 30 by the metal layer 34 also offers several mechanical advantages. The presence of the braid prevents tearing of the foil when the cable 20 is bent. Furthermore, the braid offers a degree of elasticity, permitting the cable to have a higher operating temperature than an otherwise comparable semirigid cable incorporating a shield of copper tubing. The prior art cable is limited to an operating temperature of about 150°C because the tubing has minimal elasticity so that any substantial expansion of the dielectric must be in the axial direction. Operation of this prior art cable at higher temperatures can result in damage to the tubing and/or to other components of the cable. The cable 20 of the present invention has a maximum operating temperature of about 200°C because the braid provides a greater degree of elasticity, allowing some radial expansion of the dielectric layer 26. The dielectric layer 26 is preferably formed of a flexible thermoplastic polymer such as Teflon (a registered trademark of DuPont for synthetic resins containing fluorine), polyethylene, polypropylene and cellular forms thereof. The layer of metal 34 is applied by passing the incipient cable through a molten bath of tin or solder. This causes the molten metal (which is drawn in by wicking action - capillary attraction) to fill the braid openings and to close any hairline opening between the overlapping edges 36. During the application of the molten tin or solder component, the copper foil 30 functions as a heat barrier to insulate the dielectric material from the high temperature of the molten metal. But for the foil, the molten metal would directly contact the core insulation material. The use of the foil 30 allows polymers having less heat resistance than Teflon to be used for dielectric layer 26 because the foil conducts heat away from layer 26. The cable 20 is flexible and can be bent without the use of special tools such as are required to prevent kinking or breaking of the cable having a copper tubing shield. Due to its flexible components, the bend radius of the cable 20 is approximately equal to the outside diameter of the cable which is preferably in the range of .047 inch to .50 inch (1.194 to 12.7 mm). Referring to FIG. 4, there is shown the application of the foil 30 and the braid 32 about the core assembly 22. After the core assembly is taken off a pay-out reel 38, it passes through a first station 40 which applies the foil wrapping 30, taken from a foil pay-out reel 42, so that the edges 36 of the foil overlap. Next the partially completed cable passes through a second station 44 which weaves strands of copper wire, taken from a plurality of wire spools 46, to form the braid over the copper foil 30. The incipient cable next is taken up on a reel 48. Idler wheels 50, 52 and 56 are provided for guiding the core assembly 22, the foil 30 and the cable with the foil wrapping and the braid, respectively. As shown in FIG. 5, the reel 48 can be used as the pay-out reel for the tin or solder application. The foil wrapped, braided incipient cable passes through a bath 56 of molten solder or tin. Because the incipient cable is submerged in the molten metal, the interstices of the braid 32 are filled, the braid is bonded to the copper foil 30, and the hairline opening due to the presence of the overlapping edges 36 of the foil is closed. Finally, the shielded cable 20 passes through a cooling station 58 and then is taken up on a reel 60. It is not economically feasible to combine the foil wrapping station, braiding station and tin or solder application in a single, continuous process because the several stations operate at greatly differing speeds. The braid application station, with its weaving function, is by nature the slowest. However, the cable 20 is made in very long continuous lengths compared to semirigid cable with the solid copper tubing shield, which is limited because a length of dielectric core must be pushed into the copper tubing prior to swaging. Referring to FIG. 3, an alternate embodiment of the cable of the present invention is shown by reference character 20A. Components of cable 20A corresponding to components of cable 20 are indicated by the reference numeral applied to the component of the cable 20 with the addition of the suffix "A". The primary difference between cable 20A and cable 20 is that the foil 30A is applied helically so that the overlapping edges 36A of the wrapped foil form an arcuate path. The presence of this arcuate path, along which current tends to flow, may result in undesirable inductive signal coupling through the shield 28A reducing shield performance at higher frequencies. Another alternative embodiment of the cable of the present invention is shown by reference character 20B in FIG. 6. Components of the cable 20B corresponding to components of cable 20 are indicated by the numeral applied to the component of the cable 20 with the addition of the suffix "B". In the cable 20B, the core assembly 22B is made up of several conductors 24B, which could be either solid or formed of a number of strands. Each of the conductors has a jacket 62 of flexible insulation. Encompassing the conductors 24B is a flexible layer 26B of dielectric material tightly holding the conductors which may run in parallel relationship or may be cabled, twisted about the axis of the cable. The remainder of the cable 20B is substantially identical in construction to cable 20.

Claims: A flexible shielded cable (20) comprising at least one elongate, flexible metal conductor (24); a layer of a flexible dielectric material disposed about the conductor; and a flexible metallic shield (28) disposed about the layer, the shield including a copper foil (30) having overlapping edges (36), and a copper braid (32) about the foil; characterised in that the shield also includes a layer (34) of metal which closes any opening between the overlapping edges (36), bonds the braid (32) and the foil (30) and closes interstices of the braid (32) whereby the shield is flexible and has no openings therein. A cable according to claim 1, wherein the overlapping edges (36) of the foil (30) extend longitudinally. A cable according to claim 1, wherein the overlapping edges (36A) are helical. A cable according to any one of the preceding claims, wherein the layer (34) of metal is solder. A cable according to any one of claims 1 to 3, wherein the layer (34) of metal is tin. A cable according to any one of the preceding claims, having an outer diameter in the range of 1.194 to 12.7 mm. A cable according to any one of the preceding claims, wherein the dielectric material (26) is a thermoplastic. A cable according to any one of the preceding claims, wherein the conductor (24) and the shield (28) are coaxial. A cable according to any one of claims 1 to 7, wherein a plurality of the flexible conductors (24B), each insulated from the other conductors, are encompassed by the layer (26) of flexible dielectric material. A method of forming a metallic shield (28) about at least one flexible metal conductor (24) encompassed by a layer (26) of dielectric material to form a flexible coaxial cable, the method comprising wrapping a copper foil (30) about the layer (26) of dielectric material so that the foil has overlapping edges (36); and applying a copper braid (32) over the foil; characterised by passing the braided cable through a bath (56) of a molten metal which bonds to the braid (32) and the foil (30) so that any opening between the edges (36) of the foil is closed and interstices of the braid (32) are closed.

Inventors: VISSER, LEONARD JAMES
Applicants: COOPER INDUSTRIES, INC.
Application year: 1987
Main classification: H01B11
IPC codes: H01B11, H01B7, H01B13