The present invention relates broadly to electromagnetic interference (EMI) shielding, and more particularly to a heat-shrinkable sheathing or other jacket for shielding a wire, cable, data or signal line, antenna, or other electrical conductor or elongate article.
The operation of electronic devices such as computers, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry and transmission lines of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed "electromagnetic interference" or "EMI" as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other "target" devices from other source devices. In this regard, the shielding is provided as a barrier which is inserted between the source and the other devices. For the electronic device itself, the shielding typically is configured as an electrically conductive and grounded housing which encloses the device. For the electrical wire, cable, cord, or other conductor which supplies power to the device or transmits signals to or from the device, shielding has been provided in a similar manner by enclosing the conductor within an electrically conductive conduit or other housing.
A more flexible and less cumbersome shielding has been provided by sheathing the conductor, which typically includes an inner conductive core and an outer layer of an electrically-insulating, dielectric material, within a tubular shielding layer woven or braided of a metal wire or other electrically-conductive fiber. For example, Hardie et al., U.S. Pat. No. 5,483,020 discloses a parallel pair cable for high data signal transmission. The cable has a pair of parallel conductors which are surrounded by an insulating dielectric layer. The dielectric, in turn, is surrounded by a braided metal shield of a plated electrical conductor, which shield itself may be covered with an optional jacket.
Singles et al., U.S. Pat. No. 5,477,011 discloses a low-noise signal transmission cable which employs an insulative layer that is bonded to a surrounding shield layer via an adhesive. The shield may be a braided metal, conductive polymer, or wrapped foil layer.
Gebs, U.S. Pat. No. 5,293,001 discloses a flexible, shielded cable assembly. The assembly includes a flexible metal conductor, a dielectric layer positioned about the conductor, and a flexible metallic shield disposed about the dielectric. The shield preferably employs a thin metallic foil and a metallic braid, ribbon, or tape disposed about the foil.
Davies, U.S. Pat. No. 5,043,530 discloses a shield cable which includes an internal conductive core with 1-4 wire leads, each of which are insulated with a wrapping of an insulative tape. The voids between the leads are filled with an amorphous elastomer. A shield layer is provided by braiding a silver-copper alloy wire over the elastomer-covered conductive core. The strands of the shield become embedded in the elastomeric material which thereby fills the spaces in the braided structure. A barrier of an insulative jacket surrounds the elastomer-covered shield and conductive core.
Vaupotic et al., U.S. Pat. No. 5,015,800 discloses a controlled impedance transmission line which consists of a flexible cable having a side-by-side pair of conductors. The conductors are surrounded by respective inner and outer dielectric layers. A braided wire shield surrounds the dielectric layers, which shield, in turn, is surrounded and penetrated by an exterior jacket.
Pithouse et al., U.S. Pat. No. 4,639,545 discloses a conductor which is surrounded by a dielectric. A fabric in the form of a tubular sleeve is woven or positioned around the conductor. The fabric may include a conductive metal warp and a recoverable polymeric weft, which weft is recovered to secure the fabric through engagement with the dielectric.
Maul et al., U.S. Pat. No. 4,376,229 discloses a flexible, shielded electrical conduit. The conduit includes a flexible tubing, a flexible electrical shielding disposed within the tubing, and an axially compressed, radially expanded elastic woven retainer which forces the shielding into continuous contact with the tubing. The shielding may be provided as a weave of electrically conductive filaments.
More recently, proposals have been made to provide shielding for wires, cables, lines, and the like in the form of a tubular, heat-shrinkable outer layer within which is received a conductive inner layer. For example, Derby, U.S. Pat. No. 3,576,387 discloses a heat-shrinkable shield formed of an outer layer of a heat-shrinkable tubing having an thin layer of a metal-filled polymeric matrix bonded to the inner surface thereof. The shield may be sheathed over an insulated wire or cable, and then heated to shrink the outer layer of the shield over the insulation of the wire or cable.
Lau et al., U.S. Pat. No. 5,106,437 discloses a conformable electromagnetic radiation suppression cover for a reflecting structure. The cover includes a tubular outer layer of an electromagnetic radiation absorber formed of a nonconductive composite with one or more kinds of dissipative particles dispersed in a shrinkable dielectric binder. An inner sealant layer is employed to fill any voids between the absorber and the structure. A thin metallic foil may be bonded between the sealant and the absorber as a ground plane.
Nakamura et al., U.S. Pat. No. 4,555,422 discloses a magnetic shielding article which includes a heat-shrinkable outer layer of a thermoplastic polymeric material and an inner layer of a magnetic shielding layer. The shielding layer may be formed of a thermoplastic material filled with a powdered ferrite.
Landry et al., U.S. Pat. No. 4,915,139 discloses a another heat-shrinkable tubing article. Such article is formed of an outer layer of a tubular, heat-deformable material having a thermoplastic melt liner which is bonded to the inner surface thereof. A fiber reinforcement layer is disposed between the melt liner and the shrink tubing.
A representative commercial heat-shrinkable tubing shielding is marketed under the name "CHO-SHRINK.TM." by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass. The shielding is formed by coating a conductive compound onto the outer surface of a heat-shrinkable tube. Another such product is manufactured by Raychem Corp., Menlo Park, Calif., as a heat-shrinkable tube electroplated with an outer layer of a conductive metal.
The above-described references and products have garnered acceptance for general use, and heretofore have constituted the state of the art with respect to the EMI shielding of wires, cables, data or signal lines, antennas, electrical conductors, and the like. It will be appreciated, however, that continued improvements in such shields would be well-received by the electronics industry. A preferred shielding would be of low cost and economical to manufacture, and would provide a uniform surface which resists cracking and other mechanical damage within rigorous service environments.