The field of invention is coaxial cables having an inner conductor, a foam dielectric material formed about the inner conductor, and a shield formed about the dielectric material.
Coaxial cable is commonly used for many applications, such as transmission of radio frequency signals, cable television signals and cellular telephone broadcast signals. A coaxial cable of the type with which this invention concerns includes an inner conductor, a foam-type dielectric around the inner conductor, an electrically conductive shield surrounding the dielectric foam and serving as an outer conductor, and a protective jacket which surrounds the shield. The foam dielectric electrically insulates the inner conductor from the surrounding shield.
Commercially available coaxial cables which address the cost-sensitive mass market (exclusive of special purpose cable products) comprise basically four types: 1) braided shield cable; 2) smooth-walled cable; 3) annular corrugated cable; and 4) helical corrugated cable.
Braided shield cable is the lowest cost product and has excellent flexibility, however, it suffers badly in electrical properties. The braided shield has poor shielding effectiveness due to the porous woven nature of the shield, and typically requires the addition of a conductive foil under the braided shield to achieve even marginally acceptable shield effectiveness. Further, braided shield cable is ineffective in resisting intrusion of fluids, as the braid will actually “wick” fluids through the cable. The water blocking properties of braided shield cable can be improved by impregnating the braid with heavy grease, however this step raises the cost of the product. The braided shield is a loose braid that results in inconsistent contacts that creates non-linear joints. The effect of this is intermodulation, which is a type of noise or interference that is injected into the cable. Furthermore as noted, “waterproofing” of braided cable requires the addition of a grease type material with the braid. However, this is a drawback in that it results in difficulty is attaching connectors to the cable, because the grease is emitted by the cable during attachment of the connector. Also, over time the cables are known to leak grease due to cracks or damage to the cable, and create an environmental problem.
“Smooth-walled” cable, as it is termed, typically comprises an aluminum tube as a shield and outer conductor. It is more costly than braided shield cable, however, because the shield is a solid tube, the shield effectiveness of this cable type is excellent. This product, however, has poor flexibility, requiring special tools to bend it, and suffers from intolerable kinking if the bends are not formed properly. Any such kinking dramatically impairs the electrical properties of the cable. Smooth-walled cable shields are welded using an HF (high frequency) welding process, as HF welding permits much faster line speeds than the TIG (tungsten inert gas) welding process universally used in the manufacture of helical and annular corrugated cable (to be described).
Near the high end of commercial coaxial cable is helical corrugated cable. Helical corrugated cable has a shield composed typically of copper. To form the shield, copper sheet, is wrapped around a foam dielectric core and welded. The welded copper tube is then corrugated using a corrugating die, which spins around the tube and imparts the corrugations as the tube is advanced. This “single lead” corrugation process necessitates much slower line speeds than is possible with smooth-walled cable, but results in a much more flexible product than smooth-walled cable.
The use of copper as the shield material and the typically slow corrugation process drive up the cost of helical corrugated cable, however, its superior electrical and mechanical properties compensate in many applications for the increased cost. Helical corrugated cable suffers, however, by having less-than-optimum water blocking properties. Because the helical convolutions formed in the cable shield inherently create an uninterrupted passageway along the cable between the shield and the foam dielectric, water or other fluids entering the cable easily migrate along the cable. For this reason, helical corrugated cable is not recommended for use underground or in other aqueous environments.
At the high end of the four basic types of mass-marketed foam cable is annular corrugated copper cable. This product has all the attributes of helical corrugated copper cable, and in addition has improved water-blocking capability. Conventional copper annular corrugated cable with a foam dielectric, during its manufacture, has a tubular shield welded around foam dielectric with a space provided between the shield and the dielectric. The space is needed to permit the “gathering” of the tubular material, as in the manufacture of conventional copper helical corrugated cable. This space commonly leads to the capturing of air within the annual corrugations formed. However, despite the air gaps thus formed, because the corrugations are annular, like 360-degree rings, which contact the dielectric foam, each ring acts as a sort of seal, resists water migration. The superior water blocking ability of annular corrugated cable, relative to helical corrugated cable, permits it to be used underground and in more demanding aqueous environments than helical corrugated cable. Further, for a given depth of corrugation, annular corrugated cable is somewhat more flexible than helical corrugated cable.
However, there is a price to be paid for the improved water blocking and flexibility of annular corrugated cable compared with helical corrugated cable. The process of forming annular corrugations is much slower than the process of manufacturing helical corrugations. The resulting slower line speeds add significant manufacturing cost. For example, typical industry line speeds for corrugating annular shield cable may be 50 percent slower than industry line speeds for corrugating helical shield cable. Furthermore, the annular corrugating process does not lend itself to producing high pitch-to-depth ratio cable. Accordingly, annular corrugated cable tends to be less flexible than helical corrugated cable.
Until the present invention, we know of no product which meets all four of the desired foam coaxial cable attributes: 1) low cost; 2) electrical properties including shield effectiveness and intermodulation suppression comparable to that of solid tubular shielded cable; 3) mechanical properties, primarily flexibility, comparable to corrugated cable; and 4) water blockage comparable to annular corrugated cable.