(a) Field of the Invention
This invention relates to electrical cables, and more particularly, to coaxial electrical cables, for use where space is limited or where a thin flat cable cross section is preferred and where the cable is likely to be exposed to mechanical loads, either tensile or compressive. Typically, coaxial cables embodying this invention are envisioned for use under floor carpeting in areas where furniture is to be placed or where human or equipment traffic is anticipated.
(b) Background Art
Modern concepts in building construction have spurred a search for sturdy under-carpet cabling of all types. In response to the development of a format for safely installing electrical power wiring between floors and carpets, national electrical codes have been revised to permit electrical conductors to be located under carpets. However, until the advent of this invention, the particular requirements of the wiring needed to interconnect a significant class of office equipment had not been met with regard to under-carpet deployment.
Modern office operations are increasingly reliant for the performance of their accounting, library, and word processing functions upon the information handling and storage capacities of large central computers. To maximize the flexibility and potential of such costly machinery, multiple access is afforded to these computers through a system of peripheral individual terminals, each interconnected to the main computer by electrical cables. The preservation of the integrity of the information passing as electrical impulses upon such cables is a crucial requisite for the successful operation of such an extended system. This high fidelity transmission has been achieved in the past by making the interconnecting cables sufficiently sturdy to preserve their uniform impedance characteristics and by providing the conductor with coaxial shielding from external electromagnetic interference.
When an attempt is made, in conformity with current construction trends, to lay such cables under carpets, several difficulties arise. First, coaxial cables are generally of sufficient size that they will not permit a carpet covering them to lie flat. When previously produced in a small size, these cables, though fitting inconspicuously between carpet and floor, have been vulnerable to damage from mechanical stress applied to them due to bends in routing or to the ordinary use of the floor area that they serve. Loads set upon or traffic traveling over these thin cables tend to compress their cross section, while the twisting and bending required by their routing and subsequent movement of their ends or the floor covering produce tensile forces that also endanger their structural integrity.
Two types of resulting structural damage are common. First, deformation of either the dielectric surrounding the conductor core or of the coaxial shield enclosing the dielectric can change the electrical impedance characteristics in the area so affected. Such local distortions, even if temporary, can alter electrical signals then passing through the cable. Surprisingly, temporary deformation, as for instance, due to traffic on the carpet over the signal carrier, may be more troublesome in a computer system than is permanent damage to a cable. The irregularity of the loss of fidelity that occurs in a coaxial cable being subjected to intermittent temporary deformations may alert users that the system is unreliable without permitting a conclusive determination of the cause of the problem.
A second form of damage which mechanical loading can cause in under-carpet coaxial cables is the separation of either the coaxial shield or the conductive signal-carrying core. This will result in no transmission if the broken portions do not again contact each other. However, it is common that the broken parts do reengage one another, establishing erratic transmission, the cause of which is difficult to locate.
It is one object of the present invention to produce a flat coaxial cable thin enough to be installed beneath a carpet under current and proposed national electrical wiring codes.
A second object is to afford to such a cable sufficient flexibility within its transverse plane as to permit its easy routing and to insure that any such routing does not alter the electrical characteristics of the conductor.
A final objective of the present invention is to protect miniaturized conductors beneath carpets from damage due to compressive loads upon the installed cable.