Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1998-05-26
2003-07-29
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S383000, C428S902000, C428S920000
Reexamination Certificate
active
06599626
ABSTRACT:
TECHNICAL FIELD
This invention relates to a coaxial cable construction, and, particularly, the dielectric insulation layer thereof
BACKGROUND INFORMATION
Coaxial cable is comprised of an inner conductor, typically copper or copper clad steel or aluminum; a dielectric insulation layer; and an outer conductor, for example, aluminum foil with aluminum or copper braid or tube. Signal attenuation in coaxial cables is a direct function of dissipation factor and dielectric constant of the dielectric layer, as described in the following equation:
&agr;=0.002387
[e
0.5
/(log
D
o
/D
i
)][
P
o
0.5
/D
o
)+(
P
i
0.5
/D
i
)]
f
0.5
+1.506
f
(
df
)
e
/(log
D
o
/D
i
)
wherein:
&agr;=attenuation in db/100 feet
D
o
=outside diameter of insulation in inches (inside diameter of outer conductor)
D
i
=inside diameter of insulation in inches (outside diameter of inner conductor)
P
o
=resistivity of outer conductor in micro-ohm-cm
P
i
=resistivity of inner conductor in micro-ohm-cm
e=dielectric constant of insulation
f=frequency in megahertz
df=dissipation factor of insulation in radians
Since polyethylene has excellent electrical properties, i.e., low dielectric constant and very low dissipation factor, it is one of the few materials that can be used as dielectric insulation in a coaxial cable. As the performance of coaxial cable continues to be pushed to higher frequencies where attenuation losses become more significant, small differences in insulation dissipation factor are increasingly critical to optimum cable performance.
In the most demanding coaxial cable applications, where it is desirable to transmit the electrical signal with as little loss or signal attenuation as possible, it is necessary to replace a portion of the dielectric insulation layer material with gas. This is normally achieved by injecting an inert gas such as nitrogen or argon during extrusion to create a foamed dielectric. With time, the inert gas may be slowly replaced by air through diffusion. Alternatively, a polymer dielectric comprising a tube with spacer disks or spiral spacers can be incorporated between the inner and outer conductors to provide gas (usually air) containing compartments, and hence reduce the dielectric constant. In the present case, the term “dielectric insulation” is used to describe all variations containing a mixture of gas and solid in the dielectric insulation layer.
Coaxial cables containing polyethylene or another resin in the dielectric layer usually require antioxidants to provide protection against loss of physical properties over time caused by oxidative degradation. Inclusion of antioxidants in the insulation has been considered a trade-off since there is usually a negative impact of such additives on the dissipation factor of the insulation, adversely affecting the initial cable electrical properties. Coaxial cables with dielectric insulation are typically stabilized with primary antioxidants, preferably those which were non-polar since it was believed that polarity was one cause of this negative impact. In any case, industry is seeking a coaxial cable construction, which provides long term thermal stabilization, which is at least as good as currently available coaxial cable containing typical primary antioxidants, together with substantially better electrical properties particularly low dissipation factor.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a coaxial cable construction, which is thermally stable over long periods of time and has a low dissipation factor. Other objects and advantages will become apparent hereinafter.
According to the present invention, the object is met by a coaxial cable construction comprising (i) an inner electrical conductor comprising a single electrical conductor or a core of two or more electrical conductors; (ii) dielectric insulation comprising an inert gas or air and a solid, said solid comprising (a) a polymer selected from the group consisting of polyethylene, polypropylene, fluoropolymers, and mixtures of two or more of said polymers and (b) an alkylhydroxyphenylalkanoyl hydrazine; and (iii) an outer electrical conductor.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The coaxial cable of the present invention can be designed in various ways. One design includes an inner conductor coated with a foam dielectric insulation layer and an outer conductor covering the dielectric layer. An alternate design can be referred to as a disc and air design. In this case, the dielectric insulation layer is comprised of spaced solid polymeric discs molded onto the inner conductor. Typically, there are about six discs per foot of cable. The discs are about two inches apart thus forming adjacent compartments about two inches in length. A solid polymeric tube is extruded over the discs to hermetically seal the air space from adjacent compartments.
Both of these cable designs are used in applications where their low signal loss at high frequency provides a particular advantage. These applications include CATV cable for drop, distribution, and trunk; radio frequency cable for mobile telephones and two way radio; and various other communication cables.
Optionally, the coaxial cable can also contain an outer jacket, one or more layers of adhesive material, one or more flooding compounds, one or more braids, an armor layer, and a support member.
The inner (or core) conductor is usually a single electrical conductor, but can be several electrical conductors stranded together. The core conductor ranges in diameter from about 0.01 to about 2 inch for a single conductor. The inner conductor is typically made of copper, aluminum, copper clad aluminum, or copper clad steel and can be a solid or hollow tube, corrugated or smooth.
The dielectric insulation can be a solid or semi-solid expanded by chemical or physical means to produce a material that has a reduced dielectric constant. Conventional processes can be used to prepare foamed or expanded dielectric insulation. Such processes are described in U.S. Pat. Nos. 3,968,463; 3,975,473; and 4,107,354. The insulation outer diameter ranges from about 0.1 to about 4 inches. Materials which have outstanding electrical properties are preferably used in this application, i.e., polyethylene, polypropylene, fluoropolymers, and blends of these materials. The dielectric insulation is expanded by chemical or physical means, with the latter preferred for superior electrical properties. It is uniformly applied over the inner conductor and preferably has a uniform cell distribution with cells that fall in the range of about 1 micron to about 100 microns. Alternatively, the cable design can be such that high levels of air or other gas are incorporated into the design as in the disc and air design referred to above. The same materials are used for the dielectric insulation in the disc and air design or other coaxial cable designs as are used for the coated design.
Using certain simplified approximations, the velocity of propagation, Vp, for a coaxial cable is estimated using the following equation:
V
p
=
1
DC
*
100
%
wherein DC is the dielectric constant of the insulation layer. The velocity of propagation, which provides an indication of the degree to which the insulation material is expanded, ranges from about 75 to about 90 percent for the cables of interest. It is essentially a measure of how fast the signal travels in the cable versus how fast it would travel in a vacuum.
The outer conductor is normally a thin metal layer approximately 0.001 to 0.2 inch in thickness. It must conduct electricity and is usually made of copper or aluminum. The outer conductor can be made by welding or extruding aluminum or copper tape to form a tube and can then be corrugated for additional cable flexibility. Alternatively, it can be comprised of an aluminum or copper braid or foil/braid combination. The braid is used to provide flexibility and some radio frequency shielding. The outer conductor is bonded with an a
Cogen Jeffrey Morris
Maki Sandra Germaine Mary
Kelly Cynthia H.
Shewareged B.
Union Carbide Chemicals & Plastics Technology Corporation
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