Coaxial electrical high-frequency cable

In a coaxial electrical high-frequency cable, a spacer is provided between the internal conductor and the external conductor in order to maintain the operation of the cable even in the case of elevated ambient temperatures. The spacer can be a helical profile or of individual shaped pieces made of glass or ceramic materials.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a coaxial electrical high-frequency cable 
with spacing means arranged between the internal and external conductors. 
2. Description of the Prior Art 
A coaxial high-frequency power cable is already known, in which a spacing 
is provided between the internal conductor and external conductor which is 
made of individual pieces consisting of ceramic. These individual pieces 
are arranged on the internal conductor staggered with respect to each 
other and individually screwed to the internal conductor (German Patent 
Application DE-OS 33 04 957). Although such a known cable is suitable for 
being largely operative under extreme conditions such as at the elevated 
ambient temperatures encountered during a fire, this design is very 
expensive in terms of production engineering. In addition, the fastening 
screws used to fasten the individual pieces to the internal conductor are 
frequently undesirable for electrical reasons. 
SUMMARY OF THE INVENTION 
Starting from the state of the art, an object of the invention is to create 
a coaxial high-frequency cable which is operative even under extreme 
conditions such as a fire, has low manufacturing costs and is free of 
additional materials, particularly metals, in the dielectric that have a 
harmful effect on the transmission quality. 
According to the invention, the cable is provided with a spacer having a 
helical profile, consisting of glass or ceramic materials, or 
predominantly contains these materials. A helical profile of this type can 
be applied with the customary machine systems presently available in cable 
technology, and electrical effect on the transmission properties by 
foreign materials is eliminated, and the cable according to the invention 
is operable under the extreme conditions of a fire. This is particularly 
advantageous, for example, in supplying power to an emergency call 
transmitter or a radiating high-frequency cable used for control or 
message purposes, such as is used for train monitoring, where the cable is 
arranged in a tunnel. It is also useful for supply or control cables that 
are installed on off-shore oil platforms and must be operable even in the 
event of a fire. 
The same advantages are obtained in another embodiment of the invention, in 
which the spacers are ceramic shaped pieces, which are held on the 
internal conductor by virtue of their shape. 
The helical profile made of refractory materials according to the invention 
can have any form, for example, it can be a profile strand, which is 
placed helically around the internal conductor of the high-frequency 
cable. In order to maintain a large air space between the internal and the 
external conductor of the fire-protected cable, the profile strand can be 
made up of braided individual strands of circular cross-section or of 
individual fibers. Several such profiles can, in turn, be stranded or 
braided with each other, for example, when it is desirable to bridge major 
distances between the internal and external conductors of the cable 
without disadvantageously affecting the flexibility of the cable. 
In certain situations, it may also be advantageous to provide an additional 
refractory coating if the helical profiles are made of the refractory 
materials. The additional refractory coating could be in the form of a 
braiding or woven fabric. Such an additional refractory coating is 
particularly advantageous when the helix is formed of individual strands 
stranded or knitted with each other. 
In another embodiment of the invention, the helical profile is provided by 
a plurality of profile bodies made of the glass or ceramic materials 
arranged in series on a support. The profile bodies themselves can be 
spheres, rollers or rolls which are fastened on an appropriate refractory 
support fiber or strand or have refractory fibers braided around them. 
If, as is also provided according to the invention, ceramic shaped pieces 
are used instead of the helical profile described, then these will be 
advantageously mountable to the internal conductor. Profile bodies of this 
type can be, for example, hollow cylinders with longitudinal slots leading 
from the ends to the center and displaced with respect to each other by 
more than 90.degree.. A preferable structure is one in which the 
longitudinal slots are displaced with respect to each other by 
180.degree.. This means that the profile bodies, which are prefabricated 
elements, are mounted in a transverse direction on the internal conductor 
of the coaxial high-frequency cable and are then rotated into alignment 
with the longitudinal axis of the cable. Once mounted in this fashion, the 
position of the profile body on the internal conductor can be fixed by the 
external conductor of the cable. 
Another embodiment of the shaped pieces is provided by radially-slotted 
disc-shaped spacers which are mounted transversely on the internal 
conductor, in which case it may be appropriate for the stability of these 
spacers on the internal conductor, to rotate adjacent discs uniformly with 
respect to each other. The spacers thus reliably hold the internal 
conductor in its central position and are finally fixed in their position 
on the internal conductor by means of the external conductor. 
Another appropriate form of the invention is obtained when the shaped 
pieces are injection-molded pieces completely surrounding the internal 
conductor. As in the case of other known cable constructions of the 
above-mentioned type, in which discs of plastic are sprayed 
discontinuously onto the internal conductor during manufacture, in the 
execution of the invention, for production of the ceramic injection molded 
pieces, an injection molding machine filled with the appropriate ceramic 
mass can be used to initially spray the spacers in disc form onto the 
internal conductor passing through. Following this, each disc is sintered, 
hardened or otherwise optionally finished. The usual further processing of 
internal conductors prepared in this manner and provided with the spacers 
to produce a cable can then be carried out in a known manner. 
As a material for the spacers according to the invention, known ceramics, 
in addition to glass, can be used. Such ceramics are also optionally 
permeated by glass fibers for mechanical reinforcement purposes. Also 
suitable in this connection are foamed ceramics, such as are known, for 
example, under the trademark POROTON. Since these materials are 
hygroscopic, it is recommended that the shaped pieces be provided after 
sintering with a moisture-repellant coating in the form of a varnish or a 
glaze. 
As already mentioned, the refractory cables according to the invention find 
a particularly advantageous use in the form of so-called radiating 
high-frequency cables, i.e., coaxial electrical cables whose outside 
conductors contain regular openings in the form of slots, holes, recesses 
and the like. If the external conductors are metal foils that are placed 
around the refractory spacers in a longitudinal manner, then these, as a 
rule, are not self-supporting, particularly if the tape edges leave a free 
longitudinal slot between them. In these or similar cases, it has 
therefore proven appropriate to fill the spaces between the internal and 
external conductors free of the spacers completely or in part with an 
insulating material. For a refractory product, the insulating material is 
then advantageously covered by a refractory foil or a tape, i.e., a 
glass-mica tape. Alternatively, the refractory spacers, helical profiles 
or shaped bodies according to the invention can be surrounded with a 
plastic tube, which is covered toward the outside by a refractory foil or 
an appropriate tape. The external conductor of the cable, also in a 
non-self-supporting form, is then arranged above this. 
However, even for cables with a self-supporting external conductor, where 
supporting elements in the form of insulating materials, such as a 
supporting tube, can thus be dispensed with, it may frequently be 
advantageous to surround the spacers, helical profile or shaped bodies 
with a closed layer of a refractory foil or a corresponding tape. This 
measure is of particular importance when the self-supporting external 
conductor is later provided with slots or holes and there is a danger that 
metal splinters, shavings and the like could pass into the dielectric.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, an internal conductor 1 of a coaxial high-frequency cable has a 
helix 2 placed therearound. The helix 2 is a glass cord of simple glass 
yarn, which is stranded, twisted or braided, in order to arrive at the 
cross-sectional dimensions required for a specific spacing between the 
internal and external conductors. This glass cord, as is indicated 
schematically, is surrounded by an outer braiding 3, for example, to 
increase the mechanical strength of a helical profile of this type. 
If, for example, large distances between the internal and external 
conductors of the high-frequency cables have to be bridged, then, in 
accordance with FIG. 2, it is also possible to group together several of 
the cords shown in FIG. 1, with or without braiding, to form a strand of 
larger cross-section. For this purpose, for example, in accordance with 
FIG. 2, three individual cords 2 are stranded or knitted together to form 
the shaped strand 4, which then, in turn, is applied to the internal 
conductor 5 of the high-frequency cable as a spacer for the concentric 
external conductor. 
It is important for the invention that, in contrast to previously 
conventional spacers made exclusively from polymeric materials, the 
materials used withstand elevated temperatures and do not melt, so that 
the operating properties of the cable are retained, at least for a certain 
time, even in the event of a fire. 
In addition to the helical profiles described in FIGS. 1 and 2, embodiments 
such as are exemplified in FIG. 3 are therefore also suitable. In FIG. 3, 
for example, spherical glass or ceramic bodies 8 are fastened on or to a 
support fiber 7. This combination is placed as a helix around the internal 
conductor 6 to maintain the spacing of the external conductor (not shown). 
As an alternative to this, FIG. 4 shows a helical spacer, provided by 
spherically shaped glass or ceramic bodies 9, which, in turn, is 
surrounded by a braiding 10 of glass or ceramic fibers. 
The areas between the internal conductor and external conductor (not shown) 
of the high-frequency cable free of the spacers can, particularly if the 
external conductor is no self-supporting, i.e., if it is, for example, a 
metal tape placed around the spacers, be filled with the customary 
materials, for example, with materials formed of polyethylene foam or 
other extrudable materials. 
As already explained, coaxial high-frequency cable is, for example, also 
used in supplying an emergency call transmitter or as a high-frequency 
cable used for control or message purposes. Such cable is used for 
monitoring trains and is located in tunnels. Cables of this type must 
still be operable if, after the outbreak of a fire, high ambient 
temperatures appear in the vicinity of the cable. While the operating 
properties of the known cables with spacers made of polymeric materials is 
not assured, because the spacers between the internal and external 
conductors melt at elevated temperatures, the spacers made of ceramic 
materials provided according to the invention assure that, even in the 
event of a fire, the spacing between the internal and external conductor 
required for cable operation is maintained. 
To provide reliable operation of the cable even at elevated temperatures 
encountered during a fire, the FIG. 5 embodiment includes a hollow 
cylindrical shaped piece 11 serving as a spacer, fastened to an internal 
conductor 12 of the high-frequency cable. In order to achieve this, the 
shaped piece 11 defines two longitudinal slots 13 and 14 running from the 
ends to the center. The slots 13, 14 end in a central recess 15. By means 
of the longitudinal slots 13, 14 and the central recess 15, it is possible 
to mount the shaped piece 11 transversely on the internal conductor and 
then, by rotation to align the shaped piece 11 in the axial direction of 
the internal conductor 12, to bring it into the position shown. Internal 
conductors prepared in this way, i.e., already equipped with the spacers, 
can then, in the usual manner, be surrounded by the external conductor in 
the form of a longitudinal copper band or a corrugated tube. The external 
conductor is applied to the shaped pieces 11 concentrically with the 
internal conductor 12. 
Turning now to FIG. 6, an embodiment of the invention is shown in which 
disc-shaped spacers 17 (only one shown) are mounted on the internal 
conductor 16 of the refractory coaxial high-frequency cable. Each spacer 
17 has a slot or opening 18, which is appropriate for the size of the 
internal conductor 16. The ends 19 and 20 of the spacer 17 formed by the 
slot or opening 18 are staggered in the direction of the internal 
conductor 16. This holds the spacer 17 securely on the internal conductor 
of the high-frequency cable. 
A further embodiment of the present invention is shown in FIG. 7. Here, 
disc-shaped spacers 22 are applied to the internal conductor 21 of the 
refractory high-frequency cable at intervals. The spacers 22 completely 
surround the internal conductor 21 and are applied by a spraying or 
injection-molding process. For this purpose, one can use known techniques 
to apply the spacers 22, such as simultaneously spraying a plurality of 
these disc-shaped spacers 22 onto the internal conductor 21. The thus 
formed spacers 22 are then sintered or hardened and possibly 
after-treated, while, at the same time but in a discontinuous process, an 
equal number of new shaped pieces is sprayed onto the internal conductor 
21 at a different location. 
In the event that the ceramic material used for the spacers according to 
FIGS. 5-7 is too hygroscopic to guarantee the required transmission 
properties of the cable according to the invention, the profile bodies 
already prefabricated according to FIGS. 5 and 6, and the profile bodies 
fastened to the internal conductor by a spray process according to FIG. 7, 
can be provided with a suitable coating. The profile bodies of FIGS. 5 and 
6 can be provided with such a coating before or after the application 
thereof to the internal conductor. 
Several embodiments for a high-frequency cable according to the invention 
with an external conductor that is not dimensionally stable are shown in 
FIGS. 8 and 9. 
According to FIG. 8, the internal conductor 23 is surrounded by the helix 
24 of refractory material and the spaces between the winds of the helix 24 
are filled, for example, with polyethylene foam 25. This insulating 
material simultaneously serves as a support for the external conductor 26, 
which is, for example, a slotted metal foil. In order to prevent an escape 
of molten polyethylene from the external conductors 26 in the event of a 
fire during which high temperatures prevail, a layer 27 is located between 
the insulating polyethylene 25 and the external conductor 26. The layer 27 
can be one or several strata of a refractory band, such as a mica-coated 
band of glass fabric. 
As an alternative to the embodiment according to FIG. 8, FIG. 9 shows a 
cable design in which the external conductor 30 is not dimensionally 
stable. An internal conductor 28 is surrounded by a helix 29 of refractory 
material, and the support of the external conductor 30 is provided by the 
extruded plastic tube 31 made of a suitable polyethylene. To assure that, 
at elevated ambient temperatures, molten polyethylene will not seep 
through radiation openings (not shown) in the external conductor 30, a 
refractory layer 32 is provided between the plastic tube 31 and the 
external conductor 30. The layer 32 can be a closed set of mica or 
glass-fabric tapes. 
Finally, in FIG. 10, a cable according to the invention with the property 
of radiating high-frequency signals and with a self-supporting external 
conductor is shown. A helix 34 of refractory material surrounds an 
internal conductor 33. On the helix 34 there is supported an external 
conductor 35 in the form of a closed and corrugated metal sheath. The 
metal sheath has radiation openings 36 which are produced later by milling 
along the surface of the external conductor 35. According to the 
invention, one or more layers 37 of mica tapes or coated glass-fabric 
tapes can be provided between the helix 34 and external conductor 35 to 
prevent the penetration of the metal residues formed during the production 
of the radiation openings 36 into the space 38 between the internal 
conductor 33 and the external conductor 35. 
In the embodiments of FIGS. 8-10, it is, of course, possible to use the 
spacing shaped bodies according to FIGS. 5-7 instead of the refractory 
helices. The important feature of the invention is that, in the case of 
elevated ambient temperatures, e.g., in the event of a fire, the 
transmission of high-frequency signals is possible. 
Thus, it can be seen from the foregoing specification and attached drawings 
that the present invention provides a unique means for maintaining the 
operation of the cable even in the event of elevated ambient temperatures 
caused by a fire or the like. 
The invention as described above admirably achieves the objects of the 
invention; however, it will be appreciated that the departures can be made 
by those skilled in the art without departing from the spirit and scope of 
the invention, which is limited only by the following claims.