US 3771934 A
A submersible coaxial cable has a plurality of longitudinal air spaces symmetrically spaced in the dielectric material around the inner conductor with sections collapsed at regular intervals along each air space to form water blocks in the passageways. Apparatus is provided to extrude the dielectric in the desired form and collapse the walls at selected intervals.
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Description (OCR text may contain errors)
United States Patent 1191 Delves-Brougliton 1 Nov. 13, 1973  APPARATUS FOR EXTENDING 2,708,176 5/1955 Rhodes 264/209 X WATER-BLOCKED CARTWHEEL CABLE  Inventor: Paul N. Delves-Broughton, FOREIGN PATENTS OR APPLICATlONS S h w l l d 16,542 7/191 1 Great Britain 425/1 13 811,703 4/1959 Great Britain 425/1 13  Ass1gnee: International Standard Electric Corporation, New York, N.Y.
Filed: Jan. 11, 1971 Appl. No.: 105,598
Related U.S. Application Data Division of Ser. No. 800,178, Feb. 18, 1969, Pat. No. 3,588,313.
U.S. Cl 425/135, 264/209, 425/155, 425/380, 425/465 Int. Cl B29f 3/03 Field of Search 425/113, 380, 465, 425/135, 155; 264/173, 209
Schanz 264/209 X Primary Examiner-R. Spencer Annear Attorney-C. L. Johnson, Jr.
 ABSTRACT A submersible coaxial cable has a plurality of longitudinal air spaces symmetrically spaced in the dielectric material around the inner conductor with sections collapsed at regular intervals along each air space to form water blocks in the passageways. Apparatus is provided to extrude the dielectric in the desired form and collapse the walls at selected intervals.
5 Claims, 10 Drawing Figures SHEET 2 OF 3 PATENTEU NOV 13 I975 PATENTEBNUV 13 ms SHEET 3 OF 3 This application is a division of an earlier pending ap- I plication Ser. No. 800,178, filed Feb. 18, l969and now issued as U.S. Pat. No. 3,588,313.
BACKGROUND OF THE INVENTION.
1. Field of the Invention This invention relates to coaxial cables having an air spaced dielectric.
2. Description of the Prior Art In a coaxial cable of a given characteristic impedance and attenuation, a smaller overall diameter of the dielectric and hence of the outer conductor and sheath can be obtained by using cellular polythene or known air-spaced cart wheel designs instead of a solid extruded polythene construction.
Cellular polythene, if used for buried installations, absorbs moisture unless a high integrity welded metal outer conductor is used, which adversely affects the transmission characteristics. Likewise the air-spaced configuration is prone to flooding of the intersticial air passageways should a joint fall-or should damage occur to the sheath when using a non-welded outer conductor.
SUMMARY OF THE INVENTION According to the present invention there is provided.
a coaxial cable having a center conductor, an outer conductor surrounding the center conductor and spaced therefrom by an extruded dielectric, and a plurality of longitudinal passageways in the dielectric each.
The invention also provides apparatus for manufac-.
turing a coaxial cable having a dielectric formed by extrusion around a moving center conductor, including an extrusion tool having a plurality of members each for the formation of alongitudinal passageway in the extruded dielectric, a vent tube in each member which normally communicates at one end with the atmosphere and communicates during extrusion at the other end with the passageway extruded around the member, and means to close or partially evacuate each vent tube during extrusion so that the associated passageway becomes blocked with extrudate overa region adjacent the end of the tool.
Embodiments of'the invention will now be described by way of example, with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) shows a portion of air-spaced coaxial cable;
FIG. 1(b) shows the portion of cable of FIG. 1(a) formed with a water block; 7
FIG. 1(c) shows a section through the water block of FIG. 1(b);
FIG. 1(d) is a longitudinal. cross-sectional view through the water block of FIG. 1(b);
FIG. 2(a) shows an extrusion tool for forming cable having a dielectric cross-section as shown in FIGS. 1(a) to 1(d);
FIG. 2(b) is a longitudinal cross-section of the extrusion tool of FIG. 2(a);
FIG. 3 is a cross-sectional view of an alternative type of air-spaced coaxial cable;
FIG. 4 is an extrusion tool for forming the cable of FIG. 3;
FIG. 5 shows a system of vent valves for forming the water blocks in the cable of FIGS. 1(a) to 1(d); and
FIG. 6 shows in detail the construction of the vent valves shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1(a) the air-spaced coaxial cable includes a center conductor 1, an outer tubular conductor 2, shown as a broken line, and a dielectric 3 of cart wheel cross-section between the conductors. The spokes of the cart wheel define air passageways 4 therebetween which extend along the length of the cable. As mentioned above there is a danger of flooding of these passageways should damage occur to the cable.
FIGS. .l(b), 1(c) and 1(d) show how a passageway can be blockedto restrict the passage of water therealong in the event of such damage. A portion 5 of the outer wall of the dielectric is collapsed over a limited region onto the two spokes 6.and 7 forming the other two sides of the passageway. This operation is carried out during extrusion of the dielectric while the latter is still soft, as will be described below. Each passageway is blocked at regular intervals along its length; thus longitudinal ingress of water beyond a certain amount in each direction is prevented.
Referring now to FIGS. 2(a) and 2(b), the extrusion tool for forming the dielectric includes an extended nose 8 having five segments such as 9. The slots between each segment serve to form the spokes of the cart wheel dielectric. A central bore 10 is provided in the nose 8 along which the center conductor passes durin'gextrusiomThe bore increases in diameter over a region 11 adjacent the end ofthe nose and communicates with the slots over thisregion. The inner tubular wall of the cart wheel dielectric is formed in this region by extrusion between the center conductor and the inner surfaces of the segment. The outer tubular wall of the dielectric isformed by extrusionbetween the outer surfaces of the segments and the collar 12.
During operation, extrudate flows into the extrusion tool in the direction shown by the arrows in FIG. 2(a). At the same time the center conductor is passed at a predeterined rate along the bore 10, and the cart wheel shaped dielectric is formed as described above. A vent tube, such as 13, communicating with the atmosphere at the rear of the extrusion tool is provided in each segment. By this means the air pressure in the passageways formed by the segments is equalized to the pressure of the atmosphere.
To form a water block in a passageway, the corresponding vent tube is closed for a short time. The pressure of air in the passageway thus decreases as extrusion continues and the pressure of the external atmosphere collapses a portion of the freshly formed outer tubular wall of the dielectric onto the two spokes forming the outer two sides of the passageway. The wall collapses immediately beyond the end of the nose 8, since this is the softest region of the extruded dielectric. The length of the collapsed region can be controlled by varying the length of time for which the vent tube is closed. Eventually, as the collapsed region travels away from the tool, the dielectric cools and the outer walls becomes firmly sealed to the spokes.
' Since the volume of the vent tube and the unblocked air passageway may tend to act as a reservoir of air tending to smooth out fluctuations of pressure when the vent tube is closed, it is preferable to apply a vacuum pump to the vent tube thus obtaining a large pressure difference in a short time.
During continuous extrusion the passageways are blocked in sequantial order at regular intervals along the length of the cable. Apparatus for carrying this out will be described below with reference to FIGS. 5, 6 and 7.
FIG. 3(a) shows an alternative to the cartwheel configuration for an air-spaced coaxial cable, in which the dielectric 14 has a plurality of symmetrically spaced circular passageways such as 15 positioned round the center conductor 16.
FIG. 4 shows the extrusion tool for forming a dielectric of such a cross-section. Extrudate flows into the tool between the collar 17 and the core 18 in the direction of the arrows. At the same time the center conductor l9 emerges at a predetermined rate from a central bore in the core 18. Tubular members as 20 emerge from the conical nose of the core and these give rise during extrusion to the air passageways in the dielectric.
The members20, which are hollow and thus serve as their own vent tubes, communicate in pairs with respective vent chambers in the core, only one vent chamber 21 being shown as a broken line. When a given vent chamber is closed at the atmosphere or evacuated, the two air passageways formed by the tubular member associated with that vent chamber become filled with extrudate over a region adjacent the end of the extrusion tool, thus forming the required water blocks. Such a pair of blocked passageways are shown symbolically at 30 in FIG. 3(b).
Apparatus will now be described which during extrusion automatically forms the water blocks in the passageways sequentially and at regular length intervals. This apparatus will be described in conjunction with the extrusion tool shown in FIG. 2; however, it is to be understood that such apparatus can be easily modified for use with the tool of FIG. 4 to form the water blocks in pairs.
Referring to FIGS. 5, each vent tube at the rear of the extrusion tool is connected by means of a small bore pipe such as 22 to the inlet of a respective camoperated vent valve 23. The vent valves are arranged in turn by a rotating cam 24.
The construction of each valve is shown in FIG. 6. Each valve comprises a housing 25 in which a springloaded position 26 is sliding fit. The piston has a portion of reduced diameter 27 provided with a piston valve 28 When the piston is in the lower position as shown, the small bore piping 22 is in communication with the atmosphere via the outlet 31, but when the piston is actuated by the cam 24 the pipe 22 is cut off from the atmosphere and brought solely into communication with a closed stopcock or with a vacuum pump via the outlet 29. When this occurs a water block is formed in the associated air passageway in the manner described.
During continuous extrusion the cam 24 is operated by means of a pulley, rotated by the center conductor which passes round the pulley as it travels towards the extrusion tool, and change gears. Thus the rotary speed of the cam is proportional to the speed of the center conductor, and can be preset by means of the change gears and the diameter of the pulley. By this means the length interval between water blocks may be changed.
What is claimed is:
1. Apparatus for manufacturing a coaxial cable having a dielectric formed by extrusion and a moving center conductor, comprising an extrusion tool having a plurality of longitudinally disposed members spaced about said center conductor, each member forming a longitudinal passageway in the extruded dielectric, a vent tube in each member which normally communicates at one end with the atmosphere and communicates during extrusion at the other end with the passageway formed in said dielectric extruded around the member, and means for sequentially interrupting the air flow and reducing the pressure in each vent tube at different times fromreachv other vent tube during extrusion to block each associated passageway with extruded dielectric over a region adjacent the end of the tool at longitudinal and circumferential locations along the cable different than each other passageway.
2. Apparatus as claimed in claim 1 wherein said means includes a plurality of vent valves, said vent tubes being connected to the inlets of respective valves, each vent valve being a first outlet in communication with the atmosphere and also normally with the inlet, a second outletin which air flow is interrupted and pressure reduced during extrusion, and actuating means which when operated connects the inlet solely with the second outlet.
3. Apparatus as claimed in claim 2, wherein the passageway-forming members are arranged symmetrically about the moving center conductor, the apparatus further including means to operate the vent valves in sequential order and at time intervals dependent upon the rate of extrusion.
4. Apparatus as claimed in claim 3, wherein the vent valve actuating means are cam-operated and the valves are arranged in a circular configuration with their actuating means facing the center of the circle, the apparatus also including a cam mounted for rotation about an axis at the center of the circle to actuate each valve in turn and driven at an angular speed proportional to the speed of the center conductor.
5. Apparatus as claimed in claim 4, wherein the cam includes drive means driven by the center conductor.