US3927247A

US3927247A - Shielded coaxial cable

Info

Publication number: US3927247A
Application number: US085810A
Authority: US
Inventors: Frank E Timmons
Original assignee: Belden Corp
Current assignee: Cooper Industries LLC
Priority date: 1968-10-07
Filing date: 1970-10-30
Publication date: 1975-12-16
Anticipated expiration: 1992-12-16

Abstract

An electric cable is described including a longitudinally folded foil shield having a plurality of drain wires wrapped helically around it along the outer surface of the foil shield and in electrical contact therewith.

Description

United States Patent 1191 Timmons Dec. 16, 1975 22 Filed;

[ SHIELDED COAXIAL CABLE [75] Inventor: Frank E. Timmons, Richmond, Ind.

[73] Assignee: Belden Corporation, Chicago, 111.

Oct. 30, 1970 [21] Appl. No.: 85,810

Related U.S. Application Data [63] Continuation of Ser. No. 765,286, Oct. 7, 1968,

abandoned.

[52] U.S. Cl. 174/36; 174/107; 174/115 [51] Int. Cl. H01B 11/06 [58] Field of Search 174/28, 29, 36, 102 R, 174/103 R, 107 R, 108 R, 109 R, 110 F, 113

[56] References Cited UNITED STATES PATENTS 1/1885 Clark 174/115 X 10/1941 Peterson 174/115 3,032,604 5/1962 Timmons 174/115 3,051,771 8/1962 Lee 174/36 3,274,329 9/1966 Timmons 174/108 X 3,315,025 4/1967 Tomlinson.... 174/107 3,327,050 6/1967 Barrie 174/107 3,351,706 11 1967 Gnerre et a1. 174/108 x 3,474,186 10/1969 Hale 174/103 FOREIGN PATENTS OR APPLICATIONS 762,534 4/1934 France 174/28 720,126 10/1965 Canada 174 36 Primary Examiner-Arthur T. Grimley Attorney, Agent, or Firm-Fitch, Even, Tabin & Luedeka [57] ABSTRACT An electric cable is described including a longitudinally folded foil shield having a plurality of drain wires wrapped helically around it along the outer surface of the foil shield and in electrical contact therewith.

3 Claims, 4 Drawing Figures vLAYER U.S. Patent Dec. 16, 1975 3,927,247

CELLULAR POLYMERIC MATERIAL METAL INVENTOR FRANK E. TIMMONS SHIELDED COAXIAL CABLE This application is a continuation of earlier copending application entitled Shielded Coaxial Cable, Ser. No. 765,286 filed Oct. 7, 1968, now abandoned.

This invention relates to electrical cables and, more particularly, to coaxial cables.

Often, when it is desired to transmit high frequency signals, especially television signals, with a low loss, coaxial cables are employed. A coaxial cable is constructed with an inner conductor and an outer coaxial conductor insulated from the inner conductor which prevents fields, established by extraneous electrical signals, from affecting the signal carried by the inner conductor. In order to provide flexibility, prior coaxial cables have utilized a braided outer conductor or shield of copper or similar conductive material. However, the braided shields, because of the spaces between the wires, have the disadvantage of obtaining less than 100 percent coverage of the cable core. The effectiveness of a braided shield may be increased by more fine braiding or by a double layer of braid. Unfortunately, such an expedient frequently results in the cable being excessively stiff, large or heavy, and the shield may be difficult to cut and terminate in a suitable electrical connection. Moreover, the greater amount of material required in such a construction increases the cost of the cable.

An alternative construction for a coaxial cable is to utilize a shield of thin flexible metallic foil spirally wrapped around the inner conductor or conductor. Such a shield provides total shielding effectiveness, stopping signals which would frequently leak through a braided shield. Flexible foil, however, may be subject to rupture upon flexure of the cable. This can result in a discontinuity in the ground connection. Also, the spirally wrapped foil acts as a winding, increasing the inductance of the cable. These problems can be reduced by the utilization of a drain wire extending along the length of the cable in electrical contact with the foil. A cable construction utilizing a drain wire in contact with a foil shield is disclosed in the US. Pat. No. 3,032,604, assigned to the assignee of the present invention- The particular construction as above described and as shown in the cited patent may be successfully utilized in many instances. Where the cable is subject to extreme flexure, however, the drain wire may be stressed sufficiently as to break, resulting in a discontinuity. Alternately, the discontinuity may result during the manufacturing process when the reel supplying the drain wire empties. Also, it is sometimes difficult to terminate the shield with a single drain wire. The single drain also results in an out-of-round configuration which may be undesirable in many applications.

It is an object of the present invention to provide an improved coaxial cable.

Another object of the invention is to provide a coaxial cable of high shielding effectiveness and having the ability to withstand extreme flexure.

A further object of the invention is to provide a coaxial cable having a foil shield and drain wires in which damage to the drain wires under sever conditions of flexure is avoided.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view, with parts broken away, of a portion of a coaxial cable constructed in accordance with the invention;

FIG. 2 is a perspective view, with parts broken away, of another embodiment of the invention;

FIG. 3 is a perspective view, with parts broken away, of still another embodiment of the invention;

FIG. 4 is an enlarged, fragmentary, diagrammatic, cross-sectional view of a strip comprised of a polymeric tape sandwiched between thin metal foils.

Very generally, the electric cable of the invention comprises a flexible conductor surrounded by a flexible insulating layer. An elongated metallic strip is wrapped around the layer and has its longitudinal opposite marginal portions overlapped to form an elongated seam along the length of the cable. A plurality of flexible drain conductors are wrapped generally helically around and along the outer surface of the foil strip in electrical contact therewith. The construction is completed by an outer surrounding jacket of insulating material.

Referring now more particularly to FIG. 1, the electric cable illustrated therein includes an inner conductor 11 of copper or other suitable conductive material. The inner conductor is illustrated as a solid conductor, however, the inner conductor may be a stranded conductor.

The inner conductor 11 is surrounded by a layer 12 of a suitable flexible insulating material. Preferably, the material of the layer 12 is a low loss cellular polyethylene. For a central conductor size of 18 AWG annealed copper, a suitable layer size may be a 0.070 inch wall thickness having a 0.180 inch outer diameter.

The layer 12 is surrounded by an outer conductor or shield 13 in the form of an elongated metallic foil strip which is wrapped around the layer and has its longitudinal opposite marginal portions overlapped to form an elongated seam 14 extending along the length of the cable. The overlapped construction of the foil ensures complete shielding, preventing harmful outside signals from affecting the signal carried by the inner conductor 11. The longitudinal wrap of the foil strip provides the cable with radio frequency properties which are much superior to that provided by a spiral wrap. The foil strip itself may be comprised of a layer of a suitable flexible insulating tape such as polyethylene terephthalate sandwiched between two thin foils of aluminum. Such a construction provides good shielding with particularly effective rupture resistance. The foil strip may also be formed by laminating the thin foil of aluminum to only one side of the insulating tape. In such an embodiment, the tape would be wrapped about the insulating layer with the metallic side outermost and the outer marginal edge of the tape would be folded under to form a shorted turn.

In order to provide electrical continuity throughout the entire length of the foil shield 13, a plurality of spaced drain wires 16 are provided. In the embodiment illustrated in FIG. 1, four drain wires are used, each having a substantially flat cross section, such that the wires constitute ribbon type wires. A suitable material for the wires 16 is tinned copper. Such strip drain wires provide a high D.C. conductivity which pennits low voltage DC. current to be transmitted by the cable as well as radiofrequency signalsl Also, the four drain wires permit easy termination of the shield and provides a smaller diameter, more flexible cable than one with a single drain wire. Moreover, the four drain wires ensure that continuity of the shield.

In order to prevent excessive strain and possible dam age to the drain wires 16 upon extreme flexure of the cable, the drain wires are wrapped generally helically around and along the outer surface of the foil strip in electrical contact therewith. Thus, when the cable is flexed, the drain wires on the side under compression can readily move toward each other axially along the cable. Similarly, the drain wires along the side of the cable under tension can readily move away from each other along the axial direction of the cable. This avoids placing excessive tensile stress on the drain wires which could'cause damage thereto. Accordingly, the cable may be flexed easily without stress or damage to the drain wires. The periodicity of the helical wrapping should be sufficiently great as to avoid placing excessive tensile stress on the drain wires. On the other hand, the periodicity should not be so great as to result in a wasteful utilization of material due to an excessive number of turns in the drain wires. The possiblity of the drain wires having an effect on the inductance of the cable is minimized because of the interposition of the foil strip 13 and the shielding effect thereof.

The cable is completed by the provision of an outer tubular jacket 17 of the insulating material. The jacket 17 surrounds the assembly of the foil strip 13 and the helically wrapped drain wires 16 to protect and insulate same. The jacket may be comprised of vinyl or other suitable insulating material which also provides protection for the elements which it surrounds. In a cable having the conductor and sheath sized previously indicated by way of example, a satisfactory size for the jacket may be a 0.030 inch wall with a 0.242 inch outer diameter.

Referring now to FIG. 2, a further embodiment of the invention is illustrated. The construction of FIG. 2 is identical with that of FIG. 1 except for the configuration of the drain wires. Accordingly, the various elements of the cable of FIG. 2 have been given reference numbers identical to the similar parts in FIG. 1, preceded by a 1. In the embodiment of FIG. 2, the crosssection of the drain wires 1 16 is round, rather than flat as was the case in the previous embodiment. Under many conditions, round drain wires provide satisfactory electrical contact, and may be of advantage in that they are easier to solder and include less material than the ribbon-type wires 16 in FIG. 1.

Referring now to FIG. 3, a still further embodiment of the invention is illustrated. The construction of the embodiment of FIG. 3 is identical with that of FIG. 1 except for the addition of a plurality of flexible insulated conductors 218. Accordingly, the various elements of the cable of FIG. 3 have been given reference numbers identical to the similar parts in FIG. 1, preceded by a two. In the' embodiment of FIG. 3, a plurality of flexible insulated conductors are wrapped generally helically around and along the outer surface of the foil strip 213 in the spaces between the drain conductors 216.

As many insulated conductors 218 as required may be utilized, and these conductors may be used to power various electrical devices or control circuits. Thus, the cable may be utilized to not only transmit wide band RF signals in the main conductor 211, but with no diameter increase, may function to control other electrical circuits. For example, color TV sets used in hotels are costly devices and are subject to theft. By utilizing a cable in accordance with FIG. 3, wide band signals may be supplied to the TV set while at the same time the insulated flexible conductors 218 may function together with a signalling annunciator system to signal an immediate alarm when theft of the color TV set is attempted by severing of the cable. Another example of a use of a cable of the type illustrated in FIG. 3 is in hospitals where a very small TV set is the center of the patients entertainment, instruction, monitoring of body conditions, and communication with the nurses station. Accordingly, all functions can be controlled by a single cable design.

It may therefore be seen that the invention provides an improved electrical cable having a solid foil shield for I00 percent shielding effectiveness. At the same time, the arrangement of drain wires applied helically around the foil shield provides full electrical contact with the shield for complete electrical continuity, while permitting the cable to be flexured easily without stress or damage to the drain wires. 1

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

.What is claimed is:

1. A bendable, flexible coaxial cable for carrying RF signals such as high frequency TV signals comprising an elongated central flexible conductor for carrying the RF signals, a flexible insulating layer of low loss cellular material surrounding said conductor, an elongated, longitudinally extending foil strip encircling and engaging said insulating layer and having its opposite longitudinally extending marginal portions overlapped to fonn an elongated longitudinally extending seam along the length of the cable, said foil strip having an inductance compatible with the carrying of high frequency RF signals by said central conductor, said foil strip having a flexible polymeric tape layer and a layer of metal on at least one side of said polymeric layer whereby said conductor-may be shielded by said metal layer, said polymeric layer engaging said layer of insulating layer of cellular material, a plurality of flexible drain conductors wrapped generally helically around and along and in electrical contact with said metal layer of said foil strip, said drain wires being spaced from said central conductor by said insulating layer and said foil strip, said drain conductors being helically wrapped with a predetermined periodicity and spaced from each other uniformly, said portions of the drain wires on the side of the cable under compression during flexure of the cable being free to move toward each other axially along the spaces between drain wires and along the cable, during flexure of the cable said drain wire portions under tension being free to move away from each other axially along the cable, said drain wires providing tensile strength for said cable while allowing said cable to bend, and an outer tubular jacket of flexible insulating material surrounding said helically wrapped drain wires and holding the same against said metal layer of said foil strip, said cable having a substantially right circular cross section.

2. A flexible coaxial cable in accordance with claim 1 in which at least one insulated, flexible control conductor is spiralled in the helical space between adjacent drain conductors and along the outer surface of said foil strip thereby providing both a spiral control conductor and a longitudinal central conductor for carry- 6 ing the RF signals without increasing the diameter of strip and facing said insulating layer of cellular matethe cable. rial, said inner metal layer being disposed in metal-to- 3. A flexible coaxial cable in accordance with claim 1 metal contact with said outer metal layer at said longiin which said internal foil strip has another layer of 5 tudinally extending seam. metal on a second side of said polymeric layer of said

Claims

1. A bendable, flexible coaxial cable for carrying RF signals such as high frequency TV signals comprising an elongated central flexible conductor for carrying the RF signals, a flexible insulating layer of low loss cellular material surrounding said conductor, an elongated, longitudinally extending foil strip encircling and engaging said insulating layer and having its opposite longitudinally extending marginal portions overlapped to form an elongated longitudinally extending seam along the length of the cable, said foil strip having an inductance compatible with the carrying of high frequency RF signals by said central conductor, said foil strip having a flexible polymeric tape layer and a layer of metal on at least one side of said polymeric layer whereby said conductor may be shielded by said metal layer, said polymeric layer engaging said layer of insulating layer of cellular material, a plurality of flexible drain conductors wrapped generally helically around and along and in electrical contact with said metal layer of said foil strip, said drain wires being spaced from said central conductor by said insulating layer and said foil strip, said drain conductors being helically wrapped with a predetermined periodicity and spaced from each other uniformly, said portions of the drain wires on the side of the cable under compression during flexure of the cable being free to move toward each other axially along the spaces between drain wires and along the cable, during flexure of the cable said drain wire portions under tension being free to move away from each other axially along the cable, said drain wires providing tensile strength for said cable while allowing said cable to bend, and an outer tubular jacket of flexible insulating material surrounding said helically wrapped drain wires and holding the same against said metal layer of said foil strip, said cable having a substantially right circular cross section.

2. A flexible coaxial cable in accordance with claim 1 in which at least one insulated, flexible control conductor is spiralled in the helical space between adjacent drain conductors and along the outer surface of said foil strip thereby providing both a spiral control conductor and a longitudinal central conductor for carrying the RF signals without increasing the diameter of the cable.

3. A flexible coaxial cable in accordance with claim 1 in which said internal foil strip has another layer of metal on a second side of said polymeric layer of said strip and facing said insulating layer of cellular material, said inner metal layer being disposed in metal-to-metal contact with said outer metal layer at said longitudinally extending seam.