US2614172A - High impedance shielded twin conductor cable - Google Patents

Description

Oct. 14, 1952 E. w. GREENFHELD ET AL 2,514,172

HIGH IMPEDANCE 5HIELDED TWIN CONDUCTOR CABLE Filed June 12, 194.8

TORNEYS Patented Oct. 14, 1952 HIGH IMPEDANCE SHIELDED TWIN CONDUCTOR CABLE Eugene Willis Greenfield, Hastings-on-Hudson,

and Alfred Emil Widmer, Ossining, N. Y., assignors to Anaconda Wire and Cable Company, a corporation of Delaware Application June 12, 1948, Serial No. 32,564

2 Claims.

This invention relates to high impedance shielded twin conductor cables, and is directed particularly to the provision of an improved cable of this sort, such a a television lead-in cable, for carrying high frequency electric current with low power attenuation. The new cable is characterized in that each of the twin conductors comprises a wire having a core of ferromagnetic material (such as iron) surrounded by a layer of metal of high electrical conductivity (such as copper).

Lead-in cables for television sets, and for other high frequency radio receiving apparatus, should have a high characteristic impedance so as to reduce attenuation to a minimum. Furthermore such cables should have a characteristic impedance which matches as nearly as possible that of the receiving apparatus, which (especially in the case of television sets) generally is quite high. The lead-in cable heretofore most commonly used for television receivers consists of two parallel unshielded conductors embedded in the side edges of a spacer ribbon of polyethylene. The impedance of such cable depends almost entirely on its geometry and on the dielectric constant of the spacer ribbon, and consequently it can be made economically with a characteristic impedance up to 300 ohms, which is about as high as is required commercially for television purposes. This simple cable is subject to a number of disadvantages, however, which stem from the absence of any shielding. In the first place, it picks up highfrequency radiation (other than desired signals) from such sources as automobile ignition systems, are or spark discharges, X-ray apparatus, etc., which causes distortion of the desired signal and spoils the quality of the picture produced on a television screen. In the second place, it collects films or deposits of dirt, water, etc., that raise the effective dielectric constant of the spacer tape and increase the attenuation of the cable. For example, in seacoast communities such television lead-in cable soon acquires such a deposit of salt from ocean spray that in rainy weather, when this deposit becomes well wetted, the signal supposed to be conducted by the cable from the antenna to the receiving set becomes so much attenuated that no picture can be produced on the screen. In the thirdplace, the polyethylene spacer tape is quite highly inflammable, and cannot be provided with a protective covering of flame-resistant material without greatly increasing its attenuation.

All of these disadvantages can be overcome by enclosing the cable in aconducting shield, but this at once leads, in any cable of reasonable size, to a marked lowering of it characteristic impedancel' The shield so much increases the capacitance per unit length between conductors that even with the best-dielectric materials the characteristic impedance (which is an inverse function of capacitance per unit length) is lowered to the point where it is impossible to make a reasonable match between it and the impedance of the present commercial television receivers. Furthermore, the increased capacitance between conductors leads at once to an increase in the characteristic attenuation of the cable, i. e., makes it characteristically more lossy. It has been proposed to make the capacitance as low as possible by axially supporting the conductors in a tube of good high-frequency dielectric, such as polyethylene, so that the conductor is largely surrounded by air. This has the effect of lowering the effective dielectric constant of the insulation about each conductor, and so lowering the capacitance per unit length between the conductors, but it does not do so enough in a cable of reasonable size to counteract the effect of the shield in increasing such capacitance.

It is known that the impedance of a cable is a function of the inductance per unit length of the cable, and that this in turn depends on the magnetic permeability of the material in the field of the conductors. It might be supposed, therefore, that by making the shield of a magnetic metal, or by interposing a magnetic material in the region between conductors, the impedance of a shielded pair of conductors could thereby be increased. Such indeed could be done, but all practical mag netic materials suitable for this purpose have such high hysteresis losses at high frequencies that the use of such materials for the shield, or in the space between conductors, so increase the attenuation of the cable as to make it of no practical value. We have discovered, however, that by providing each conductor with a core of ferromagnetic material, it is possible to attain a surprisingly large increase in the effective permeability of the space in the field of the conductors without at the same time disproportionably increasing the attenuation of the cable. In this manner, therefore, we are enabled to produce a shielded twin conductor cable of reasonable size which has a much higher characteristic impedance than it has been possible to produce heretofore, and that at the same time has reasonably low attenuation.

The high impedance shielded twin conductor cable of our invention, which is based on the foregoing discovery, comprises a pair of conductors each of which is insulated by being axially supported in and spaced from the inner surface of a tube of dielectric material, and a metallic shield surrounds the pair of thus-insulated conductors. The characteristic feature of the new cable is that each conductor comprises a wire having a core of ferromagnetic material and a surface layer of metal of high electrical conductivity surrounding the core. The new cable is described below with reference to the accompanying drawing, the single figure of which shows the new cable in cutaway perspective.

The cable comprises a pair of wire conductors I each of which has a ferromagnetic core 2 surrounded by a layer of metal 3 of high electrical conductivity. The core 2 preferably is iron or steel, but it may be any other ferromagnetic material, such as nickel or cobalt, if desired. It

is advantageous that the core material have as high magnetic permeability as is reasonably practical. The surface metal layer 3 preferably is copper or silver because these metals have the best electrical conductivity of any known, but some other metal of good conductivity, such as aluminum, may be used if desired.

Each wire I has a thread 4 of dielectric material wrapped around it in the form of a longpitch helix. This thread serves to support the wire axially in a tube 5 of dielectric material, but to keep it spaced from the inner surface of the tube, so that the wire is largely surrounded by air, thus making what is termed as air-spaced" cable. The dielectric material of which the thread 4 and tube 5 are composed should be one having as low dielectric constant as possible. Polyethylene, which is essentially a pure, saturated hydrocarbon, is probably the most satisfactory material now available, both on account of its low dielectric constant (about,2.3) and its natural flexibility and chemical stability. Other dielectric materials may also be used, however. Also, other means than the helically wrapped thread 4 may be used to air-space the wires I in the tubes 5. For example, polyethylene or other plastic dielectric may be extruded with a star-shaped cross section about the wire to hold it centered in the tube. The air-spacing results in making the effective overall dielectric constant of the insulation about the conductors substantially less than if they are embedded in a solid dielectric, and so reduces the capacitance between them.

The pair of insulated conductors is surrounded by a metallic shield 6 which most advantageously is in the form of a braid of fine wires of copper or other metal of good electrical conductivity. However, the shield 6 may alternatively be made of a metallic foil (of aluminum, copper, silver or other metal of good electrical conductivity) which is either folded longitudinally or wrapped helically (preferably with some overlap) about the pair of insulated conductors. If such foil is very thin it may be backed with paper or other reinforcing material. Over the shield a jacket "I of protective and preferably flame-resistant material such as polyvinyl chloride is provided. The metallic shield isolates the cable conductors from high-frequency radiations which otherwise would be picked up and conducted to the television set or other device to which the cable is connected, and thus the shield protects the desired signal from distortion. It also enables the protective jacket I to be of any desired material for making the cable weatherproof, flame-resistant and otherwise durable, withoutparticular concern that such material have good high-frequency dielectric properties. The braid also, however, lowers the capacitance per unit length between the wires 1, and so reduces the characteristic impedance of the cable. But by virtue of the fact that the wires each have a ferromagnetic core, the

permeability of the space about each conductor is substantially increased, as compared with a similar cable having plain copper wire conductors.

Consequently the inductance of the cable is increased, and surprisingly the extent to which it is increased (even though each ferromagnetic core occupies but a very small volume of the space in the field of the other conductor) is great enough to offset to a very useful extent the increase in capacitance brought about by the provision of the shield. A shielded cable made in accordance with the invention with ferromagnetic-core conductor wires spaced apart a given distance therefore has a characteristic impedance almost as high as an unshielded cable having conductors embedded in a solid dielectric and spaced apart the same distance, and it has a substantially higher characteristic impedance than a similar shielded cable having plain copper wire conductors. For example, a shielded cable made in accordance with the invention having overall cross-section dimensions across the outer jacket of only ,4 by A", in which the conductors were spaced apart about r n", was found to have a characteristic impedance of 225 ohms. This impedance is high enough for the cable to match reasonably well with commercial television receivers. A similar cable, of the same crosssection dimensions but with solid copper conductors, has a characteristic impedance of about ohms-too low for a proper match with commercially available television sets. The high impedance of the new cable also makes its attenuation considerably less than that of a similar cable having plain copper wire conductors, especially at frequencies high enough so that substantially all of the current is carried by the surface layer of copper outside the ferromagnetic core.

We claim:

1. High impedance shielded twin conductor cable for carrying high frequency electric current with low power attenuation comprising a pair of conductors, each of said conductors being insulated by being axially supported in and spaced from the inner surface of a tube of dlelectric material, and a metallic shield of nonmagnetic material surrounding the pair of thusinsulated conductors, characterized in that each conductor comprises a wire having a core of ferromagnetic metal and a layer of metal or" high electrical conductivity surrounding said core, and further characterized in that the region between conductors inside the shield is substantially free of any ferromagnetic material.

2. High impedance shielded twin conductor television lead-in cable comprising a pair of conductors, each of said conductors being insulated by being axially supported in and spaced from the inner surface of a tube of dielectric material, and a metallic shield of non-magnetic material surrounding the pair of thus-insulated conductors, characterized in that each conductor com-v prises a wire having an iron core surroundedby a layer of copper, the region between conductors inside the shield being substantially free from any ferromagnetic material.

EUGENE WILLIS GREENFIEID. ALFRED EMIL WIDMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,032,658 lement c July 16, 1912 2,034,026 Curtis Mar. 17, 1936 2,118,952 Strieby May 31, 1938

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