US2622152A - High attenuation coaxial cable - Google Patents

Description

Dec. 16, 1952 s. J. ROSCH HIGH ATTENUATION COAXIAL. CABLE Filed Sept. 21, 1946 INVENTOR SAMUEL J. R H

[ JLQW Wn' uE/ ATTORNEY Patented Dec. 16, 1952 HIGH ATTENUATION COAXIAL CABLE Samuel J. Rosch, Yonkers, N. Y., assignor to Anaconda Wire and Cable Company, a corporation of Delaware Application September 21, 1946, Serial No. 698,498

2 Claims. 1

This invention relates to electric cables, and more particularly to high-attenuation coaxial cables for conducting electric currents at frequencies above 100 megacycles. The new cable is of conventional coaxial construction in that it comprises a central metallic conductor, a surrounding layer of dielectric material, and an outer cylindrical metallic conductor. The characteristic feature of the new cable is the provision of a separat thin-walled cylindrical conductor 0.001 to 0.010 inch in thickness having high electrical resistance positioned between the outer conductor and the dielectric. It is this separate high resistance conductor that imparts to the cable its high-attenuation characteristics.

Coaxial cables find important uses in making connections between high-frequency electronic devices, because such cable is particularly efflcient as a conductor of high-frequency electrical energy-that is, it conducts such energy with minimum power attenuation. Occasions arise, however, when a coaxial cable having high-attenuation characteristics is required. For example, it is often desirable to tap off a fraction of the power being conducted by a main highfrequency circuit without introducing disturbing influences into the main circuit. If low-attenuation cable is used for this purpose, the amount of power drained from the main circuit may seriously disturb proper operation of the main circuit. In such case, therefore, a high-attenuation coaxial cable is ordinarily preferred.

As another example, it is often necessary to connect two mismatched high-frequency electronic devices by means of a low-attenuation coaxial cable. In such case the mismatching of the two devices may cause standing waves to develop in the cable connecting them, with the result that only a small fraction of the power fed in at one end of the cable isusefully delivered at the other end. It is often possible to correct this condition by properly connecting a matching terminating section of high-attenuation coaxial cable to the receiving device supplied with power by the low-attenuation cable. The terminating section of high-attenuation coaxial cable dissipates the power that would otherwise be reflected back from the mismatched power-receiving device to create standing waves in the connecting cable.

High-attenuation coaxial cables of known attenuation characteristics are also useful for measuring the attenuation of other devices (such 'as'high-frequency power lines) having the same characteristic impedance as the cable used for 2 making the measurement. Such cable also may be used for connecting electric measuring instruments into high-frequency circuits, especially where the instruments used have scale readings too low to permit being connected directly into the circuit by low-attenuation connections, or where the instrument would create excessive disturbances if thus connected directly to the circuitto be measured. Multiplying the instrument reading by, or adding to it, an appropriate conversion factor derived from the known attenuation characteristics of the high-attenuation cable by which it is connected in the circuit then gives a proper indication of the quantity desired to be measured in the main circuit.

The standard high-attenuation coaxial cabl heretofore known and employed has been of conventional coaxial cable construction, except that the central conductor has been composed of a metal having rather high resistance. Nichrome wire (composed of an alloy of nickel and chromium) has been used almost exclusively as the central conductor in such cables. High-attenuation coaxial cables of this heretofore known construction have a number of serious drawbacks; The Nichrome generally used for the central conductor is difiicult to solder, and accordingly it is not easy to make good electrical connections to it. The resistance of Nichrome" varies appreciably with changes in temperature, and consequently the attenuation characteristics of coaxial cables employing a Nichrome central conductor vary markedly with temperature. Since the use of high-attenuation cables involves dissipation of power, and since the power is invariably dissipated as heat, the dependence of the attenuation characteristics of Nichrome-core coaxial cable on temperatur is particularly serious. Most of the heat generated in Nichrome-core high-attenuation coaxial cables is produced within the Nichromecore wire itself, and must be dissipated by conduction through the dielectric between the two conductors and through the outer insulating jacket (if any) to the atmosphere. The insulating materials most commonly used for making the dielectric and the outer insulating jacket are poor conductors of heat, and moreover become damaged if heated excessively. Consequently the heat (power) dissipation rating of such cables must below, and long lengths of quite large cable of this character must be employed if any very large amount of power is to be dissipated.

The present invention provides an improved high-attenuation coaxial cable that for themost.

part is free of the objections inherent in the Nichrome-core type of high-attenuation cable heretofore commonly employed, and that at the same time possesses distinctive advantages of its own. Basically, the new cable comprises a central metallic conductor, a layer of dielectric material surrounding the central conductor, and an outer metallic conductor surrounding the dielectric, and is characterized by the provision of a conductor in the form of a thin-walled cylinder having high electrical resistance positioned between the outer conductor and the dielectric. The outer metallic conductor immediately surrounds the high-resistance conducting element,

and is in contact with it over substantially its entire outer surface. Consequently these two elements together constitute the complete outer conductor of the cable.

The high-resistance conductor advantageously comprises a non-conducting material made conductive by incorporation therein of finely-divided. carbon. Paper impregnated with a finelydivided carbon such as carbon black is particularly satisfactory. Such paper in the form of tape may be wound helically about the dielectric material prior to applying the outer conductor, to form a thin-walled conductive cylinder underlying the outer metallic conductor. In place of carbon-impregnated paper, however, the high-resistance conductor may comprise some other material such as rubber, polyethylene resin, polyvinyl chloride resin, or other plastic composition made conductive by being impregnated with finelydivided carbon. The resulting conductive plastic composition may be extruded or otherwise formed into a thin-walled cylinder surrounding the dielectric and underlying the outer metallic conductor, or surrounding the inner conductor and underlying the dielectric.

If desired, the new cable may be provided with an outer insulating jacket of any suitable material.

'The invention is described in greater detail below in connection with the accompanying draw ing, the single figure of which is a perspective showing the construction of a particularly advantageous embodiment of the new cable.

The cable shown in the drawing comprises a central metallic conductor I surrounded by a layer of dielectric material 2. Wrapped helically about the dielectric is a tape 3 of paper impregnated with carbon black to render it conductive. The helically wrapped tape forms a thin-walled conducting cylinder having high. electrical resistance: Generally it is preferable to wrap the paper tape with edges abutting, but if desired the' edges may overlap, or may even be separated somewhat. The type of wrap used maybe chosen for the purpose of varying the electrical resistance somewhat. Alternatively to helically wrapping the paper about thev dielectric, it may be tubed longitudinally thereabout. A braid 4 of metallic wires is applied immediately over the carbon black paper tape 3 and in contact therewith. The braid and the carbon-black paper together form the outer conductor of the cable, and are held in. coaxial relation with the central conductor by the intervening layer of dielectric material. An outer insulating jacket 5 completes the cable assembly.

Both the central. conductor l and the outer metallic braid 4 are of copper or other good conducting metal. The dielectric 2 thatseparates the inner and outer conductors and maintains them in their coaxial relationship advantageously is tivity of. the carbon-black paper.

a material (such as a polyethylene composition) which possesses good dielectric properties at high frequencies. The outer insulating jacket 5 may be composed of a rubber composition, or of some other plastic composition such as a composition of a polyethylene or polyvinyl chloride. Provision of such an outer insulating jacket often is desirable but is not always necessary.

The new cable conducts direct current and lowfrequency alternating current in substantially the same manner as conventional coaxial cable not having the wrapping of carbon-impregnated paper tape 3. In conducting such currents, the outer conductor may be considered as composed of a low-resistance conducting element (the metallic braid 4) and a high-resistance conducting element (the carbon impregnated paper 3) connected in parallel. In such case most of the current carried by the outer conductor flows through the outer braid 4. In conducting currents at high frequencies, however, sufficient loss is developed through the impregnated paper 3 to result in ap preciable attenuation. As the frequency is increased, slrin erlects become more and more pronounced, so that an increasing amount of current is shifted from the metallic outer braid to the inner spirally wrapped conductive element. Since this medium possesses high electrical resistance, substantial power loss and high attenuation. resultin the range of high frequencies atwhich the new cable is designed to operate.

It is possible to control the frequency at which maximum attenuation characteristics first develop in the cable by controlling the thickness of the carbon black paper. The thicker the carbon black paper wrapping, the lower will be the frequency at which high attenuation begins to appear. For most uses, the thickness of the carbon-black paper or other high. resistance conductive element is advantageously between about 0.001 inch and 0.010 inch, although thicknesses above or below these limits are sometimes desirable. Further, since the extent to which the cable attenuates depends upon the resistivity of the conducting pathairorded by the carbonblack paper, it is possible to control (within limits) the amount of attenuation that occurs in the cable by controlling the electrical resis- This in turn may be controlled readily by controlling the amount of carbon black impregnated into the paper- The extent to which the cable attenuates at high frequencies increases (within limits) as the amount of carbon impregnated in the paper (and hence the conductivity of the paper) de creases.

The following tabulation shows the attenu ating characteristics of cableconstructed as described above, in comparison with other types of coaxial cables. In the following tabulation, cable A was a standard. RG8/U coaxial cable havinga copper central conductor and a copper outer conductor. Cable B was of the same construction as cable A., except that carbon-black paper was wrapped helically about the dielectric separating the central and outermetallic conductors- Cable 0 was a standard type RG-Zl/U coaxial cable having. a Nichrome wire central conductor and a copper outer conductor; Gable D was identical with cable C, except that the central conductor was copper, and paper impregnated with carbon black was wrapped hell cally about the dielectric immediately beneath the outer metallic conductor. In both cables B and D the carbon-black paper on the cable possessed a D. C. resistance of 3.80 megohms per foot. The figures in the body of the table give the attenuation of the cables in decibels per 100 feet at 26 C.

Comparing the attenuation characteristics of cables A and B in the above table, it will be observed that the new cable (cable B) has substantially greater attenuating characteristics than standard copper conductor coaxial cable. At the lower frequency the new cable attenuates approximately three times as much per unit of length as does the regular cable, and at a frequency of 3000 megacycles it attenuates almost four times as much. Comparing the new cable with the heretofore known Nichrome-core attenuating cable, it will be noted that at the lower frequencies the new cable (cable D) does not attenuate quite so much as the Nichrome-core cable (cable C). At the high frequency of 3000 megacycles, however, the new cable has substantially greater attenuating characteristics than the Nichrome"-core cable.

Other thin-walled cylindrical conductors having high electrical resistance may be used in the new cable in place of the paper tape impregnated with carbon black and wrapped helically about the dielectric as particularly described above and shown in the drawing. For example, rubber made conductive by being heavily loaded with finely-divided carbon may be extruded as a thin jacket about the dielectric. Similarly, other non-conducting plastics such as polyethylene compositions or polyvinyl chloride compositions made conductive by incorporation therein of finely-divided carbon may be extruded about the dielectric, or tapes or strips of such compositions may be wrapped about the dielectric.

The new cable possesses a number of advantageous characteristics not possessed by the Nichrome-core high-attenuation coaxial cable,

heretofore frequently used. Not the least of these advantages is the ability of the new cable to conduct direct current or low-frequency alternating currents eificiently (i. e., without substantial attenuation). The new cable, therefore, may

be employed advantageously where it is desired to conduct such currents over the same conductor in which high-frequency currents are to be attenuated. Thus the new cable may be used in some circuits as a low-pass filter.

Another advantage of the new cable when the high resistance element surrounds the dielectric is that the heat generated in consequence of its attenuation characteristics is liberated for the most part in the high-resistance conductor immediately underlying a metallic conductor. This heat is readily and rapidly conducted through the outer metallic conductor, and its dissipation to the atmosphere is impeded only by whatever outer insulating jacket is applied to the cable. Consequently, its heat dissipating capabilities are substantially greater than is that of Nichromecore coaxial cable, in which the heat is generated inlthe central conductor and must be conducted through the dielectric as well as through an outer insulating jacket. The new cable, therefore, may safely carry a higher heat-dissipation rating than the Nichrome-core coaxial cable.

1 claim:

1. A high-attenuation coaxial cable for conducting electric currents at frequencies above megacycles comprising a central metallic conductor, a layer of insulation having good dielectric properties at frequencies above 100 megacycles surrounding the central conductor, and an outer metallic conductor surrounding the dielectric, characterized in that a tape of paper I about 0.001 to 0.010 inch in thickness impregnated with carbon black is wrapped helically about the dielectric beneath the outer metallic conductor, whereby heat produced in consequence of power dissipation during operation of the cable at a high frequency is developed substantially entirely outside the high-frequency insulation where it is readily transferred to the surrounding atmosphere.

2. A high-attenuation coaxial cable for conducting electric currents at frequencies above 100 megacycles comprising a central metallic conductor, a layer of insulation having good dielectric properties at frequencies above 100 megacycles surrounding the central conductor,

v a tape of paper about 0.001 to 0.010 inch in thickness impregnated with carbon black wrapped helically about the dielectric and forming thereabout a cylindrical conductor having high electrical resistance, and an outer metallic conductor immediately surrounding and in contact with the carbon-black-impregnated paper, whereby heat produced in consequence of power dissipation during operation of the cable at a high frequency is developed substantially entirely outside the high-frequency insulation where it is readily transferred to the surrounding atmosphere.

SAMUEL J. ROSCH.

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

UNITED STATES PATENTS OTHER REFERENCES Principles of Radar, Radar School, M. I. T., McGraw-Hill Book Company, Inc., N. Y., 1946. (Page 8-5 relied upon.)

Images