US1818027A - Concentric conductor system - Google Patents

Description

H. A. AFFEL ET AL CONC'ENTRIG CONDUCTOR SYSTEM Aug. 11, 1931.

Filed May 23. 1929 2 Sheets-Shee 1 5, Elecdntlield 1?. {11mm Field Jud/zed I I 17112? I Aug. 11, 1931. H. A. AFFE L ET AL 1,818,027 CONCENTRIC CONDUCTOR SYSTEM Filed m 23. 1929 2 Sheets-Shee 2 Cor-e0 o'lleraf anysuj table mater INVENTORS f/fel will 6mm ATTORNEY Patent ed Aug. 11, 1931 UNITED STATES PATENT OFFICE HERMAN A. AI'I'E'L, OI RIDGE-WOOD, AND ESTILL I. GREEN, OF EAST ORANGE, NEW

JERSEY, ASSIGNOBS TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A

CORPORATION 01' NEW YORK CONCENTRIC CONDUCTOR SYSTEM Application filed ma 23, 1939. Serial in. 365,524.

This invention relates to a conducting system for transmitting with small attenuation a band of frequencies whose upper limit extends well above'the frequencies now em- 5 ployed in carrier transmision.

Modern developments in the art of communication render it highly desirable to have available for transmission purposes a system which will transmit without undue attenuation frequencies extending from the audio frequency range well up into the radio frequency range. For example, high grade circuits are now required for the transmission of programs over telephone lines to broadcasting stations. In order to transmit musical programs it is necessary to rovide circuits that will transmit a band 0 frequencies extending well up toward 10,000'cycles, as compared with the voice range ordinarily em- 'ployed in telephony, which does not exceed about 2,750 cycles. For the best quality of transmission of music it might be desirable to transmit over telephone lines frequencies up tothe audio limit, which would be in the neighborhood of 15,000 or 20,000 cycles. Modern cable circuits are not adapted for 1 transmission of such high frequencies, and -the only commercial circuits now available which would be capable of transmitting frequencies of this order would be the open wire circuits which have heretofore been employed for high frequency multiplex carrier transmission. 7

- Even for carrier transmission, 0 enwire lines have been found uncommercia for the transmission of frequencies much above 30,000 cycles. If, therefore, an open wire line is used for the transmission of a high grade audio frequency rogram involving frequencies in the neigh orhood of 15,000 or 20,000 cycles, the remanent frequency range above such audio band would be so narrow as to be of little use for carrier transmission. From this standpoint, therefore, it would be highly desirable to have available a circuit which would transmit frequencies much higher than 30,000 cycles. a

The modern development of television also introduces a new factor. Existing television systems which have been experimented with have been limited to the transmission of a small image of a few square inches in area, and in such image the elements of the picture making up the entire image have been rela-. tively large, so that the picture is not well defined. The mechanical problems involved in designing a television transmitting and receivin apparatus capable. of picking up and receiving with excellent definition a representation of some large scene, such as a ball game or a theatrical performance, are capa: ble of solution by known means, but the transmission of such a picture electrically involves the transmission of frequencies from zero u to the neighborhood of 500,000 cycles, an 7 there is no telephone circuit now available which would commercially transmit any such range of frequencies. It therefore becomes desirable to have available a conducting s- .tem which would transmit without un ue attenuation a wide range of frequencies including the extremely high frequencies necessary for television, the circuit being at the same time available if necessary for the transmission of a very large number of carrier channels or for any desired number of bands of suflicient width for the transmission, without undue distortion, of high grade programs.

In accordance with the resent invention, a circuit havin these desirable characteristics is obtained y employing concentric conductors of relatively large diameter, one acting as a return for the other. The two conductors, which may preferably be in the form of hollow cylinders, are insulated from each other and held in proper concentric relationby an suitable means, suchas spacing washers 0 some dielectr c of low loss angle and low dielectnc constant so as to introduce a minimum leakage loss 'between the conductors. By spacing the washers relativel far apart the principal part of the dielectric between the two conductors will be air, which, as is well known, involves substantially no leakage loss.

Inaccordance withthepresentinventiomthe attenuation due to the skin effect is decreased by formingthe hollow cylindrical conductors of a plurality of fine wires insulated from each other by some suitable coating and braided together in such a manner as to form a hollow tube. Various methods of braiding hollow tubes are known. For example, a hollow tube may be constructed by the use of a basket weave, or .the tube may be constructed by weaving in the manner common in the construction of the fabric covers for telephone cords and electric light cords. A method of braiding has also been employed for producingv hollow tubes of braided wire to be used as. metallic shields for groups of conductors in a cable. Any ofthese methods may be used in producing the hollow cylinders of the present invention, the essential requirement being that the wires, in braiding, be weaved-inwardly and outwardly toward and from the center of the cylinder. If this is done, the current at high frequencies cannot confine itself to the skin or surface of the cylindrical conductor but will, as it passes I along, flow through all portions of the cylincler wall. The inner conductor, instead of being a hollow tube of stranded wires, may be a solid strand of insulated wires which are interwoven in the manner described above.

A conducting system such as above outlined has a number of advantages. It may be made waterproof, with the result that the leakage losses between the conductors (which in the case of ordinary open wire construction vary greatly with weather conditions and at high frequencies contribute very substantially to the attenuation) may be made small and constant. Furthermore, the increase in conductor resistance with frequency due to the skin effect is relatively small, so that the increase in the attenuation com onent due to resistance is much less rapid t an for ordinary open wire construction. Also, the form of construction is such that interference from nearby circuits and noise coming from external sources may be made practically negliible- Also, the velocity of transmission will substantially uniform for all frequencies. The invention will now be more fully understood from the following description when read in connection with the accompanying drawings, of which Figures 1 to 5 are curves illustrating the principles of the invention, and Fig. 6 is a broken section of a portion of the conducting system in accordance with the present invention.

' Referring to Fig. 6 of the drawings. 10 designates an outer conductor cylindrical in form which may be' made up of a large number of relatively small wires each insulated from the other in some suitable manner, as,

for instance, by coating the wire with anfor the other and not as a mere shield, this fact being indicated by the conventional representation of a source of alternating electromotive force G with its terminals connected to the two conductors. The conductor 12 may be a hollow tube as shown, with a core or filler 15 of any suitable material, or it may be a solid strand of insulated interwoven wires.

In order that the attenuation may be small at high frequencies, the leakage loss between the conductors must be as small as possible. As the leakage loss is due to the nature of the dielectric interposed between the conductors, the dielectric should be principally gaseous. Accordingly, the two conductors are held in proper concentric relation and out of electrical contact with each other by means of spaced dielectric washers 14. These washers should be separated from each other a suitable distance and should be made as thin as possible consistent with the required mechanical strength. They should also be composed of some dielectric of small loss angle and low dielectric constant, since if these conditions are obtained, the leakage loss (which in the ordinary open wire system comprises a large part; of the attenuation) may be made so small as to be practically negligible. For example, hard rubber, or preferably pyrex glass or other good insulting material, may be used for the insulating washers 14. In this connection it should be noted that as the outer shell may be made watertight by a suitable waterproof cover 16, the insulating washers will be maintained dry and free from dirt or contamination so that the leakage loss will not increase or change with time. In ordinary open wire construction where the insulators are exposed to the air and to the action of'the elements, the insulators become coated with a film of relatively high resistance conductive material which introduces large leakage losses, and these leakage losses are enormously increased when the external surfaces of the insulators become wet. If it were possible to maintain an open wire line with its insulators in the dry and clean condition which characterizes them when they come from the factory. th attenuation component due to leakage would be so small as to be negligible.

It is possible to mount the concentric con-' ductorarrangement upon the metallic support of an ordinary overhead cable construction or to permit the arrangement to be buried directly in the ground or laid in a conduit such as might be employed for underground cable and the flexible construction of the system due to the braided arrangement 7f the conductors especially adapts it for mounting in overhead cable hangers or in underground cable conduit. 1

In the ordinary type of conductor system, either open wire or cable, where one solid wire acts as a return for another solid wire, the component of the attenuation which is due to-the skin effect is of great importance at high frequencies. As is well known, where a.

solid conductor is employed, more and more of the current tends to flow at or near the surface of the conductor as the frequency becomes higher, so that the conductive material nearthe center of the conductor takes but little part in the action at high frequencies.

the conductor is usefully employed in transmitting current, In a system of concentric conductors in which the cylindrical walls are composed of continuous conductin material, the two conductors, bein in the orm of thin hollow shells, offer a muc 1 less resistance at high frequencies due to the skin effect forthe same amount of conductive material than in the case of an ordinary transmission circult consistlng of two SOlld wires. In fact,

with such a system of concentric conductors, .the current at higher frequencies tends to flow more and more at the inner surface of the outerconductor and the outer surface of the inner conductor, due to the skin efiect.

By forming the cylindrical conductors of a plurality of braided wire strands each insulated from the other in accordance with the present invention, the skin effect will be still further reduced. The individual wires making up a given cylindrical conductor may be braided together in any one of a number of known ways to roduce a hollow tube. For example, the met-ll basket weave'may be used, or the wires may be braided together in the manner commonly employed in the construction of the fabric covers for telephone cords, electric light cords and the like. Another method of braiding the wires is that employed for the construction of the braided wire tubes which are used as metallic shields for a group of conductors in a cable. In short, any known method of braid- 55 ing the wire may be employed so: long as the 0d of braiding known as individual strands are woven in and out to.- ward and from the center of the resulting hollow cylinder. If this is done and the strands are insulated from each other by suitably coatin them with insulating enamel or the like, t e current at high frequencies can no longer flow at the skin or surface of the cylinder but is forced to follow the path taken by the individual wire. The result is that the current flows at high frequencies just as at low frequencies substantially throughout the cross-sectional area of the cylinder.

By means of the construction above de vscribed, therefore, we have the one.compo-' reduced to practically negli ible proportions by reason of the fact that t e dielectric between the conductors is very largely of air, and such other dielectric as is employed introduces but little leakage. The result is that a concentric conductor arrangement employing the braided or woven wire as herein described can be used for the transmisison of exceedingly high frequencies without substantial attenuation, and the braided construction, furthermore, has the advantage of rendering the conductor flexible so that it may be readily inserted in cable ducts or overhead cable hangers. I

The form of construction herein disclosed also has the advantage that it does not produce material interferencein a neighboring. circuit and, conversely, may be madesubstan-- tially free from interference from nearby circuits and noise coming from external sources.

In order to understand this more clearly it should be remembered that the interference between any two circuits is due to the fact that the one circuit lies within either the electric field-or the magnetic field or both, of the other circuit. Considering first the magnetic field, let us consider two conductors a and b circular in cross-section and arranged side by side, one acting as a return for the other. These conductors are shown in section in Fi 2. The lines of force due to the cross-talk from'the conductor'system wb.

If, now, We have two conductors 10 and 12, as shown in F ig. 1, in the form of hollow shells concentrically arranged and the one acting asa return for the other, each con- .ductor has lines of magnetlc force surrounding it, each successive llne of force being of larger radius and all of the lines, due to the current flowing in the particular conductor, such as 12, being external thereto. As the current flows in one direction through thc conductor 12 and in the opposite direction through the conductor 10, the lines ofmagnetic force due to the current through the conductor 12 are in one direction, as indicated by the arrows, while those due to the current flowing in the conductor 10 are in the opposite direction. Now, an inspection of Fi 1 shows that some of the .lines of force due to the current in the conductor 12 are within the conductor 10, but none are within the conductor 12. On theother hand, all of the lines of force due to the current flowing in the conductor 10 are external to said conductor, and the two magnetic fields produced by the currents flowing in the two conductors tend to oppose each other outside of the conductor 10. The resultant field of magnetic force external to the conductor 10 is, therefore, very small, and the only eflective magnetic field lies within the space between the two conductors. Since the external magnetic field is very small it is obvious that another conductive system external to the conductor 10 will not receive any appreciable amount of cross-talk interference from the conducting system 10- 12.

In so far as the electric field is concerned, the distribution of the field in the case of two parallel conductors a and b" is as indicated in Fig. '4, so that any external conductor which is' cut by the lines of electric force between a and b will have cross-talk induced therein. In the case of thetwo concentric conductors 1012, however, the electric field set up due to currents flowing in the two conductors is entirely between the adjacent surfaces of the two'conductors, as indicated in Fig. 3. No external conductor can possibly be cut by any of the lines of the electric field-d'ue to current flowing in the conductor .12' and returning in the conductor 10, or vice versa, andhence so far as the electric field is concerned, no possible external interference can take place.

The concentricarrangement not only has the advantage that it produces substantially no external field to interfere in other circuits, but it may be practically freev from interference due to any external source. For example, referring to Fig. 5, let us assume that some external force produces a field as represented by the arrows. The lines of force cutting the two concentric conductors produce differences in potential between points of the two conductors. For example, consider the points 0 and d, the one onthe other surface of theconductor 12 and the other on the inner surface of the conductor 1Q.

,The lines of force cutting the two conductors produce an induced E. M. F. between these points in the direction and having the I equals value indicated by the arrow a-d. Since the same number of lines of force out the two conductors on the opposite side of the diagram, a .diflI'erence in potential indicated by the arrow c'd will be produced between the two points 0 and d. The induced potential c-d, however, tends to produce a current flow equal to and opposite that induced by the difference of potential at cd, so that a balance is obtained. Due to the symmetry of the conductin system with respect to the cutting lines o forcefall differences in potential induced between any other two points of the two conductors will be balancedby similar differences 'of potential induced at corresponding points on the opposite side, so that if the interfering field is evenly distributed through the cross-sectional area of the conducting system (as would be the case where the interfering source is not too near the system) substantially no interfering efi'ect would result in the conducting system 10-12.

While the foregoing explanation only applies to fields perpendicular to the axis of the conducting r system, field components parallel to theaxis may also be prevented from causing interference.

In orderthat a conducting system such as herein disclosed may have as'small attenuation as possible at high frequencies, the diameters of the two concentric conductors should be made as large as possible. However, due to practical considerations it may be desirable that the system should be of such character that it might be used in existing cable ducts or in connection with present securing proper electrical characteristics and,

mechanical strength.

As the outer conductor is, or at least can be made watertight, the leakage losses can be reduced to verylowvalues by the use of pyrex or other insulation where mechanical support isnecessary, with the largest possible air space between thetwo conductors.-

Under these circumstances the leakage loss will not change withweather conditions. For zero leakage (a condition which would be approximately obtained) the attenuation /U/L at high frequencies, where R represents the resistance, C the capacity and L the inductance. From this expression it is evident that the values of R and C should be as small as possible. For a given thickness of conlarger the diameter of ductor the value of R at high frequencies is inversely proportional to the diameter of the conductor, and hence the attenuation will be smaller at any given frequency the capacity C also is an inverse function of the diameter and decreases as the difference be tween the diameters of the inner and outer conductors increases. Consequently if the diameter of the outer conductor is fixed, as the diameter of the inner conductor increases from some small value the resistance of the conducting system decreases, while at the same time the capacity increases. The decrease in resistance tends to reduce the attenuation, while the increase in ca acity tends to increase the attenuation. or a given diameter of the inner conductor these two effects balance and the attenuation becomes a minimum. 4

At 500,000 cycles the attenuation er mile of a concentric conductor system as escribed above will be very much less than the attenuation of a 165-gauge open wire circuit. There is a further advanta e in using the former on account of the lower levels to which the current may be attenuated before a repeater is necessary.

It follows, therefore, for the transmission of frequencies-up to 500,000 cycles an open wire circuit would be quite unsuitable, whereas the concentric conductor system would carry frequencies as high as 1,000,000 to 2,000,000 cycles or even higher, without undue attenuation. A carrier telephone system could be operated over such a conductor with as man as one to two hundred two-wa channels, a lowing 5,000 c cles for eac channel in each direction. his is comparable to the number of circuits which might be obtained from the pairs of wires in a cable of equivalent size; Any particular circuit in the cable could notbe used for the transmission of frequencies much above the ordinary telephone range, and hence could not be employed for the transmission of musical pro ams involving frequencies up to the audlo without using a very expensive system. It is impracticable to arrange a cable circuit to transmit frequencies high enough for good television transmission. The concentric conductor system, on the other hand, may be employed for either program transmission or television.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely dlfi'erent from those illustrated without departm from the spirit of the invention as define in the following claims.

What is claimed isi 1. In a conducting system for the communication of intelligence, two concentric conductors,each conductor being in the form of a shell of conductive material comprising a e conductor. The

plurality of insulated wires so interwoven that each wire is passed back and forth toward and from the center of the shell, and insulating means for separating the conductors electrically and for maintaining them in concentric relation, said insulatin means being so formed that the dielectric etween adjacent surfaces of the conductors will be principally gaseous.

2. In a conductive system for the communication of intelligence, two concentric conductors, one of said conductors being in the form of a shell of conductive material comprising a plurality of insulated wires so interwoven that each wire is passed back and forth toward and from the center of the shell, and insulating means for separating the conductors electrically and for maintaining them in concentric relation, said insulating means being so formed that the dielectric between adjacent surfaces of the conductors will be principally I gaseous.

3. In a conductive system for the communication of intelligence, two concentric conductors, each conductor being in the form of a shell of conducting material comprisin a plurality of insulated wires braided toget er to form a hollow tube, the wires being so braided as to be woven inwardly and outwardly toward and from the center of the tube, and insulating means for separating the conductors electrically and for maintaining them in concentric relation, said insulating means being so formed that the dielectric between adjacent surfaces of the conductors willbe prlncipally gaseous.

4. In a conducting system for the communication of intelligence, two concentric coning them in concentric relation, sa'id insulating means being so formed that the dielectric between adjacent surfaces of the conductors will be principally gaseous.

5. In a conducting system for the communication of intelligence, two concentric conductors. one of said conductors comprising a plurality of insulated wires so interwoven that each wire is passed back and forth toward and from the center of the system, and insulating means for separating the conductors electrically and for maintaining them in concentric relation, said insulatin means being so formed that the dielectric etween adjacent surfaces of the conductors will be principally gaseous.

6. In a conducting system for the communication of intelligence, two concentric conductors, each of said conductors comprising a piuiahity of insulated wires so interwoven that each wire is passed back and, forth towardl and from the center of the system, and insuiatin means for separating the condoctors e ectricaily and for maintaining them in concentric relation, said insulatin means being so formed that the dielectric adjacent surfaces of the conductors will be principally gaseous.

In testimony whereof, we have si ed our names to this specification this 2% day of May, 1929. Y 1 HERMAN A. AFFEL.

' ESTILL I. GREEN.

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