US1885168A - Concentric conducting system - Google Patents

Description

H. A. ,AF-FEL l CONCENTRIC ONDUCTING SYSTEM Filed May 23, 1929 Sweats-Sheet fm@ @www EL@ l x /d* n? baza? Ezduced EME' 2% n INVENTOR E/r//je ATTORNEY Nov. 1, 1932. A. AFFEL 1,885,168

GONCENTRIC CONDUCTING SYS TEM Filed May 25, 1929 3 Sheets-Sheeis 0"- gz gg/$529119 525i/gg S/oira 10 INVENTOR ATTORNEY substantiall no leakage loss. The conductors themse ves maybe made up of sections of copper pipe and the sections may be connected to each other `by suitable pipe cou-A` plings, thereby enabling the conductor to be mounted upon the usual supports for overhead cable or to be pushed section by section through an ordinary underground cable conuit.

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 hig frequences 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 component due to resistance is much less rapid than for ordinary open wire construction. Also, the form of construction is such that interference from nearby circuits and noise coming from external sources will oe practically negligible. Moreover, the nature of the circuit is such that even though the outer conductor 'be ounded it will not be subject to interference 3o rom ground currents. This enables the conductor to be laid directl on the metallic supports of an overhead ca le system or underground in a conduit without any external insulation., Also the velocity of transmises sion will be substantially uniform for all frequencies.

The invention will now be more fully understood from the following description when read in connection with the accom anying drawings of which Figures 1 to are curves illustrating the rinciples of the inventon; Fig. 8 is a bro en section of a por-` tion of the conducting system in accordance with the present invention; Fig. 9 is a per- .45 spective view of the end of a conductor sect1on together with a connector for Joining two sections of conductor together' Fig. 10

is a broken section showing a modiiied orm of conductor system.; Fig. 11 shows an ar- 5o rangement for crimping the spiral dielectric used in Fig.- 10. c

Referring to Figs. 8 and 9 of the drawings,

10 designates an outer conductor which may be made up of sections of pipe of cop r or other conductive material screw threa ed at 'the ends, as indicated at 11, to enable the conductor to be extended by sections to any desired length. A second conductor 12 1s mounted concentrically with the outer con- 00 ductor 10, the conductor 12 bein also in the form of sections of cop er or ot er conductlve ipe screw threade at the ends, as indi- Ycate at 13. One of the conductors acts as a return 'for the other `and not as a mere shield,

c: this fact being indicated by the'conventionalA representation of a source of alternating electromotive force G with its terminals connected to the two conductors.

In order that the attenuation may be small at high frequencies the leakage loss between wise thereto so that the outer conductor 10 may be slipped over the outer edge of the spiral. The successive coils of the insulator 14 should be separated from each other as far as possible consistent with mechanical strength. The spiral insulator 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, the s iral insulating member 14 might be forme by shaving a spiral ribbon from the edge of a hollow cylindrical mass of hard rubber of such dimensions that when stretched out, as shown in Fig. 8, the ribbon will assume the desired spiral shape. Ur if preferred, thei insulating member 14 may be constructed by.` molding pyreX glass or other ood insulating material 1n the spiral form lndicated, so that the spiral member maybe relatively rigid.

In this connection it should be noted that as the outer shell may be made watertight the insulating member 14 will be maintained dry and free from dirt or contamination so CII ICI)

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 insulatorsbecome coated with a film of relativel high resistance conductive material whic introduces large leakage losses, and these leakage losses are enormously increased when the external surfaces of the insulators become wet. If it were ossible to maintain an open wire line with 1ts insulators in the dry and clean condition which characterizes themvwhen they come from the factory the attenuation component due to leaka e would be so small as to be negligible. It will therefore be ap arent that even if the dielectric material o which the insulatin member 14 is composed is not the most per ect from the standpoint-of low loss angle and small dielectric constant, the attenuation due tothe insulatin member may still be practically neligi le. i

, ince, as will be explained later, a conducting system of this type will be practically free from external interference even though the outer conductor is grounded, it is possible to mount the concentric conductor arrangement upon the metallic supports 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. In any case the transmission system might have to be made up of a plurality of sections of concentric tubular conductor material, but particularly in the case of underground conduit construction the sectional construction would be desirable. In this case the sections may be made of such length that a section may be inserted through a manhole and pushed into the conduit, the next section being then brought through the manhole and used to push the preceding section further into the conduit, after which it should be attached to the preceding section.

In order to accomplish this result an arrangement somewhat resembling a pipe coupling, as shown in Fig. 9,A may be employed. This consists of two short pieces of tubing 10 and 12 concentrically arranged and interiorly threaded, as shown at 11 and 13', at each end. In order to maintain the two pieces of tubing in-proper concentric relation a spacing insulator 14 of spiral form similar to that of the insulatingmember 14, may be employed. As each section of the concentric conductor is pushed into the conduit one of the coupling combinations 10- 12-14 Will be screwed to the end of the section, with the threads corresponding to 11 and 13 of the coupling mating with the external threads 11 and 13 of the conductor section. As the next section of conductor is pushed into the manhole its external threads corresponding to 11 and 13 will b caused to engage with the internal threads of the coupling and the joint will be pulled up until the joint between the outer shells of the coupling and the conductor sections will be made watertight. If desired, the outer coupling may be made watertight at the joints by the use of any well known waterproofing material. Also the joints may be -soldered or welded if necessary, and unthreaded sleeves may be used for coupling the sections of pipe.

As has been previously pointed out, no insulation between the outer conductor and any external conductor is necessary in order to prevent interference. The insulation of the system,`so far as it affects transmission,l

` is confined entirely to the space between the two concentric conduct-ors. Consequently, by making the external conductor waterproof the leakage due to the dielectric of which the insulating member 14 is composed will not change with wet weather, and the surfaces thereof will not deteriorate with time due to which will be due to the dielectric material.

of which the spiral member 14 is composed when it is new, clean and dry. If reasonably good dielectric material is employed the leakage loss due to the insulating member 14 will be practically negligible, and if a material of very low loss angle and dielectric constant is used, such for example as pyrex glass, the factor of attenuation which is d'ue to leakage will be so small as to be practically negligible. In ordinary open wire construction (which has the lowest attenuation vat high frequencies of any type of construction now employed in telephone practice) the attenuation due to vleakage losses has been very large, and in wet weather becomes enormous. With the type of construction herein disclosed this factor of-the attenuation becomes of little importance, and any attenuation due to this factor is fixed and unchangeable with variations in weather conditions.

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, as the frequency becomes higher more and more of the current tends to flow at or near the surface of the conductor, so that the conductive material near the center of the conductor takes but little part in the action at high frequencies. As a consequence, the conductor resistance increases with frequency as a.

smaller and smaller part of the cross-section of the conductor is usefully employed. If the same amount of conductor material is arranged in the form of a relatively thin shell, the resistance at any given high frequency is very much reduced because now more nearly all of the material of the conductor is usefully employed in transmitting current. With a system of concentric conductors, such as described in connection with the present invention, both conductors, being in the form of thin hollow shells offer a much less resistance at high frequencies due to the skin effect for the same amount of conductive material than in the case of an ordinary transmission circuit consisting of two solid wires. In factwith a system of concentric conductors such as herein disclosed. the current at higher frequencies tends to flow more l rate of increase is veryl much less than in the case of an open wire line. By means of the construction above described, therefore, we have the one com onent of the attenuation which is due to lea age losses or the so-called shunteflz'ect reduced to practically ne l1- gible proportions by reason of the fact t at the dielectric between the conductors is very largely of air and such other dielectric as'ls employed introduces but little leakage, while the other component of attenuation, namely, that due to the conductor resistance or socalled series effect is very much reduced as compared with the Ordinar type of conducting systeln for any given requency.

The form of construction herein disclosed also has the advantage that it does not produce material interference in a neighboring circuit and, conversely, is substantially free from interference 'from nearby circuits and noisev 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. Consideringr 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 1n section 1n Fig. 2. The lines of force due to the magnetic larger radius and all of the lines, due to the site direction.

current flowing in the particular conductor, such as 12, being external thereto.- ,As the current flows in one direction through the con-v ductor 12 and in the opposite 'direction through the conductor 10, the lines of magnetic force due to the current through the conductor 12 are in one direction, as indicated by the arrows, while those due to the current fiowing in the conductor 10 are in the oppo- Now, an inspection of Fig. 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 the other hand, all of the lines of force due to the current flowing in the conductor 10 are external tosaid conductor, and the two magnetic fields produced by the currents fiowing in the two conductors tend to oppose each other outside of the conductor 10. The resultant field of magnetic force g external to the conductor 10 is, thereforevery assunse small, and the only effective magnetic field lies within thefspace between the two conductors.' Since the external magnetic field is very small it is obvious that another conductivesystem external to the conductor 10 will not receive any appreciable amount of Acrosstalk 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 crosstalk induced therein. InA the case of the two concentric conductors 10-12, however, the electric field set up due to currents fiowin in the two conductors is entirely between te 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 due to current flowing in the conductor 12 and returning in the conductor 10, or vice versa, Land hence so far as the electric field is concerned, no possible external interference can take place.

The concentric Varrangement not only has the advantage that it produces substantially no external field to interfere in other circuits, but it is practically free from interference due to any externalsource. For example, referring to Fig. 5, let Vus assume Athat some external force produces a field as represented by the arrows. The lines ofl force cuttin the two concentric conductors produce ifferences in potential between points of the two conductors. For example, consider the oints c and d, the one on the outer surface o the conductor 12 and the other on the inner surface of the conductor 10. The

lines of force cutting the two conductors pro- Aduce an induced E. M. F. between these points in the direction and having the value indicated by the arrow c`d. Since the same number of lines of force cut the two conductors on the opposite side of the diagram, a difference in potential indicated byfthe arrow c-d will be produced between the two points c and d. The induced potential od, however, tends to produce a current How equal to and opposite that induced by the difference of potential at c-d, so that a balance isobtained. Due to the symmetry of the conducting system with respect to the cutting lines of force, all differences in potential induced between any other two points of the two conductors will be balanced by similar differences of potential induced at corresponding points on the opposite side,

so that if the interfering field is evenly distributedvthrough the cross-sectional area of the conducting system (as would be the case where the interfering sourceis not too near the system) substantially no interfering. ef-

feet .would result inthe conducting system l10-12.l

seams While the foregoing explanation only applies to fields perpendicular to the axis of the conducting system, held components parallel to the axis are also prevented from causing interference. This is because the skin eii'ect in the outer conductor furnishes protection against such fields.

As has been previously stated, the concentric conducting system is free from external interference even though the outer conductor be grounded, and hence there is no necessity for insulating the outer conductor from metallic supports in case it is :mounted like an overhead cable, or from yground in case it is placed in a conduit. The reason for this is that a ground return circuit is noisy, due to the fact that a wire supported above ground forms with the ground a loop to pica up stray elds. But from the diagram of Fig. it is evident that if the outer conductor such as is grounded so that it in effect becomes a ground return for the tube 12, it is only the s ace between the two concentric conductors t at-acts as the loop to pick up stray fields. Hence, as has been just explained in connection with Fig. 5, substantially no interfering currents are induced in the conductors 10-12.

In order that 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 aerial cable construction. For these reasons,

in practice, it is convenient to make the diameter of theexternal conductor not much greater than about two and five-eighths inches, if the conductor is to be used in theexisting telephone plant. For mechanical reasons the thickness of the conductors should be made as small as is consistent with securing proper values of electrical resistance and mechanical strength. In general, ithas been found that if the tubing is made thick enough to satisfy the mechanical r uirements, thel electrical resistance is not a imiting factor in the attenuation at high frequencies. This isdue t0 the skin eect or proximity effect, which, as previously described, causes the current to crowd to the outer surface of the Ainner conductor and the inner surface of the v 12 inches mission units, which com ares with 0.43

ameter of the conductor. The capacity C also is an inverse function of the diameter and decreases as the difference between the diameters of the inner and outer conductors increases. Consequentl if the diameter of the outer conductor is xed, 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 reduce thel'attenuation, while the increase incapacity tends to increase the attenuation. For. a given diameter of the inner conductor these two effects balance and the attenuation becomes a minimum.

Fig. 6.1s a curve showing how the attenuation varies with diameter of the inner conductor at 500,000 cycles, with the inner diameter of the outer conductor fixed at two and one-half inches. This curve shows a minimum attenuation of .43 transmission units per mile for an inner conductor diameter of about .7 inch. As will be clear from the curve, either an increase or decrease of the diameter of the inner conductor from the foregoing value results in an increase in the attenuation. In Fig. 7 is showna curve of the attenuation at various frequencies of a concentric conductor system whose outer conductor has a diameter of two and one-half inches and the inner conductor has the o timum diameter of about .7 inch. It will be o rved from this curve that while the attenuation increases with frequency, the slope of the curve is not stee and the increase in attenuation is very muc less than would be the case for an open wire line.

At 500,000 cycles the attenuation per mile of a 165 o n wire circuit with a spacing of btween wires is about 1.67 transtransmission units per mile or the concentric conductor system. The advantages of using thelatter are even greater than would apar from these fi res on account of the owerlevels to which the current may be attenuated before a repeater is nry. This is due to the absence of coupling to external elds and results in a very low noise level. On an open wire circuit the level could lll l nels, al

notbe allowed to go below 50 transmission units, while with the concentric conductor system it might be permitted to fall as low as -80 transmission units. If the repeaters are adjusted to give an output of +10 transmission units this would result in a repeater s acing of thirty-six miles for the open wire circuit and 210 miles for the concentric return circuit. It appears to be impractical to devise a transmission system for an open wire circuit at such 'high frequencies, and the high frequency crosstalk would limit its use to one circuit on a given lead. Due to the absence of couplings to other circuits this limitation would not apply to the tubular conductor s stem, and any desired number of such con uctor systems might be mounted upon the same pole line or carried in adjacent conduits without undue interference.

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 'sucli a conductor with as manI as one to two hundred two-way chanowing 5,000 cycles for each channel in each direction. Thisis comparable to the number of circuits which might be obtained from the pairs of wires in a cable ofv equivalent size. Any particular circuit in the cable could not be used for the transmission of frequencies much above the ordinary telephone range, and hence could not be employed for the transmission of musical programs involving frequencies up to the audio limit without using a very expensive loading system. A cable circuit could not conceivably be loaded to transmit frequencies high enough for good television transmission. The concentric conductor system, on the other hand, may be em loyed for either program transmission or te evisiou.

Fig. 10 illustrates a modified form of concentric conductor arran t employing a spiral dielectric o f s orm that the amount of dielectric m rial other than air between the conduct s will be even less than in the case of thearrangement of Fig. 8. Here the dielectr ribbon 24 is made thinner than in the ca of Fig. 8 and is spirally wound about the inner conductor ,12' so as to be interposed edgewise between the inner lconductor 12 and the outer conductor 10. In order to give the spiral ribbon the necessary rigidity it is crimped as shown in the figure. As the outer edge of a single spiral has a reater length thanthe inner edge which is a jacent to conductor 12, the convolutions or folds are more widel s aced at the outer edge and gradually iminish in spacing towards the inner ed i crimped ribbon adapted to be spirally Wound as indicated in Fig. 10 may be produced as shown in Fig. 11. Here a very thin plain ribbon 23 of some suitable dielectric material is run between two rollers 25 and smaller part of the roller. The corrugations in the two rollers mate each other so that as the ribbon 23 passes between the rollers it is crimped as shown at 24.

, It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without de arting from the spirit of the invention as degned in the following claims.

What is claimed is:

1. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being c lindrical in form and the twoconductors being arranged concentrically, means to separate the two conductors electrically and maintain them in concentric relation, said means comprising a strip of insulating material coiled spirally edgewise about the inner conductor with its outer edge supporting the outer conductor, said strip being of such thin cross-section and the successive coils thereof being so spaced that the dielectric in the space between the conductors is largely gaseous.

2. In a conductin system for the communication of intelligence, two conductors' connected one as a return for the other, each conductor being hollow and cylindrical in form and having a diameter large as comy pared with its wall thickness so that its attenuation will be relatively small at high frequencies, said conductors being arranged concentrically, means to separate the two conductors electrically and maintain them 1n concentric relation, said means comprising a strip of insulating material coiled spirally edgewise about the inner conductor with its outer edge supporting the outer conductor, said strip being of ksuch thin cross-section and the successivey coils thereof being so s aced that the dielectric in the space between t e conductors is largely gaseous.

3. In a conducting system for the communication of intelligence, two cylindrical electrical conductors having a common axis andA separated by insulating material arran ed in the form of a thin corrugated helix Elietween the said conductors.

4. In a conducting system for the communication of intelligence, two conductors connected one as a return for thel other, each conductor being cylindrical in form and the two conductors being arranged concentrically, means to separate the two conductors electrically and maintain them in concentric relation, said means comprising a thin strip of insulating material corrugated transversely and coiled spirally about the inner conductor, said strip being of such cross-section and the successive coils thereof being so spaced that the dielectric in the space between the conductors is largely gaseous.

5. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being cylindrical in form and the two conductors being arranged concentrical- 1y, means to separate the two conductors electrically and maintain them in concentric relation, said means comprising a thin strip of insulating material corrugated transversely and coiled spirally edgewise about the in? ner conductor with its outer edge supporting the outer conductor.

6. In a conducting system for the communication of intelligence, two conductors `connected one as a return for the other, each conductor being cylindrical in form and the two conductors being arranged concentrically, means to separate the two conductors electrically and maintain them in concentric relation7 said means'comprising a thin strip of insulating material coiled spirally edgewise about the inner conductor with its outer edge supporting the outer conductor, said strip being corrugated transversely, the corrugations being more-widely spaced nearer the outer edge and the s acin-g gradually diminishing as the edge a jacent the inner conductor is approached. l

7. In a'conducting system forthe communication of intelligence, two conductors connected one as a return for the other,"each conductor being cylindrical in form and the two conductors being arranged eoncentrical- 1y, means to separate the two' conductors electrically and. maintain them in concentric relation, said 'means comprising a thin flat strip of insulating material coiled s irally about the inner conductor anddispose edge wise thereto at a plurality of points about said conductor, the inner edge of said strip supporting the inner conductor at some of said points, and said strip being of such thin cross-section and the successive coils thereof being so spaced .that the dielectric in the space between the conductors is largely gaseous.

8. In a conducting system for the communication of intelligence, two conductors c onnected one as a return for the other, elch conductor being cylindrical inform and the two conductors being arranged concentrical.-

ly, means lto separate the two conductors electrically and maintain them in concentric :jelation, said means compri a thin Hat stnp of insulating material coil spirally about the inner conductor, said strip having its lon er cross-sectional dimension arranged radially between said conductors at a plurality of points along said inner conductor and being adapted to support said inner conductor at a plurality of points about its circuinference, said strip being of suchthincross-section and the successive coils thereof being so f nected one as a return for the other, each conductor being cylindrical in form and the two conductors being arranged concentrically, means to separate the two conductors electrically and maintain them in concentric relation, said means comprising insulating material in the form of a flat strip disposed about the inner conductor, said material being arranged endwise to saidI inner conductor at a luralityof points along said conductor and ing ada ted to support said inner conductor at a uralit of points about its surface, and sai material bein of such thin crosssection and so spaced t at the dielectric in the space between the conductors is largely gaseous.

10. In a system for the transmission of 1ntelligence, a transmission circuit comprising two cylindrical conductors arranged one as a returnfor the other, said conductors being arranged in coaxial relation to'one another, and means to separate the two conductors electrically-and maintain them in coaxial relation,'said means comprising insulating material coiled spirally about said inner conductor, the cross-sectional dimensions of said insulating material being less when measured lin certain directions than when measured in other directions, said insulating material being so arra ed that it supports said inner conductor o y at points where the greatest cross-sectional dimension of the insulating material lies in a radial position, said insulating material occupying a relatively small portion of the space tween conductors, and

the material occupyin the remainder of said space being substantie ly gaseous.

In testimony whereof, I have signed my name to this specification this 20th day of` May 1929.

y A. AFFEL.

Images