|Publication number||US1781124 A|
|Publication date||Nov 11, 1930|
|Filing date||May 23, 1929|
|Priority date||May 23, 1929|
|Publication number||US 1781124 A, US 1781124A, US-A-1781124, US1781124 A, US1781124A|
|Inventors||Nein Harry R|
|Original Assignee||American Telephone & Telegraph|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (6), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Nov. 11, 1930. NEIN 1,781,124
CONCENTRIC CONDUCTING SYS TEM Filed May 23. 1929 2 Sheets-Sheet l I? Q a 0 ,0
A 1 10 1 ma! EMF INVENTOR ATTORNEY Nov. 11, 1930. H. R. NEIN 1,781,124
CONCENTRIC CONDUCTING SYSTEM Filed May 23. 1929 2 Sheets-Sheet 2 .50- EJO- w pervade 1 L157. 9 2 INVENTOR ATTORN EY Patented Nov. 11, 1930 UNITED STATES PATENT OFFICE HARRY B. m, WESTIIELD, NEW JERSEY, ASSIGNOB TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION 01' NEW YORK CONCENTBIC OONDUC'I'IN G SYSTEM Application fled 1:137 23, 1929. Serial Io. 865,514.
This invention relates to a conducting s stem for transmitting with small attenuation a band of frequencies whose upper limit extends well above the frequencies now em- 5 ployed in carrier transmission.
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 provide circuits that will transmit a band of frequencies extendin well up toward 10,000 cycles, as compared with the voice range ordinarily employed in telephon which did not exceed about 2,500 cycles. or the best qual ty of transmission of music it might be desirable to transmit over telephone lines frequencies up to the audio limit, which would be in the neighborhood of 15,000 or 20,000 cycles. Modern cable circuits are not adapted for transmission of such high frequencies, and the only commercial circuit 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.
Even for carrier transmission, open wire lines have been found uncommercial for the transmission of frequencies much above 30,-
v 000 cycles. If, therefore, an open wire line is used for the transmission of a high grade audio frequency program involving frequencies in the neighborhood 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 hi hly desirable to ha e available a clrcuit w ch comes desirable to have available a conduct- "the same time available would transmit without undue attenuation frequencies much higher than 30,000 cycles.
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 relatively large, so that the picture is not well defined. The mechanical problems involved in designing a television transmitting and receiving 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 readily capable of solution by known means, but the transmission of such a picture electrically involves the transmission of frequencies from zero up to the neighborhood of 500,000 cycles, and there is no telephone circuit now available which would commercially transmit any suchrange of frequencies because of the enormous attenuation involved at frequencies above about 30,000 cycles. It therefore being system which would transmit without undue attenuation a wide range of frequencies including the extremely high frequencies necessary for television, the circuit being at 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 present invention a circuit havin these desirable characteristics is obtained y employing concentric conductors of relativel large diameter, one acting as a return for tlie other. The conductors may be insulated from each other and held in pro r concentric relation by means of spaced was ers of some suitable dielectric of small loss angle and low dlelactnc constant so as to introduce a minimum leakage loss between the conductors. By spacin the washers relatively far apart the principal part of the dielectric between the two conductors will be air which, as is well known, involves sub-.
stantially no leakage loss. The conductors themselves may be made up of strips or ribbons of copper or other suitable conducting material helically wound to form a substantially continuous cylindrical shell. These strips or ribbons of conducting material are crimped at their edges and otherwise pressed into the general form of the outer metallic protective tape used in so-called B. X. cable. One ribbon is helically wound about a suitable core with the edges of successive coils having their crimped portions overlapping to form the inner conductor. The dielectric washers are then mounted upon the inner conductor and another conducting tape or ribbon'similarly coiled upon the outer surfaces of the washers to form the outer concentric conductor. The whole construction may then be covered externally by a suitable waterproof covering such as is used in ordinary telephone cable or submarine cable.
A conducting system such as above outlined has a number of advantages since it may be made waterproof, with the result thatthe 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 eflect 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 be practically negligible. Moreover, the nature of the circuit is such that even though the outer conductor be grounded it will not be subject to interference from ground currents. This enables the conductor system to be laid directly on the metallic supports of an overhead cable or un-.
derground in,a conduit without any external insulation, even where the waterproof covering is composed of a conductive lead or other metallic sheath. Also, the velocity of transmission will be uniform for all frequencies.
Theinvention will now be more fully understood from the following description when read in connection with the accompanying drawing, of which Figures 1 to 7 are curves illustrating the principlesof the invention; and Fig. 8 is a perspective view partially broken away of a portion of the conductor arrangement of the invention.
Referring to 1 Fig. 8 of the drawing, 10 designates an outer conductor, 12 an inner conductor mounted concentrically therewith,
14 dielectric washers for separating the inner and outer conductors and maintaining them in concentric relat1on,.15 a center coreupon which the inner conductor is mounted, and
In order to form the conductors, strips orv ribbons of conducting material, such as copper, are pressed into the eneral shape of the metallic ta e employed or protecting socalled B. X. ca le. These strips are crimped or formed at the edges as shown at 17, and one of the strips is spirally wound upon the core 15 with its crimped edges overlapping, as
shown, to form the inner conductor 12. The dielectric washers 14 are then mounted upon the inner conductor 12 in suitable spaced relation and another tape or ribbon is similarly helically wound over the outer surfaces of the washers to form the outer conductor, the whole being rendered waterproof by a suitable external covering.
In order that the attenuation may be made 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 of air, since air introduces no leakage loss. Accordingly, the spaced dielectric washers 14 which hold the two conductors in proper concentric relation and out of electrical contact with each other, should be separated from each other by suitable distances, 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, may be used for the insulating washers 14.-
In this connection it should be noted that as the outer shell may be made watertight, 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 be so small as to be negligible. It will be the outer'jconductor is grounded, it is posevident, therefore, that the use of spacing washers in a manner above described introduces substantially no leakage loss.
Since, as will be explainedlater, a conducting s stem of this type will be practicall free rom external interference'even though sible to mount the concentric conductor arrangement upon the metallic supports of an ordinary overhead cable construction, or to permit the conductor arrangement to be uried directly in the ground or laid in a conduit such as might be employed for under.- ground cable. The concentric conductor arrangement herein described is particularly adapted for these purposes as the helical arrangement of the conducting strips ermits.
of a flexible construction similar tot at obtaining in ordinary tele hone cable so that it may readily. be pushe through the ducts of an underground conduit.
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, is confined entirely to the space between the two concentric conductors. Consequently, b makin the external conductor waterproo the lea age due to the dielectric of which the washer 14 is composed will not change with wet weather, and the surfaces of the dielectric washers will not deteriorate with time due to accumulations of dirt or other foreign substances. The leakage loss of the system will therefore be confined to that leakage loss which will be due to the dielectric material of which the washers are composed when the washers are new, clean and dry. If reasonably good dielectric material is employed, the leakage loss due to the supporting washers 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 due to leakage will be so small as'to be practically negligible. In ordinar open wire line construction (which has t e lowest attenuation at high frequencies of any type of construction now employed in telephone practice) the attenuation due to leakage losses has been very large, and in wet weather becomes enormous. With the resent type of construction this factor of t e attenuation becomes of little importance, and any attenuation due to this factor is fixed and unebangeable 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 a 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 hi her more and more of the current tends to ow at or near the surface of the conductor, so that the conductive ma- I terial near the center of the conductor takes but little part in the action at high frequencies. As a con uence, the conductor resistance increases wit 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 arran d in the form of a relatively thin shell, t e resistance at any given high frequency is verfi much reduced because now more nearly a 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, bothconductors, being in the form of thin hollow shells, offer a much less resistance at high frequencies due to-the skin efiect for the same amount of conductlve material than in the case of an ordinary transmission cir cuit consisting of two solid wires. In fact,
with a system of concentric conductors such as herein disclosed, the current at higher frequencies tends to flow more and more at the inner surface of the outer conductor and the outer surface of the inner conductor, due to the well known proximity efiect. The result is that while that component of the attenuation which is due to the conductor resistance increases with freqllliency, the rate of increase is very much less t an in the case of an open wire line. By means of the construction above described, therefore, we have the one component of the attenuation which is due to leakage losses or the so-called shunt effect reduced to practically negligible proportions by reason of the fact that thedielectric between the conductors is very largely of air and such other dielectric as is employed introduces but little leakage, while the other component of attenuation, namely,
that due to the conductor resistance or socalled series efiect is very much reduced as compared with the ordinary type of conducting system for any given frequency.
The form of construction herein disclosed also has the advantage that it does not'produce material interference in the neighboring circuit and, conversely, is substantially 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 Fig. 2. The lines of force due to the magnetic field surround each conductor and are crowded together in the space between the two conductors. Any other conducting system introduced at a point where the conductors of such other system will be cut by these lines of force will have induced therein cross-talk from the conductor system 0-6. If, now, we have two conductors 10 and 12, as shown in Fig. 1, in the form of hollow shells concentrically arranged and the one acting as a return for the other, each conductor has lines of magnetic force surrounding it, each successive line of force being of larger radius and all of the lines, due to the current flo ing in the particular conductor, such as 12, ing external thereto. As the current flows in one direction through the conductor 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 flowing in the conductor 12 are in' the opposite direction. Now, an ins ection of Fig. 1' shows that some of the lmes 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 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 effective 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 1012.
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 wil have cross-talk induced therein. In the case of the two 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 due to current flowing in the conductor 12 and returning in the conductor 10, or vice versa, and hence .so far as the electric field is concerned, no possible external interference can take place.
The concentric arrangement not.only has the advanta e that it produces substantially no external field to interfere in other circuits, but it is practically free 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 betweenv points of the two conductors. For example, consider the points 0 and d, the one on the outer surface of the conductor 12 and the other on the inner surface of the conductor 10. The lines of force cutting the two conductors produce an induced E. M. F. between these points in the direction and having the value indicated by the arrow c-ri. Since the same number of lines of force out the two conductors on the opposite side of the diagram, a difference in potential indicated by the arrow c'0l' will be produced between the two points 0 and d. The induced potential cd, however, tends to produce a current fiow equal to and opposite that induced by the difference of potential at c'd, so that a balance is obtained. 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 difi'erences of potential induced at corresponding points on the opposite side, so that if the interfering field is evenly distributed through the cross-scectional area of the conducting system (as would be the case where the interfering source is not too near the system sub stantially no interfering efiect would result in the conducting system 10-12.
While the foregoing explanation only applies to fields perpendicular to the axis of the conducting system, field components parallel to the axis are also prevented from causing interference. This is because the skin effect in the outer conductor furnishes pro- -tection against such field.
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 ground 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 pick up stray fields. But from the diagram of Fig. 5 it is evident that if the outer conductor such as 10 is grounded so that it in effect becomes a ground return for the conductor 12, it is only the space between the two -concentric conductors that acts as the loop to pick up stray fields. Hence, as has been just exp ained 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 ma have as small attenuation as possible at igh frequencies, the diameters of the two concentric conductors should be made as large as ossible. However, due to practical consi erations 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 dimeter of the external conductor not much greater 'than about two and five-eighths inches, if the conductor is to be used in the existing telephone plant. For mechanical reasons the thickness of the conductors should be made as small as is consistent with securing ro r values of electrical resistance and mac anical stren h. In general, it has been found that if e concentric conductor is made thick enough to satisfy the mechanical requirements, the electrical resistance is not a limiting factor in the attenuation at high frequencies. This is due to the skin effect or proximity effect which, as previously described, causes the current to crowd to the outer surface of the inner conductor and the inner surface of the outer conductor as the frequency increases, thereby rendering the remaining cross-sectional area of little utility for carrying current.
As the outer conductor is or at least can be made watertight, the lealrage losses can be reduced to very low values by the use of pyrex insulation where mechanical support is necessary, with the largest possible air space between the two conductors. Under these circumstances the leakage loss will not change with weather conditions. For zero leakage (a condition which would be approximately obtained) the attenuation equals R/2 /zl/L at high frequencies, where represents the resistance, 0 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. At high frequencies R is inversely proportional to the diameter of the conductor, and hence the attenuation will be smaller at any given frequency the larger the diameter of the conductor. 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 om 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 attennation while the increase in capacity 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 is a curve showing how theattenuation varies with diameter of the inner conductor at 500,000 cycles, with the diameterof the outer conductor fixed at two and onehalf 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 shown a 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 optimum diameter of about .7 inch. It will be observed from this curve that while the attenuation increases with fre uency, the slope of the curve is not steep1 an the increase in attenuation is very muc less than would be the case for an open wire line.
At 500,000 cycles the attenuation r mile of-a 165-gau open wire circuit wit a spacing of 12 inc es between wires is 1.67 transmission units, which compares with .43 transmission units per mile for the concentric conductor system. The advantages of usingthe latter are even greater than would a pear from these fi res on account of the lbwer levels to whic the current may be attenuated before a repeater is necessary. This is due to the absence of coupling to external fields and results in a very low noise level. On an open wire circuit the level could not be allowed to go below transmission units, while with the concentric conductor system it might be permitted to fall as low as transmission units. If the repeaters are adjusted to give an output of +10 transmission units, this would result in a repeater spacing of 36 miles for the open wire circuit and 210 miles for the concentric return circuit. It appears to be impractical to devise a transmisslon system for an open wire circuit at such high frequencies, and the high frequency cross-talk 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 concentric conductor system, and any desired number of such conductor 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 I could be operated over such a conductor with as many as one to two hundred two-way channels, allowin 5,000 cycles for each channel in each directlon. This is comparable to the number of circuits which might be obtained from the pairs of wires in a cable ofequivalent 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 musicalprograms 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 00d television transmission. The concentrlc conductor system, on the other hand, may be employed for either program transmission or television.
It will be obvious that the general rinciples herein disclosed may be embo ied in many other organizations widely different from those illustrated without departin from the spirit of the invention as define in the following claims.
What is claimed is:
1. In a conducting system for the communicationof intelligence, two conductors connected one as a return for the other, each conductor being in the form of a shell of conductive material and the two conductors being arranged concentrically one inside the other, means to prevent moisture from entering the interior of the outer conductor, the inner conductor being formed by spirally winding a thin strip of conducting material so that the edges of successive spirals overlap, 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 gaseous, and said outer conductor bein formed by spirally winding a thin strip 0 conducting material over said insulating means so that the edges of successive spirals overlap.
2. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being in the form of a shell of conducting material and having a diameter large as compared with its wall thickness so that its attenuation will be relatively small at high frequencies, said conductors being arranged concentrically one inside the other, means to prevent moisture from entering the interior of the outer conductor, the inner conductor bein formed by spirally winding a thin strip 0 conducting material so that the edges of successive spirals overlap, and insulating means for separating the conductors electrically and for maintaining them in concentric relation, said insulating means being so formed that the dielectric will be principally gaseous, and said outer conductor being formed by spirally windin a thin strip of conducting material over said insulating means so that the edges of successive spirals overlap.
3. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being in the form of a shell of conductive material and the'two conductors being arranged concentrically one inside the other, means to prevent moisture from en-,
adjacent surfaces of the conductors will be principally gaseous, and said outer conductor being formed by spirally winding a thin between adjacent surfaces of the conductors strip of conducting material with crimped edges over said insulating means so that the crimped edges overlap.
4. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, each conductor being in the form-of a shell of conducting material and having a diameter large as compared with its wall thickness so that its attenuation will be relatively small at'high frequencies, said conductors being arranged concentrically one inside the other, means to prevent moisture from entering the interior of the outer conductor, the inner conductor being formed b spirally windin a thin strip of conductmg material wit crimped edges so that the crimped edges of successive spirals overlap, 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 gaseous, and said outer conductor being formed by spirally winding a thin strip of conducting material with crimped edges over said insulating means so that the crimped edges overlap.
5. In a conducting system for the communication of intelligence, two conductors connected one as a return for the other, the outer conductor being in the form of a shell of conductive material and the two conductors being arranged concentrically one inside the other, means to prevent moisture from entering the interior of the outer conductor, and insulating means for separating the conductors electrically and for main-.
electric between adjacent surfaces of the conductors will be principally gaseous, and said outer conductor being formed by spirally winding a thin strip of conducting material over said insulatlnf means so that the ed es of successive spira s overlap.
n testimony whereof, I have signed my name to this specification this 20th day of May, 1929.
-HARRY R. NEIN.
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|U.S. Classification||174/28, 174/108, 333/243, 174/131.00R|