US2119853A - Electric wave transmission system - Google Patents

Description

June 7,' 1938. H. E. CURTIS 2,119,853

ELECTRIC WAVE TRANSMISSION SYSTEM Filed Oct. 50, 1955 2 Sheets-Sheet 1 11; I 7;2.27 I F/G.

//v l EN 70/? H. E. CUR 775 A T TORNEV June 7, 1938. H. E. CURTIS ELECTRIC WAVE TRANSMISSION SYSTEM 2 Shets-Shwt 2 Filed Oct. 30, 1935 FIG. 6

FIG. 7

N m R 0 n m m m w E H u Fall IL W .1 l 0 MR ym T T vu a h r m F I|L //v l/ENTOR H. E. CUR T/S ATTORNEY Patented June 7, 1938 UNITED STATES ELECTRIC WAVE TRANSMISSION SYSTEM Harold E. Curtis, East Orange, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 30, 1935, Serial No. 47,376

4 Claims.

This invention relates to electrical transmission circuits and especially circuits derived from a number of pairs of wires enclosed in a common conducting shield.

An object of the invention is to obtain transmission circuits which have the property of low attenuation over a wide band of frequencies.

In accordance with the present invention it is proposed to enclose a number of pairs of conductors in a conducting shield so as to obtain a number of independent electrical transmission circuits. The shield acts to prevent external electromagnetic and electrostatic high frequency disturbances from causing disturbances in any of the circuits.

In order to reduce the high frequency attenuation of each individual circuit, it is proposed to secure low shunt losses by employing a dielectric having a small power factor and to reduce the series losses in the conductors by employing an insulating medium having a low dielectric constant. Accordingly, it is proposed in one embodiment of the invention to utilize a substantially gaseous dielectric between the conductors of the pairs and the shield. The invention comprehends also, however, the use of non-gaseous dielectric material between the conductors and the shield.

A feature of the invention is the provision of a particular conductor arrangement which will substantially minimize the high frequency attenuation of each circuit.

The invention is concerned especially with systems in which circuits derived from a number of pairs of conductors enclosed in a single shield are utilized for the transmission of high frequencies or wide bands of frequencies.

The satisfactory transmission of television images with good definition requires the transmission of a frequency band which may extend from zero frequency to hundreds or perhaps thousands of kilocycles. If, for example, it is desired to transmit, with a total of 24 reproductions per second, an image containing 40,000 picture elements, there is required a frequency band of approximately 500 kilocycles in width. Still wider frequency bands may be necessary for reproducing with adequate detail such scenes as a theatrical performance or an athletic event. A television band of such width may be transmitted directly over a shielded pair constructed in accordance with the principles of the invention or it may be shifted to a higher frequency position in order to avoid the necessity of transmitting the extremely low television frequencies over the line.

Moreover, by the application of multiplexing the wide frequency bands obtained from such pairs which are constructed in accordance with the invention may be used to provide substantial numbers of narrower frequency bands suitable for other communication purposes, as, for example, for telephone circuits which may require bands of about 2500 cycles in width, for high quality program circuits which may require bands extending up to 10,000 cycles or higher, for high speed facsimile transmission or for other services.

Other objects and features of the invention will be apparent from the following description and claims.

Fig. 1 is a cross-sectional diagram of a pair of conductors effectively isolated in space, proportioned in accordance with the invention;

Figs. 2 to 5, inclusive, represent various transmission structures embodying forms of the invention, each of these structures comprising a number of pairs of wires enclosed in a circular conducting shield; and

Figs. 6 and 7 show diagrammatically transmission systems embodying the principles of the invention.

Considering at first a single pair of wires l and 2 as shownin Fig. 1, the size of wires which will make the high frequency attenuation a minimum for any given separation can be determined as shown below. It will be assumed that the frequency is well above the audible range and that the leakage is zero (a condition that may be approximately obtained). All units are in the c. g. s. electromagnetic system. The wires are assumed to be solid conductors, i. e., to be of solid material.

The high frequency attenuation of an electrically smooth line with zero leakage is given very closely by the expression where R represents the resistance, C the capacitance, L the inductance and G the leakance of the system.

The total high frequency resistance of the circuit is given by the expression 2P pf 4 cosh' (4) where is is the dielectric constant 1() for air).

At high frequencies L=4 cosh g) (5) Placing these expressions in Equation (1) and assuming for the present that the leakance is zero we obtain For any given high frequency and given wire separation, assuming the dielectric constant, conductivity and permeability to be fixed, the attenuation may be minimized by differentiating the expression L ter -e) with respect to the ratio and equating it to zero. Performing these operations we find that should equal approximately 2.27 for minimum attenuation.

The foregoing derivation of the proportioning of an open wire circuit in order to obtain minimum high frequency attenuation has been di rected toward the case where the insulating medium is largely gaseous so that the dielectric constant is substantially unity and a leakage conductance substantially zero. It can be shown, however, that the optimum proportioning will remain substantially unchanged for other types of dielectric. If the conductors are embedded in a homogeneous non-gaseous dielectric as, for example, rubber or oil, extending to infinity the ratio giving minimum high frequency attenuation is theoretically the same as for a gaseous dielectric. For practical purposes if the dielectric only substantially covers the conductors, the optimum ratio Will be the same as for a substantially gaseous dielectric. This will also be the case when a mixture of dielectric is employed, for example, a combination of gaseous and nongaseous dielectrics provided that the arrangement of the dielectric is such as not to distort the path which would be assumed by the dielectric flux if the dielectric medium were entirely gaseous. Where a combination of dielectrics is employed in such a manner as to produce such distortion of the flux both the ratios for optimum proportioning may be changed to some extent but, in general, characteristics approaching the optimum will be obtained for the values which have previously been set forth. In a practical structure the dielectric should be as nearly com pletely gaseous as possible, a minimum of solid dielectric being used in order to reduce the dielectric loss of the circuit.

Some of the fundamental principles of the invention having now been set forth, further consideration may be given to types of structures in which these principles may be incorporated. Fig. 2 represents a view of a transmission structure consisting of seven pairs of conductors enclosed in a. circular shield. In this figure, i and 2 represent two solid conductors which may be held in position with respect to one another and the metallic shie1d'3 by insulating spacers 4 or other suitable devices. Wrapped spirally around the insulators 4 is the paper tape 5, thereby forming an insulating tube separating it from the shield and other pairs. The conductors of the pair are connected one as a return for the other as is indicated conventionally by the generator G.

The conductors i and 2 are of such a type that currents of frequencies well above the audible range travel substantially on the outer surfaces of the conductors. For example, the conductors may be solid wires or may be tubular. If tubular conductors are employed, their wall thickness will ordinarily depend upon mechanical rather than electrical considerations, since only a very thin wall is required for the conduction of the high frequency currents.

In such a structure the size of the shield is generally limited by economic or practical reasons and accordingly so is the separation of the pairs which are included in the shield. The problem arises as to (1) what should be the wire size and wire spacing to obtain minimum attenuation for a given number of wires to be enclosed in a sheath of given size or (2) what is the minimum size of shield which will give a fixed attenuation for a given number of wires. The first problem resolves itself thus. The size of the shield and the number of pairs to be included being given by some economic or practical reason, the separation of each pair can be determined within reasonable limits. Several pairs being included within a shield, the separation of the conductors as well as the size of the wires will be small relative to the size of the shield. Therefore the shield Will contribute relatively little to the attenuation of each circuit. The presence of the other pairs also contributes a relatively small amount providing the conductors are small. Therefore to a fair approximation each pair will have the same capacity and resistance which it would have if it were isolated in space, and accordingly, to obtain minimum F high frequency attenuation for each circuit, the ratio of interaxial separation of the wire to the diameter of the wires should be approximately 2.3. Accordingly the wire size for minimum attenuation can immediately be determined. The knowledge of the wire size thus obtained will allow a more precise determination of the maximum separation of wires possible in each pair. This more precise value of separation will in turn give a more precise value of wire size for minimum attenuation.

The second problem is solved in a similar manner. In each case the ratio of the interaxial separation to the diameter of the conductors must be approximately 2.3.

In connection with the structure of Fig. 2 the enclosed pairs of conductors may be transposed at frequent intervals in order to reduce the possibility of interference into or from the other pairs enclosed within the shield and the possibility of interference from sources external to the shield at low frequencies where the shield is less effective. Such transpositions can readily be accomplished in the structure shown in Fig. 2 by twisting the two conductors of each pair helically about one another. Fig. 3 shows such a structure with the conductors helically twisted.

Several pairs may also be included within the same common shield with other conductors such as coaxial conductors, voice frequency quads or shielded pairs. Fig. 4 shows a structure containing a number of pairs proportioned according to the invention together with a coaxial conductor system 1 all included in a conducting shield 3.

Generally it will be desirable that the amount of insulating material be a minimum in order that the dielectric between the conductors may be largely gaseous. In some cases, however, it will be found advantageous to use a dielectric which is wholly or partly non-gaseous, as, for example, rubber insulation. A structure with a dielectric of this kind is shown in Fig. 5, wherein rubber insulation is indicated at 8. For the insulation arrangements that would ordinarily be employed in practicing the invention, the structures which have been illustrated in Figs. 2, 3, 4, and 5 may be employed as a transmission medium for various types of transmission systems. Two such systems are illustrated in Figs. 6 and '7, in which each transmission line is a single pair of a structure such as shown in Fig. 2.

Fig. 6 is a diagram of a multiplex carrier telephone system including the channel modulating and demodulating equipment, the filtering apparatus required for segregating the different channels and the amplifying apparatus at the terminals and at intermediate points along the line. In this figure voice frequency currents derived from the instruments SS are applied to individual modulators, as indicated by CM, which convert them to carrier frequencies. The wanted side-bands are selected by channel filters CF and may, after passing through amplifiers such as TA, be applied to the cable section LC comprising one or more pairs of wires l, 2 constructed in accordance with the invention. At suitable points in the pairs repeaters such as IR may be inserted. At the receiving end the incoming carrier channels may, after being amplified in the receiving amplifiers such as RA, be separated by means of the channel filters SF and be brought again to voice frequencies in channel demodulators as indicated by CD. The arrangement as shown serves for transmission in one direction and a duplicate arrangement would be provided for the opposite direction of transmission.

Fig. 7 is a diagram of a television system in which the transmission cable LC comprises one or more pairs of conductors l, 2 designed in accordance with the invention. In this diagram 'IT represents the television transmitting apparatus by means of which the television signals are applied to the cable LC. The transmitting apparatus may be such as to furnish to the line or conductor pair a band of frequencies extending from approximately zero frequency to a high frequency determined by the degrees of image definition which it is desired to obtain. If desired, however, this apparatus may also include modulating equipment whereby the television band of signals is shifted to a higher position in the frequency spectrum. At the receiving end the television receiving apparatus TR. takes the band of signals delivered by the line and converts it into the desired image, this apparatus including whatever demodulating apparatus may be required to shift the frequency position of the television band in a manner reverse to that employed at the transmitting end. The arrangement illustrated serves for a single direction of transmission and may be duplicated for the opposite direction of transmission. If desired, other signals, as, for example, those from voice channels, may be combined with the television signals for transmission over the line.

The terminal apparatus and amplifiers which may be used in connection with a transmission line such as described above may be shielded from electrical interference from outside sources by surrounding them with sheet metal compartments. These compartments may be connected to the shield of the transmission line if desired. Such compartments are illustrated in Figs. 6 and 7.

The general principles disclosed herein may be incorporated in many other organizations different from those shown by way of example, without departing from the spirit of the invention as defined in the claims.

What is claimed is:

1. An electrical transmission system comprising a conducting shield and multiplicity of pairs of conductors therein, each of said pairs comprising two conductors arranged side by side, said conductors being insulated from one another and from said shield, one of said conductors of each pair being connected as a return for the other conductor of the pair to form therewith a transmission path, each of the paths thus formed having connected thereto apparatus for applying thereto and receiving and utilizing therefrom a band of signal frequencies extending from approximately zero to a frequency many times the upper limit of the audible range, each of said paths acting to transmit without excessive attenuation a band of frequency so applied, the ratio of the interaxial separation to the diameter of said conductors of each of said'pairs being approximately 2.3, and saidseparation and diameter of the conductors being sufiiciently small compared to the size of the shield to leave the capacity and resistance of each pair substantially as if the pair were isolated in space.

2. An electrical transmission system. comprising a multiplicity of pairs of conductors, each of said pairs comprising two conductors arranged side by side, one of said conductors being connected as a return for the other, a conducting shield surrounding all of said conductors, all of said conductors being insulated from one another and from said shield by a substantially gaseous dielectric, the ratio of the interaxial separation to the diameter of said conductors of each of said pairs being approximately 2.3, and the diameter of the shield being as great as several times said interaxial separation.

3. An electrical transmission system comprising a multiplicity of pairs of conductors, each of said pairs comprising two conductors arranged side by side, one of said conductors being connected as a return for the other, a conducting shield surrounding all of said conductors, all of said conductors being insulated from one another and from said shield by a substantially non-gaseous dielectric, the ratio of the interaxial separation to the diameter of the conductors of each of said pairs being approximately 2.3, and the interaxial separation of each two of said pairs exceeding the interaxial separation of each pair of conductors.

4. An electrical transmission system comprising a multiplicity of pairs of conductors, each of said pairs comprising two conductors helically twisted about one another within a cylindrical space exclusive of the space occupied by the other conductors, one of said conductors of each pair being connected as a return for the other, a conducting shield surrounding all of said conductors, all of said conductors being insulated from one another and from said shield by a substantially non-gaseous dielectric, the ratio of the interaxial separation to the diameter of the conductors of each of said pairs being approximately 2.3 and the interaxial separation of each two of said pairs exceeding the maximum intera-Xial separation of each pair of conductors.

HAROLD E. CURTIS.

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