High-frequency Cables

Abstract

Cables are provided in which components of the cable having high-frequency conductors, such as coaxial units or waveguides, are combined with each other or other elements of the cable in a spiral configuration wherein the pitch of the spiral configuration is varied throughout the length of the cable. The variation of pitch of the spiral reduces the potential for the development of regularly spaced impedance discontinuities which can cause high reflection losses of energy in the coaxial or waveguide-type components in situations where the cable has been slid or rolled across a rigid surface. Undesirable secondary effects associated with a periodicity of variation in the spiral configuration can be reduced by employing a sinusoidal pattern in varying the pitch of the component in the cable.

Parent Case Text

This is a division of application, Ser. No. 756,541, filed Aug. 30, 1968.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to cables wherein regularly spaced impedance discontinuities are reduced in a range of intervals of length which correspond to one-half-wave lengths of a desired bandwidth of operation by varying a pattern in which components are spirally combined into the cable.

2. Description of the Prior Art

If a coaxial line is not uniform in impedance throughout its length, and particularly if changes in impedance occur abruptly, a portion of a transmitted signal will be reflected at the point of impedance discontinuity and returned to a transmitter. The magnitude of this reflected wave (usually called an echo or reflection) is proportional to the magnitude of the impedance discontinuity at the reflection point.

Return loss is perhaps a more meaningful term for expressing the magnitude of a reflection. It is defined as the loss that the reflected portion of the incident signal experiences in being reflected, expressed in decibels.

Since reflections are caused by an impedance discontinuity in a coaxial structure, any abrupt change in either the physical or electrical characteristics of the coaxial may produce a reflection. Dimensional change, change in dielectric material, and short increments of resistance, capacitance or inductance, for example, may produce reflections. Those reflections which arise from some structural change in a transmission line can be referred to as structural return loss, or SRL, for simplicity.

A multiplicity of reflection points may exist on a transmission line. The most typical of this latter type is a more or less random distribution of very small reflections occuring throughout the length of the line. These assorted reflected waves travel back to the transmitting end and form a vector sum at that point. Since the reflected waves arrive with random phase angles, the probabilities are that just as many reflected waves arrive with random negative phase angles as with positive. Thus, the vector sum is a random function or frequency. The effect on the transmission of information is negligible.

It is conceivable that a series of evenly spaced discontinuities may occur on a transmission line. This would occur if there were some periodic effect in the manufacturing process of the transmission line or some inherent periodicity in the structure of the transmission line.

For such evenly spaced discontinuities, there would be a fundamental frequency for which the inter-discontinuity spacing would be just one-half-wave length. At this frequency, the reflections from each discontinuity would add in phase at the transmitting end and would combine with the transmitted signal to produce a substantial impedance change from the desired impedance.

Whereas a single discontinuity, though large, may have little effect on transmission, the impedance change due to multiple discontinuities is a direct function of length. Each reflection contributes to the total, so the longer the line, the more contributions, therefore the greater the total return.

The effect of impedance discontinuities on the transmission of a signal down a transmission line is twofold in nature. Delay distortion, one effect, is caused by double reflection of the transmitted signal such that a reflected portion of the original wave arrives at the receiver delayed in time from it parent wave. On telephone lines, this distortion is manifested as an annoying echo to the telephone users. On a television screen, it is seen as the well-known "ghost", a second and fainter image of the transmitted picture appearing to the right of the original.

Gain distortion, the second effect, is caused by a frequency-dependent change in input impedance as the transmitted wave and reflected wave combine at the transmitter. Since a load which varies with frequency is being presented to the transmitter, varying power will be sent down the line, and so received at the far end.

In non-regenerative repeatered communication systems, the transmission degradation in each repeater section (caused by impedance discontinuities) is preserved in subsequent sections and is so received at the termination of the line. Thus, this degradation is cumulative, with each section adding its inherent contribution. It can be seen that the minimization of this degradation is vitally important to ensure a high quality transmission facility.

When the distance between impedance variations becomes a half-wave length of any of the signal frequencies, the small reflections add together in phase and produce a large total reflection at that particular frequency.

There has been recognition, in the past, that the structural characteristics of individual transmission lines such as a coaxial unit or a tubular waveguide should be controlled from both a manufacturing and design point of view in order that repetitive spacing of impedance discontinuities did not occur in patterns which would adversely effect transmission of signals in the desired bandwidth of the individual transmission line. There has not been, however, a recognition that when a plurality of the individual transmission lines are combined together in a cable or when other types of transmission components are combined with the coaxial or waveguide-type transmission lines in a cable that there is created, in the combination, a potential for producing regularly spaced impedance discontinuities within the coaxial or waveguide-type transmission lines. This potential is created because repetitive deformation can occur which deformation can act as impedance discontinuities to distort a desired signal.

SUMMARY OF THE INVENTION

It is an object of the invention to provide new and improved cables in which the effect of impedance discontinuities introduced by any periodicity associated with a pattern of combination of components of the cables is substantially reduced.

It is another object of the invention to provide new and improved cables stranded in a manner which reduces a potential for the development of regularly spaced impedance discontinuities in the transmission components thereof.

It is still another object of the invention to provide a cable having members with tubular conductors stranded into a spiral configuration wherein the pitch of the spiral configuration is varied throughout the length of the cable.

It is a further object of the invention to provide a cable in which components of the cable, having members with tubular conductors, are stranded together into a spiral arrangement wherein the pitch of the spiral arrangement is varied according to a predetermined program throughout the length of the cable.

A cable embodying certain features of the invention may include an electrical transmission cable which comprises a plurality of elongated elements, at least one of which is conductive, the elements being combined with each other in a pattern which varies along the length of the cable and reduces the number of occurrences of impedance discontinuities having intervals which fall in at least one group of discontinuities having intervals of length between successive discontinuities thereof which correspond to a half-wave length of a frequency which has its fundamental or harmonics in the desired bandwidth of the conductive elements so that in-phase addition of reflections of energy associated with said group of discontinuities is reduced and any degradation of a transmitted signal caused by said group of discontinuities is decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will be more readily understood from the following detailed description of specified embodiments thereof when read in conjunction with accompanying drawings in which:

FIG. 1 is a cross-sectional view of a cable embodying certain principles of the present invention;

FIG. 2 is a perspective exploded view of a coaxial unit which forms a component of the inventive cable of FIG. 1;

FIG. 3 is a graphical representation of a relationship between structural return loss in decibels versus frequency of transmission in MegaHertz as it existed in prior art cables;

FIG. 4 is a graphical representation of the relationship between the same parameters shown in FIG. 3 but is illustrative of the characteristics of a cable embodying certain principles of the present invention;

FIG. 5 is a schematic illustration of the relationship which exists between a coaxial unit and a portion of the cable which underlies the coaxial unit before and after a part of a revolution of the coaxial unit about its own axis which occurs during stranding. The figure shows the amount of revolution of the coaxial unit which takes place to bring a seam into engagement again with the underlying portion of the cable;

FIG. 6 is a graphical representation illustrating the relationship between the number of rotations of the coaxial units in a given layer per unit length of cable versus the cable length, and

FIG. 7 is a graphical representation of the relationship between helix take-up in percent and lay length in inches for a layer of stranded elements having a mean diameter equivalent to the mean diameter of an outer layer of coaxial units produced by one embodiment of the invention herein.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is illustrated a coaxial cable, designated generally by the numeral 20, which includes an outer layer, designated generally by the numeral 22, of conductors and an inner layer, designated generally by the numeral 24. The outer layer 22 includes a plurality of coaxial cable units, designated generally by the numerals 26--26. There are twelve of the coaxial units 26--26 included in the outer layer 22. The inner layer 24 includes eight of the coaxial units 26--26. The number of the coaxial units in each of the layers is not critical to the invention; the particular configuration shown in FIG. 1 simply being illustrative of a working embodiment of the invention.

Also included within the outer layer 22 are two stranded conductor units 28--28 and a plurality of interstitial conductors 30--30. Some conductors similar to the interstitial conductors 30--30 are also included within the inner layer 24. A center core 32 of stranded conductors is situated inwardly of the inner layer 24.

Surrounding the outside of the cable 20 is an extruded plastic dielectric jacket 34, preferably of a material such as low-density high-molecular-weight polyethylene. Extruded over the dielectric jacket 34 is a lead sheath 36, preferably of an alloy made up of 1 percent antimony and the remainder lead. Covering the lead sheath 36 is a plastic abrasion-resistant jacket 38, preferably of high-molecular-weight,, low-density polyethylene with a high carbon black content for an increased resistance to ultraviolet ray damage.

An enlarged, detailed view of one of the coaxial units 26--26 is shown in FIG. 2. Each of the coaxial units 26--26 includes an inner conductor 40, preferably of a copper wire having a diameter of 0.1 inch and an outer conductor 42 formed into a tubular shape, preferably from a tape of copper 1.31 inches wide and 0.012 inch thick which is positioned concentrically about the inner conductor. The outer conductor 42 is closed with a serrated seam 44. The inner conductor 40 is supported in the desired location within the outer conductor 42 with disc insulators 46--46, preferably punched from high-molecular-weight, low-density polyethylene tape 0.085 inch thick. Surrounding the outer conductor 42 are two steel tapes 48--48 which are applied helically with a left-hand wrap in overlapping relationship with each other. The steel tapes 48--48 are preferably low-carbon steel 0.006 inch thick and 0.3125 inch wide. A coaxial unit of the type described here is disclosed in U.S. Pat. NO. 2,471,299 issued to E. Bertalan et al., on May 24, 1949.

The steel tapes 48--48 which are wrapped helically about the outer conductor 42 of each of the coaxial units 26--26 are tightened in the manner described in U.S. Pat. No. 2,182,330, issued to A. S. Windeler on Dec. 5, 1939. The coaxial units 26--26 and the interstitial conductors 30--30 are stranded on the core 32 with a spiral pattern with a given length of "lay" or "lay length." The "lay length" imparted of the inner layer 24 is 20 inches, and backtwist imparted to each of the coaxial units 26-- 26 within the layer is 37.5.degree. After the core 32 and the components of the inner layer 24 are bound together with a strand material, a paper tape is applied helically over the outside of the inner layer in an overlapped relationship

It is of some consequence in the design of cables, like the cable 20, to have the lengths of the coaxial units 26--26 in the inner layer 24 equal to the lengths of the coaxial units in the outer layer 22 so that pairing of the coaxial units can be accomplished between coaxial units in the inner layer and coaxial units in the outer layer without experiencing problems associated with unequal transmission delay caused by unequal lengths of the coaxial units. The coaxial units 26--26 within the inner layer 24 are stranded into the layer with a mean circumference of 3.46 inches and a lay of 20 inches. The coaxial units 26--26 of the outer layer 22 are stranded with a mean circumference of 6.22 inches and a lay of 36 inches. The particular set of mean circumference and lay lengths of the coaxial units 26--26 in the inner and outer layers 24 and 22 are such that the length of the coaxial units are substantially all the same.

Referring now to FIG. 3, there is a graphical representation of the relationship between structural return loss as a function of the frequency of a transmitted signal within one of the coaxial units 26-13 26. The graphical representation of FIG. 3 is typical of one of the coaxial units 26--26 of the outer layer 22 where a fixed lay length of 36 inches was used in stranding the outer layer. A large spike, representing a high loss, develops at approximately 155 to 157 MHz. A frequency of 157 MHz. has an associated half-wave length of approximately 36 inches, and the fact that a severe loss develops at that frequency suggests that some repetitive discontinuity of the impedance within the coaxial unit 26 is developing at intervals of 36 inches.

A particular group of impedance discontinuities may have harmonics associated with itself, which harmonics may manifest themselves as in-phase additive disturbances to frequencies which are harmonics of the fundamental frequencies which are affected by the fundamental repetitive impedance discontinuities.

If, during the construction of the cable 20, the cable is allowed to slide or roll over a rigid surface, then each of the coaxial units 26--26 within the outer layer 22 will contact the rigid surface at intervals of 36 inches because the coaxial units are stranded with a lay of 36 inches. The contact causes deformation of the outer conductors 42--42 which, in turn, results in impedance discontinuities.

FIG. 5 is a graphical representation of the relationship of the serrated seam 44 with respect to the underlying portion of the cable 20 during a part of a revolution of the coaxial unit 26 about its own axis which occurs during a stranding operation. It can be seen that, at an angle of 360.degree. minus A, the seam 44 assumes the same relationship to the underlying portion of the cable 20 as that which existed at the starting point of the revolution in the graphical representation of FIG. 6. In the particular example of the backtwist being 37.5.degree. and the mean circumference about which the coaxial unit 26 is stranded being 6.22 inches, the angle A becomes 34.degree.; and the associated periodicity of the interaction phenomenon becomes 32.5 inches with an associated transmission loss developing at 172 to 173 MHz.

The graphical representation of FIG. 3 shows a rather large loss developing at 172 to 173 MHz. is approximately 32.5 inches. Thirty-two and one-half inches is the length of one of the coaxial units 26--26 between a point on the cable 20 where the serrated seam 44 is directly adjacent to the underlying portion of the cable shown graphically in FIG. 5 and a corresponding point in the same coaxial unit where the serrated seam is again directly adjacent to the underlying portion of the cable. A periodic contacting of the serrated seam 44 with the underlying portion of the cable 20 will tend to introduce a periodic change in impedance of the coaxial unit 26. It is thought that the periodic impedance change associated with the interaction of the seam 44 and the underlying portion of the cable 20 is manifested in the large structural return loss at a frequency of 172 MHz.

FIG. 4 is a graphical representation of structural return loss as a function of a frequency being transmitted through one of the coaxial units 26--26. The coaxial unit 26 which is represented in FIG. 4 has been stranded as a component of an outer layer 22 with a variable lay having an average lay length of 36 inches. The lay length of the coaxial unit 26 represented in FIG. 4 was varied continuously between some lay length less than 36 inches and some lay length greater than 36 inches. As one can readily see, the sharp loss associated with frequency of 157 MHz. and 172 to 173 MHz. no longer exists, but rather a general smearing of losses within the broad range of frequencies has occurred. The periodicity associated with a lay length of 36 inches and its related impedance changes and the periodicity associated with a 37.5.degree. backtwist used with the lay length of 36 inches are no longer evident as sharp losses because the effect of the periodicities have been greatly reduced.

For example, if the lay of the outer unit 22 is allowed to vary linearly from 32 to 40 inches and back again from 40 to 32 inches during the stranding of one length of cable of say, 1,000 feet; then, a lay of 36 inches would exist within each of the coaxial units 26--26 only two times within that particular length of 1,000 feet of the cable 20. Thus, an impedance change associated with a frequency, having a wave length of 36 inches, would develop only twice within that 1,000 feet long length of the cable 20; and these changes might very well be out of phase with each other. Similarly, an impedance change having an associated frequency with a half-wave length of 37 inches would also develop twice within that same 1,000 foot length of the cable 20 and so on. This should be contrasted to the case illustrated in FIG. 3 where a 1,000 foot length of the cable 20 could have an impedance variation associated with a frequency having a wave length of 36 inches occurring over 300 times, and the discontinuities are in phase with each other.

Extending the above comparison even further, it might appear that the optimum pattern for variation of lay length in any particular cable would consist of varying the lay length continuously from one end of a length of the cable 20 to the other without changing the direction of variation. If this were the practice, a lay length of 36 inches, for example, would only occur once within a length of the cable 20.

However, because of manufacturing convenience, the cable 20 is made in various lengths; in other words, not all lengths which are made are 1,000 feet. The program by which lengths of the cable 20 are varied is substantially random in nature. In other words, during the course of any one day of operation, lengths of the cable 20 of 1,000 feet, 500 feet, 1,700 feet and 1,100 feet may follow one another.

In order to strike an appropriate balance between convenience of manufacture and decrease of the structural return loss within the coaxial units 26--26, a repetitive pattern of lay variation has been chosen as the technique by which the cable is manufactured.

The pattern is illustrated graphically in FIG. 6 where it can be seen that the rotation of the coaxial units 26--26 per unit length of cable varies substantially linearly with respect to length of cable, and the direction of variation reverses periodically through the length of the cable 20. The variation in direction occurs in intervals of approximately 150 feet of the cable length. The length of 150 feet is chosen because it is sufficiently small so that if a cable length does not correspond to an even multiple of 150 feet, there will not be a very significant variation in the length of the coaxial units 26--26 from the desired length that occurs with an average lay length of 36 inches. The length of 150 feet is sufficiently large that a repetition of any particular lay length, for example 36 inches, will occur only approximately six times in a length of the cable 20 which is 1,000 feet as compared to 300 times in a cable 1,000 feet long having a fixed lay length.

Since it is important to provide the same length for the coaxial units 26--26 in the outer layer 22 as the coaxial unit in the inner layer 24 by maintaining an average lay length of 36 inches, it becomes significant to realize that the relationship between "percentage takeup" or the percentage amount of excess length of the coaxial unit taken up by the unit traversing a spiral path about the cable 20, is not linearly related to the particular lay length with which the unit is being applied.

The relationship between "percentage takeup" and lay length for the particular mean circumference of the outer layer 22 is illustrated in FIG. 7.

Be referring to a graph like the one shown in FIG. 7, it is possible to establish arbitrarily a limit of maximum lay length, greater than 36 inches. A first area under the curve between 36 inches and the maximum lay length can be determined and a determination of the minimum lay length can be made inwardly along the curve until an area equivalent to the first area is attained. When the two areas are equal, the limits of lay variation will provide a cable 20 in which the coaxial units 26--26 have the same excess length or takeup as they would have if they were stranded with a lay of 36 inches.

Of course, it is also possible to establish the limits of lay variation by choosing the lower limit first and determining the upper limit as described above.

If the direction of variation of lay occurs at some regular interval such as 150 feet as used in the embodiment described herein, there is a possibility that some transmission loss might develop at a frequency with a half-wave length order of 150 feet or at frequencies which are higher order harmonics thereof. This secondary phenomenon, as one can call it, may become significant when numerous lengths of the cable 20 are combined together into a transmission system which may extend for thousands of miles. The suffering of a loss of transmission capability with a frequency associated with a half-wave length of 150 feet would probably not be significant to the use of the cable 20 as a transmission system because the frequency is outside the bandwidth of the cable. However, third, fourth or higher order harmonics of the frequency associated with a 150 feet half-wave length might very well be within the bandwidth of the cable 20.

Thus, it is desirable in making the cable 20 for a transmission system where the effect of the secondary phenomenon is important to use a technique for varying the lay in the cable wherein the generation of harmonics is held to a minimum.

Any secondary, periodic disturbance which occurs because of the use of a sinusoidal relationship between lay length and cable length will be limited to the fundamental frequency associated with the period of the sinusoidal variation. A purely sinusoidal variation in lay, per se, will have no harmonics associated with it.

A reduction, but not a complete elimination in generation of harmonics of the type described above, can be accomplished by reducing the sharpness or abruptness in reversals of direction of lay variation. An introduction of some curvature at the area of reversal will reduce the number of harmonics.

A reduction in the adversity of effects in the secondary periodicity might also be accomplished by varying the period of the reversals of the direction of lay change. For example, a cable might be made with its lay varying upwardly for 150 feet; then, downwardly for 100 feet; then, upwardly for 72 feet and etc.

In reducing the adverse effects of the secondary phenomenon, it is important to remember that any departure from a simpler linear shape with substantially instantaneous reversals leads to complications in choosing parameters which will result in a proper matching of electrical-delay time between the coaxial units 26--26 in the outer layer 22 and the coaxial units in the inner layer 24 of the cable 20.

When the lay of the outer layer 2 is varied, the sharp spike at 155 to 157 MHz. on FIG. 3 is reduced so that the "SRL" pattern is as shown in FIG. 4. The spike at 172 to 173 MHz. which is associated with the repetitive discontinuity involved in the backtwist is also reduced when the lay of the outer layer 22 is varied. The reduction of the spike associated with backtwist occurs because the discontinuity associated with the backtwist has been varied in varying the lay of the outer layer 22.

A decrease in the size or even an elimination of the spike associated with backtwist can also be achieved by varying only the amount of backtwist as the cable 20 is being stranded. This variation in backtwist can be accomplished even if the stranding of the outer layer 22 is performed with a fixed lay. The technique of varying the backtwist can be useful if for some reason the variation of lay of the outer layer 22 is not practical or possible and an improvement of transmission capability at the associated frequency of the backtwist is important.

An alternate embodiment of the inventive cable 20 (not shown) may include a central, tubular conductor, such as one of the coaxial units 26--26, around which one or more additional conductors might include, for example, conventional twisted pairs of wire or other coaxial units. Varying the pattern with which the additional conductors are placed around the central tubular conductor can have a beneficial effect in reducing the introduction of periodic impedance discontinuities in central tubular conductors and the additional conductors.

The term "cable" as employed in the specification and the following claims will be understood to refer to any form of multiple-conductor transmission media including those multiple-conductor transmission media known in the industry as "wire" or "cords."

The term "group" as used in the specification and the following claims in intended to include any series of impedance discontinuities which have equal spaces therebetween. It is intended that other impedance discontinuities with unequal spacing therebetween may exist between the discontinuities which are members of the group, but the discontinuities which are members of the group, but the discontinuities with unequal spacing are not to be considered members of the group.

Impedance discontinuities which are effective in producing peaks in return loss are those which are in groups wherein the spaces between adjacent discontinuities of the group are equal. Introduction of other impedance discontinuities within the same physical space that the group occupies will not necessarily have an effect on the peak return loss unless the other impedance discontinuities of the group. If there is some phase relationship between the discontinuities of the first-defined group and the other discontinuities, then there will exist another group incorporating these other discontinuities; and this group will, in turn, have an effect on peak return loss at some other frequency.

The terms "pattern" as used in the specification and the following claims is intended to include a pattern generated by stranding some elements of a cable about other elements of the cable and a pattern generated by revolving individual elements about their own axes as the elements are combined into a cable.

Claims

What is claimed is:

1. An electrical transmission cable which comprises a plurality of elongated elements, at least one of which is conductive, the elements being combined with each other in a pattern which varies along the length of the at least one conductive element and reduces the number of occurrences of impedance discontinuities which fall in the at least one group of discontinuities having intervals of length between successive discontinuities thereof which correspond to a half-wave length of a frequency which has its fundamental or harmonics in the desired bandwidth of the associated conductive element so that in-phase addition of reflections of energy associated with said group of discontinuities is reduced and any degradation of a transmitted signal caused by said group of discontinuities is decreased.

2. The cable of claim 1, wherein at least one of the elements is substantially free of lay and another of the elements is stranded around the substantially lay-free element.

3. The cable of claim 1, wherein at least one of the elements of the cable is an element having at least one tubular conductor.

4. The cable of claim 3, wherein the element with a tubular conductor is a coaxial conductor having an outer, hollow, tubular conductor and an inner conductor coaxial with the tubular conductor and insulated therefrom.

5. The cable of claim 3, wherein a plurality of the elements having tubular conductors are arranged in at least two concentric layers, and wherein the effective electrical length of said elements in the outermost layer is substantially the same as the effective electrical length of said elements in other layers of the cable to reduce distortions of signals transmitted through a pair of said elements made of a first element from a first layer and a second element from another layer.

6. The cable of claim 1, wherein the pattern is formed in the cable by backtwist being imparted to at least one of the elements of the cable.

7. The cable of claim 6, wherein the amount of backtwist varies with a predetermined pattern with respect to the length of the cable.

8. The cable of claim 6, wherein the amount of backtwist varies with a predetermined pattern with respect to said element.

9. The cable of claim 1, wherein the pattern is formed in the cable by a lay being imparted to at least one of the elements of the cable.

10. The cable of claim 9, wherein the variation in lay occurs sinusoidally with respect to cable length.

11. The cable of claim 9, wherein the variation in lay of each of the elements occurs sinusoidally with respect to the length of the associated ones of said elements.

12. The cable of claim 9, wherein the elements are stranded into the cable with backtwist in said elements, and the amount of backtwist varies along the length of said elements.

13. The cable of claim 9, wherein the variation in lay occurs linearly with respect to the length of the cable.

14. The cable of claim 13, wherein the direction of linear variation alternates through the length of the cable.

15. The cable of claim 14, wherein the period between alternation of linear variation is not greater than 300 feet or less than 75 feet of the cable length.

16. The cable of claim 14, wherein the length of cable between changes of direction of linear variation is not greater than 20 percent of the cable length or less than 5 percent of the cable length.