U.S. patent number 3,651,243 [Application Number 05/032,987] was granted by the patent office on 1972-03-21 for high-frequency cables.
This patent grant is currently assigned to Western Electric Company, Incorporated. Invention is credited to Eugene M. Hornor, William J. Hyde.
United States Patent |
3,651,243 |
Hornor , et al. |
March 21, 1972 |
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.
Inventors: |
Hornor; Eugene M. (Rosedale,
MD), Hyde; William J. (Baltimore, MD) |
Assignee: |
Western Electric Company,
Incorporated (New York, NY)
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Family
ID: |
26709143 |
Appl.
No.: |
05/032,987 |
Filed: |
April 29, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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756541 |
Aug 30, 1968 |
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Current U.S.
Class: |
174/34; 174/28;
174/113R; 174/27; 174/103; 174/115 |
Current CPC
Class: |
H01P
11/005 (20130101); H01B 13/016 (20130101); H01B
11/20 (20130101) |
Current International
Class: |
H01B
11/18 (20060101); H01B 11/20 (20060101); H01B
13/016 (20060101); H01B 13/00 (20060101); H01P
11/00 (20060101); H01b 011/02 () |
Field of
Search: |
;174/34,27,103,113,115,107,27,28,33 ;333/84,96,97 ;57/6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,167,964 |
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Oct 1969 |
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GB |
|
764,056 |
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Dec 1956 |
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GB |
|
537,354 |
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Oct 1931 |
|
DD |
|
Primary Examiner: Askin; Laramie E.
Assistant Examiner: Grimley; A. F.
Parent Case Text
This is a division of application, Ser. No. 756,541, filed Aug. 30,
1968.
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.
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.
* * * * *