US3671662A

US3671662A - Coaxial cable with flat profile

Info

Publication number: US3671662A
Application number: US98787A
Authority: US
Inventors: Calvin Max Miller; Robert Charles Sacks
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1970-12-16
Filing date: 1970-12-16
Publication date: 1972-06-20
Anticipated expiration: 1989-06-20
Also published as: ES398356A1; BE776638A; IT945822B; AU468809B2; CA937648A; SE383432B; AU3673971A; DE2161904A1; JPS4712683A; FR2119974B1; GB1369459A; CH547553A; FR2119974A1

Abstract

A flattened coaxial cable structure has been found to exhibit advantages over conventional circular coaxial structures for some applications. This structure is a bifurcated conventional coaxial cable with the two semicircular segments joined by flat solid metal. A wide range of impedances can be realized without any significant penalty in added loss. Furthermore, tolerances to achieve certain design objectives, such as attenuation deviation, may be appreciably relaxed with the new structure.

Description

United States Patent Miller et a1. [45] June 20, 1972 154] COAXIAL CABLE WITH FLAT 3,121,136 2/1964 Mildner 2174/28 PROFILE 1,912,794 6/1933 Peterson .....333/96 X 3,106,600 10/1963 Crosby ..174/28 X [72] Inventors: Calvin Max Miller, Millersville; Robert 2 865 979 12 1958 74 R Charles Sacks Emcon y both of M d Klassen ..l /117 [73] Assignee: Bell Telephone Laboratories, Incorporated, FOREIGN PATENTS OR APPuCATlONS Murray Berkeley Heights, 956,334 1 1950 France 174/28 [22] Filed: Dec. 16, 1970 Primary E.\'aminerLewis H. Myers PP N05 981787 Assistant Examiner-A. T. Grimley Attorney-R. J. Guenther and Edwin B. Cave [52] US. Cl. ..174/28, 174/16 B, 174/99 B,

174/117 F, 333/96 R ABSTRACT [51] Int. Cl. ..H0lb9/04 A flanened coaxial cable Structure has been found to exhibit held of Search g 1 advantages over conventional circular coaxial structures for l 17 1 17 I Q b g ggg gg 5 some applications. This structure is a bifurcated conventional l coaxial cable with the two semicircular segments joined by flat solid metal. A wide range of impedances can be realized [56] References Cited without any significant penalty in added loss. Furthermore,

UNITED STATES PATENTS tolerances to achieve certain design objectives, such as attenuation deviation, may be appreciably relaxed with the new 2,356,166 8/1944 Lee et a1. ..174/l17 R structure. 3,136,965 6/1964 Lunden ..174/27 X 2,297,202 9/1942 Dallenbach et a1. ..174/ 28 UX 6 Claims, 16 Drawing Figures PATENTEDJum m2 3, 6T 1 ,662

SHEET 10F 4 PRIOR ART ,8 PRIOR ART F/G. 1c

CM. MILLER /NI/E/V7OR$ RC SACKS (MQ M ATTORNEY PATENTEDaunzo m2 3, 671 .662

SHEET U 0F 4 "IIIIIIIIIIIIA rllll COAXIAL CABLE WITH FLAT PROFILE FIELD OF THE INVENTION This invention relates to coaxialcable structures in' general and particularly to such a structure comprising flattened top and bottom portions.

BACKGROUND OF THE INVENTION Numerous coaxial cable designs exist which attempt to take advantage of the skin effect. The skin effectconsiderations prompted applicants to consider designs thatwould increase the effective surface area of the conductors at high frequencies.

One object of the invention is to achieve savings in material costs in a coaxial cable to be used at high frequencies. I

A further object of the invention is to provide a coaxial cable structure that at low frequencies exhibits a self-equalizing property.

SUMMARY OF THE INVENTION As will be shown in detail in the description to follow, our coaxial cable structure is characterized by an outer conductor having rounded sides joined by flat, elongated top and bottom portions. This flat coaxial cable transmission line (FCC), is further characterized by an inner and an outer conductor whose opposing surfaces are maintained at a uniform, constant mutual separation. This separation may be denoted b-a where a is the radius of curvature of the outside surface of the inner conductor and b is the radius of curvature of the inside surface of the outer conductor. The opposing surfaces consist of the two semicircular end portions joined by flat portions of a width w. The advantageous operating range of the overall structure is specified by the relationships: b/a and w/b 1.5.

A feature of our invention, therefore, resides in the flattened, elongated top and bottom portions of a coaxial cable.

A further feature of the invention is in spacing arrangements for such a coaxial cable structure.

The invention, and its further objects, features, and advantages will be readily discerned in full from a reading of the description to follow of an illustrative embodiment taken in conjunction with the drawing.

DESCRIPTION OF THE DRAWING FIGS. 1A, 1B and 1C are schematic diagrams showing the structural evolution of coaxial cable into the flat coaxial line 'ofFIG. 2, and FIG. 2A.

FIGS. 3-8 are graphical portrayals of various comparative relationships exhibited in circular coaxial versus flat coaxial lines.

FIGS. 9 and 10 are schematic cross-sectional diagrams of multiple jacketed flat coaxial structures; and

FIGS. ll, I2, and 13 are various schematic diagrams of spacers designed for the present invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT FIG. 1A shows a cross section of a standard coaxial line 1, and FIG. 18 a cross section of a parallel transmission line 2. FIG. 1C depicts the combining process of two

parallel transmission lines

2a and 2b in a bifurcated coaxial line whose sections are designated la and lb.

FIG. 2 depicts in cross section the resulting flat coaxial cable line designated generally 10. The line 10 consists of an outer conductor 11 having an exterior surface 12 and an interior surface 13. The inner conductor is denoted 14, with exterior surface 14b.

As seen in FIG. 2, the top and bottom portions of both outer conductor 11 and inner conductor 14 are flat within the region denoted w. The radius of curvature of the two end portions of inner conductor 14 is denoted a, and the radius of curvature of the interior surface of the two end portions of outer conductor 11 is denoted b. At either end, these curvature radii are struck from a common center, denoted 15 at the left-hand end and 16 at the right-hand end. The resulting structure is shaped like a racetrack in which the

opposing surfaces

13, 14b experience a constantuniform separation along all points of the structure.

The flat coaxial cable structure may also be thought of as a circular structure in which the inner conductor has been hollowed out and then both inner and outer conductors flattened as described above. It will be appreciated that the flattening operation for cables having a diameter greater than a few tenths of an inch, does not increase the attenuation nearly as much asit decreases the areaof the inner conductor. The material saved can be usedin part to increase the total area embraced by the outer. conductor, which decreases the attenuation to less thanthe figure which obtained before the flattening. Hence, a lower attenuation and a net savings in cost of materials results. Of course, both flattening and hollowing out achieve a further savings in materials. FIG. 2A shows a flattened coaxial cable structure with the inner conductor hollowed out, the hollowed out portion being denoted by the numeral 14a.

The below equations are directed to high-frequency relationships that show attenuation increasing as the square root of frequency. At lower frequencies where f (2.6l4/a) and the conductors are electrically thin, flattened coaxial cable exhibits a self-equalizing property. That is, the attenuation is constant with frequency up to a transition frequency. This characteristic is potentially advantageous in digital systems where a flat response would yield. good pulse transmission. The transition frequency can be increased by decreasing the thickness of the inner and outer conductors.

Multicoax cables consisting of plurality of flattened coaxial cables, as described above, are depicted in FIGS. 9 and 10. In FIG. 9, a plurality of flattened coaxial cable units or structures, each designated 20, are disposed in a line with their inner conductor centers lying in substantially the same plane. Advantageously, the edges of adjacent structures are not in contact. An extruded jacket 21 of, for example, polypropylene, is placed over the flattened coaxial cable structures 20. The resulting ribbonlike structure will bend more readily than a similar structure consisting of circular coaxial cables since for the same a and 2 the thickness is less. Further, crosstalk is normally reduced because the inner conductor midpoints are spaced a greater distance than for the corresponding conventional circular coaxial cable.

FIG. 10 depicts a multiplicity of flattened coaxial cable structures, denoted 20a, in a diamond configuration that approximates a circle in cross section. The structures 200 are not in edge contact. An extruded jacket 21a is placed around the assembly. In the configuration of FIG. 10, the flattened portions of structures 20a advantageously are parallel to one another.

Spacing of the inner and outer conductors as well as support for the inner conductor are achieved pursuant to the following further inventive embodiments.

In FIG. 11 the inner conductor 14 is spaced from the outer conductor 13 by means of an insulating layer 30 which has been longitudinally undulated or crimped in six places as shown. The layer is continuous advantageously along the entire length of the structure, its cross section being depicted as in FIG. 11. Specifically, two end crimps 30a, 30b, contact the ends of the inner conductor 14. The opposing crimps 30c, 30d, contact the top and bottom surfaces respectively of inner conductor 14 adjacent to the crimp 30a. Similarly, the crimps 30e, 30f, contact the top and bottom surfaces of inner conductor 14 adjacent to the crimp 30b. The insulating material advantageously is a plastic such as polypropylene.

A lattice-type spacer is depicted in FIG. 13. This spacer, denoted 40, comprises basically a pair of

legs

41, 42 intersecting in lattice fashion at their lengthwise midpoints. The respective ends 43, 44 and 45, 46 are semicircular in shape, and are ofiset with respect to the legs 41 so that the ends 43, 45 fall in a common plane and the

ends

44, 46 fall in another common plane which is parallel to that of ends 43, 45. Lengthwise down the entire midsection of the

legs

41, 42 are

slits

47, 48. These are slightly wider than the width of inner conductor 14.

As seen in FIG. 13, a number of spacers 40 are combined as end-to-end units and mounted upon the inner conductor 14. The

legs

41, 42 each straddle-mount the inner conductor 14, with their top and bottom sides normal to the top and bottom surfaces of inner conductor 14; and are oriented obliquely, rather than perpendicularly, to the central axis. By virtue of the oblique orientation of each

leg

41, 42 with respect to the inner conductor central axis, it is seen that inwardly directed forces on the

ends

43, 45 and ends 44, 46 bring the ends of

slits

47, 48 into firm contact with the edges of inner conductor 14. In this position, the ends 43-46 are substantially perpendicular to the central axis and also to the flattened surface of inner conductor 14. The spacer provides honeycomblike support of the outer conductor, as well as positive central spacing of the inner conductor with respect thereto.

The benefits of the inventive flat coaxial cable structure were further revealed from the following analysis.

For coaxial and parallel transmission lines, the high p resistivity of conductors in ohm meters f frequency in Hz t magnetic penneability of conductors in henrys/meter e dielectric constant of insulation in farads/meter m 21rf Z, characteristic impedance R resistance in ohms/meter G conductance in siemens/meter L inducatance in henrys/meter C capacitance in farads/rneter e, relative pennittivity a total attenuation in neperslmeter a, loss due to conductors in nepers/meter a loss due to dielectric in nepers/meter A'I'I'ENUATION To exhibit the attenuation characteristics as a function of dimensions, Equation (3) is normalized with respect to banda respectively. Thus The asymptotic conditions of 20:0 and w: a: result in These formulas represent the cases of coaxial line (w=0) and infinite parallel strip line w= are plotted respectively for the conditions e, 2.28 (for polyethylene) and p= 1.741 by 10" ohm meters (for copper). FIG. 3 particularly demonstrates the interaction between the coaxial and the parallel lines in the intermediate dimension case. Notice that the standard optimization conditions for coaxial are exhibited. The null region broadens until the hyperbolic form dominates. This is equivalent to varying b/a holding b constant. FIG. 4, on the other hand shows a simple family of hyperbolics with no local minimal or maxima analogous to varying b/a holding a constant.

IMPEDANCE The normalized equations for impedance from Equation 1 are:

FIGS. 5 and 6 are graphical representatives of Equations I0 and 11. As in attenuation, these represent the case where b and a, respectively, are held constant. The conditions w=0 and w= lead to and coax in its minimum loss region fixes its impedance to approximately 50 (I (for polyethylene). In contrast, due to the broadening of the minimum loss region for L0 w/b 1.5, a wide range of impedances can be obtained with FCC without a significant penalty in added loss.

EXACT REPLACEMENT CABLE teristic impedance given in Equation (I), on the other hand,

has no frequency characteristic and can be considered the infinite fi'equency asymptotic value.

DIMENSIONAL SENSITIVITIES It should be noted from FIGS. 3 and 5 that operating a circular where

N

2 or 3. The implicationsof relation (16) can be seen in F IG. 7. The heavy curve represents the normal operating range for circular coaxial cables. The specific sensitivity coefficients are:

in: S/(39 m normalized equations a, and a equations (4) and (5) may also be found. The relationships are:

ker K l k a K l (21) bla n b/ n b) where k, km, and k,, k To lend some additional support to FIG. 7, it may be argued that the total percent uncertainty in attenuation is given by (-t.)= e) (th s Since the three processes are independent, we plot the power sum'of the coefficients assuming that the specification percent tolerances of all three dimensions will probably be equal.

From the foregoing, it is readily seen that the advantage gained using the FCC concept depends on its use in the operating range of b/a and w/b 1.5. This is contrasted to the usual circular case where 3 b/a 5.

It is significant to note the results of dimensional tolerances on characteristic impedance. In many applications, computer cables for example, impedance uniformity is much more important than attenuation.

Applying definition (14) to Equation (1) with Z, r) substituted for 11(7) we obtain the following:

6, ere) In this case,

( In W (26) and N Lim 2102,:2 for N=2 or 3 Following the line of reasoning culminating in Equation (23), FIG. 8 is a plot of the r.s.s. values of the sensitivity coefficients. For w 0, only the region representing coaxial in its normal operating range of b/a values is shown. Consequently, as before, it can be seen that it is reasonable to operate FCC in regions (b/a l0 and w/b 1.5) where sensitivities are reduced both in attenuation and impedance.

In summary, the consequences of flattening coaxial cable in terms of attenuation, impedance and manufacturability have been demonstrated. The structure possesses a versatility owing to the range of impedances that can be obtained while the cable is operating near an optimum point. Further, crosstalk is likely to be less, in general, between two flat cables placed side by side with their inner conductors coplanar. It is seen also that manufacturing costs stand a chance of reduction because flattened coaxial cable may be made by laminating strips rather than by extrusion. Also, high frequency integrated circuitry may benefit from thin film flat cable for short runs.

It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. A coaxial cable comprising:

an outer conductor having semicylindrical side portions each having an interior surface radius of curvature b, and flat top and bottom surfaces each of width w joining said side portions;

an inner conductor having semicylindrical side portions each having an exterior surface radius of curvature a, where b a, and flat top and bottom surfaces each also of width w joining said last-named side portions;

where b/a 10 and w/b L5; and

spacing means for maintaining a uniform, constant material separation between said interior and exterior surfaces.

2. A coaxial cable pursuant to claim 1, wherein said inner conductor interior is hollow.

3. A multicoaxial communications cable comprising plural coaxial cable units constructed in accordance with claim 1, said units being disposed with their said inner conductors lying in substantially the same plane, the edges of adjacent said units being in noncontacting relation, and a unitary outer jacket surrounding all said units.

4. A coaxial cable pursuant to claim 1, wherein said spacing means comprises a continuous insulative layer disposed between said outer and inner conductors, said layer comprising plural longitudinal undulations extending from said outer conductor interior surface and contacting said inner conductor midway of the latters semicylindrical side portions, and

also inwardly of both said side portions on both said top and bottom surfaces of said inner conductor.

5. A coaxial cable pursuant to claim 1, wherein said spacing means comprises a plurality of units comprising first and second legs intersecting at their lengthwise midpoints, each leg having semicircular ends and a central lengthwise slit uniform, constant mutual separation denoted a-b, where a the radius of curvature of said inner conductor, and b the radius of curvature of the inside surface of said outer conductor; said surfaces comprising first and second semicircular end portions joined by flat portions of width w, and further characterized in that b/a 10 and that w/b 1.5.

, UNITEDSTATES PATENT OFFICE CERTTLHCATE Q]? (IQIRRECTION Patent 3, 7 2 Dated June 20", 19712" i Inventofls) Calvin M. Miller and Robert CA Sacks It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' L lyi i Column 3, lin 33 q n 1, Change 5 to (Please note that this change was requested in our I amendment received in the Patent Office on December 22, 11971.)

E I 11 2 11 2 Column 6, line 17, change 35- 00 t9 v 1 e v 1 Signed and sealed this 20th day of February 1973.

(SEAL) Attesjp:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCI-LALK 'Attesing Officer Commissioner of Patents FORM PO-1050 (10-69) 'uscoMwDc eoanmoa i 11.5 GOVEINMINI PIINYING CE 1!. O-JO6-JSI UNI EDSTATES PATENT OFFICE 5 CERTIFICATE (HF CQRRECTIQN y 1? Patent No. 3, 671, 662 Dated June 20', 1972'- Inventor s Calvin M. Miller and Robert c. Sacks 1 It is certified that error appears in the above-identified patenti and that said Letters Patent are hereby corrected as'shown below: I L 1;; 5

Column 3, line 33, equation 1, change Z C .5 to

(Please note that this change was requested in our amendment received in the Patent Office on December 22, 1 971.

v i-X Column 6, line 17, change to l 1 1 Signed and sealed this 20th day of February 1975.

(SEAL) Attes't:

EDWARD M.FLETCHER,JR. v ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (0-69) v c'50375qa5g u.s. aovenunzuv PRINTING arr-c: nu O-Jfi6-J.N

Claims

1. A coaxial cable comprising: an outer conductor having semicylindrical side portions each having an interior surface radius of curvature b, and flat top and bottom surfaces each of width w joining said side portions; an inner conductor having semicylindrical side portions each having an exterior surface radius of curvature a, where b < a, and flat top and bottom surfaces each also of width w joining said last-named side portions; where b/a > 10 and w/b < 1.5; and spacing means for maintaining a uniform, constant material separation between said interior and exterior surfaces.

4. A coaxial cable pursuant to claim 1, wherein said spacing means comprises a continuous insulative layer disposed between said outer and inner conductors, said layer comprising plural longitudinal undulations extending from said outer conductor interior surface and contacting said inner conductor midway of the latter''s semicylindrical side portions, and also inwardly of both said side portions on both said top and bottom surfaces of said inner conductor.

5. A coaxial cable pursuant to claim 1, wherein said spacing means comprises a plurality of units comprising first and second legs intersecting at their lengthwise midpoints, each leg having semicircular ends and a central lengthwise slit between the ends of each said leg for mounting said inner conductor, said ends contacting, anD extending normally from the edges of said inner conductor, said units being combined in end-to-end relation as a honeycomb support for said outer conductor.

6. A coaxial transmission line comprising: an inner and an outer conductor whose opposing surfaces are maintained at a uniform, constant mutual separation denoted a-b, where a the radius of curvature of said inner conductor, and b the radius of curvature of the inside surface of said outer conductor; said surfaces comprising first and second semicircular end portions joined by flat portions of width w, and further characterized in that b/a > 10 and that w/b < 1.5.