US 3227800 A
Description (OCR text may contain errors)
Jan. 4, 1966 L. A. BONDON 3,227,300
COAXIAL CABLE AND INNER CONDUCTOR SUPPORT MEMBER Filed June 5, 1964 INVENTOR 10m A. Bmvflm rroszwsys United States Patent 3,227,800 COAXIAL CABLE AND INNER CONDUCTOR SUPPORT MEMBER Lewis A. Bondon, 90 Yantacaw Brook Road, Upper Montclair, NJ. Filed June 3, 1964, Ser. No. 372,333 6 Claims. (Cl. 174-29) This invention relates generally to coaxial cables of the continuous (as opposed to braid or tape) concentric conductor type and in particular to a novel compressible tubular spacer and its orientation in the cable to provide very high propagation velocity characteristics.
Many forms of coaxial cable are known in the art. All have in common coextending inner and outer conductors, with means for concentrically supporting or spacing the former in the latter. The precise nature of these conductors, as well as the means spacing them, underlie the distinctions between the various forms. The spacing means employed is of particular significance because of its profound influence on the radio frequency transmission characteristics of the cable. Thus it becomes necessary not only to support the inner conductor in concentric relation to the outer or surrounding conductor and to do so in such a manner as to resist distortion upon cable flexing, but it is also necessary to provide a dielectric between the two conductors which optimizes the complex propagation constant defining the attenuation and phase velocity characteristics. Unfortunately, as far as the electrical properties are concerned, this leads to the requirement of a low dielectric constant, such as that obtainable only with a vacuum or gas. This, of course, diametrically opposes the introduction of a rigid or semirigid dielectric material for the mechanical stability of the central conductor.
Thus while cables employing a solid dielectric spacing provide a supporting structure which has the attributes of being both strong and continuous, they inherently lack optimum RF signal transmission qualities since even the best of the solid dielectrics have loss constants inferior to gases. Moreover, the employment of a solid dielectric introduces manufacturing problems in attempting to maintain: (1) the concentricity of the inner conductor during the extrusion process, and (2) the initial and subsequent intimate and continuous contact of the dielectric with all internal conducting surfaces.
A compromise conventional solution is to utilize periodically spaced dielectric rings or helically developed dielectric wedges for supporting the inner conductor. The remaining volume between the two conductors may then be air or gas filled. While these approaches minimize the dielectric supporting structure, too wide a dielectric member spacing allows inner conductor sagging and introduces electrical discontinuities and a specific frequency sensitivity at half wavelength points, while too close a spacing permits the rings or wedges to dominate the effective air dielectric, thus increasing the losses.
An array of hollow insulated tubes, compressed axially parallel between the inner and outer conductors and imparting pressure upon all conducting surfaces overcomes the aforementioned objections, particularly where the insulating cross-section provides a minimum of dielectric contact and a maximum of air space. However, room for improvement still remains since a further minimization of the supporting dielectric, particularly that in contact with the conductors, will result in improved transmission properties. It is toward this end that the instant invention is directed.
Accordingly, it is the object of this invention to provide a spacer for employment between continuous inner and outer coaxial cable conductors which inherently maximizes the free space air dielectric and minimizes the solid dielectric and its contact with conducting surfaces.
It is another object of this invention to orient a tubular spacer between the inner and outer continuous conductors of a coaxial cable in such a manner that minimizes the spacers use, while insuring the desired structural requisites.
It is a further object of this invention to provide an arrangement of spacer and inner and outer continuous conductors in which the central or coaxial positioning of the inner conductor is virtually insured during fabrication and final use.
It is a further object of this invention to provide a coaxial cable of continuous conductors which is simple and inexpensive to manufacture, and which is easily trimmed and terminated.
It is a still further object of this invention to provide a spacer arrangement between continuous inner and outer conductors of a coaxial cable in such a manner as to minimize the longitudinal displacement of the former relative the latter due to the effect of ambient temperature variations on differing coefiicients of expansion of the two conductors.
The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings wherein:
FIGS 1a and 1b illustrate in side and end views, respectively, the tubular coaxial spacer of the invention;
FlGS. 2a and 2b show traverse cut-away and end crosssectional views, respectively, of a coaxial cable employing the spacer of FIGS. la and lb, as it would appear before compression in the fabrication process; and
FIGS. 3a and 311 show traverse cut-away and end crosssectional views respectively of the finished cable, depicted in an intermediate stage in FIGS. 2a and 2b.
Referring now to the drawings, and in particular, to FIGS. 1a and 1b, thereis shown an inventive spacer for concentrically aligning an inner coaxial conductor. The spacer 10 is formed of hard but resilient dielectric material such as polyethylene, Teflon or other material which possesses a minimum dielectric constant consistent with the physical strength and resiliency desired. Examples of such other material are natural or synthetic rubber, neoprene, copolymers of butadiene and styrene or acrylonitrile, polyisobutylene, isoprene, polystyrene and vinyl compounds such as polymers and copolymers of vinyl chloride, vinyl acetate and vinylidene compounds.
The spacer, for maximum phase velocity, takes the configuration of a helically fluted cylinder, which may be either tubular or solid (in which case it must be of compressible material) and is preferably tubular (as shown). A consideration bearing upon the interior of the spacer is the specific resiliency of the material employed and its elasticity with and without a central opening. The helical fiuting produces a crest of major diameter D, having a pitch p, and a trough of smaller diameter d. Suitable values will be given below, however, it would be appreciated that these dimensions depend upon the material employed, the inner and outer conductor spacing, and the final configuration of the spacer within the cable (which will be described further). The diameter of the coextending hole 11 is sufiiciently less than that of the minor diameter a to preclude fracture at the trough during use. Although the spacer is shown with flat crests, this need not necessarily be the case, since the crest width is a function of the fiuting depth. However, since the propagation velocity is affected by the contact area between spacer and conductor surface, crest minimization is preferable.
In the following the employment of this spacer is described with reference to FIGS. 2a, 2b, 3a and 3b, in which a particular spacer orientation is shown. It is to be clearly understood, however, that this description is not necessarily limited to the helically fluted spacer of FIGS. 1a and 112, but may be adapted to any continuous tubular spacer without fluting. Where the fluting is omitted, it will be appreciated that while a greater band width will be achieved, there is an attendant sacrifice of decreased propagation velocity. In the following description certain details, as will be apparent, pertain only to the high velocity fluted arrangement.
FIGS. 2a and 2b illustrate the spacer of FIGS. la and 1b, employed in a coaxial cable having an external jacket or outer conductor 12 of a continuous conductive material such as copper .or aluminum. Other highly conductive materials of a semirigid nature, which may be flexed without loss to their electrical or mechanical properties may also be used. concentrically located within the jacket 12 is a center conductor 14 formed of a suitable conductive material such as one of those delineated for the jacket. Although shown solid, the inner conductor 14 may also be hollow where either weight or material saving considerations dominate, or where it is desirable to place passive or active elements therein.
The concentricity of the inner conductor 14 is ensured by the disposition or orientation of the spacer 10, which is helically wound thereon in a pitch (hereinafter called overall pitch as distinguished from the fluting pitch) to maximize the air space and at the same time prevent the lateral displacement of the inner conductor during use. FIGS. 2a and 2b illustrate the arrangement intermediate in the manufacturing process, before a reduction in diameter of the external jacket 12. As may be seen, the spacer is at this time without deformation, and the outer jacket is ten to fifteen percent larger than the sum of the inner conductor and major spacer diameters.
FIGS. 3a and 3b illustrate the final cable configuration after the reduction in diameter of the outer jacket; which may be accomplished by extrusion, tube reduction, form welding, or any suitable process known in the art. The spacer has now undergone the compressive deformation as shown, to impart transverse and opposite forces to the inner and outer conductors, rigidly sustaining their relative positions by preventing lateral displacement and frictionally impeding longitudinal displacement of one relative the other. Since the spacer may first be helically wound upon the inner conductor, and the combination then inserted into the initially oversize outer conductor, the arrangement is self-centering during the sheathing operations; assuming, of course, suflicient rigidity of the inner conductor to withstand the lateral pressures inherent in this process.
Perhaps diflicult to visualize in FIGS. 2 and 3 is the reversal of helices between the self-contained helix of the spacer, and its helical orientation in the cable. That is, the arrangement shown is the preferred one, with the spacer helix advancing counterclockwise to the right, and the overall helix advancing clockwise to the right. This arrangement precludes the trough of the spacer helix from contacting the inner conductor and insures that the con tact obtains at the minimum area crest, regardless of the overall pitch; thus maximizing the air space. Other arrangements may also result in a crest-inner conductor contact. Thus, for example, while both helices may advance counterclockwise or clockwise to the right, the desired contact may be obtained by tightening the pitch of the spacer helix and/or lengthening the pitch of the overall helix. The pitch reversal method, however, allows an independent relationship between the two.
The manner in which various characteristics of the cable, such as the frequency bandwidth, attenuation constant, and characteristic impedance are controlled by the dimensions and materials of the various elements is well known in the art, and will not be discussed here. Exemplary dimensions with resultant characteristics will, however, be given below.
Assuming the following dimensions for a tubular Teflon dielectric spacer:
Cm. Major diameter D 1.155 Minor diameter d 0.763 Fluting pitch 2 0.635 Hole diameter h 0.381
cables having the following delineated characteristics may be constructed by utilizing the following design dimensions:
Inner conductor diameter cm 1.50 Outer conductor diameter cm 4.130 Overall helix pitch cm 14.0 Characteristic impedance ohms 53.4
While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.
What is claimed is:
1. A continuous spacer for coaxial cable of the type having continuous concentric inner and outer conductors consisting of a helically fluted cylinder of dielectric material.
2. The spacer claimed in claim 1, in which said cylinder is tubular.
3. The spacer claimed in claim 2 in which the helical fluting pitch of the spacer and its minor diameter is less than one cm.
4. A coaxial cable comprising continuous inner and outer concentric conductors with the spacer claimed in claim 2 continuously and helically disposed in compression therebetween.
5. The coaxial cable claimed in claim 3 in which the helical fluting pitch of the spacer is reversed with respect to the overall pitch of the spacer within the cable.
6. The coaxial cable claimed in claim 3 in which the overall pitch of the spacer within the cable is greater than ten cm. and the helical fluting pitch of the spacer is less than one cm.
References Cited by the Examiner UNITED STATES PATENTS JOHN F. BURNS, Primary Examiner.