US3287490A - Grooved coaxial cable - Google Patents

Description

M. WRIGHT Nov. 22, 1966 2 Sheets-Sheet 1 Filed May 21, 1964 Invenior. M4110? Wadi 5.133. by 7/ M. WRIGHT Nov. 22, 1966 2 Sheets$heet 2 Filed May 21, 1964 w wmwvfi \WMW an m a u M United States Patent M 3,287,490 GROOVED COAXIAL CABLE Malor Wright, Lexington, Mass., assignor to United-Carr Incorporated, Cambridge, Mass., a corporation of Delaware Filed May 21, 1964, Ser. No. 369,198 1 Claim. (Cl. 174102) This invention relates generally to the metaiizmg or plastics and more specifically to metalizing a plastic conduit or tube having a conducting core.

An object of the present invention is to provide a method of plating a plastic tube having a conducting core.

Another object of the present invention is to provide a method of forming flexible miniature precision coaxial cables.

A further object is to provide an article of commerce having a continuous sheet of outer conductor applied over a dielectric.

A still further object is to provide a method of applying a continuous sheet of material over a grooved dielectric.

Another object is to provide a method which permits the application of a continuous outer coat upon a bendable support.

Still another object of the present invention is to provide a tube, cylinder or square which is coated with a different material from that which comprises the tube, etc., and which can be bent to an arcuate configuration without destroying the continuity of the coating even if stressed to the point where some cracks will appear.

In the past, it was very diflicult to provide a microwave coaxial cable whose outer conductor would not develop cracks, when bent drastically, thereby disturbing or destroying proper electrical conductivity. The present invention by depositing a layer of various ductile metal plates or corrugations formed on the outer surface of a plastic support which provides stress reduction during bending and increases flexibility thereby alleviating the mentioned cracking and splitting. Flexibility is one of the major considerations in the design of miniature precision coaxial cables. During bending and flexing, the outer conductor is stressed much more than the inner conductor and should therefore be constructed to reduce the stresses.

In the drawings:

FIG.1 is a side elevation of the plastic core having the necessary convolutions formed therein prior to plating;

FIG. 2 is an end elevation of the plastic core shown in FIG. 1;

FIG. 3 is an expanded view of a portion of the cable shown in FIG; 1 with the sheath in section;

FIG. 4 is a figure similar to FIG. 3 with the plating applied;

FIG. 5 is a figure similar to FIG. 4 showing a portion of the cable under bending stress;

FIG. 6 is a side elevation of a variation of the cable shown in FIGS. 1 through 5; and

FIG. 7 is a section taken on line 7-7 of FIG. 6.

Referring to the drawings and particularly to FIGS. 1-5, there is shown a miniature cable comprising an inner conductor 10 and an outer insulating sheath 12, which is formed about the conductor 10 in an abutting relationship therewith. The sheath 12 has a series of grooves 14 formed in its external surface which are spaced from each other and which in horizontal cross section provide a configuration approximating a sinusoidal wave which is repeated for the length of the sheath 12. The root of each groove 14 is rounded as is the crest 16 of what may be referred to as the 3,287,490 Patented Nov. 272, 1966 thread which is formed between each groove. The inner conductor 10 is maintained at a constant diameter.

The outer conductor 18 comprises a thin layer of ductile metal or conductive material which is applied on to the plastic sheath 12 utilizing methods to be set forth hereinafter. The conductor 18 can be made very thin (for instance .001 inch) at a saving in cost over presently available methods. Any metal or semiconductor can be used as long as the material can be applied to the outer surface of the sheath 12. For best electrical performance, the sheath 12 may be formed of polytetrafluoroethylene, polyethylene, polypropylene or a copolymer of tetrafluoroethylene and heXafiuoro-' propylene.

There are several methods of applying the plate to the plastic sheath. One which the applicant has found to be highly satisfactory is commenced with a series of steps for preparing the surface, including cleaning, which includes a chemical etch to make the surface hydrophilic, sensitising, which consists in having the sheath or surface absorb a material which is readily oxidised, and finally catalysing, which consists in exposing the absorbed material to the ions of a noble metal which is readily reduced, thereby reducing the noble metal on the surface, thus creating a catalytic surface for initiating further metal deposition. After surface preparation is completed, approximately one micro inch of silver is deposited on the catalysed surface from a 10 g.p.l. solution ammoniacal silver nitrate and a one percent solution of glyoxalin water. The final step is to deposit a thin copper layer on the silver layer. A solution of 250 g.p.l. copper sulphate, g./ liter sulphuric acid at a current density of 1 amp/dm. was found satisfactory as a: plating bath for this latter purpose.

Turning again to the physical construction of the grooves formed on the sheath 12, when the cable is bent stretching stresses which would occur in a cylindrical sheath, occur in the herein disclosed cable mostly as bending stresses somewhat akin to the bending of a beam along the flat area 20 which is formed on each side' of the thread from the crest 16 to the root of the groove 14. The flats 20 are in angular relationship to each other and also in angular relationship to the axis of the sheath 12. As shown in FIG. 3 the crest 16 and the root of the groove 14 are both arcuate in configuration.

The relationship of the pitch from crest 16 to c1 est 16 should be chosen as a function of the diameter of the cable and the depth of the grooving 14 should be chosen on the same basis for optimum results. Pitch from crest 16 to crest 16 should be approximately four times shorter than the shortest microwave that is to be propagated through the cable. When the plated core is bent around a mandrel, a tensile force or stress occurs at the outer surface. If a coaxial cable of a cylindrical section is bent over the mandrel, a series of downward force vectors result, which are all substantially equal and all directed toward the center of the mandrel thereby causing distortion of the dielectric, and there is also a stretching of the metal surface as mentioned heretofore. In these circumstances, the crests 16 of the herein disclosed cable tend to flatten and this flattening in turn relieves stresses thereby reducing strain amplitude. The maximum stretch will be at the crest and less stress will occur at the flat areas. Also as bending occurs the distance from crest to crest of the outside layer will increase. Since adherence between the metallic layer and the plastic layer is not percent perfect, the metallic layer will come up slightly from the root of the grooves 14.

There is shown in FIGS. 6 and 7 a variation of the cable shown in FIGS. 1-5. The threads or corrugations should be chosen to suit the particular application involved. If flexibility is the chief requirement, the depth 3 of the thread and the relationship of the size of the thread to each other would dictate for maximum results the configuration disclosed in FIGS. 6 and 7. The difference between this variation and the other embodiment is that the flats 20a of this variation are in substantially parallel relationship to each other. This particular configuration allows extremely deep grooves 14a, which in turn will increase the flexibility of the cable.

In general, metals will take a little more strain amplitude in compression than an extension or stretching. Therefore, we have been concerned mainly with the problerns regarding the extension caused by the bending of the cable. Furthermore, substantially as much strain is found in compression as in extension about the neutral axis, in other words, one type of strain may be considered pretty much the mirror image of the other type.

While there has been illustrated and described a pre ferred embodiment of the invention it should be understood that the invention is best defined by the following claim.

What is claimed:

A coaxial cable comprising a flexible inner conductor, a solid dielectric of homogeneous polymeric material having convolutions preformed on its outer surface, said dielectric surrounding said inner conductor, and a continuous uninterrupted electroless deposit of a conductive material on the external surface of said dielectric.

References Cited by the Examiner UNITED STATES PATENTS 2,663,754 12/1953 Bianco l74l02 2,690,496 9/1954 Perls 33396 2,808,450 10/1957 Peters 174102 2,890,263 6/1959 Brandes et al. 174l02 3,130,256 4/1964 Milkner 174-28 LEWIS H. MYERS, Primary Examiner.

H. HUBERFELD, Assistant Examiner.

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