Method Of Forming Coaxial Conductors Of Small Diameters

Abstract

A method of forming coaxial conductors of small diameters.

Description

BACKGROUND OF THE INVENTION

Coaxial conductors and cables are conventionally formed by converting a thin metal tape into tubular form with a longitudinal slit defined by side edges of the tape; the inner conductor being introduced within the freshly formed outer conductor by way of the slit, which is then sealed by solder, welding, folding or the like; all in a manner known in the art. The cable is then further processed to convert the outer conductor to a desired cross-sectional design.

A solid dielectric is utilized to maintain the inner and outer conductors in suitably spaced relation to each other. Since the damping factor of the cable or transmission line is increased with the addition of dielectric to the cable construction, the amount of dielectric used is held to a minimum.

The introduction of the solid dielectric material during the fabrication of coaxial cables or lines presents no difficulties where the cables are of large or medium cross-sectional dimensions. Thus, the dielectric in the form of a helix with a long lay is applied to the inner conductor before the inner conductor is introduced into the longitudinally slit outer conductor. The helix in this case may be cut out or recessed axially or radially thereof, to further reduce the dielectric content of the cable.

In the case of coaxial lines of somewhat smaller diameters, dielectric in the form of discs are mounted on the inner conductor with maximum possible spacing therebetween. The resultant assembly of inner conductor and dielectric discs is then introduced into the outer conductor via the longitudinal slit therein. In another procedure, a foamable synthetic resin is applied to the inner and/or outer conductors, and the resin is then subjected to conditions for foaming the same after the assembly of the two conductors.

However, such known procedures are not applicable for coaxial lines of relatively small cross-sectional dimensions. Thus, with an inner conductor having a diameter of 0.5 mm. or less, it is impractical to apply dielectric in helical form thereto, because of torsional or twisting stresses incident to the winding operation of applying the helix to the conductor. This tends to open up the helix so that the inner conductor is no longer held centrally with respect to the outer conductor.

The use of dielectric discs for small coaxial lines has also found disadvantages. Thus, the inner conductor tends to break during the attachment operation and the inner conductor may sag between the axially spaced discs. Finally, the high flexibility of the inner conductor interferes with foaming operations where foamable synthetic resins are used, as the inner conductor cannot be maintained in a true centrally related position during the assembly of the inner conductor with the outer conductor via the longitudinal slit in the latter.

Accordingly, an object of this invention is to provide an improved method of introducing dielectric into coaxial cables or lines of very small cross-sectional dimensions.

Another object of this invention is to provide a method of the character described wherein the dielectric is applied directly to one surface of a metal tape which is to be converted into the outer conductor of the coaxial cable or line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A thin metal tape which may have the thickness of about 0.1 mm. is coated on one surface thereof with a suitable dielectric such as polystyrene or other synthetic resins used as dielectrics. The coating of dielectric material may have a thickness of the order of 0.1 mm.

The coated metal tape is converted to tubular form with a longitudinal slit corresponding to the adjacent side edges of the tape. An inner conductor, which may have a diameter of about 0.2 mm. is continuously introduced into the tubular outer conductor via the slit therein; the coating on the inner surface of the outer conductor providing a dielectric spacer between the conductors.

The longitudinal slit in the outer conductor is then sealed as by soldering, welding, edge folding, adhesive, or the like. The outer conductor is then drawn down to the cross-sectional dimensions required by the desired characteristic impedance. Such drawing operation produces a solid homogeneous dielectric layer between the conductors, providing a line with relatively high damping.

If a line with low damping is preferred, the cable or line is passed through the corrugating dies, known in the art, which corrugate the outer conductor, in which case, the cable has a mixed dielectric of air and solid synthetic resin. The outer conductor is suitably dimensioned to allow for corrugating the same; in which case the inner dielectric-coated valleys of the corrugated outer conductor bear on the outer surface of the inner conductor.

As an alternative, the outer conductor may be coated on its inner surface with the dielectric material, in longitudinally extending, spaced strip form. This will provide a transmission line with a further reduction in damping. Thus, with the line in corrugated form, air spaces will be provided axially thereof, as well as transversely thereof, further reducing the total amount of dielectric material used, and increasing the proportion of air dielectric. The spacing between the dielectric strips is selected to not exceed the outer diameter of the inner conductor; and the strip width also does not exceed such outer diameter.

Thus, the dislocation of the inner conductor into the between the dielectric strips, is avoided during the fabricating operations. In coating the metal tape forming the outer conductor, preferably the marginal side edges of the tape are left free of coating so that there will be no interference with the soldering or welding operation which closes the seam and further, dielectric coating will not melt and thus prevent uneven centering of the inner conductor relative to the outer conductor.

Further, with low-melting-point dielectrics, the metal tape may also be coated on its outer surface; thus providing means for sealing the longitudinal seam of such conductor by way of the outer coating of dielectric. The heat requirement for such seaming operation is very small, thus avoiding fusion of the inner dielectric coating which spaces the conductors. It has been found that the longitudinal slit which remains when sealed with dielectric as described, in no way influences the transmission characteristics of the coaxial line. Moreover, the slit can be kept so small that external fields cannot extend into the coaxial system by way of coupling.

As various changes might be made in the hereindescribed embodiments of the invention without departing from the spirit thereof, it is understood that all matter herein set forth is by way of illustration and not limiting except as set forth in the appended claims.

Claims

I claim:

1. A method of forming coaxial transmission lines of small diameters comprising an outer conductor, an inner conductor and dielectric between the conductors, comprising: providing a metal tape; applying longitudinally extending, spaced strips of dielectric to one surface of said tape and in parallel relation to the side edges thereof; converting the tape to tubular form with the dielectric strips on the inner surface thereof, the side edges of said tape being adjacent each other to form a longitudinally extending slit, introducing a longitudinally extending inner conductor within said tubular outer conductor by way of said slit, sealing the adjacent edges of said tubular outer conductor together, and deforming the assembled outer conductor to a selected cross-sectional configuration, the spacing between the strips being no greater than the diameter of the inner conductor.

2. A method as in claim 1 wherein the outer conductor is corrugated transversely thereof.

3. A method as in claim 1 wherein a low-melting-point synthetic resin is applied to the outer surface of said tape to provide means for sealing the seam in the outer conductor.