Corrugated Coaxial Cable

Abstract

The bending life of coaxial cable with a helically corrugated copper outer conductor is greatly increased, without impairment of other important mechanical or electrical characteristics, by employing specific relations of corrugation pitch and depth to each other and to overall cable diameter.

Description

This invention relates to coaxial cable of the type having a helically corrugated copper outer conductor, and more particularly to cables of this type having a foam dielectric.

Helically corrugated foam-dielectric cable, particularly of the type of construction shown in U.S. Pat. No. 3,173,990 of Robert P. Lamons, is now in widespread use in many applications wherein older types of cable, normally with braided outer conductors, were previously employed. Because of the superior resistance to crushing or other cross-sectional deformation, together with exclusion of moisture and similar mechanical advantages which permit operation under conditions which would produce prohibitive degradation of the performance of older cables, the corrugated sheath of outer conductor of solid copper provides substantially lower attenuation and, at the same time, complete containment of leakage radiation. Wherever high standards of cable performance are required, particularly where conditions of use produce a hazard of crushing, etc., the corrugated cable is normally advantageous. However, a notable exception has heretofore existed in the type of use wherein the cable is exposed to frequent flexing. In permanent fixed cable installations, the corrugated cable is more or less freely interchangeable with older types of cable, the flexibility being generally fully adequate even though substantially less than that of the braided cable. However, the corrugated cable known before the present invention has not been suitable for applications involving repeated bending, as in coupling items of equipment frequently moved with respect to each other or in a movable test equipment and similar uses wherein the required bending force and the limited bending life which are of little significance in fixed installations become important.

A typical corrugated foam cable is half-inch 50-ohm cable with a dielectric of low-loss polyethylene foam. Such cable has been manufactured for a number of years and is often used in fixed runs where braided cable would have been previously used. Such cable, however, had heretofore had very limited bending life. The outer conductor of such cable normally fails after about a hundred or so cycles of bending back and forth to a radius of the neighborhood of 5 inches on a mandrel. Such mandrel bending is of course not fully representative of actual conditions of use, in which the end of the cable is normally affixed to some item of equipment, and the bending motion is some form of back-and-forth movement of a remote portion of the cable, thus producing nonuniform bending which is maximized at the point where the cable is secured, i.e., its point of connection to an end connector. (The point of stress need not, of course, be at the end of the cable, since passage through a panel-mounted or wall-mounted feed-through bushing will have the same effect). Accordingly, the bending life may be specified in terms of a test more closely approximating actual use conditions than cycles of "radius bends." One simple form of test "rocks" the free end of a test specimen back and forth to apply reverse bending about a rigidly clamped portion until the point of failure. Such a test is readily automated by reciprocatory motion of a support ring or fork about a central position aligned with the clamped portion of the cable. A back-and-forth stroke of about 10 inches (5 inches in each direction from the neutral position) at about 9 inches from the point of clamping of the cable produces failure points (in terms of full-bending cycles) fairly accurately predicting cable performance under most conditions of use for a half-inch cable. The corrugated cables of the prior art are found to fail after a number of cycles of the same general magnitude as in the reverse mandrel bending, i.e., of the order of 100 to 150 cycles.

It has been found that a large improvement can be effected in the bending life of corrugated copper foam-dielectric cables previously known by proper relation of the pitch of the helical corrugations to their depth and to the overall cable diameter. Not only is this improvement accompanied by no important loss or diminution of other features of mechanical or electrical performance, but indeed the performance features are substantially improved in a number of respects beyond the increase in bending life. Resistance to hydrostatic pressure is increased by a substantial factor and there is also increase of the strength against impact. The cable is much more flexible in terms of the force required for bending and the minimum bending radius is substantially reduced.

The manner of achievement of these objects is best described in connection with the drawing, in which:

FIG. 1 is a view, partially in side elevation and partially broken away in longitudinal section, of the foam-dielectric cable of the invention; and

FIG. 2 is a transverse sectional view of the cable.

Except for the dimensioning established by experimentation, the illustrated cable is of conventional construction. The inner conductor 12, of stranded wire, is surrounded by a foam-dielectric sleeve 14 extruded thereon and the outer conductor 16, formed from a strip and welded at 18, is helically corrugated, the root or inner diameter 20 of the corrugation compressing the foam dielectric, but the crest 22 being spaced from the dielectric. If so desired, the void 24 thus formed may be provided with moisture barriers (not illustrated) as described in U.S. Pat. No. 3,394,400 of Robert P. Lamons. The cable illustrated also employs, when so desired, a suitable plastic jacket. Where such a jacket is applied by extrusion, however, care must be used to insure that the plastic does not extend to any substantial depth in the corrugations.

The primary object alteration of prior art constructions required for achievement of the improved performance of the invention is, shown by legend in the drawing, employment of a corrugation depth d.sub.c and pitch P such that the ratio of the former to the latter is between 0.55 and 0.70. The outer diameter D.sub.o is from 3.5 to 4.5 times the pitch, and the thickness T of the copper sheet forming the outer conductor is between 0.05 P and 0.20 P.

An exemplary embodiment of the invention employs an inner conductor 12 of No. 8(AWG) seven-strand copper wire, of which is extruded a foam polyethylene dielectric of approximately 0.325 outer diameter. The outer conductor 16 is formed from copper strip of 0.010-inch thickness and helically corrugated, with generally sinusoidal corrugation configuration, to a depth of approximately 0.075 inch with a helix pitch of approximately 0.120. The bending life of the half-inch outer diameter cable is a large multiple of that of a conventional corrugated half-inch cable. The simulated actual use-test oscillation earlier described produced an average bending life of well over 1500 cycles. The reverse bending on a 5-inch radius produced no failures within the lifetime thus indicated by the other test.

Claims

What I claim is:

1. In a coaxial cable comprising an inner conductor, a foam dielectric surrounding the inner conductor, and a helically corrugated copper outer conductor surrounding the dielectric, the improved construction having the ratio of the corrugation depth to the corrugation pitch of the copper outer conductor substantially in the range of 0.55 to 0.70 with the copper outer conductor having a ratio of thickness to corrugation pitch between 0.05 and 0.20 and having a ratio of outer diameter to pitch at least equal to 3.5, and having an inner conductor of stranded wire.

2. The coaxial cable of claim 1 having the ratio of the outer diameter to corrugation pitch between 3.5 and 4.5.