US 2556244 A
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June 12, 1951 W. K. WESTON COAXIAL CABLE WITH HELICALLY WOUND SPACER Filed Oct. 22, 1946 7am can INVENTOR- AM K4 WESTON ATTORNEY Patented June 12, 1951 COAXIAL CABLE WITH HELICALLY WOUND SPACER William Kirby Weston, London, England, assignor to International Standard Electric Corpora tion, New York, N. Y.
Application October 22, 1946, Serial No. 704,968 In Great Britain September 7, 1945 2 Claims.
This invention relates to high frequency electric cables of the coaxial conductor type.
In some instances it is advisable to form the inner conductor of a coaxial cable as a hollow tube, as for example when the coaxial cable is made of large diameter, in which case a solid central conductor would unduly increase the weight of the cable. It may be desirable to use the hollow tube functioning as an inner conductor as the outer conductor of a second coaxial cable the inner conductor of which is supported centrally within the hollow tube.
This inner conductor may be formed of a single tape or of a plurality of tapes, but in the interest of minimum attenuation it is desirable that any discontinuities formed by the abutting edges of the tapes should run longitudinally of the cable or as nearly so as possible and thus that the tape or tapes should be folded longitudinally. The edges of these tapes should be held together under pressure, and this may be accomplished by a binding of insulating tapes. This arrangement has been proposed in our U. S. application No. 667,272 filed May 4, 1946, entitled Improvements in or relating to concentric conductor electric cables, now abandoned, which also proposes as a preferred alternative that a layer of a dielectric material be extended over the inner hollow conductor to maintain the latter in circular form. It is also proposed in that application to fill the-whole space between inner and outer conductors with solid dielectric material.
It is, however, desirable to keep the volume of solid dielectric between the inner and outer conductors to a minimum as is well known, and in our U. S. application No. 695,027, filed September 5, 1946, now Patent No. 2,480,170, entitled Improvements in or relating to electric cables for high frequency, it is proposed to use spaces composed of dielectric material formed in two or more portions, which when placed together to form a complete washer exert a wedging action upon the tape over the greater part or the hole of the length of the inner conductor. These wedge spacers may be held in place by means of insulative tape or by a thin wall of insulating material extended around them, or, again, by forming the outer conductor immediately around them.
According to the present invention there is provided high frequency electric cable of the coaxial conductor type comprising a hollow inner conductor formed of one or more metallic tapes folded longitudinally in which spacing means between said inner conductor and the outer conductor is provided by one or more helices of low loss dielectric material wound about said inner conductor. These helical spacers may of themselves hold together the edges of the tape or tapes of said inner conductor or alternatively these edges may be held together under pressure by a distinct helix or group of helices of low loss material. This material must not be confused with the insulating tape mentioned above, but would be of substantial thickness compared thereto. Further, it is preferred that the material should be hollow or corrugated so as to form a cross-section the area of which is only partially occupied by solid dielectric.
The invention will now be described in more detail with reference to the accompanying drawings in which Fig. 1 illustrates a cross-section of an electric cable embodying features according to the present invention.
Figure 2 is a cross-sectional view of a high frequency electric cable embodying the present invention in a preferred manner.
In Figs 1' and 2 the inner conductor l of a large diameter coaxial cable consists of a single copper tape folded longitudinally, the edges of the tape abutting as shown at 2. Around thi hollow inner conductor are wound spirally hollow tubes 3 of low loss dielectric material and such as a solid ethylene polymer.
These tubes may be wrapped with insulating tape, 4, of similar material. In the cable illustrated in Fig. 1 the hollow tubes are spaced about the inner conductor without adjacent turns touching, a further group of helical spacers, 5, are wrapped about the first group. The spacers of this second group are shewn as having a corrugated cross-section. Low loss insulating material of any convenient cross-sectional shape may be used in place of those shewn at 3 and 5, but it is to be preferred that the cross-sectional area defined thereby should only partially be occupied by the solid dielectric. It is also to be preferred that the lay of thesecond set of helices be in the opposite sense to that of the first group. This second group of spacing elements may also be wrapped with insulating tape 6 if desired. The first group may be considered functionally as means for holding together under pressure the edges 2 of the inner conductor, while the insulators 5 serve primarily as spacing members in addition to 3.
In both Figs. 1 and 2, outer conductors I are provided by any of the usual arrangements. The spacers inside the hollow inner conductors i may each accommodate, for example, a further conductor 8 separated from conductor I by means of the usual washers 9.
In Fig. 2 it will be observed that six helices 3 are provided and that the insulators are of the same diameter as the inner conductor l. Neglecting the thickness of tape 4 this arrangement would provide a cable with a ratio outer/inner conductors of 3:1. It is well known, that in order to obtain a minimum attenuation constant, this ration should be approximately 3.6 to 1. With a multiple grouping of spacing helices as in Fig. 1 there is no difficulty in arranging this, but there is much to be said for having the insulators touching one another and forming a single group to lock the inner conductors edges 2 and simultaneously to provide the necessary spacing means. Accordingly, in a further preferred embodiment, the insulating members 3 of Fig. 2 are made over size and squeezed into position in contact with adjacent members and with the inner conductor; their cross-section is thereby deformed from being circulated with the result that they may be made to take up the extra radialspace to provide the correct ratio of diameters for the two conductors.
Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, We .declare that what we claim is:
l. A high frequency coaxial cable that comprises a tubular inner conductor formed of a metaltape longitudinally folded whereby its longitudinal edges are disposed in opposed relationship; a plurality of tubular spacers, formed of insulating material and wound side by side helically around the inner conductor; and a tubular outer conductor surrounding the helically wound spacers, having a diameter related to the diameter of. the inner conductor in approximately the ratio of 3.6 l, the'diameter of the tubular spacers being such that they are compressed and distorted by the outer conductor and press against the inner conductor in a manner causing the 0pposing longitudinal edges of the folded tape to press against each other.
2. A high frequency coaxial cable according to claim 1 further characterized in that a layer of insulating tape is helically wound around the helically wound spacers, the lay of the tape being in an opposite sense to the lay of the spacers.
WILLIAM KIRBY WESTON.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,859,390 Green May 24, 1932 1,978,418 Dudley Oct. 30, 1934 2,116,267 Klimmer May 3, 1938 2,381,003 Ryan Aug. 7, 1945 FOREIGN PATENTS Number Country Date 526,702 Great Britain Sept. 24, 1940