US2468091A - Electric cable - Google Patents

Description

April 26, 1949. M DDDD CK 2,468,091

5. I \lifi Patented Apr. 26, 1949 ELECTRIC CABLE Bruce H. Maddock, Fanwood, N. J., assignor to Federal Telephone and Radio Corporation, New York,'N. Y., a corporation of Delaware Application January 18, 1945, Serial No. 573,350

1 Claim. (01. 17424) This invention relates to an electric cable in Y which a large part of the surrounding dielectric,

particularly that in close proximity to the conductor, is formed of air, the conductor being spaced from its surroundings by solid material which occupies only an extremely small proportion'of the intervening space.

It has been recognized already for several decades that it is essential to design electric cables with high insulating qualities for the conductors and with a low inductive capacity. It was also well known that the provision of air spaces around the conductor served both purposes, namely, to'provide extra insulation for and to reduce the capacity of the cable. In order to achieve the desired results, it has been proposed to surround the conductor with a covering containing longitudinal or radial air spaces. Thus, it can be seen that there was a clear tendency in the construction of transmission cables utilized in connection with conductors, microphones, television and radio frequency distribution to lower the capacity and, therefore, to enlarge the air spaces in the covering of the cables and to provide predetermined portions of the interior of the cable with a lower average dielectric constant. This tendency brought about a cable having a cross-section shaped to provide-a plurality of arms. Whereas these previous constructions proved to have a great proportion of solid material in close proximity to the conductor, and in some cases surrounded the conductor even entirely with solid material, with the result that the desired low capacity of the cable could not be achieved, the later development in the art went into the direction of providing a large part of the dielectric surrounding the conductor, particularly in close proximity to the latter, of air. The conductor was in these latter constructions spaced from its surrounding by solid material which occupied only a small proportion of the intervening space, whereby the spacing was effected by a cord or string of insulating material wound over the conductor as an open helix. Although this later development appeared to be an improvement over the older constructions, it still lacked entirely satisfactory results, apparently due to the limited radial extension of the air space around the conductor.

The present invention overcomes all drawbacks of these known cable constructions and provides a cable of lowest capacity. Whereas the oldest constructions for electric cables provided air spaces in more or less greater distances from the cable and the space in close proximity to the conductor was occupied in a large proportion by solid material, thus lacking sufficient air space in close proximity to the conductor, the later constructions provided a larger proportion of air space in close proximity to the conductor only, which air space had far too limited radial extension. Both kinds of constructions did not provide, therefore, entirely satisfactory results.

The present invention is based on a newly found principle, according to which suflicient air space in close proximity to the conductor and also suificient radial extension of that air space are essential in order to achieve the desired results for high frequency cables.

It is, therefore, the object of the present invention to provide an electric cable which is constructed for particularly low capacity to be used substantially for high frequency purposes.

It is another object of the present invention to provide an electric cable in which the amount of solid dielectric surrounding the cable is substantially reduced.

It is still another object of the present invention to provide an electric cable which possesses a very low average dielectric constant particularly in close proximity of the conductor.

It is a further object of the present invention to provide an electric cable in which the condoctor is spaced from its surroundings by solid material which occupies only a small proportion of the intervening space, thus the large proportion of the intervening space being occupied by air which air gap is extended radially into the surrounding solid material.

With these and other objects in view the present invention will become apparent in the following specification and drawing, in which:

Figure 1 is a cross section of an electric cable; and

Fig. 2 is a longitudinal section along the lines 2-2 of Fig. 1.

As shown in the drawing, the cable comprises a central wire conductor l around which is wound insulating spacing 2. The latter has preferably theform of an open helix of relatively long pitch, through any other form can be used for the spacing means, and consists of any convenient insulating plastic material as preferably of the lowloss type such as polyethylene, polystyrene, or compounds which may include these, or equivalent materials. Around the spacing member 2 is disposed a cylindrical jacket 3 having inwardly directed radial webs 4 each of which engages the insulating helix 2 merely at a few points which are distanced by the predetermined pitch of the helix 2, whereas the remaining portions of the inner ends of the webs I are spaced by an air gap 5 from the conductor I. The helix 2 engages any point of the conductor I only on a part of its periphery. The cylindrical jacket 3 and the webs l are preferably of the same insulating plastic material as described before for the helix 2. Whereas four webs are shown, it is obvious that a fewer or greater number can be employed.

The construction according to the present invention substantially increases the radial length of the average air gap around the conductor, which air gap comprises two connected air zones, namely, one inner zone achieved by the spacing means and an outer zone which is achieved by a plurality of air spaces formed by the cylindrical jacket 3 and each pair of adjacent webs 4. This increase of the radial length of the average air gap around the conductor is achieved without in creasing the portion of solid insulating material in close proximity to the conductor while at the same time adequately mechanically supporting the center conductor. Thus the provision of two interconnected air zones in the construction of the electric cable resulted in a particularly low capacity characteristic for the cable.

While I have disclosed the principles of my invention in connection with several difierent embodiments, it will be understood that these embodiments are given by way of example only and not as limiting the scope of the invention as set .forth in the objects and the appended claim.

4 WhatIclaim is: In an electriccable of the type including a central conductor, electrically insulating means surrounding said conductor that comprises a cord of low loss insulating material wound therearound in the form of an open helix of relatively long pitch, and a tubular member of low loss insulating material surrounding said central conductor and the cord wound therearound but retained in spaced relationship thereto by longitudinally extending ribs, formed integrally with and extending radially inwardly from the interior wall of said tubular member, said ribs tapering inwardly and pressing against the helically wound cord but not against the central conductor.

BRUCE H. MADDOCK.

REFERENCES crrEn The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain, 1911 Number Number

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