US2226928A - Electrical transmission line - Google Patents

Description

Dec. 31, 1940. l R L HARVEY ETA-L v 2,226,928

ELECTRICAL TRANSMIS S ION LINE Filed sept. 12, 1939 ,sacaran-'Lem y Patented Dec. 31, 1940 UNITED STATES PATENT OFFICEv ELECTRICAL TRANSMISSION LINE Delaware Application September 12, 1939, Serial No. 294,420

2 Claims.

This invention -relates to electrical transmission lines and has for its principal object to provide a simple, inexpensive, flexible, covered transmission line which exhibits an extremely low attenuation for currents of radio frequency.

concentric lines, parallel lines, twisted pairs and other types of lines for transmitting radio frequency energy are known wherein the spacing between the conductors is maintained by a solid yet yieldable insulating material such, for example, as paragutta, or other rubber-like substance. Materials of this general nature exhibit a relatively high power factor and a relatively high dielectric constant. T his latter characteristic. results in a higher capacitance rating, for a given physical structure, than is desirable where radio frequency currents are involved.

Attempts to use solid embedments formed of thermoplastic materials capable of exhibiting better electrical properties than rubber have not met with any degree of success because o! the iniiexible nature of the proposed substitute materials. 'I'he addition of plasticizing ingredients to any of the proposed substitute thermoplastic embedment materials in an effort to achieve the degree of flexibility desired results in a sacrifice in the heat resistant and moisture repellent characte'ristics of the coating. These mechanical weaknesses may become manifest in electrical losses in the line and augment the losses due to the sacrifice in dielectic properties of the embedment which are incident to the use of plasticizing ingredients.

As above indicated, high frequency transmission lines contained in the solid embedments of the prior art have not proven entirely satisfactory. This has led to the almost universal use of various forms of discrete supports, such asinsulating discs, mechanical spiders, etc.. arranged in spaced relation along the conductors for maintaining them in a desired physical and capacitive relation, while permitting the requisite degree o! flexibility or curvature in the line. Such spaced supports. however, are expensive, particularly in their installation, where difficulties may arise not only in attaching them to the conductors, but also in fitting them within the outer conductor and in maintaining them in upright operative alignment therein.

The above and other disadvantages inherent in the radio frequency transmission lines of the prior art are obviated in accordance with the present invention by the provision of a solid embedment of especial form and constituted of styrol, polystyrene or equivalent substance which may be molded, in a continuous process, without the use of plasticizing agents. More specifically, instead of the circular solid embedments of the prior art, the present invention contemplates, and its practice provides, a ribbon-like embedment or casing having a thickness which is no greater and is preferably less than the diameter of the wire or wires embedded therein. The ribbon form of the embedment is essential where a substantial degree of flexibility is required. Flexibility is enhanced when the surfaces of the ribbon intersect the coated surfaces of the wire at a sharp angle. Stated another way, an added mass of material along a line or lines where the ribbon meets the coated wire or wires will constitute a reinforcing rib which will inhibit flexing.

Other details of construction, together with additional objects and advantages, will be apparent and the invention will be best understood by reference to the following specification and to the accompanying drawing, wherein:

Figure l is a longitudinal sectional view and Figure 2 is a cross-sectional view of a concentric transmission line embodying the invention..

In Figs. 1 and 2, W designates a copper or phosphor bronze wire embedded in a ribbon R which is reinforced adjacent its edges by a pair of oppositely located cotton or similar strings or cords DD. This single wire W may constitute the central conductor of a concentric line, in which case the ribbon R and the cords DID comprise abrasion-resisting means for maintaining the inner conductor in the desired predetermined embedment or ribbon R should be no greater and,

as shown, is preferably less than the diameter of the wire W. Considering the wire and cords as being covered by separate coats C, C1, C of the thermoplastic material, it will be noted that the lines L along which the ribbon R joins these coats are sharp and well defined. This results in a construction which permits of maximum flexibility in the direction of the extension of the wires and yet resists any force tending to alter their original spacing. As previously set forth. any rounding or filling along thel lines L where the ribbon R and coats C meet will constitute a reinforcement which will limit flexing in the direction of the extension of the wires W.

In the manufacture of the transmission lines of the invention, either liquid styrol or granulated polystyrene may be employed in the molding process. When styrol is employed, its polymerization may be effected as b y means of a relatively high powered ultra-violet light mounted adjacent the point where the ribbon leaves the mold. When granulated polystyrene is employed, it may be rendered plastic by heating either or both, preferably both, the injector and the mold.

In the ease of wires or cables of large diameter, the thickness of the ribbon may be much less than the diameter of the wire. Thus. in one embodiment of the invention employing gauge Il (B and S) cable, entirely satisfactory results have been achieved with a polystyrene ribbon of 20 to 80 one-thousandths of an inch in thickness. This ratio of ribbon thickness to wire gauge cannot ordinarily be maintained with very fine wires. Thus, where 36 gauge (B and S) wires are employed, the thickness of the ribbon should correspond substantially to the diameter of the wire.

It is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting sense, except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. An electrical transmission line comprising a hollow metal tube containing an inner conductor embedded in a substantially at insulating ribbon having a width substantially equal to the inside diameter of said tube, said ribbon being provided along its edges with an abrasion resisting reinforcement.

2. The invention as set forth in the precedins claim and wherein said reinforcement comprises a pair of non-conductive cords encased in the material of which said ribbon is constituted.

ROBERT L. HARVEY. CHANDLER WENTWORTH.

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