US 2510358 A
Description (OCR text may contain errors)
June 6, 1950 WOLF 2,510,358
ART OF MAKING CONCENTRIC TRANSMISSION LINES Filed March 20, 1946' IN! IN TOR.
465561 J 1461/ BY @QM irrakwsy Patented June 1950 ART OF KING GONGENTBIC TRANS- MISSION LINES Lester J. Well, Audubon,
Corporation oi America, ware N.J., assignmaeorporaflon to Radio of Dela- Application March 20, 1946, Serial No. 655,697 4 China. (Cl. 174-28) making electrical transmission lines, particularly concentric transmission lines, and has special reference to the provision of improvements in so called button type insulators for maintaining the separate conductors of such a line in spaced relation.
The prior art shows both elastic and non-elastic button' type insulators for supporting an inner conductor in coaxial relation with respect to an outer conductor. Elastic buttons, constituted of paragutta or other rubber-like materials, possess the advantage of being tight-fitting (in that they are self-biased into engagement with the outer conductor) but are seldom used because such materials exhibit a higher power factor and a higher dielectric constant than is desirable where currents of radio frequencies are involved. On the other hand, spacers constituted of many of the common plastic or ceramic materials possess the requisite insulating properties but, because of their non-elastic nature, are not tight-iitting. This latter disadvantage is especial- 1y objectionable when the line is called upon to handle currents of substantial voltage since, in this case, even a relatively minute air-space around the periphery of the button (1. e. between the button and the inner surface of the outer conductor) augments the possibility of corona discharge.
Accordingly, the principal object of my present invention is to obviate the foregoing and other less apparent objections to present day transmission lines.
A related object of my invention is to provide a coaxial cable which shall exhibit an extremely low attenuation factor for currents of radio frequency, and one wherein the possibility of corona discharge is minimized.
Another and specific object of my invention is to provide an improved button-type insulating spacer for the inner conductor of a coaxial cable, and one which is capable of being maintained in intimate, non-yielding contact with the inner surface of the outer conductor.
Still another object of my present invention is to provide a simple yet reliable method of manufacturing a concentric transmission line and one which in its practice ensures the correct permanet and rigid alignment of the inner conductor with respect to the outer conductor.
Stated generally, my invention is predicated upon an appreciation of the fact, and its application to the art of making electrical transmission lines, that certain non-elastic yet distortable 2 synthetic plastic materials, when distorted, exhibit a "plastic memory," i. e. the ability, especially when heated, to return to the shape and dimensions which they possessed when cast, or prior to being distorted. Among the materials possessing this property, as well as the insulating properties required for the purposes of this invention, are (1) polyethylene, (2) polytetrafluorethylene, (3) cross-linked polystyrene, (4) cross-linked polymethyl methacrylate resin (5) polyamide resins and (6) solid silicones.
In carrying my invention into effect I either cast, or cut from a sheet of such material, a desired number of buttons each of a diameter slightly larger than the inner diameter oi the outer conductor of the transmission line and string said buttons in spaced relation on the wire or tube which is to comprise the inner conductor of the line. Prior to the "stringing operation I distort the said buttons to permit the assembly to be entered into the bore of the outer conductor and, finally, I heat the spacers, either by conduction or with an electric field, to cause them to assume substantially the original shape and dimensions dictated by their plastic memory.
Other details of construction together with additional objects and advantages will be apparent and my invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein:
Fig. l is a view in perspective of a button-type spacer which is constituted, in accordance with my invention, of a distortable insulating material, having a plastic memory;
Fig. 2 is a similar view of the button of Fig. 1, when distorted;
Fig. 3 is a broken-away view of a concentric line showing the manner in which the inner condoctor and its distorted spacers are assembled within the outer conductor;
Fig. 4 is a sectional view of a button distorted in an alternative form and Fig. 5 is a broken-away view of the transmission line of Fig. 3 after the button-like spacers oi. Figs. 2 and 3 have been caused to assume the original shape and dimensions dictated by their plastic memory.
In the drawing wherein I have used like reference characters to designate the same or corresponding parts in all figures I designates the inner conductor, and 2 the outer conductor of a concentric transmission line or cable. The dimensions of the cable which are pertinent to my invention are the inner diameter D of the outer conductor and the outer diameter d of the inner conductor. In the completed cable shown in Fig. the inner conductor i is supported within the outer conductor 2 by insulating button-like spacers 3 which are formed in accordance with my invention of a material (e. g. polyethylene) having a plastic memory.
As shown in Fig. 1 the button 3 in its original form, has parallel major faces a and b and has an outer diameter D slightly larger than the inner diameter D of the outer conductor 3, and an inner diameter d slightly less than the outer diameter d of the inner conductor i. As previously indicated this button 3 may conveniently be cut or punched from a thin sheet of material, though it may be formed by casting in a mould. Subsequently, preferably prior to their assembly upon the inner conductor, the buttons 3 are placed in a coining die or pressure mold (not shown) and forced to change their shape, for example in one of the patterns shown in Figs. 2, and 3, or 4. In Figs. 2 and 3 the button has been shaped by the die in the form of a convex cone and, in the embodiment of Fig. 4 comprises a hollow right circular cone wherein the thickness of its walls is greater adjacent the apex of the cone than it is at the base or lower perimeter. I prefer the latter form for high voltage work since the thickness of the material is greatest adjacent to the inner conductor where the lines of force of the field are the more concentrated. Irrespective of the exact shape for the button the coining die should operate to compress the blanks in such a way that the outer diameter is made slightly less than the inner diameter D of the outer conductor and the inner diameter of central aperture is made slightly larger than the outer diameter d of the inner conductor i which it is designed to accommodate.
When the insulating blanks of Fig. 1 have been distorted into one of the forms shown in Figs. 2 or- 4 they are assembled or strung in spaced relation on the inner conductor 51 which is then placed, with the spacers properly positioned, within the outer conductor 2. As shown in Fig. 3, at this time the insulators 3 are loose in the outer conductor and on the inner conductor. Thereafter the insulating buttons or, alternatively, the entire assembly is heated to a temperature (say 300 C. or higher) sufficient to activate the plastic memory of the insulating material so that the buttons are urged to change their shape in accordance with their past history. In the instant case the buttons tend to go back to the shape shown in Fig. 2. As a result the buttons are urged into intimate contact with both the inner and outer conductors l and 2, though because of their original oversizeddimensions they will not quite achieve their original shape.
It will now be apparent that my invention atfords a simple yet reliable method of manufacturing a concentric transmission line having an extremely lowattenuation factor for currents of radio frequencies and one wherein the possibility of corona is minimized.
1. A spacer for supporting an inner conductor within an outer conductor, said spacer comprising a button having a curved face of a diameter less than the inner diameter of said outer conductor and formed by distorting a blank of a relatively larger diameter constituted of a distorteble insulating material of the type having a plastic memory.
2. The invention as set forth in claim 1 and wherein said button comprises a cone having a concave face.
ing said spacers to permit said assembly to be' entered into said outer conductor, and then heating said spacers to cause them to assume the original shape and dimensions dictated by their plastic memory.
LESTER J. WOLF.
REFERENCES omen The following references are of record in the file of this patent:
UNI'Im STATES PATENTS