US2967900A

US2967900A - Coaxial transmission line

Info

Publication number: US2967900A
Application number: US729102A
Authority: US
Inventors: Stanley A Pryga
Original assignee: North American Aviation Corp
Current assignee: North American Aviation Corp
Priority date: 1958-04-17
Filing date: 1958-04-17
Publication date: 1961-01-10
Anticipated expiration: 1978-01-10

Description

Jan. 10, 1961 s. A. PRYGA comm. TRANSMISSION LINE 3 sheets -sheet 1 Filed April 17, 1958 INVENTOR. STANLEY A. PRYGA BYgaWm AGENT Jan. 10, 1961 Filed April 17, 1958 (OHMS) s. A. PRYGA 2,967,900

COAXIAL TRANSMISSION LINE 3 Sheets-Sheet 2 D (INCHES) FIG. 3

INVENTOR. STANLEY A. PRYGA AGENT Jam 10, 1951 s. A. PRYGA ,967,900

COAXIAL TRANSMISSION LINE Filed April 17, 1958 3 Sheets-Sheet 3 INVENTO STANLEY A. PR

dQ/m' AGENT United States Patent O 'COAXIAL TRANSMISSION LINE Stanley A. Pryga, Paramount, Calif., assignor to North American Aviation, Inc.

Filed Apr. 17, 1958, Ser. No. 729,102

4 Claims. (Cl. 174-28) This invention relates to coaxial transmission lines and more particularly to a coaxial transmission line having low losses and a low reflection coefficient and which will operate at extremely high temperatures. There is a great need for transmission lines which will stand temperatures in excess of 250 F., especially in high speed aircraft and in missiles. The problem of finding a suitable cable for such applications becomes even more difficult where the cable is to carry microwave radio signals.

There are many coaxial lines which will function satisfactorily at temperatures below 250 F. These include cables using solid dielectrics such as tetrafiuoroethylene plastic, or similar material having a low dielectric constant. Such a dielectric material, however, if it does not deteriorate completely, will expand considerably at elevated temperatures and such expansion will significantly change the cables electrical characteristics. This makes such cables entirely unsuitable for high temperature applications. ceramic dielectric material such as unfired magnesium oxide suffer from serious resonance effects when utilized in the microwave frequency range resulting from dimensional defects inherent in their manufacturing processes. Another transmission line which might satisfy the high temperature requirements is one utilizing ceramic bead separators in a gas dielectric line. Such beads of ceramic, while they will stand the high operating temperatures, have high dielectric constants and when utilized in the conventional cylindrical form as separators will produce intolerable losses and reflections in a line used in the microwave frequency range.

Basically, therefore, it is difiicult to achieve low line reflections and losses in a coaxial line operating at temperatures in excess of 250 F. with signal frequencies in the neighborhood of 2.5 kilomegacycles. The device of this invention solves this problem. In a preferred form of the invention, a substantially flat strip is utilized as a center conductor. This fiat strip is separated from an outer tubular conductor by ceramic supporting pins positioned along the center line of the strip.

It is therefore an object of this invention to provide an improved coaxial transmission line for use at elevated temperatures.

It is another object of this invention to provide a coaxial transmission line having low losses and a low reflection coefiicient which will operate at high temperatures.

It is still another object of this invention to improve the operation of coaxial transmission lines in the microwave range under high temperature operating conditions.

It is a further object of this invention to provide an efficient coaxial transmission line for operation at extremely high temperatures.

Other objects of this invention will become apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is an isometric cut-away drawing of a preferred embodiment of the invention;

Other cables available which utilize solid 2,967,900 Patented Jan. 10, 1961 Fig. 2 is a cross sectional view taken as represented by the line 22 indicated in Fig. 1;

Fig. 3 is a graphical representation of the variation of the characteristic impedance of the device of the invention with variations in the width of the center conductor;

Fig. 4 is a schematic representation of the electric and magnetic fields in the device of this invention; and

Fig. 5 is an isometric cut-away drawing of a second embodiment of the invention.

Referring to Fig. 1, a center conductor 14, preferably fabricated of a suitable highly conductive material such as copper, which is twisted through its entire length, is surrounded by a tubular outer conductor 11 of similar conductive material. The center conductor 14 should preferably be given a uniform twist throughout its entire length. The amount or uniformity of twist, however, is not critical and will not affect operation substantially. Pins 12 may be attached to center conductor 14 at one end portion of the pins while each pin has its opposite end portion in supporting contact with the inner surface of outer conductor 11. Referring to Fig. 2, the spacing elements, or separator pins, may be attached to the center conductor 14 by swaging through perforations 16 in the center conductor or they may be cemented or otherwise fixed in place in any suitable fashion. The pins 12 should preferably be made of a high temperature ceramic such as, for example, fired natural lava. Lava A, manufactured by the Alsimag Company, has been found to be I satisfactory.

The center conductor can be slightly concave or corn vex; however it is desirable to keep it as close to a fiat configuration as possible. The transmission line may operate with air as its principal dielectric, or it can be filled with a suitable gas such as hydrogen. The separator pins 12 could conceivably be mounted on the outer conductor or engaged at their opposite end portions with the center and outer conductors in some other suitable fashion, the only requirement being that they are able to support and center the inner conductor within the outer conductor, and at the same time electrically insulate the two from each other. In this respect, it is desirable to attach. the pins to one of the conductors using pins of such a length that when the inner conductor is inserted within the outer conductor, they will be contiguous to the conductor to which they are not attached. The pins are mutually spaced and consecutively positioned along a helical path between the inner and outer conductors.

Referring to Fig. 5 which is a pictorial drawing of a second embodiment of the invention, spacing elements 13 which extend outwardly in opposite directions from the center conductor 14 are used to separate the center conductor 14 from the outer tubular conductor 11. The spacing elements 13 preferably may be press fitted through holes 16 in the center conductor and may be permanently mounted thereon with cement or other suitable means. The spacing elements preferably should be contiguou to opposite sides of the inner wall of the outer conductor. Other than for the different type of spacing elements utilized, the embodiment shown in Fig. 5 is similar in all respects to the preferred embodiment. It has the advantage of greater structural support for the center conductor but the disadvantage of having more high dielectric material between the conductors than the first embodiment. These considerations are discussed infra.

The amount of twist used for the center conductor should be suflicient to allow for adequate centering and support of the center conductor Within the outer conductor. It has been found that in a transmission line having an outer conductor with an inner diameter of approximately of an inch that approximately one full twist every two inches is satisfactory. In such a design, ea

inch spacing between separator pins will furnish adequate support and centering for the center conductor. It is to be noted'that it is 'desirableto use as few pins'as' possible compatible with adequate center conductor support and centering within the outer conductor. Pins approximately .090 inch in diameter will function satisfactorily .istic impedance varies in accordance with'the width D of the center conductor. The characteristic impedance of this device does not appear to vary as a regular function of the parameters involved. The indicated curve, however, may be utilized in the design of all similar coaxial lines having a ratio of inner diameter of the outer conductor to thickness of the center conductor of or 15.6, by appropriately interpolating the widths D so that they bear the same proportionality to the inner diameter of the outer conductor as those indicated in Fig. 3. Other characteristic impedance ranges can similarly be empirically derived.

-It is to benoted that the twist of the center conductor is not absolutely essential and has little effect on the electrical characteristics of the transmission line. A

transmission line having a center conductor with either slight twist or no twist at all will electrically function equivalently to one having a moderate amount of twist. Twisting the center conductor, however, allows for better support and centering of this conductor within the outer conductor for a given number of ceramic separator pins.

.As it is desirable to use as few ceramic separator pins as is possible commensurate with adequate support and centering of the center conductor, twisting the center conductor by allowing for fewer separator pins in achieving these ends is conducive to a higher performance-transmission line. The .pins should be as thin as possible or :may .be tapered. in accordance with these same considerations. This is because it is desirable to have aslittle high dielectric material between the center and outer conducftors if there are to be minimum reflections and minimum losses in the line.

The ceramic separators: should be arranged along the center line of the center conductor for best results. The reasons for this can best be explained by reference to Fig. 4 whichshows the electric field E and the magnetic field H between the center and outer conductors. As can be seen, the'electric field is concentrated at the ends of the center conductor which are closest to the outer conductor, or at the points of highest capacity between these two elements. Consequently, the electric field is least between the center line of the center conductor and the outer conductor. As the reflection coefiicient will tend to be greatest if the ceramic pins are located'at the point of highest electric field and least if they are located at the point of minimum electric field, less reflections will occur if the pins made of high dielectric constant material are located along the center line which is the position of the minimum electrical'field.

Comparative tests made at frequencies between 2.5 and 3.5- kilomegacycles of 8 foot sections of the device of this "frequency range enumerated (2.5't0 3.5 kilomegacycles) an. 8-foot section of the device 'ofthis invention at no point had a voltage standing wave ratio in excess of 1.1. The high temperature cable utilizing the solid dielectric material had a standing wave ratio in excess of 1.4 at several points within this frequency range, while the line using cylindrical ceramic bead separators had standing wave ratios which averaged approximately 1.4 within this frequency range and towards the high end of the range approached 4. It is therefore "apparent that the device of this invention enables the'utilization-of a-coaxial line in the microwave range operating at temperatures well in excess of 250 F. which has a considerably lower reflection characteristic and lower losses than existing type high temperature coaxial lines.

Although the invention has been described and illustrated in detailyit is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. A coaxial transmission line comprising a flattened center conductor, said center conductor being twisted, a tubular outer conductor surrounding said center condue tor, a minimum electric'field being located between the center line of said center conductor and said outer conductor and a plurality of non-conductive separator pins, one end of each of said separator pins being attached to said center conductor along the center line thereof, the other end of each of said separator pins being contiguous to the inner wall of said outer conductor wherebysaid center conductor'is supported within and electrically insulate'd from said outer conductor andsaid separator pins are located at points of minimum electric field.

2. In a coaxial transmission line, a tubular outer conductor, "a center conductor of flattened cross-section hav .ing uniform thickness and width throughout its entire length, said center conductor being uniformly twisted intermediate its ends and symmetrically centered 'within said outer conductor, a minimum electric field being located between the center line of said center conductor and said outer conductor said center conductor further having uniformly spaced perforations symmetrically located along its center line, and high temperature separator pins, one end of each :of said separator pins being swaged to said center conductor through an associated one of said perforations, the other end of each of'said pins being contiguous to the'inner wall of said outer conductor, whereby said center conductor is'supported within and electrically insulatedfrom said outer conductor and said separator pins are located at points of minimum electric field.

3. In a coaxial transmission line, a tubular outer conductor, a substantially flat strip-center conductofhaving uniform thickness and width throughout its entire length, said center conductor having uniformly s'pacedperforations symmetrically located along its center line, said center conductor further being uniformly twisted intermediate its ends, and ceramic separator pins, one end of each of said separator pins being swagedto said center conductor through said perforations, the other-end of each of said pins being contiguous to the inner wall of said outer conductor, a minimum electric field being located between thercenter line of said center conductor and said outer conductor whereby said center conductor is supported within and electrically insulated-from said outer conductor and said separator pins are'located at f points of minimum electric field.

4. A transmission line comprising an elongated outer conductor, an elongated inner conductor positioned Within said outer conductor, said inner conductorcomprising a helically twisted strip of rectangular cross section having one pair of mutually opposite surfaces of relatively large dimension as compared with the other pair of=-mutually opposite surfaces, whereby electric field lines between said inner and outer conductors in said transmission line are concentrated about said other pair of surfaces and electric field lines are of minimum density at central portions of said one pair of surfaces, and a plurality of elongated separator pins fixedly attached to said inner conductor positioning said inner conductor within said outer conductor, each of said pins being spaced from its adjoining pins, said pins being consecutively positioned along a helical path within said outer conductor, each pin having the opposite end portions thereof in supporting oontact with the inner surface of said outer conductor and with a central portion of one of said inner conductor surfaces of relatively large dimension, the dimension of each pin in a direction transverse to its longitudinal extent being substantially less than the extent of said relatively large dimension surface of said inner conductor, whereby said pins are substantially spaced from said concentrated electric field lines.

6 References Cited in the file of this patent UNITED STATES PATENTS 1,900,976 Carpe Mar. 14, 1933 2,180,722 Rust Nov. 21, 1939 2,280,200 Smith Apr. 21, 1942 2,462,887 Muller Mar. 1, 1949 2,796,589 Adams June 18, 1957 2,847,499 Peterson Aug. 12, 1958 FOREIGN PATENTS 482,912 Great Britain Apr. 7, 1938 OTHER REFERENCES Andrew Co. Publication: Design Data for Beaded Coaxial Lines, reprinted from Electronics, May 1946. pages 130-136.