US2585484A - Method of making high-frequency transmission line - Google Patents

Description

Feb. 12, 1952 H. c. MENES 2,585,484

METHOD 0F MAKING HIGHFREQUENCY TRANSMISSIONVLINE Filed June 25, 1947 7 W AK2A! .b

INVENTOR. HERMAN c. MENES NEYS.`

Patented Feb. 12, 1952 METHOD OF MAKING HIGH-FREQUENCY TRANSMISSION LINE Herman C. Menes, Wyckoff, N.. J., assignor to Fairchild Camera & Instrument Corporation, Jamaica, N. Y., a corporation of.' Delaware.

Application June 25, 1947, SerialNo. 756,873

9 Claims. l.

This invention relates to high frequency transmission lines and methods of producing such lines; More particularly it relates to low capacity coaxial transmission line and highly eflicient continuous methods of production thereof.

Coaxial lines are useful particularly as transmissionrchannels for transferring radio frequency energyffrom: one point to another because of the high eiiiciency and low radiation characteristics of suchV lines'. Such linesv are also useful as 'tuned circuits and as` reactive circuit elements for matching impedances between various circuit components in high frequency systems.

There are two maior sources of energy loss in such coaxial lines; (1) loss in the dielectric, and Y (129 "copper loss.

Thecopper loss is minimized by suitable choicer of materials and designs forl the inner andouter conductors, and by preventing the reilection of' radio frequency energy, such as would bei caused by abrupt discontinuities along the line. Such discontinuities, occuring either along thelength of the line or at the termination, produceY standing voltage Waves on the line. The eiciency of a coaxial line is higher when the standing waves are 10W, i. e. less radio frequency energy is dissipated and the input impedance to the line is less sensitive to small changes in irequency: or length of the line. In addition, the

power capacity of the coaxial line, as limited by the break-down voltage of the insulating material supporting the central conductor, is decreasedI by the presence of the standing waves.

The dielectric loss may be minimized by re ducing the amount of solid dielectric material whichxsupports the central conductor in the line, Aand by' proper choice of the dielectric material.

vexcessive reflection of the radio frequency energy, particularly at the higher frequencies. In addition, when such beads are merely iittedlooselyraround theinner conductor, the non-linear potential. gradient within the line increases the likelihoodof break-down at the air-gap between the insulating-material and the inner conductor.

For many applications it is desirable that the capacity between the inner and outer conductors be as low as possible. Solid dielectric materials have an appreciably higher dielectric constant than does, for example, air; it is advantageous, therefore, to reduce the dielectric material to a minimum. However it is important, also, to incorporate sufficient solid dielectric to insure proper concentric alignment of the inner conductor.

Thus; for many reasons, it is desirable to use the minimum. amount of. dielectric material within the transmissionline and to avoid abrupt discontinuities that would produce excessive reiiection of radio frequency energy, and to so arrange the dielectric material that the breakdown voltage is retained at the required level.

If the transmission line is to be of the flexible type, it is necessary that the dielectric material be so constructed and arranged as not to interfere with fiexing of the line and to maintain the inner conductorin its central position even when the line is subjected to relatively sharp bends, Furthermore thel transmission line should be capable ofmanufacture by high speed, preferably continuous, methods of fabrication.

It is, accordingly, an object of this invention to provideA a` low capacity coaxial transmission line and an improved .continuous production method of fabricating it rapidly f and economically.

It is another object of this invention to provide a continuous method of fabricating low capacity coaxial transmission line.

A further object is to produce, continuously, coaxial cableby the'successive steps of surroundingl an inner: conductor with extruded dielectric material to form longitudinally along the conductor successive areasl of solid and fluid dielectric and'extruding therearound a sheath of solid dielectric material.

Still another object is to` provide a method of producing coaxial cable wherein dielectric material surrounding the inner conductor is cooled to a temperature below normal room temperature and immediately surrounded, by extrusion, with a sheath of insulating material.

Another: object is to produce such cable by braidingy strands of dielectric material around a central conductor, cooling the strands to a ',temperaturebelownormal room temperature and immediatelysextruding a sheath of dielectric vmaterial aroundk thevbraided strands.

It is anA additional object to.; provide, in a pro- ,cess for.' producingy coaxial cable,. accelerated cooling of thermoplastic material prior to a subsequent extruding operation.

A further object is to produce a low-capacity coaxial cable by braiding strands of flexible insulating material about a central conductor, covering the braided strands with a continuous sheath of insulating material, and surrounding the sheath with an outer conductor.

It is still another object of this invention to provide, as an article of manufacture, a coaxial transmission line having braided strands of .dielectric material surrounding and supporting the inner conductor.

Another object is to provide a coaxial cable having a continuous center conductor surrounded and supported by braided strands of dielectric material Within a continuous dielectric sheath.

The invention, accordingly, consists in the features of construction, combination of elements, arrangement of parts, and methods of construction as will be exemplified in the sequences and series of steps to be hereinafter indicated and the scope of the application of which will be set forth in the following claims.

A more complete understanding of the invention may be had from a consideration of the following ,description and the accompanying drawings in which similar numerals refer to similar parts throughout the several views.

Figure l illustrates diagrammatically, the successive steps of one method of continuously producing coaxial transmission line;

Figure 2 is a fragmentary cut-away perspective view of a low-capacity coaxial cable produced in accordance with the steps illustrated in Figure l; and

Figure 3 is a cross-sectional view taken on line 3 3 of Figure 2.

The low-capacity cable, shown in Figures 2 and 3, has a filamentary inner conductor 2 surrounded by braided strands of flexible dielectric material 4, of polyethylene or other suitable material, encompassed within a sleeve or sheath 6 which may be formed of similar insulating material. The sheath 8 is surrounded by an outer conductor 8 which is provided with a suitable protective coating I2 of insulating material.

In order that the capacity of the cable may be low, the Wire, which forms the inner conductor 2, is of small diameter. Because of the small diameter of the inner conductor, it is desirable that it be formed of material having high-tensile strength, for example, Phosphor bronze. To reduce the resistance of this conductor to the fiow of high frequency current, it is plated, desirably, with silver or other material having similar lowresistance characteristics.

The strands 4 of dielectric material, advantageously, have a larger cross-sectional area than the inner conductor 2 and may vary in number from 3, the minimum number which may be used to produce the desired braid, to or more strands, The number of strands to be used in a particular application depends upon the conditions under which the cable is to be used; it being desirable to use a number sufficient to provide proper centering of the inner conductor. These strands, being braided, then maintain the center conductor 2 in its correct position and prevent it shifting from that position, even with long periods of continuous use and flexing.

The final capacitance of the completed cable will be determined to a large measure by the 4tightness of the braided strands 4 and the angle and number of cross-overs thereof, as Well as the gauge of the strands. By forming the sheath 6 of flexible dielectric material and braiding the outer conductor 8 from small filaments, as indicated in Figure 2, the cable may be made quite flexible; proper centering of the inner conductor 2 being assured by the braided strands 4 even when the radius of curvature of the flexed cable is quite small.

The spaces between the strands 4 represent air spaces (or for particular` applications may be filled with other inert gases) which act to reduce the average dielectric constant of the area surrounding the inner conductor, and thus, to reduce the capacity between the inner and outer conductors. For example one cable constructed in this manner had a capacity of 5.5 mmfd, per linear foot and extremely low losses. The center conductor was formed of number 32 Wire and the strands 4, of which there were eight, had a diameter of 0.03 inches.

The steps by which this cable may be constructed are illustrated diagrammatically in Figure l. The filamentary inner conductor 2 passes continuously from a roll I3 through a braiding machine, generally indicated at I4, where several reels, for example, reel I6, holding the supply of dielectric strands 4, rotate circumferentially around the conductor 2 with the proper relative speeds and transverse movements to cause the strands 4 to form, continuously, a braid with the conductor 2 running longitudinally through the center.

This braided structure may be drawn into a pre-formed sheath of dielectric material or, advantageously, the dielectric material may be extruded directly around the braided strands. If the extrusion of the thermoplastic material takes place at a temperature higher than the softening temperature of the strands 4, these strands would be altered or deformed during the extrusion process. In order to overcome this difficulty, the braided structure is passed through a cold chamber I8, which is maintained at a sufilciently low temperature that the strands 4 in passing therethrough Will be cooled at least to the temperature necessary to prevent heat distortion during the extrusion of the sheath 6.

The braided structure emerging from the cold chamber I8 passes immediately through a jacketing die, indicated generally at 22. The thermoplastic dielectric material 24 is heated by suitable means (not shown), and forced into the jacketing die by suitable means, as for example screw 26, and extruded to form the sheath 6 around the pre-cooled braided strands 4. This extrusion process is completed before the temperature of the strands 4 has increased enough to result in serious heat distortion. The jacketed structure passes from the extrusion step through a second braiding machine, generally indicated at 28, Where the braid 8, formed of metallic strands 'or wires, is served about the sheath 6.

For many applications it is desirable that the outer conductor 8 be protected by a coating of rubber or other insulating material which is applied, advantageously, by a second extruding machine, indicated generally at 32, which extrudes suitable thermoplastic insulating material 34, to form the protective covering I2 around the outer conductor 8.

The various steps of the process are synchronized, advantageously, so that the inner conductor 2 moves at constant speed through the various operating steps, the finished cable, generally indicated at 36, accordingly emerging continuously from the extrusion machine 32.

From the foregoing it will be observed that the process and apparatus embodying my invention are well adapted to attain the ends and objects hereinbefore set forth since the separate features are Well suited to common production methods and are subject to a variety of modifications as may be desirable in adapting the invention to different applications.

As many possible embodiments may be made oi the above invention and as many changes might be made in the embodiments set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limited sense.

I claim:

1. The method oi continuously producing lowcapacity coaxial transmission line comprising the steps of continuously surrounding an electrical conductor with continuous solid dielectric material so arranged as to form longitudinally along said conductor connected areas of iluid and solid dielectric material, continuously extruding a continuous sheath of dielectric material therearound, braiding an outer conductor on said sheath, and extruding an outer protective coating of insulating material around said outer conductor.

2. The method as defined in claim 1 wherein the first step of surrounding comprises braiding at least three strands of iiexible dielectric material around said electrical conductor.

3. The method of continuously producing coaxial transmission lines comprising the steps of braiding a plurality of strands of dielectric material around an electrical conductor, extruding a substantially cylindrical sheath of polyethylene around said braided strands, and serving flexible conductive material about said sheath to form an electrically continuous outer conductor.

4. The method of producing coaxial transmission line comprising the steps of braiding strands of dielectric material around an electrical conductor, extruding thermoplastic insulating material to form a continuous sheath about said braided strands, braiding metallic strands around said sheath to form an outer conductor, and extruding a continuous protective coating of insulating material over said outer conductor.

5. A method of producing low-capacity coaxial transmission lines comprising the steps of entwining stranded dielectric material about an electrical conductor, cooling said stranded dielectric material to a degree suiilcient to preclude fusing with a subsequent layer of similar material, and immediately extruding a sheath of dielectric material around. said stranded dielectric material.

6. The method of producing low-capacity coaxial cable, comprising the steps of braiding strands of dielectric material around a wire, cooling the braided strands to a degree suincient to preclude fusing with a subsequent layer of similar material, immediately extruding thermoplastic insulating material therearound to form a continuous cylindrical cover, and surrounding the sheath so formed with an outer metallic conductor.

iii

7. The method of continuously producing lowcapacity coaxial radio frequency transmission line comprising the steps of braiding at least three strands of high polymer insulating material about an inner conductor, passing the braided strands continuously through a reirigerating medium and thereby reducing its temperature to a degree sufiicient to preclude fusing with a subsequent layer of similar material, extruding a sheath of high polymer thermoplastic insulating material around the braided strands While at the sub-normal temperature to prevent heat distortion of the thermoplastic strands, braiding an outer conductor of metallic strands around said sheath, and applying a protective coating of insulating material to said outer conductor.

8. The method of continuously producing lowcapacity coaxial transmission line, which includes the steps of continuously surrounding an electrical conductor with continuous solid dielectric material so arranged as to form longitudinally along said conductor connected areas of gaseous and solid dielectric material, cooling said dielectric material to a degree suiiicient to preclude fusing with a subsequent layer of similar material, continuously extruding an uninterrupted sheath of dielectric material around said first-mentioned dielectric material, braiding an outer conductor on said sheath, and extruding an outer protective coating of insulating material around said conductor.

9. The method of continuously producing lowcapacity coaxial transmission line, which includes `the steps of plating a conductor with a metal having low resistance characteristics, continuously surrounding said conductor with continuous solid dielectric material so arranged as to form longitudinally along said conductor connected areas of gaseous and solid dielectric material, cooling said dielectric material to a degree suilcient to preclude fusing with a subsequent layer of similar material, continuously extruding a continuous sheath of dielectric material therearound, braiding an outer conductor on said sheath, and extruding an outer protective coating of insulating material around said outer conductor.

HER-MAN C, MENES.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Yolles Sept. 5, 1950

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