US2498335A - Termination for coaxial lines - Google Patents

Description

Feb. 2l, 1950 Filed May 5, 1945 IN VEN TOR.

,ai Wifi/nf? @Trop/Vey -to 1000 mc./s.

Patented Feb. 21, 1950 TERMINATION FOR COAXIAL LINES Edward William Hunt, Chesham, England, assignor, by mesne assignments, to Radio Corporation of America, New York, N, Y., a corporation of Delaware Application May 3, 1945, Serial No. 591,731 In Great Britain February 1, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires February 1, 1964 (Cl. Z50-31) Claims.

This invention relates to improvements in coaxial transmission lines, particularly in the termination thereof. It is more especially concerned with a termination device for a coaxial transmission line which is required to operate in the very high frequency range extending from 1 The invention, however, resides in the principle involved in the device itself and not in any particular range of frequencies in connection with which the device may be required to be used.

The principal object of the invention is to provide an essentially resistive termination for a coaxial transmission line having a characteristic impedance lying between 30 to 120 ohms.

Another object of the invention is to provide a device for the detection or measurement of the voltage across the termination.

According to the invention in its broadest aspect, a substantially exclusively resistive termination for a coaxial transmission line includes a centrally apertured carbon disc having at least the peripheral surface and the internal surface of the aperture metallised to constitute terminals with which, in use, the interior surface of the outer conductor of the transmission line (or of an extension thereof constituting part of the terminating device) and the exterior surface of the inner conductor of the transmission line (or-of an extension thereof constituting part of the terminating device) make continuous circumferential and width electrical connection, and wherein the resistance between the edges of the two metallised surfaces through the disc has the desired value.

- In addition to the-peripheral surface, an outer annular surface on one or both sides of the disc maybe metallised, the metallisingof the peripheral surface and of the annulus or annuli being continuous, and the resistance between the edge of the annulus or edges of the annuli and the edges of the metallised surface of the aperture through the carbon disc having the desired value.

Again, in addition to the said peripheral surface or peripheral and outer annular surfaces, an inner annular surface on one or both sides of the discvmay be metallised, the metallising of the inner surface of the aperture and of the last named annulusror annuli being continuous, and the resistance between the edges of inner and outer annuli having the desired value. According to theinvention in a more' restricted aspect, a terminatingdevice fora coaxial transmission line includes an outer tubular conductor,

va coaxially disposed inner conductor, a carbon disc having a concentric aperture, the said outer conductor being electrically connected to the said disc continuously around the outer cylindrical surface, or continuously around an outer annular surface adjacent thereto on one or both faces of the disc, or around both, and the said inner conductor being electrically connected to the said disc continuously around the inner cylindrical surface of said aperture, or continuously around an inner annular surface adjacent thereto on either or both faces of the disc, or around both, the resistive termination being constituted by the carbon disc between the surfaces of connection and being disposed symmetrically about the centre line of the transmission line.

In order to provide a device for the detection or measurement of the voltage developed across the terminating resistance, a crystal element is situated, within the outer tubular conductor, on the side of the disc remote from that on which the coaxially disposed inner conductor is situated and has one of its terminals connected to the said inner conductor, or continuously around an inner annular surface on the face of the disc adjacent to the said crystal.

The end of the outer tubular conductor may be provided with a conductive plug having a coaxial bore through which extends a conductive shank insulated from but in capacitative relationship with the said plug, the inner end of the shank being provided with a terminal to which the other terminal of the crystal is connected, the capacitance between the shank or terminal and plug constituting a by-pass condenser for high-frequency energy.

An indicatingV instrument, for example, a micro-ammeter, may be connected to said shank and to said outer tubular conductor or to the said plug, or to any other conductive member which is in conductive relationship to the outer conductor of the transmission line.

In order to ensure good electrical connection with the carbon disc, those parts (outer cylindrical, inner cylindrical, and outer or inner annular surfaces) to which connection is to be made should be metallised.

The invention is illustrated in the accompanying drawings in which Figure l is a sectional View of an embodiment of the invention; Figure 2 is an end view of a metallised disc; Figure 3 is a schematic diagram of the circuit, and Figure 4 is a simplified circuit diagram.

In Figure l, an outer conductive tubular memoer T, internallyv'threaded at both ends is provided with an abutment at 8. Into one endV of .shank and f Vice.

tubular member "i is screwed a tubular member 3 having an internal diameter equal to the internal diameter of ,the r transmission line jin use (man experimental model Coaxially supported, by insulating material 9 is an inner conductor 4 having an external diameter equal to that of the inner conductor of the transmission line. members 3 and 4 serve as means whereby vthe terminating device may be connected to ythe con ductors of the transmission line.

A terminating resistor, consisting of a carbon disc, a method of producing which will be described, is indicated at I. This Vdisc is ,metallised as at a and b, Figure 2, on both faces and onits outer and inner cylindrical surfaces c and d, Figure 1. The disc 1 is threaded "upon conductor 4 so that its metallised surface d makes connection with that conductor. The member 3, con ductor 4 and disc I are then screwed into tubular `member 'I so that connection is madeat a (on "line.

A crystal element 2, is inserted in the tubular member'l so asto makeconnection, by means of a shank I which enters a recess in conductor .4, with the said'conductor 4, and thus with the in- .ner terminal of the disc resistor I.

Into the right-hand end of tubular member TI is screwed `a conductive plug .I,I, having a coaxial bore. Through the bore in plug I I extends a shank I2 `having on `its Ainner end a terminal .Ii for connection tothe other end of `the crystal element, shank I2 'being coaxially supported within metal plug Il by a plug of keramot or other Vsuitable insulating material. The outer face of terminal T and the inner end of plug Il are insulated from each other by means of adisc 5 of,

`for example, mica and together constitute the relectrodes of `a by-pass condenser. Shank I2 serves for the external connection to a microammeter I4, Figure 4, which serves as a Voltage indicating device, being connected between the the earthy outer member of the de- In .Figures 3 and 4, the partscorresponding to .those of Figure 1 ar.e,.so far as may be, indicated ...by .the same reference numerals as are used in Figure 1. It is thought unnecessary to describe the circuit.

The production of resistor I may be eiected as follows: A carbon resistor of the 2 watt rating (e. g. vErie fR. SM. A. `2l) nominal `value vi00- 1,000 ohms, is `,drilled through the centre and a section `about wide parted 01T. The entire surface of this section .is then rendered conductive by copper-electrplating vin an electrolyte of copper sulphate solution. A `plating current of 100 ma. should not be exceeded, otherwise the layer of copper deposited on rthe carbon will be too .soft and will peel off too readily in the subsequent operation.v lAn .annular ring `of the den posited copper is now removed :from each end surface of the section until the resistance from 'centre to outer edge is of the'required Value (in fthe case of the experimental model, '75 ohms). The completed section is then `wax impregnated.

It is found that the resistance is fairly stable The,

in operation, the total drift vbeing Aof the order Y of 1% over a period-cfa week. The circular nature `of the resistor `makes it possible to preserve absolutely symmetrical construction of the unit.

The fact that the internal diameter of member 3 is the same as that of the outer conductor of the transmission line, and that the external diameter of member 4 is the .same as that of the inner conductor of the line preserves the physical properties of the coaxial cable in use, and .therefore since the terminating resistor I is placed directly across the end of this composite line, no rise in voltage occurs between the source Vof Avoltage and the `termination. The arrangeelectrically connected tothe 'outer cylindrical'survface yof the-inner conducto-r adjacent one face of the carbon ydisc and retained to the'housing by a bushing threaded in said metallic housing, :the o uter conductor `oi said line land Ysaid metallic housing forming `part of the Vterminating device, the resistive termination being constituted by said solid carbon ldisc -between the surfaces Vof connection to said 'housing 'and being disposed symmetrically aboutthe inner conductor o'f the transmission line,

2. A terminating device for a coaxial transmission line, comprising an youter tubular conductor, a metallic housing having a shoulder portion and `threadedmeans to join said outer tubular conductor thereto, a coaxially'disposed inner conductor insulatingly arranged to extend within said housing, a solid carbon disc having a concentric aperture, the said outer conductor being continuously electrically connected by a threaded bushing and `retained in'position at the-shoulder portion of said housing 'to said-disc aroundthe outer annular surface of said Vdisc adjacent to said outer conductor on both .faces of the disc, and the 'said `inner conductor being -continuously electrically connected to the said disc around an inner annular surface of said `disc adjacent yto `said outer conductor on both faces of the disc,

the resistive termination of said device being constituted by `connections between the disc surfaces which are disposed symmetrically `about the center-line ofthe transmission line.

' 3. A device as claimed -in claim 2, wherein "in order to vinsure good electrical connection with said metallic housing and said inner conductor 'the carbon disc has its outer cylindrical, inner cylindrical and outer and :inner annular surfaces metallized `with copper. 4. A device as claimed 1in claim 2,'wl'1erein -in order to insure good electrical connection with said housing and said inner conductor the lcarbon disc has its outer cylindrical, inner cylindrical vand outer and inner annular surfaces metall lized with copper, 'and the carbon disc'is Awax impregnated,

.5. A coaxial transmission line terminating device comprising ametallic housing member having a threaded porton'and a shoulder portion. an outer conductor, an inner conductor, ymeans to electrically connect Asaidmetallic housing mein- -ber to said outer conductonfa centrally apertured 'carbon disc `having 'at least the peripheral-sue. face andthe internal surface of the :aperture metalli'z'edto constitute terminals :for said'fdiSc,

u'means including" a threaded bushingto retain said disc within said housing at the shoulder portion thereof and to make contact with the peripheral surface of said disc, and insulating means within said bushing to support said inner conductor` within said housing.

6. A coaxial transmission line terminating device comprising a metallic housing member having a threaded portion and an internal abutment portion, an outer conductor and an inner conductor, means to electrically connect said metallic housing member to said outer conductor, a centrally apertured carbon disc having at least the peripheral surface and the internal surface of the aperture metallized to constitute electrical terminals for said disc, means including a threaded bushing to retain said disc within said housing and to make contact with the peripheral metallized surface of said disc against said abutment portion, and insulating means within said bushing to support said inner conductor within said housing.

'7. A coaxial transmission line terminating device comprising a metallic housing member, an outer conductor, an inner conductor, means to electrically connect said metallic housing member to said outer conductor, a centrally apertured carbon disc serving as a terminating resistance for said transmission line, a crystal detector elen ment centrally located within said housing and connected in series with said inner conductor for detection of the voltage developed across said terminating resistance, means to retain said disc within said housing and to make contact with the peripheral surface of said disc, and insulating means to support said inner conductor and said crystal element within said housing.

8. A coaxial transmission line terminating device comprising a metallic housing member, an outer conductor, an inner conductor, means to electrically connect said metallic housing member to said outer conductor, a centrally apertured carbon disc serving as a terminating resistance for said transmission line, a metallic shank connected to one end of said inner conductor and arranged to pass through the centrally apertured portion or said carbon disc, a crystal element connected to said shank so as to be in series with said inner conductor for the detection of voltage developed across said terminating resistance, means to retain said disc within said housing and to make contact with the peripheral surface of said disc, and insulating means to support said shank and said crystal element within said housing.

9. A coaxial transmission line terminating device comprising a metallic housing member, an outer conductor, an inner conductor, means to electrically connect said metallic housing member to said outer conductor, a centrally apertured carbon disc serving as a terminating resistance ior said transmission line, a metallic shank connected to said inner conductor and arranged to pass through the centrally apertured portion of said carbon disc, a crystal detector element connected to said shank so as to be in series with said inner conductor for the detection of voltage developed across said terminating resistance, an insulating washer interposed between said crystal element at the end opposite said shank and a portion of said housing to serve as a by-pass condenser for said transmission line, means to retain said disc within said housing and to make contact with the peripheral surface of said disc, and inn sulating means to support said shank and said crystal element within said housing.

10. A transmission line device comprising inner and outer conductors, a metallic housing member having a shoulder portion at one end thereof and threaded at both ends, a threaded tubular metallic member, an apertured solid carbon disc having its outer periphery connected to said metallic housing member at the shoulder portion thereof and retained in position by said threaded tubular metallic member, a threaded metallic plug retained in said housing at an end opposite said shoulder portion, a crystal detector located centrally within said housing member, said crystal detector having a metallic shank extending within the aperture of said solid carbon disc and electrically connecting said carbon disc to a portion of the inner conductor, an insulation member retaining said portion of the inner conductor within said threaded tubular metallic member, a mica disc having a central aperture therein, said mica disc being interposed between the crystal detector and said threaded metallic plug at the end opposite said shank to serve as a bypass condenser between the inner and outer conductors, and insulating means to permit another portion of the inner conductor to pass through said metallic plug and through the aperture in said mica disc to connect the crystal detector in series with said other portion of the inner conductor the latter connection being made at an end opposite said shank.

EDWARD WILLIAM HUNT.

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

UNITED STATES PATENTS Number Name Date 1,093,947 Rice Apr. 21, 1914 2,273,547 Radinger Feb. 17, 1942 2,294,881 Alford Sept. 8, 1942 2,414,456 Edson Jan. 21, 1947

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