US2504494A

US2504494A - Transmission line seal for electron discharge devices

Info

Publication number: US2504494A
Application number: US537855A
Authority: US
Inventors: Bull Cabot Seaton
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1940-03-05
Filing date: 1944-05-29
Publication date: 1950-04-18
Anticipated expiration: 1967-04-18

Description

April 18, 1950 c. s. BULL TRANSMISSION LINE SEAL FOR ELECTRON DISCHARGE DEVICES Flled May 29 1944 IN VEN TOR.

[vazrrimmwfiuu BY Mg ATTORNE v Patented Apr. 18, 1950 "TRANSMISSHON LINE SEAL FOR ELECTRON DISCHARGE DEVICES Cabot Seaton Bull, Hiilingdcn, England, assignor to Electric and Musical Industries Limited,

Middlesex, England Applicationli Iay 29, 1944, SerialNo. 537,855 In Great Britain March 5, 1940 Section 1, Public "Law 690, August'S, 1946 Patent expires March 5, 1960 15 Claims.

My in'vntion relates to electron discharge de- "vices and is concerned with such devices which have at least a pair of conductors passing from the exterior to the interior of the device and which are constructed to carry high frequency currents.

Invordert'o maintain a vacuum or gas-tight envelopeforthejdevice, it is necessary that the conductors be associated with the device in a .vacuum or gas-tight'manner and for this purpose a seal is 'providedih Tcontaict'with the conductors, the seal inmost cases 'beirig the material of the envelope, if the latter is of insulating material, .such as g1ass,fdrif'theenvelope of the device is of metal a separate seal of insulating material is employed.

The conductors forhigh frequency work are inthe form'ofatransmission line, for example, iarconcentric line or a parallel wire system, and it .is found with such :a-line that due to the dielectric constant of jthematerial forming the seal being greaterthan the medium surrounding the conductors externally and internally of the enve1ope,;the v,camicity:perunit length in the seal of the conductors is not the same as that outside the seal. Thisfcausesthecharacteristic impedance of the -part of the line forming the seal to be different from-thecharacteristic impedance ,of other partsofthe line, which gives rise to undesired reflections.

The object of the :present invention 'is to f provide an improved electron discharge device with a view to reducing the change in the characteristic impedance which occurs in the vicinity of the lead-in conductor seals.

According to the invention, an electron 'discharge device is provided having a transmission line comprising a'pair of conductors passing from "the exterior to the-interior of the envelope of said device and in which'the envelope is maintained "Ivacuum or gas-tight by a seal in contact with said conductors and wherein, for'the'purpose of reducing the change of the characteristic impedance of said transmission'line which would otherwise occurowing to the presence of said seal, one or Eboth "ofsaidconductors is or are distorted in the region-of said seal.

In one form of the invention one or both of said conductors is or are reduced in size in the region of the sealso as to offset the increase in capacity due to the presence of the seal, or, alternatively, 'said conductors l are spaced farther apart-in the region'of said'seal. The invention *may be appliedto'conductorswhich form a con- -centric line or to conductors in the form of a Lecher wire system, i. e., where the conductors comprise parallelwires or strips.

In'order that the said invention may be clearly understood and readily carried into efiect, it will now be more fully described with reference to the accompanying drawings in which:

Figure l is a longitudinal section of an electron discharge device embodying 'theinvention,

Figure 2 is a longitudinal section taken 90 to Figure 1,

Figure 3 is a transverse section along the line "3S of Figure 2,

Figure 4 illustrates a modification of the arrangement shown in Fig. 2, and

Figures 5, 6 and 7 illustrate the invention as applied to an electron discharge device in which the conductors are in the form of concentric l'ines, the three figures illustrating different forms of seals.

The characteristic impedance Z0 of a concentric transmission line is given by where "K is the dielectric constant, 11 the inner radius of the outer conductor and 2-2 is the radius of the inner conductor. It will be seen that if the dielectric constant K changes the characteristic impedance is no longer constant from point to point'with the result that reflections and other undesired effects occur. The above equation applies only to concentric lines, but similar equations are known for conductors in the form of Lecher wire systems either where the conductors "ofthe system are circular wires or in the form of flat strips.

According to the invention it is possible to offset the change or" the dielectric constant that occurs owingto the presence of the seal by distorting the conductors in the region of the seal eitherby reducing the dimensions of one or both =of-s'aid conductors or by spacing the conductors farther apart.

Referring now toFigures 1, 2 and 3 of the draw- -ings, the invention as shown in these figures is "applied toconducto'rs in the form of a Lecher wire system, the conductors being in the form of fiat strips. The reference numeral 3 indicates the envelope of the device through which conductors 3 and ill pass, these conductors being sealed through the envelope 8, the latter being assumed to be of glass, in any of the manners known in the art. The conductors 9 and I0 are in the form of fiat-strips, the conductor 9 being connected to agrid ll comprising two support rods i2 and it around which the grid wire It is wound. The conductor IE) is connected to a fiat cathode l5. The grid and cathode are of course arranged inside the envelope and high frequency currents are fed thereto by the conductors 9 and it from the exterior of the envelope, these high frequency currents being applied to the conductors from a concentric line comprising an inner conductor It connected to the conductor 9 and an outer conductor ll connected to the conductor ill. An anode H is of course also part of the electrode assembly.

Since the characteristic impedance of the parallel wire system comprising the conductors Q and it changes at the seal owing to the fact that the dielectric constant of the seal is different from the dielectric constant of the medium inside and outside the envelope, the increase in the capacity per unit length of the conductors which would otherwise occur is offset, according to the invention, by distorting one or both of the conductors where they pass through the seal.

In Figures 1 to 3, the conductors are distorted by reducing the area of both conductors 9 and iii as indicated at iii. If desired, only the area of one of the conductors may be changed although it is preferred to decrease the area of both conductors. In this manner it is possible to make the characteristic impedance of the part of the transmission line which passes through the seal substantially the same as other parts of said line. Hence, reflections can be eliminated or substantially reduced.

Instead of reducing the area of the conductors, as shown in Figures 1 to 3, the spacing of the conductors can be increased in the vicinity of the seal and, as shown in the example of the invention illustrated in Figure 4, the conductors 9 and it, where they pass through the envelope 8, are spaced wider apart compared with their separation both internally and externally of the envelope.

Although in the example shown in Figures 1 to i the Lecher wire system shown comprises flat strips, it will be understood that the invention can equally be applied to systems in which the conductors are in the form of wires of circular or other cross-section.

Figures 5 to '7 illustrate the invention as applied to a concentric line which is provided with a vacuum or gas-tight seal. Referring to Figure 5 of the drawings, the reference numeral 8 again indicates the envelope of an electron discharge device the concentric line passing through the envelope, the exterior of the outer conductor i9 of the line being sealed to the envelope 8 whilst between the inner conductor 20 and the interior surface of the outer conductor 59 a seal 2! is provided which serves to seal the interior of the concentric line thus ensuring a vacuum or gastight envelope. To ofiset the change of characteristic impedance that would otherwise occur due to the presence of the seal 2!, either the internal diameter of the outer conductor 19 is increased, i. e.; the wall of the outer conductor is reduced in thickness, or the external diameter of the inner conductor is reduced, or both inner and outer conductors are so distorted. In the example shown in Figure 5 both the outer and inner conductors are so distorted and, as will be seen from the figure, the reduced portions of the inner and outer conductors project beyond the envelope 8 to the exterior thereof and these reduced portions are arranged to form one element of a plug and socket connection, thus fa cilitating the connection of a complementary concentric line to be attached thereto. Since with concentric lines high frequency currents are confined to the interior of the outer conductor, the dielectric constant of the envelope 8 which is sealed to the exterior of the outer conductor [9 does not affect the characteristic impedance of the line. The distortion of the conductors l9 and 23, therefore, only takes into account the change of characteristic impedance that is cocasioned by the presence of the seal 2|. The seal 2! need not lie in the same plane as the portion of the envelope 8 shown, but can be disposed in the manner shown, for example, in Figures 6 and 7.

Referring now to Figure 6 of the drawing, the seal 22 extends obliquely between the outer and inner conductors is and '26 and is in the form of a cone of insulating material suitably connected to the outer and inner conductors. The parts or" the outer and inner conductors within the planes bounding the length of the seal 22 are reduced, as shown, so as to avoid the change in capacity per unit length which would otherwise occur. Since the capacity effect of the seal 22 is greater at points nearer the inner conductor 23 than at points nearer the outer conductor l9, this eilect can be compensated by progressively changing the thickness of the conductors I9 and 2&2 in the region of the seal. In the example shown, the diameter or" the inner conductor 25 may be progressively reduced towards the smaller end of the seal 22 or, alternatively, or in addition, the internal diameter of the outer conductor l9 may progressively diminish towards the smaller end of the seal 22. Alternatively, the seal 22 may be of non-uniform thickness, whilst the conductors l9 and 20 may be reduced uniformly. It will be seen from Figure 6 that the seal 22 is displaced from the envelope 8. In Figure 7 the seal 22 is of substantially bell-shape and the outer and inner conductors i9 and 2B are progressively reduced, as shown. Further reduced

portions

23 and 24 are provided where the seal 22 is actually connected to the outer and inner conductors.

Although in the example shown in Figures 5 to 7, the outer and inner conductors are reduced, it will be appreciated that the invention can also be applied where the conductors are not reduced, but in which the outer conductor is arranged farther from the inner conductor analogous to the arrangement shown in Figure 4. The concentric lines shown in Figures 5 to '7 may be con nected interiorly of the envelope to conductors in the form of Lecher wires similar to those shown in Figures 1 to 3. Where it is desired to feed more than two electrodes in an electron discharge device by a concentric line as, for example, in the case where a screening grid is provided in addition to the control electrode and cathode shown in Figures 1 to 3, the outer conductor of the concentric line shown in any of the figures may be split longitudinally, the control electrode being connected to the inner conductor, the oath ode to one of the sections of the outer conductor and the screening grid to the other section of the outer conductor. If desired, the conductors may be plated with silver or copper, including the portions of the conductors passing through the seal 50 as to reduce losses. It is preferred in all cases that the length of each conductor passing through the seal is substantially the same.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific applicar tion io 'iwiiithjiiivenncn may be employed, it 'wiube fssparentthat my ,ri-v'ennon is by no means limited to" the.,exact1 forms illustrated "or the use'indi'cat'd, but that many variations may said envelopein whichlenvelope a vacuum is .-;insured by a seal-incontact with "said conductors, one of said conductors being "formed so that the spacing betweensaid essential-s from one surface of the seal to the other surface of the seal is greater than'at'other portions of said conductors for reducing the change in the characteristic impedance of the transmission line through the seal.

2. A device foi' -u'se at 'ul'tr-a high frequencies and having an envelope and a transmission line :comprising-a pair of conductors passing from the interior to ,the exterior-of the envelope of said device, both of said conductors extending through said envelope in which envelope a vacuuinis insured by a seal in contact, with said conductors,

:one of said conductors having a portion of re- 16111686 size extendin from one surface of the seal to the other surface of 't'he seal for reducing the change in the characteristic impedance of the transmissionl-ine through the seal.

3."A;;device for use atultra high frequencies :and having an envelope anda transmission line comprising a pair of conductors passing from the interior to the exterior of the envelope of said device, both of said conductors extending through said envelope in which envelope a vacuum is insured by a seal in contact with said conductors, said conductors being spaced farther apart in the region of the seal for reducing the change in the characteristic impedance of the transmission line through the seal.

4. A device for use at ultra high frequencies and having an envelope, and a transmission line comprising a pair of conductors passing from the interior to the exterior of the envelope of said device, both of said conductors extending through said envelope in which envelope a vacuum is insured by a seal in contact with said conductors, said conductors comprising a concentric line, one of said conductors being distorted in the region of the seal for reducing the change in the characteristic impeance of the transmission line through the seal.

5. A device for use at ultra high frequencies and having an envelope containing electrodes, and a transmission line comprising a pair of conductors connected to said electrodes and passing from the exterior to the interior of the envelope of said device, both of said conductors extending through said envelope in which envelope a vacuum is insured by a seal in contact with said conductors, said conductors comprising a concentric line, the inner of said conductors being reduced in diameter and the inside diameter of the outer conductor being increased at said seal, and a cooperating coaxial line having an inner conductor recessed to receive the inner conductor through said seal, and a recessed portion on the outside of the outer conductor for receiving the outer conductor passing through said seal.

ajtcanse A device for use at ultrahigh frequencies and having an envelope containin electrodes, and a transmission line comprisinga pair'of conductors connected to said' electrodes and passing from the exterior to'the interior of the envelope "of said device, in which envelope a'v'acuum is insured by a seal in contact with said conductors, 'said conductors comprising a concentric line, said seal between the outer and inner conductors "extending obliquely "therebetween, one of said conductors bein progressively reduced 'indiam- 'e'ter toward said seal for deducing the change in the characteristic impedance of the transmission line through the seal.

'7. A device for use at ultra high frequencies and having an envelope containing electrodes,

and a transmission line comprising a pair of'conductors connected to said electrodes and passing from the exterior to the interior of the envelope seal between the outer and inner conductors extending obliquely 'therebetween, one of said conducto'rs being progressively reduced in diameter toward said seal, andthe other'of said conductors being progressively increased in diameter inversely to said 'first'conductor.

8. A device for use at ultra high frequencies and having an envelope, and a transmission line comprising a pair of concentric conductors passing from the exterior'to the interior of the envelope both of 'said conductors extendingthrough p'said envelope, in which envelope a vacuum is "as insured by a seal in contact with said conductors, the inner of said conductors'bein'g'reduced in diameter and the inside diameter of the outer conductor being increased at said seal.

'9. A device for use at ultra high frequencies and having an envelope containing electrodes, a transmission line comprising a pair of conductors connected to said electrodes and passing from the interior to the exterior of said envelope, said envelope being evacuated and a seal in contact with said conductors, said conductors comprising a concentric line, the inner of said conductors being reduced in diameter and the inside diameter of the outer conductor being increased at said seal, said seal comprising a cone-shaped member, the small inner end being sealed to the inner conductor at the reduced diameter and the larger end being sealed to the outside conductor at the increased inner diameter.

10. A coaxial terminal assembly comprising a dielectric means, tubular coaxial line portions approaching the dielectric means at opposite ends, a central rod through the dielectric means axially disposed with respect to both said coaxial line portions, said dielectric means having one part thereof sealing one of the coaxial line portions and having a part of greater diameter than said one part, and conductive means overlying said dielectric means and connecting said tubular coaxial line portions, said dielectric means with said overlying conductive means and portion of rod therein having a surge impedance substantially equal to the surge impedance of the coaxial line portion adjacent said dielectric means.

11. A coaxial terminal assembly having a tubular portion and a coaxial rod, a bead sealing said tubular portion with the rod projecting through said bead, and providing therewith a surge impedance discontinuity in the line, and means for increasing the effective diameter of the bead and surge impedance to substantially equal the surge impedance between the tubular portion and rod.

said tubular coaxial line portions, and conductive means overlying said dielectric means and connecting said tubular coaxial line portions, said dielectric means with said overlying conductive means and portion of rod therein having a surge impedance substantially equal to the surge impedance of the coaxial line portion adjacent said dielectric means.

13. A coaxial line assembly comprising a tubular member, a rod coaxially mounted therein, a dielectric member sealing said tubular member with said rod extending through said dielectric member, and means for increasing the effective diameter of said dielectric member and the surge impedance of the assembly at said dielectric member to substantially equal the surge impedance between the tubular member and the rod adjacent said dielectric member.

14. A coaxial terminal assembly comprising a dielectric member, two tubular conducting members extending in opposite directions from the dielectric member, a central conductor sealed through said dielectric member and axially disposed with respect to both said tubular members,

said dielectric member sealing one of said tubular members, said one of said tubular members having a portion of increased internal diameter extending over said dielectric member and connected to the other of said tubular members,

said dielectric member with said extending portion and the portion of said central conductor enclosed therein having a surge impedance substantially equal to the surge impedance of said one tubular member and said central conductor adjacent said dielectric member.

15. A device for use at ultra high frequencies and having an envelope and a transmission line comprising a pair of conductors passing from the interior to the exterior of the envelope of said device, both of said conductors extending through said envelope in which envelope a vacuum is insured by a seal in contact with said conductors, both of said conductors having a portion of reduced size extending from one surface of the seal to the other surface of the seal for reducing the change in the characteristic impedance of the transmission line through the seal.

CABOT SEATON BULL.

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

UNITED STATES PATENTS Number Name Date 782,749 Guest Feb. 14, 1905 908,378 Bastian et a1 Dec. 29, 1908 2,165,961 Cork et a1 July 11, 1939 2,241,505 Cuttler May 13, 1941 2,289,846 Litton July 14, 1942 2,330,381 Quayle Sept. 28, 1943 2,376,725 Richardson et a1. May 22, 1945 2,408,271 Rigrod et a1 Sept. 24, 1946 FOREIGN PATENTS Number Country Date 20,948 Australia Jan. 15, 1935 of 1935