US2610249A - Dielectric wave guide system - Google Patents

Description

Sept. 9, 1952 M. D. FISKE 2,610,249

DIELECTRIC WAVE GUIDE SYSTEM Original Filed Sept. 15, 1942 2 SHEETS -SHEET l Inventor: Mi lan D. Fl'ske,

His Attorney.

Sept. 9, 1952 D. FISKE DIELECTRIC WAVE GUIDE SYSTEM 2 S!-IEETS--SHEET 2 Original Filed Sept. 15, 1942 Fig.7

Inventor:

Milan D. Fiske, flwijwiam His Attorney.

Patented Sept. 9, 1 952 DIELECTRIC WAVE GUIDE SYSTEM Milan D. Fiske, Schenectady, N. Y., assignorto. General Electric Company, a corporation ;of

New York Original application September 15, 1942, Serial No. 458,423. Divided and this application December 16, 1944, Serial No. 568,512

The present invention is a division of my copending application, Serial No. 458,423, filed September 15, 1942, and relates to dielectric wave guides of the hollow-pipe type for transmitting electromagnetic waves of ultra high frequency, and more particularly to structures associated therewith'for selectively controlling the propagation of electromagnetic waves through the guides.

It is now quite generally appreciated that electromagnetic waves may be propagated through the interior of a hollow pipe type guide which may be constructed of a conductive material and which contains a dielectric medium such as air or a gas through which the energy of the waves is transmitted. Of course, the frequency of the exciting electromagnetic waves must be greater than the critical minimum frequency established by the dimensions, principally the transverse dimensions of the guide.

There may be placed within the interior of a dielectric guide a metallic wall having an aperture which is tuned to the frequency of the exciting waves to effect a concentration of the potential due to the waves within or across a substantially small dimension of the aperture. As the intensity or the magnitude of the exciting electromagnetic waves is increased, the magnitude of the potential difference appearing between opposing surface of the aperture is also increased, and when the magnitude of this potential difference increases to a predetermined value established by the configuration of the aperture and the dielectric strength of the medium surrounding the aperture, an electric discharge is established across the aperture thereby materially changing the propagation characteristics of I the guide.

One way in which a structure of this nature may be employed is in connection with a system which is subjected to electromagnetic waves of considerable range of intensity and utilizing the characteristics of the aperture to effect transmission or attenuation of the waves within the guide.

It is an objectof my invention to provide a new and improved dielectric wave guide of the hollowpipe type and associated structure whereby the transmission of electromagnetic waves or the attenuation thereof within the guide may be selectively controlled. l

'It is a further object of my invention to provide a new and improved resonant aperture within a metallic wall enclosed within or associated with a wave guide of the dielectric type, and which is enclosed within an atmosphere, such as a gas, at low pressure. I

9 Claims. (01. 178-44) It is a still further object of my invention to provide new and improved arrangements for controlling or adjusting the natural resonance frequency of an aperture mounted within a hollowpipe wave guide of the dielectric type.

It is a still further object of my invention to provide a new and improved assembly for maintaining. a resonant aperture in a dielectric wave guide in an atmosphere of desired pressure, and which is proportioned so that the reflection of the electromagnetic waves is reduced to a minimum throughout that range of intensities within which it is desired to propagate the waves.

Briefly stated, in the illustrated embodiments of my invention I provide a metallic wall having an enclosed resonant aperture, such as a resonant slot, tuned substantially to the frequency of the exciting electromagnetic waves and which i designed to effect a breakdown of the surrounding atmosphere in the event the intensity of the electromagnetic waves attains or exceeds a predetermined value. The presence of the electric discharge across the slot varies the eifective dielectric constant of the dielectric medium through which the electromagnetic waves are propagated, thereby changing the wave guide from a propagator to an attenuator of the electromagnetic waves. A structure for housing a tuned aperture in a dielectric wave guide is provided which is sealed to maintain the aperture in an atmosphere of low pressure, and which is also provided with externally accessible adjusting means to control or tune the aperture.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Fig. l diagrammatically illustrates one way in which my invention may be incorporated in a hollow-pipe dielectric wave guide, and Figs. 2 and 3 are views of the sealing structure for the resonant aperture or slot. Figs. 4 and 5 diagrammatically illustrate another modification of my invention wherein the sealing structure constitutes a dielectric window of plane construction. Fig. 6 represents a further modification of my invention wherein a bulbous vitreous dielectric window may be attached or sealed to the frame including the resonant aperture by means of a sealing ring having a temperature coeificient of expansion substantially equal to that of the dielectric material employed. Fig, 7 represents an assembly of the elements shown in Fig. 6, and Fig. MS a still further modification wherein the enclosing compartment for the resonant aperture comprises two bulbous type vitreous windows mounted on opposite sides of the metallic wall. Fig. 9 diagrammatically illustrates a still further feature of my invention wherein the resonant element is provided with means for controlling its effective dimensions, the means being externally controllable. Fig. 10 diagrammatically illustrates another manner in which externally accessible means may be employed for controlling the natural resonance frequency of the slot and wherein the adjustable means is positioned so that it is notsubjec'ted to the electric discharge whenthe magnitude of the waves exceeds a predetermined value.

Prior to a detailed description of the various embodiments of my invention, it is believed that it will be helpful to describe 'generallythe type of system to which my invention is applicable. It is now understood that ultra high frequency electromagnetic Waves may be propagated dielectrically through hollow-pipe type guides where the frequency of the exciting electromagnetic waves isgreaterthah critical minimum frequcncy'whi'ch',in turn, is determined principally by the transverse dimensions of the guide. The guide is preferablyconstructed of a conductive material such as copper or brass. I Many typeser high frequency electromagnetic waves may be transmitted dielectrically through hollow-pipe type guides. "I hese waves have been generally indicatedheretofore as being of the E andrr typesj subscripts, as indicated, E u and Hmm, have b eeriassigned to these Waves in order to indicate the nature" of the waves, where n represents the order of the wave and m represents the mode of propagation. The order of the wave isdetermined by the manner in which the field intensity varies circumferentially around the axis of the guide, wherea's'the mode is determined by the manner of its variation with radial distance from axis of the" gu de, he type Waves h aveb'oth longitudinal and a transversev component of electricv field'but only a transverse ch n or ent of magnetic field, whereas the H type waves have both a lpngitudi'nal and a transverse component ofv magnetic field but. only a transwmw ol i Qt. le t eld- Although my nvent n abp ica lsjt sy t s for transmitti s a grea var e y wavesl in c in my inv nt en re naft r ref ren w be made i cul i'lr t he H 1 e Qiwa e- Referring now to Big. 1 of the accompanying drawings, my invention is there illustrated as applied to a system including a hollow-pipe type wave guide of the dielectric type. The dielectric Wflff guide may be, of rectangular crosssection formed by conductive material, such as copper or brass, havinsa height a and a base dimension b, 'E le'ctn'magnetic waves are established within the gu de by suitable exciting, electrodes which may have various configurations, depending upon the nature of the excitation required or desired. "One form which the input electrodes or exciting means may assume is that of a concentric line including a, tubular. conductor I and a conductor}, the; former of which is conductively connected; to, the, bottom of, the wave guide and the latter 6f. Which may be conductively connected to'jthe' top, There is positioned within the wave guid a metal wall. 3,, preferably constructed. of cop ero'r brass, and which is provided with'an ap, refd, having an appreciable dimension transv'erl's to'the direction of propagation of electromagnetic: waves through the guide. Where the waveguide is'. excited byelectromagnetic waves of the type, for example, the slot affords a principal dimension transverse to the electric component of the electromagnetic field and effects a concentration of the potential due to the wave across the upper and lower edges of the slot. This accentuation or concentration of the potential due to the wave is effected by virtue of the fact that the slot is tuned to the frequency of the exciting waves. If desired, the resonant aperture may be of the configuration illustrated wherein an elongated slit 5 is provided with enlarged openings, such as circular openings 6 and l, at each end thereof, and the complete opening or aperture is tuned to be resonant at the frequency of the exciting waves.

The wall, 3 and the associated dielectric window constitute one end of the sealed chamber or section of the dielectric wave guide, the other longitudinal end of such section being provided by the end wall beyond the concentric electrode means I and 2., Of course itis to be understood that the concentric line may be sealed in any conventional way so that the region between the 'vjall t and the end wall may be maintained at a desired low pressure.

Where'the principal transverse dimension of the aperture is perpendicularto the electric com pcnent of the electromagnetic field, the dimensions of a rectangular aperture may be calculated with reference to the dimensions a and b of the wave-guide. Generally speaking, as the effective dimension of the aperture transverse to the electric component of the field is increased, the frequency to which'the aperture is resonant is decreased. Furthermore, a the effective height of the aperture approaches zero, the effective length of the aperture approaches the limit I where A is the wave length of the exciting electromagnetic waves.

' Where it is desired to seal a portion of a dielectric wave guide or to maintain the aperture within an atmosphere at the proper pressure in order to effect a selective response in accordance withthe magnitude of. the electromagnetic waves being transmitted, I provide a sealing structure for the aperture which performs the scaling function without sacrificing the wave" transmitting characteristics, of the guide, For example, in Fig. l l provideja bulbous type dielectric window '8; for the resonant aperture and which is designed to neutralize substantially, any reflection or discontinuity in the, wave propagating characteristics of the guide incident to the wall 3 and the aperture 4. Stated in other words, the bulbous dielectric window. 3 produces an effect which is substantially equal and opposite to that of the wall 3, and the aperture'so thatthe. amount cfenergy reflectedisminimized. h

The. manner in which he. dielectric windowt cooperates with the wall 3. and the associated aperture to minimize any discontinuity in. impedance of the dielectric guide may be more fully appreciated by reviewing. certain fundamental concepts. rel'ativeto, the. propagation of an H01 y e, wave in a dielectric. guide. Thetotalimpedance Z0 of a, rectangular wave guide, which may, of course, be characterized as the ratio of the applied voltage V to the current-I,mayalso be defined as follows;

.V emrc. a 'T 2 it (1) where co is the dielectric constant o f air i -the.

Wave-length of the wavewithin the guidaand c is the velocity of; light. If the total guide. im

M: constant k an dfvvhere his the wave length of the exciting Waves in free space and where the electric component of the fieldis perpendicular. to thedimension bof the guid V I Where .the transverse wall includes an aperture which is-resonant to the frequency of the exciting electromagnetic waves and where the change in ,the guide dimensions occasioned by such a structure isa small fraction of the wave length ofthe waves transmitted through the guide, the

change in thetotal impedance ofiered by the guide is also relatively small. I

Therefore, I. provide a window 8 which offers an impedance effect substantially equal and opposite to that produced by the resonant aperture, so that a wave propagated through theguide experiences substantially no reflection.

The voltage or potentialdifference appearing between the upper and lower edges of the aperture is effected by the resonant characteristics of the'aperture, and the magnitude of this voltage differenceincreases as the magnitude of the exciting Waves increases. Uponreaching a prede termined value, the dielectric material within the aperture breaks down; that is, the voltagediiference is suflicient to cause ionization of the medium which may be air, and an electric discharge takes place across the aperture. Upon occurrence of such an electric discharge, the wave transmitting characteristics of the guide are radically changed, causing rapid attenuation of the electromagnetic waves within the vicinity of the wall 3. Without attempting to enter into any detailed discussion of the reason why the waves are not transmitted beyond the wall 3, suffice it to. say that the presence of the charged particles within the vicinityof the resonant aperture, due tothe electric discharge, effectively changes the dielectric constant of the medium through which the waves are being propagated, as, for example,. air, in the present discussion, thereby substan tially increasing the critical minimum frequency fonthe particular guide employed so thatthe waves are-not propagated an appreciable distance beyondthe wall 3.

Fig-2 represents a more detailed cross sec.- tional view of that portion of the Wave guide including the wall 3 ofthe dielectric Window 8. The, dielectric window 8 may be constructed of a suitable vitreous material, such as a boronsilicate glass, and sealed .to the metal wall 3 by means of a cylindrical flangev 9. The flange 9.

may be constructed of an alloy, such as an ironnickel-cob'alt alloy, to facilitate sealing of the glass to the metal wall.

In Fig.3, an end view of the bulbous dielectric window and the resonant slot is also illustrated. Figs. 4 and5 represent two views of a different form or the dielectric window for sealing a section of a dielectric wave guide... The metallic wallincluding the resonant aperture or slotmay .be formedintegral with. the walls of the dielectric waveguide, as shown in Fig. 4, where the walls oonstituteright-angle extensions. of the top l0 and the bottom of a sectionof a rectangular wave guide. In this modification of my invention, the resonant aperture may be in the nature of. a tuned or resonant rectangular slot 12 whichis alsotuned to the frequency of the exciting means; .which establishes the electromagnetic waves in the guide andin which the height h and the length L thereof bear a. definite relationship with respect to the excitation frequency and the transverse dimensions of the guide, particularly the base dimension b. It is to be understood that various dimensions of the resonant slot may be. employed; that is, the dimensions of the slot may be varied and still maintain its resonance with respect to the exciting frequency.

I provide a sealing structure for a dielectric wave guide which not only affords mechanical strength where it is desired to seal a section of a dielectric wave guide, but is also designed and arranged so that the assembly including the slot,

tom 1 l of one section of a wave guide and a top I6 and bottom I! of another section of a dielectric wave guide. It is to be understood that similar strapsMaand l5a shown. in Fig. 5 are also placed. along the sides of the guide to afford similar bridging means. In this manner, a continuous conductive path is provided between the two sections of the guide. If it is not desired to form the wall or diaphragm #8, which includes the resonant slot l2, integral with the top and bottom members l0 and II, the diaphragm may be soldered or welded to these members.

The window 13 is designed and positioned relative to the wall 18 so that the reflection from the slot l2 cancels the reflection from the dielectric,

or vice versa, depending upon the direction of wave propagation through the guide. The window I3 may be cemented to the diaphragm by using a vacuum cement I9 so that one of the sections orregions of the wave guide may be maintained at a different pressure from an adjoining region.

I have found that the length L of the slot :2-

required to make the window [3 reflectionless depends very critically upon the proximity or spacing of the dielectric material used in the window. To establish this distance and to allow space for the cement [9, a thin metal gasket or spacer 2D is inserted. around the slot 12. This gasket may be formed as an integral part of the wall or diaphragm I8 and positioned as close to the slot-as desired. Residual reflection from the window [3 after construction may be canceled or neutralized by cementing to the outside surface of the dielectric window l3 a diaphragm l3 of properly chosen characteristics.

I have found that a window constructed in accordance with the principles of my invention de-.

acre; 249

0.3 %1offthe energy incident upon it...

Figs. 6iand i7 diagrammatically .illustratexa still further modification of ithe-WindOWl COIlStlJllCtlOll ion a a dielectric waveguide wherein; ammetallicwall member 2| positioned ina..dielectric.wave. guide and"preferablyconstructed of a;metal,..suc-h.

as brass or copper, is provideid with a resonant apertureiZZL' of the typezhavingan elongated slit'i" with circular- Openings ateitherend: Inzorder to establish awseal :of' suitable mechanical; rigidity.- and -one which is substantially reflectionlessyby' maintainingthe dielectricelement. of the -.window. preciselypositioned relative to the wall and. the

slot, I pro'videan: arrangement wherein the di electric. window-sis sealed to the :walli'by means: of.

a supporting frame or ring-23," preferablyrco'nstructed ofa metal such as an iron-nickel=cobalt alloy, thereby. facilitating the. sealin of a. .Vitres ousor: glassdielectric. window 24. whichxis of. bulbousor hemisphericalconfiguration.- The mes tallic ring 2 is preferably constructed; of a .thin sheet and may be formed by a suitable stamping operation so that it is. provided withitwo sections 24a and 24b of unequal'diameter joined by 215L113! stantially transverse section 25:1so. that a certain degree of resilience is provided to compensate for strains incident to pressure. and temperature variations. In this manner, variations in pressure do not change the position of the glass'window, and consequently do not appreciably affect the net reflection from the window. Furthermore, the glass Window 25 is constructed of a suitable glass, such as boron-silicate, having a temperature coefficient of expansion substantially equal to that of the metal constituting the ring 23. Wall 21' is provided with an annular groove 2'! inwhich the section 24a of ring 23 is seated. Ring: 23'may be welded or soldered to wall 2 l.

In the production of this type of window; the ring 23 may be sealed to the wall 2! and the glass wihdowZB may be sealed toscction 24b of the ring.

by placing thewindow 26' upon the section 24b and subjecting the rin 23 to a .heatingiagency such as a high frequency field, causing the-glass to seal around. the outer edge of section 24b,zthe

-vide externallyaccessihle means for varying the natural resonance frequency'of the slot, and which may comprise a screw QEIWh'lCh' extends into the body of the Wall 2| and intotheaperture; and as illustrated, may extend into one of the circular openings at the ends-of the elongated. slit 22 thereof. It will be appreciated that as the position of the. screw is changed, the .efiective dimensions of the aperture are also changed, thereby-adjusting or controlling the frequency at which the aperture is resonant.

If desired; the screw ZBinay he provided Withan extension 28" of substantially paddle shape which may extend into one of thecircular openings. of the resonant slot,a.and the position of which determines or, controls the natural resonance frequency of the slot.

Where it is desired to -maintain the resonant aperture in a suitable atmosphereand at.the

proper pressure, thus maintaining ahigh degree s of sensitivity ofatheaaperture tolthe intensity-of the; electromagnetic waves; being: transmitted; through the guide; the. aperture; may: be come pletely enclosed inxsucn an" atmosphere: For? example; two: arrangementszsuch' as. shown in:

Fig.1fi may be:employed one'on each sideofi the: wall 2 I, in a manner. illustrated in: Fig; 8'.'wher.e;=* in a wall 29, provided with aresonantraperturez- 30, is sealed by bulbous vitreous windows 3| and 32,,which are sealed to the wall 29 by rings 33 and 34 of a construction and material similar to ring 23 discussed :inconnection with Fig-,6.-

The atmosphere employed within-the enclosure may be any suitable fluid or gas which will sup- 3 port an electric discharge when the voltage difference between the edges of theslot or aperture attains a predetermined value; and which- -per=-'- mits a substantially rapid I extinction of the arc when the voltage difference decreasesbelowf the critical value. For example, I may employwith intheenclosure an-atmosphere-of hydrogen-and water vapor; an atmosphere of nitrogen, or an atmosphere of argon.- I

It'will be understood that in-- the application 4 of the embodiment of my invention shown in Fig. 8; to a dielectric waveguide, wall member 29 is placed in conductive contactwith the metallic or conductive walls of the guide; For-example," it -may replace the wall 3 -andassociated assembly shownin Fig. 1. 5 As-statedabove, the dimensions of a resonant' slot in a'system of this nature-are fairly critical} that is, variations in thedimensions--cliange-the frequency at which theaperture or slot *is' resona-nt. In someapplications' and in the operation of-dielctric wave-guides, v it-may-be de-- sirable to tune-or adjust the frequency at-which the aperture is resonant while maintaining-the aperture in a-region of low pressure or in a-re'g ion ofa desired atmosphere. Consequently; I' pro vide externally accessible -means for-controlling the effective dimensions-Ora tunable aperture mounted in;- ametallic wall suitable for "use ';in-- connection with-a dielectric wave guide. One; form ofmy invention is illustrated-in Fig-g 9 where in 'ametallic wall 36 is employed and may beconstructed inupper and lower sections 31 and-38 1 to facilitate the construction of theapertureand to permit the placement therein ofthe adjusting means to be described presently;-

Theaperture 33-maybe of the farm illus trated inFig. Tcompris'ing a restricted "-elong'ated section-andhaving circular openings at eac'li end thereof; Furthermore, the aperture-'33 may- Q be maintained at a -low-'pressure,- ormaybe m-aintained inasuitable atmosphere at the proper pressure by means Ora anof bulbous vitreouswindows 40 and which are sealed to and sup-- ported by the W alI SB by ;means--of metallic rings 42- and 43' constructed in the mariner explained above in connection with the apparatus shown in Figi6'." The-rings 42 and-43may be s'eate'd in annular gro'oves in th'e "end fa'cespf wall 36.

As a means for tuning the resonant aperture 39'; I provide adjustablev member; suchflas :a.v

deformable diaphragm' l i' which -may constitutes one boundary of :the .resonant aperture and may; be welded or.solderedztoitlietupper section 3170f; Wall136; i

The uppersecti'orr 3710f .Wall 36;.isprovi'ded-Zvvitlii: a recess45, which may be .of.'rectan'gulari-z-cross'zv section,.and.iscalso provided withia communicating circular copeningldfi whiclr extends. to tan outer peripheryiiof'ithe:upper'; sectionl'.3"|; l This arrangement "permits external-adjustment of .the

position of the diaphragmsothatit is notneces-i sary'to disturb the seal'ior the resonant aperture in making an adjustment; I provide ametallic block" 41 whichis connectedtothe diaphragm 44 and which=ispositionedIby means of adiffer ential screw arrangement48 which extends from the block 41 to the recess 49". The block- 41'is preferably constructed "of a 'material having a relatively high thermal conductivity and thermal capacity to facilitate the dissipationof the heat incident to the establishment of electric'discharge across the boundary ofithe resonant aperture when the intensityof the electromagnetic waves exceeds the critical'value. 1 1

For example, the block-41 may-be constructed of copper. Upon adjustment ofthe difierential screw arrangement 48, the block 41 is raised or lowered, thereby deforming the diaphragm 44 and effectively controlling the dimensions of theresonant aperture; Of course, thediaphragm 44 is sealed toKthe upper: section 31: of wall 36 so that theadjustmentmay be made without, disturbing the condition of the atmosphere with in whichthe resonant aperture is enclosed.

An alternative embodimentof my invention is. illustrated in Fig. 10 wherein the adjustable means'for controlling the natural resonance frequency of an aperture or slot 5 is positioned in the slot so as not to be subjected to the electric discharge which is produced when the magnitude of the electromagnetic waves transmitted through a wave uideattains or exceeds a predetermined value. The resonant aperture or slot 50 comprises an elongated portion or slit terminated at each end -in enlargedopenings52 and {53; One of the openings, such as opening 53, may be provided with an extension 54 in which is placed an adjustable or deformable diaphragm 55 which is sealed to the body of the wall so that the region including the aperture may be maintained at low pressure or within a desired atmosphere. A metallic member or block 56 of high thermal conductivity is attached to diaphragm 55 and may be actuated or positioned by a differential screw arrangement 51 similar to that shown in Fig. 9. In like manner, the adjusting means is made externally accessible by extending to a recess 58 in the body of the metallic wall.

While I have shown and described my invention as applied to a particular system embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention. a

What I claim as new and desire to secure by Letters Patent of the United States is: i

1. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electro-magnetic waves within said guide, a metallic wall positioned in said guide and lying in a plane substantially transverse to the direction of wave propagation through said guide and provided with an elongated slit having a pair of substantially circular openings at each end thereof, said slit and associated openings constituting an aperture tunable to the frequency of said exciting means, and a deformable diaphragm forming a boundary of said slit for control1ing the effective natural frequency of said aperture.

2. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electro-magnetic waves within said guide,

a metallic wall positionedin said guide and-lying in a plane substantially transverse'to the direction of wave propagation through said guide and provided with an aperture comprising a slot hav-'- ing its principal dimension transverse to the elec tric component of the field incident to'the propagation of waves through said guide andhavinga pair of circular openings at each end'thereof, means for providing an evacuated enclosure for that portion of said wall including said aperture; a deformable diaphragm supported by, said Wall and constituting a boundary of said slot for controlling the effective dimensions ofsaid-ap'erture'; and externally accessible means: for adjusting said diaphragm. 1 Ti H i l-f lz 3. In 'combinationa dielectric wave guide of the hollow-pipe: type, exciting means for estab lishing electro-magnetic waves within said guide; a metallic wall positioned in said guide and lying in a plane substantially transverse to the direc tion of wave propagation through said guide-and provided with an aperture comprising a slothav' ing its principal dimension transverse to the ele'c trio component of the. field incident to the propa gation of Waves through said guide and having a pairof circular openings at each end thereof, meansfor providing an evacuated enclosure for that portion of, said Well including said a'perture, said wall being provided with a recessiadjoining said slot; a diaphragm constituting -a boundary of said slot and attached to said wall, a metal block of sub'stantial' thermal capacity and conductivity in physical contact with i said dia' phragm, and externally accessiblemeans for adjusting the position of said 5 block and said dia-' phragm l ,l l

4. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for estab lishing electro-magnetic waves within said guide, a metallic wall positioned in said guide and lying in a plane substantially transverse to the direction of wave propagation through said guide and provided with an aperture comprising a slot having its principal dimension transverse to the electric component of the field incident to the propagation of waves through said guide and having a pair of circular openings at each end thereof, means for providing an evacuated enclosure for that portion of said wall including said aperture, 7 said wall being provided with a recess adjoining said slot, a diaphragm sealed across said recess and constituting one boundary of said slot, and externally accessible means for adjusting the position of said diaphragm.

5. In combination, a dielectric wave guide of the hollow-pipe type, means for establishing electromagnetic waves within said guide, a metallic wall associated with said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and including a resonant aperture comprising an elongated slit havin at one end thereof an enlarged opening, a diaphragm forming a boundary of said opening, and means extending through the body of said wall for positioning said diaphragm.

6; In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves within said guide, a metallic wall adaptable for use in connection with said guide and having a plane substantially transverse to the direction ofwave propagation therethrough and provided with an aperture tuned substantially to the frequency of said exciting means and comprising an elongated slit terminated in an opening in one end thereof,

and :means; for: controlling "thenatural frequency 01 said. aperture. icluding a member extending; into said: opening andactuatingmeans for said member-'lying'within thebody of said metallic wall.- I

*7. :In-asystem. for the transfer of high ire-- quency; electromagnetic waves including a pipe waveguide a protective electrical breakdown-device which-.includesa metallic conducting barrier located transversely of said wave guide and havinganaperture of elongated contour with its long. dimension oriented perpendicularly to the electric. vector of: oscillation transmitted in said waveguide,- the width of said aperture being sufficiently small at least for a part of its length to permit theoccurrence of. an electrical breakdown discharge across said aperture when oscillations in. said guide exceed a predetermined amplitude, said aperture. being dimensioned and shaped. to resonate. at. a frequency in the=neighw borhood of .that of oscillations desired to be transmitted through said Wave guide system, and a rotatable vane within at least alportionof said aperture for adjusting the. resonant frequency thereof.

8. Ina-wave guide system, a protective device which includes a conducting barrier positioned transversely of the direction of, propagation of electromagnetic energy in said Wave guide and havingan elongated aperture located in said barrier, said aperture bein so dimensioned in a direction parallel to the direction ofthe electric held of oscillations in said guide that a break.- down will. occur across said aperture in said direction when the amplitude of said oscillations exceeds a predetermined value, said aperture be ing further dimensioned whereby said. aperture: is resonant at: the frequency; of said oscil1atlons,

quency electromagnetic waves including a. pipewave guide, a protective electrical. break-down device whichincludes a metallic conducting: bar.-

rierv located. transversely of said wave guide and having an aperture of elongated contour with. its. long dimension oriented. perpendicularly to the electric vector of oscillations transmitted in said wave guide; the width of said aperture being sufficiently small. atleastfor. a partof itslengthf. to. permit the occurrence of an electrical break down discharge across saidaperturewhen oscillations in said guide exceeda predetermined. amplitude, said apertureqbeing dimensioned and.

shaped toresonatesat a frequency in; the neigh 'lborhood of that. of: oscillations desired.- tohe, transmitted throughnsaid wave guide, system, and.

means for tuning said.- apertured barrier for: ad.- justing the.- frequency: at which. it. resonates.

MILAN. D. FISKE.

REFERENCES CITED The following references are of record in the file of this patent? UNITED STATES PATENTS Number Name Date- 2,396,0&4 Fox .Mar. 5, 1946 $102,184. Samuel .J.une- 18, 1946

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