US2582846A - Microwave amplifier - Google Patents

Description

Jan. 15, 1952 H; v. NEHER 2,582,846

MICROWAVE AMPLIFIER Filed April 19, 1944 ELE; L

58 l ELE- lA 3mm" Patented Jan. 15, 1952 MICROWAVE AMPLIFIER HenryVictor. Neher, Pasadena, Calif., assigner, by mesne assignments, to the. United States of Americav as represented by the Secretary of the Navy Application April 19, 1944, Serial No, 531,828

25 Claims.

This invention relates to electronic vacuum tubes and particularly to. vacuum tubes having one or more grids .adaptedfor operation at extremely high frequencies.

Construction of vacuum tubes in. accordance with this invention. provides for extremely close spacing between the elements of the vacuumv tube, thus reducing electron. transit time and grid circuit loading and also provides for built-in resonant cavities of which the electrodes form a part, thereby. avoiding losses and other. undesired effects of the conventional leadsof supports of the electrodes. Although the said resonant cavities are enclosedwithin the4 vacuum system of the vacuum tube, means are. provided for tuning them from the outside. An alternate form of cathode may also be provided which is adaptedv to reducev diiiiculties arising from heating of the grids. Among the objects of the invention, therefore, are the production of a vacuum tube capable of providing amplification at extremely high frequencies, such as about 3,000.megacycles, the provision of very close spacing between the electrodes of such vacuum tube, the provision of resonant cavities into which the electrodes are built to minimize lead reactances and losses, and the provision of means for tuning such a vacuum tube.

The. invention will. be. explained in connection with the drawing, in which Fig. l is a longitudinal cross section, broken off at the top and bottom. of the vacuum tube in accordance with the present invention; Eig. 1A is a fragmentary seciionr view of a tube of the type shown in Fig. 1, but having a plurality of grids; andy Fig. 2 is a longitudinal cross section of a form of cathode particularly adapted for use in anV improved part of a vacuum tube. in accordance with this in- Vention.

As will be seen from Fig. l, a distinguishing feature of this type of vacuum tube is that its elements are formed chiefly of. two opposed quarter-wave coaxial resonators 5 and 6, the opposite and neighboring extremities oi the central columns of which form the cathode and the anode respectively, the two resonators being separated by a small space in which one or. more grids, such as the grid 'I- may be located. The vacuum tube may conveniently be constructed by iirst building the anode resonator assembly and the cathode resonator assembly and putting each together separately, then fitting these together with one or more grids and spacers and securing the electrode structure together, then enclosing the electrode structure. in a steel shell such as the steel shell used for making metal vacuum tubes oi the type 6L6, provided with` the usual base, which basemay previously have been. attached to the cathode assembly, then inserting the input and output transmission lines through the steel shell tothe. electrode assembly and sealing these lines to the. steel shell, and finally evacuating and seal.- ing the steel shell. The. forms of construction which facilitate this type oiv assembly constitute the important features oi this invention.

The anode resonator G is formed by a metallic cylinder illA which may be of copper, a flexible diaphragm, preferably oi copper, showny at II. and a copper cylinder I2 having4 a reduced lower portion I3. the lower surfaceof which is the, effectiveanode surface of the vacuum tube. There is alsdan inward iiange III.A mounted on the lower end of the cylinder in. The central portion of the. anode surface is formed by the` extremities of an Invar rod I5, which is securely fastened, as by. soldering or welding tothe reduced portion i3` of thecopperstructure I2.

The, cylinder I0 is mounted snugly in another cylinder ilv from` which it is insulated by a thin layer of insulation shown at I8 and` I9. This insulation is preferably thin mica. Thus the en tire, anode resonator isv insulated from the rest Of the structure of the vacuumy tube and may be maintained at anode potential. The elimination of by-,pass condenser structure within the resonant circuit of the resonator itself provides an important reduction of losses the device. The surfaces oil the'cylindersA IQ and I`I are close together overa sufficient area to provide good heat conduction away from the anode.

When the vacuum tube is assembled, the cylinder I'I is in a lifted position with respect to the outer steel shell 2G. The cylinder It is likewise maintained; in such a fixed position. The cylinder l'l is provided with upper and lower anges 2 I and 22 extending toward the shell 2li. On the upper flange 2l, are mounted two struts 24 and 25, from the junction of which is mounted, by meansv of a rod 2t, an insulated fixed coupling with the Invar rod I5, the` said xed coupling including a metal cup 2l and a ceramic insulator 28 cemented both to the rod I5, and to the cup 2i. Preferably struts 2!!` and 25 are so positioned that the junctions of these struts with flange 2I and the junction of these struts with each other lie substantially within a plane including the axis of resonator. 6. If now the shell 20 is squeezed in the plane of the struts 24 and 25, the junction of these struts will be pushed upward and the rod I5 will thereby be lifted, pulling the anode away from the grid and cathode. and thereby decreasing the loading capacitance across the resonator and raising the natural frequency of the resonator. In this operation the mechanical deformation is taken up by flexing of the diaphragm I I. The importance of the use of Invar as a material for the rod I5 is shown by the fact that when the anode becomes heated because of the operation of the vacuum tube, the consequent expansion of the copper cylinder I2 will cause the diaphragm I I to flex without, however, changing the position of the anode. The slight change in the volume of the resonator thus produced will have a substantially insignificant effect upon the resonator frequency, a resonator of this type being more sensitive to changes in capacitance at locations of maximum electric iields than to slight changes in volume.

If squeezing pressure on the shell 20 in the plane of the struts 24 and 25 is released, or if the shell 20 is squeezed in a plane at right angles to the plane of the struts 24 and 25, the junction of these struts will be pulled down and the position of the anode will likewise be pushed down, thereby lowering the natural frequency of the resonator 6. In order to facilitate the application of squeezing pressures of the types described upon the shell 20, a clamping ring 30 is provided around the outside of the shell 20 at an appropriate place and a screw 3| is threaded in this ring in a position adapted to squeeze the shell 20 in the plane of the struts 24 and 25. Another screw is provided 90 away around the circumference of the clamping ring 30 for obtaining squeezing in the other dimensions. Each screw cooperates with a stud such as the stud 32 mounted on the clamping ring 30 at a position opposite the screw. Other forms of squeezing mechanism might be provided instead, if desired.

Output coupling from the anode resonator 5 is provided by the loop 35 connected to the coaxial conductor transmission line 36. The transmission line 36 is mounted on the shell 20 and the cylinder I1 but is not in electrical contact with the anode resonator, passing through a hole in the wall of the cylinder I which leaves y a slight clearance between the cylinder I0 and the outer conductor of the transmission line 36. In order to maintain the vacuum in the apparatus, the outer conductor of the transmission line 35 is sealed to the shell 20 by a gas-tight joint. A seal is also provided between the inner and outer conductors of the transmission line 36. This seal is of the usual glass-to-metal type and is not shown on Fig. l, the transmission line 36 being broken off. If' desired this seal may be provided at the extremity of the transmission line 36 which is coupled to the resonator The cathode resonator comprises a metal cylinder 38, preferably maoleV of copper, an outward flange 39 at the lower extremity thereof, a flexible diaphragm 40, an inner copper cylinder 4I and the upper extremity 42 of the cathode structure which is mounted within the copper cylinder 4I. The cathode structure is tubular in form with a disk-like end surface 43 the upper surface of which is coated with a suitable electron-emitting material adapted to emit electrons when heated. A heater` 68 is provided within the structure in the usual way. An upper flange 44 is also provided at the upper end of the cylinder 38 in order to facilitate mounting of the resonator 5 and assembly of these tubes. The upper flange 44 is mounted at its upper edge on an outer cylinder 45 the functions of which are structural rather than electrical. Extensive structure by means of the rod 50.

4 portions of the cylinder 45 are cut away as shown at 46 and 4l, in order to provide access during construction of these tubes to the means for valigning the resonators '5 and 6 and the grid structure or structures.

When the tube is assembled, the outer cylinder 45 is fixed in relation to the outer shell 20. It is provided with outward flanges at top and bottom, on the latter of which are mounted two struts 48 and 4S the junction of which is adapted to control the vertical position of the cathode The operation of this strut 43 and 49 is essentially similar to that of the strut 24 and 25. A clamping ring 5I is provided in the neighborhood of the strut 48 and 4S to facilitate their actuation in practice. The tuning of the resonator 5 operates in the same manner as that of the resonator 6, except that an additional effect may be present on account of the fact that the flexing of the diaphragm 4S, by changing the narrow spaces between flange 39 and the diaphragm 40 may produce an additional effect 4from the natural period of the resonator, which may be expected to be in the same sense as the change produced by the variation of the capacitance across the open end of the resonator.

Since, as shown below, the grid may be operated at a potential other than ground potential, the cathode resonator may be grounded. The input transmission line 53 may then be brought into direct electrical contact with the cylinder 38 of the resonator 5 and, because a relatively large degree of coupling between the input and the resonator 5 has been found to be desirable, the inner conductor of the transmission line 53 may be directly connected to the inner cylinder 4I, as shown on Fig. 1, by means of the small tube 54 which is adapted to engage the inner conductor of the transmission line 53 in a circumferential ring contact. To provide spring contact, oneA or more slots not shown may be cut longitudinally in the tube '54 near its left hand extremity. Like the transmission line 35, the transmission line 53 is provided with a vacuum seal, not shown on Fig. 1, between the inner and outer conductor and, likewise, the outer conductor is sealed in a gas-tight manner to the shell 20.

The cylinder 45 is provided, on its lower flange, with a number of supports such as those shown at 55 and 55 which are adapted to hold the said cylinder in fixed relation to the base of the vacuum tube. The base, resonators and grid structure may be assembled as a unit and the electrode connections brought out to base pins before the steel shell 2% is fitted over the assembly.

The electrodes of the vacuum tube of this invention are of the parallel plane type, which is particularly well adapted for very close spacing. In Fig. 1 a single grid l is shown interposed between the cathode and anode. The grid I is formed of a mesh of very ne wires mounted on a central aperture of a thin metal disk 58.

Wires of a diameter of .0003 spaced .001" apart are preferred. The cathode resonator is designed so that the grid will be about .001 from the cathode when the resonator is tuned to the desired frequency. The small size of the grid wires is important, because it tends to prevent too great a spread in transit times of the emitted electrons when the cathode-grid spacing is small. The disk 58 is sandwiched in between thin mica sheets 59 and 50 which serve to insulate it respectively from the resonator 6 and the resonator 5. yThe ilange I4 of the resonator 6, the ilange and the mica spacing-disks59and 60^areall provided with a plurality'of perforations which aredesigned-to be brought into alignment to for-m holes such as that shown at 62. Thereare a nurnber of these, for instance, Isix orV eightv at about the same radial distance from the centerof the various pieces. -There is also an outer row of perforations adapted to receive machine screws such as screws 63v and 641v which are threaded into the flange 22' and which hold the electrode assembly together. is used for providing an electrical connection to the grid, as by the pin 65 andthe wire 66, connected to one of the base pins of the vacuum tube which serves as a grid terminal; Itv is to be noted that the outer perforations in the disk 58 are suiciently wide to avoid contact with the screws 63 and 64'.

Ifr desired, more than one grid' may be provided' in the manner shown in Fig. 1A, the grids 58"' and 58' being separated by a mica spacer 59 similar to the Ispacing disks 59 and`60 andthe upper and' lower grids being separated by similar spacers from the resonators 5 and 6 respectively. In practice it is found that the addition of a screen grid improves operation by reducing tendencies the tube may have to go into oscillation. In assembling the tube, the metal disk or disks carrying the grid or grids are assembled with the mica spacers serving to insulate them as aforesaid andare aligned by the use of temporary pins passing through the inner seriesof perforations. The aligning s desirable in order to assure that the grids are properly centered. The grid and spacer sandwich is then insertedl between the resonators 5 and 5 with the aligning pins passing through the perforations in the anges of the resonators and the screws 6 3, 64, etc., are then inserted in the outer perforations and fastened down. Thereafter, the temporary pins which served'to align the grids and spacers may be removed through the large apertures 46 and 41 in the cylinder 45. The grid connection pin 65 may then. be connected with, one of` the base, pin terminals through a suitable wire 66 and likewise the anode cylinder l2 may be connected with another of the base pins through another wire, indicated generally at 61. The presence of these wires is shown at Fig. 1 in a purely diagrammatic fashion, it being understood that in actual practice insulating bushings would be provided where such wires pass near Vother conducting structures and so on. The heater 68 and the wires connecting thereto, shown at 69 and 10, are likewise indicated in Fig. l, in a diagrammatic fashion for purpose of simplicity.

,v It will be observed that when the resonators 5. and 6 are tuned,` the grid-cathode spacing"` and the grid-anode spacing will be varied, which means that the amount of grid-loading effect, the transit time and the amplification factor may be varied within a small range during the tuning proce-ss. These effects, however, are extremely small, only a very small movement of the cathode or anode surfaces being necessary for tuning. It is to be; understood that the tuning range of the vacuum tube of this invention is relatively narrow and the device is intended to be built for service at frequencies. very close to. a particular design frequency. The tuning range is ample for following the frequency drift of transmitting ape paratus within a reasonable range, for instance, and indeedl it is sufficient for a number of other purposes also. Because of the extremely close One ofthe inner perforations spacing-of the elementspossble withvv this typeof. construction, itis possible to obtain radio-fre-V quency amplification at very high` frequencies with this device.

ciesof about 3000 mc./sec.

It will be noted that the cathode-grid resonator has alower Q than the anode circuit resonator.

This isv desirable because` thegrid= circuit is to small, about 3 square millimeters is a conven-Y ient area, in order to reducelthe numberofelectrons which. start out towards the grid and thenreturn tothe cathode- The noise effect of such electrons upon the circuits of the tubecanA be reduced by removing` the cathode properfrom the grid and providingl a virtual cathode near the grid. in the manner shown in- Fig. 2.

Fig. 2 shows a form of cathode adaptedl for use in an improved form for vacuum tube otherwise constructed inv accordance with Fig. 1. The representationof Fig. 2 is ona larger scale than Fig. 1 in order to show details of' construction. The cathodey proper is provided by a coatingl of material adapted to emit electrons when heated, which coating is shown at 10, on the end of a cylinder 1| which encloses a suitableheater and electrical connection therefore, indicated generally by a dotted line. The cylinder 1l is mounted within an outer cylinder 12- and spaced therefrom by a cylinder or bead 13 of insulating material. The cylinder 12 extends upward beyondv the end of the cylinder 1I to provide an enclosure for the4 end of the cylinder 1| and the cathode surface 10, which enclosure is open at the top of 14, the upper part of the cylinder 12 being preferably slightly constricted. If desired, grid wires might be provided across the opening. 14 in order to assure that the electric field gradient is perpendicular tothe opening Over a small area. Even without such grid wires, however, a substantially plane virtual cathode is formed at the opening. It is to be noted that the chamber between the cathode surface 10 and the opening 1 4 is also provided with an annular disk 15 having a relatively large central openingthe disk'15 serving as an accelerating or focusing electrode and in part as an additional heat shield and assisting the upper end ofthe cylinder 12 in reducing the tendency of the heat dissipated at the cathode to cause the wires of the grid to be heated.

The metallic cylinder 1I and the cathode surface 1,0 provided thereon are, by virtue of being insulated from the cylinder 12, adapted to be placed at a potential` more negative than that of the cylinder 12 so that the annular disk 15 and the rconstricted cylinders 12 will exert an elec? tricaccelerating forcein the direction of thev anode on the electrons emitted by theA cathode 10. At the virtual cathode provided at the opening 14, therefore, the electrons will already have some rvelocity toward the anode. Under such conditions the possibility is presented of obtaining a relatively high mutual conductance.

An important reason for the advantage-ofv the Power amplification of the order of 15-20vdb has been obtainedl at frequen.

cathode of the type shown in Fig. 2 for use in a vacuum tube of the type shown in Fig. 1 is that with ordinary cathodes the extremely small spacing betweenv the cathode and the grid results in a greater tendency for the grid temperature to rise than occurs in vacuum tubes where the spacing is greater. Increase of grid temperatureinvolves increased probability of electron emission from the grids. The extreme proximity of the cathode coating to the grid wires also involves ,some danger of sputtering some ofv this material onto the grid, so that the heating of the grid will involve even greater danger of electron emission. When a cathode structure of the type shown in Fig. 2 is used, however, although the virtual cathode provided at the opening 14 remains very close to the grids, the actual emitting layer 'lll is at a considerable distance from the grids, thus reducing sputtering, and the hot portions of the cathode are likewise removed from the grids, reducing the heating of the grids. It is further to be noticed that the grid-cathode capacitance isA also reduced, particularly if no wires are used across the opening 14. The cathode -structure of the type shown in Fig. 2 is therefore especially well adapted to cooperate with a vacuum tube structure of the close-spaced parallel-plane element type such as that shown in Fig. 1; Because of the reduced scale of Fig. 1, amore conventional cathode structure, which may also be used in the device of Fig. l, was more conveniently villustrated there.

' What I desire to claim and obtain by Letters Patent is:

` 1. An electronic tube for very high frequencies comprising opposed coaxial resonators having opposed parallel plane extremities of their respective inner conductors constituting a cathodeand an anode respectively, with relatively' close spacing, a grid interposed between said extremities and parallel thereto mounted on supporting means interposed, together with electrical insulation, between said resonators, adjustable means for adjusting thelongitudinal position of said extremities including a pair' of struts each mounted `at an" oblique angle 'to the axis of said resonators and having a junction substantially on said axis' mechanically connected to the inner conductorl of 'one vof said resonators, said struts being adapted'whensqueezed together to pull 'said inner conductor. of said resonator away from the other of said `vresonators and when pulled apart to propel said inner conductor towards'said other resonator,'said means including' also another pair of struts adaptedto` act similarly'o'n the inner conductor of the other of said resonators,"said resonators 'each having a flexible diaphragm at their respective axial extremities farthest from said grid, said flexible diaphragm being ladapted to permit adjustment of the longitudinal posi'- tion of said inner conductor by said'pair ofstruts, and means, including a tubular metal shell maintaining a desired gas pressure between said extremities and in said resonators.

2. An electronic tubein accordance with claim 1 having also annular adjustable clamping means externally of said means of maintaining a high vacuum, said clamping means being adapted to push together and to pull apart the struts, through the deformation of the said tubular shell of said vacuum maintaining means.

3. Apparatus in accordance with'claim 1 in which the junction ofthe pair of struts adapted to control the; longitudinal position of the anode is mechanically `.connected to the anode by an Invar rod fastened to the inner conductor of the anode resonator only in the immediate neighborhood of said anode and fastened at its other extremity to the junction of said struts through an insulated joint.

4. An electronic tube for very high frequencies comprising opposed coaxial resonators having opposed extremities of their respective inner conductors constituting an anode and a virtual cathode respectively, with relatively close spacing, a grid interposed between said extremities mounted on supporting means interposed, together with electrical insulation between said resonators, one of said inner conductors the extremity of which is adapted to operate as a virtual cathode being of hollow construction and including a chamber opening through said extremity and being slightly constricted toward said extremity, a cathode structure including heating means within said chamber and insulated from said hollow inner conductor in which it is situated, and a diaphragm having a central aperture slightly constricting said chamber between said cathode and said extremities, said cathode being adapted to be maintained at a potential more electronegative than said hollow inner conductor in which it is located and to provide a virtual cathode at the extremity of said inner conductor, means associated with said inner conductors for adjusting the longitudinal position of said inner conductors of said resonators, and means associated with said resonators for coupling electrical circuits to the respective oscillating elds of each of said resonators.

5. An electron tube for very high frequencies including first and second closely spaced electrodes, means for minimizing changes in the spacing of said electrodes due to temperature changes in saidv second electrode, said means comprising rst support means supporting said first electrode, second support means secured to said rst support means and disposed remote from said second electrode, a pair of struts each secured to said second support means at one end of said struts, the other end of said struts meeting in a mechanical junction, a rod of a material having a thermal coefficient of expansion of substantially zero, said rod being secured at one end thereof to the portion of said second electrode nearest said rst electrode, and means securing the other end of said rod to the mechanical junction of said struts.

6. An electron tube for very high frequencies including rst and second closely spaced electrodes, said second electrode having the form of a hollow cylinder, means for minimizing changes in the spacing of said electrodes due to changes in temperatures of said second electrode, said means comprising, rst support means supporting said first electrode, second support means secured to said first support means and located remotely from said electrodes, a pair of struts each secured to said second support means at one end of said struts, the other end of said struts meeting in a mechanical junction in alignment with the axis of said second electrode, a rod of material having a thermal coefficient of expansion of substantially zero disposed within said second electrode, said rod being secured at one end thereof to the end of said second electrode nearest said first electrode and means securing the other end of said rod to said junction of said struts.

7. An electronic tube for very high frequencies comprising opposed coaxial resonators having opposed parallel plane extremities of their respective inner conductors constituting a cathode and an anode, respectively, with relatively close spacing, a grid interposed Ibetween said extremities and parallel thereto, mounted on supporting means interposed together with electrical insulation between said resonators, adjustable means for adjusting the longitudinal position of said extremities including a pair of struts each mounted at an oblique angle to the axis of said resonators and each mechanically connected to the inner conductor of one of said resonators, said struts being adapted when squeezed together to pull said `inner conductor of said resonator away from the other of said resonators and when pulled apart to propel said inner conductor tolward said other resonators.

Lconductor of one of said resonators, said struts being adapted when squeezed together to pull said conductor of said resonator away from the other vof said resonators and when pulled apart to propel said inner conductor toward said other resonator, one of said resonators having a flexible 4diaphragm Iat the axial extremity 'farthest from said grid, said exible diaphragm being adapted to permit adjustment of the longitudinal position of said inner `conductor by said pair of struts.

9. vAn electronic tube for very high frequencies comprising opposed coaxial resonators having opposed parallel plane extremities of their respective inner conductors constituting a cathode and an anode, respectively, with relatively close spacing; va plurality of grids interposed between said extremities and parallel thereto mounted on supporting means 'interposed together with electrical linsulation between. said resonators, adjustable means associated with one of said inner conduc- -tors for adjusting the longitudinal position of said extremities, including a pair of struts, each mounted Vatan oblique angle to the axis of said resonators and having a junction substantially on said axis mechanically connected to the inner `vconductor of one of said resonators, said struts being adapted when squeezed together to pull said inner'conductor of said resonator away from "the other of said resonators and when pulled Iapart to propel said inner conductor toward the other of said resonators.

10. An electronic tube for very high frequencies, comprising opposed coaxial resonators havving opposed parallel plane extremities of their respective inner conductors constituting a cathode and an anode, respectively, with relatively `close spacing; a plurality of grids interposed between said extremities 'in parallel thereto,

mounted on supporting rneans interposed together with" electrical insulation between said resonators, adjustable means Iassociated with one of said inner conductors for adjusting the longitudinal vposition of said extremities including a pair of struts, each mounted at an oblique angle to the axisofA vsaid resonators and having a junction substantially on said'axs, saidjunctionbeing metors having opposed parallel plane extremities of their respective inner conductors constituting a cathode and an anode, respectively, with relatively ciose spacing, a plurality of grids interposed between said extremities and parallel thereyto mounted 4on supporting means interposed together with electrical insulation between said resonators, adjustable means associated with said inner conductors for adjusting the longitudinal position of said extremities including a pair of struts, 'each mounted at an oblique angle to the axis of said resonators and having a junction substantially on said axis mechanically connected to the inner conductor of Vone of said resonators,

said struts being adapted when squeezed together to pull said inner conductor of said resonator away -from the other-of said resonators and when pulled apart to propel said inner conductor toward said other resonator, said means including also another pair of struts adapted to act similarly on the inner conductor of the other of said resonators, said resonators each having a flexible diaphragm at their respective axial extremities furthest from said grids, said flexible diaphragms being ladapted to permit adjustment of the longitudinal `position of -said inner conductors by said pairs of struts, and means including a tubular `metal shell surroundingsaid resonators for maintaining a `desired gas pressure between said extremities and in said resonators.

12. An electronic tube lfor very high frequencies, `.comprising opposed coaxial resonators having opposed extremities of their respective inner conductors -constituting an anode and la Virtual cathode, respectively, with relatively close spacing, a grid interposed between said extremities mounted -on supporting means interposed together with electrical insulation between said resonators, one-of said inner conductors, the extremity of which is adapted to operate as a virtual cathode, being of hollow construction and including a chamber opening through said extremity and being slightly constricted toward said extremity, -a Acathode structure including heating means `within said chamber and insulated lfrom said hollow inner Iconductor in which it is ing, adjustable means for adjusting the longitudinal-position of said extremities, including-a pair of struts, each mounted at an oblique angle tothe axis of saidresonators and having a junction substantially on said axis, and a rod secured at one end thereof to the inner conductor of one van anode, respectively, with relatively close spacing, adjustable means for adjusting the longitudinal position of said extremities including a pair of struts, each mounted at an oblique angle to-the axis of said resonators and having a junction substantially on the axis of said resonators,

a rod of a material having a thermal coeiiicient expansion of substantially zero, said rod being secured at one end thereof to .the extremity of one of said inner conductors, means joining the hother end of said rod to said junction of said struts,`and means associated with said struts for varying the spacing of the ends of said struts remote from said junction whereby the spacing A of said extremities of said inner conductors is varied.

15. In an electron tube including first and second elements, one oi' said elements being 'rigidly supported, means including a rod of material having a thermal coeicient of expansion of substantially zero for maintaining the spacing of said elements constant while the other of said elements undergoes a change in temperature and.

Ymeans including said rod, and a pair of intersecting struts supporting said rod for micrometrically'varying the spacing between said two elements.v

16. An electron tube for very high frequencies f comprising opposed coaxial resonators having opposed parallel plate extremities of their respective inner conductors constituting a cathode and Van anode respectively with relatively close spacing, a rst one of said resonators including a conductive wall substantially closing the end of said first resonator adjacent said second resonator, a grid interposed between said extremities and 'parallel thereto mounted on a flat, conductive supporting sheet extending parallel to said conductive wall, a sheet of electrical insulation disposed between said conductive wall and said conductive supporting sheet, and electrical insulating means disposed between said conductive supporting sheet land said second resonator.

17.Y An electron tube as in claim 16, said elec- "tron tube further comprising means coupled to fs'aid inner conductors for adjusting the longitudinal position of said extremities.

j 18V. An electron tube as in claim 16, said elec- 1x9, An electronic tube for very high frequen- {fcies comprising opposed coaxial resonators havjrinrg opposedr parallel ,plate extremities of their --respective inner conductors constituting a cathode and an anode respectively with relatively fclose spacing, a first'one of said resonators infcludi'ng' a metallic wall substantially closing thel 'end'of said first resonator adjacent the second jA of said resonators, a grid interposed between said extremities and parallel thereto mounted on a at conductive supporting sheet extending paral- V`-lel to lsaid metallic wall, a first sheet of electric insulation disposed between said supporting sheet and said metallic wall, a second sheet of electrical insulation disposed between said supporting sheet and said second resonator, means associated with ysaid'inner conductors for adjusting the longitudinal positions of said extremities and means associated with said coaxial resonators providing a gas tight enclosure surrounding said extremities. 20. An electronic tube for very high frequency including a coaxial resonator, the extremity of the Yinner conductor of said resonator acting as a.

virtual cathode, said inner conductor being of hollow construction and including a chamber opening through said extremity and being slightly constricted toward said extremity, a cathode structure including heating means within said chamber and insulated from said hollow inner conductor and a diaphragm having a central aperture slightly constricting said chamber between said cathode and said extremity, said cathode being adapted to be maintained at a potential more negative than said hollow inner conductor to provide a virtual cathode at the extremity of said inner conductor.

21. A cathode structure for an electron tube comprising a hollow cylindrical member the extremity of which is adapted to operate as a virtual cathode, said member including a chamber opening through said extremity and being slightly constricted toward said extremity, a cathode including heating means disposed within said Lchamber and electrically insulated from said cylindrical member, and a diaphragm having a. central aperture slightly constricting said chamber between said cathode and said extremity, said structure thereby providing a virtual cathode at said extremity of said cylindrical member when said cathode is maintained at a potential more negative than said cylindrical member.

Y22. In a coaxial cavity resonator in which a first end of the inner conductor of said resonator `is disposed in spaced relationship to a iirst end of said resonator, means for varying the spacing between said first end of said inner conductor and said rst end of said resonator comprising a pair of intersecting struts each mounted at an oblique angle to the axis of said resonator and each mechanically connected to the inner conductor of said resonator, said struts being adapted when squeezed together to pull said inner conductor of said resonator away from said rst end of said resonator and when pulled apart to propel said inner conductor toward said rst end of said resonator.

23. In a coaxial cavity resonator in which a first end of the inner conductor of said resonator is disposed in spaced relationship to a rst end of said resonator, means for varying the spacing between said rst end of said inner conductor and said first end of said resonator comprising a pair of struts each mounted at an oblique angle rto the axis of said resonator and having a junction substantially on the axis of said resonator vmechanically connected to said inner conductor,

said struts being adapted when squeezed together --to pull said innerconductor of said resonator away froml said first end of saidresonator and when pulled apart to propel said inner conductor toward said rst end of said resonator.

i. 24.V In a coaxially cavity resonator in which a first end of the inner conductor of said resonator is disposed in spaced relationship to a rst end of said resonator, means for varying the spacing between said first end of said inner conductor and said first end of said resonator comprising,

Va pair of struts each secured at one end to the outer conductor of said resonator at a second end of said resonator, the other ends of said struts meeting in a mechanical junction in align- Ament with the axis of saidresonator. a rod dis- 13 posed within said inner conductor, said rod being secured at one end to said i'irst end of said inner conductor, said rod being secured at its other end to said junction of said struts.

25. 1n a coaxial cavity resonator in Which a first end of the inner conductor of said resonator is disposed in spaced relationship to a first end of said resonator, means for varying the spacing between said iirst end of said inner conductor and said first end of said resonator comprising, a pair of struts each secured at one end to the outer conductor of said resonator at a second end of said resonator, the points of attachment of said struts being located at the opposite ends of a diameter of said outer conductor, the other ends of said struts meeting in a mechanical junction in alignment with the axis of said resonator, means securing said innerconductor to said junction of said struts and means for deforming said 'second end of said resonator in a manner to change the spacing between the points of attachment of said struts to said outer conductor, said struts being adapted when squeezed together to pull said inner conductor of said resonator away from said first end of said resonator and 'i4 when pulled apart to propel said inner conductor toward said rst end of said resonator.

HENRY V. NEHER.

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

UNITED STATES PATENTS Number Name Date 2,088,722 Potter Aug. 3, 1937 2,146,365 Batchelor s Feb. 7, 1939 V2,167,201 Dallenbach July 25, 1939 2,183,215 Dow Dec. 12, 1939 2,251,085 Unk July 29, 1941 2,284,405 McArthur May 26, 1942 2,353,742 McArthur July 18, 1944 2,353,743 McArthur July 18, 1944 2,396,802 Mouromtsei et al. Mar. 19, 1946 2,418,844 Le Van Apr. 15, 1947 2,429,760 Hotine Oct. 28, 1947 2,436,700 Spielman Feb. 24, 1948 2,446,017 McArthur July 17, 1948 2,445,992 Beggs July 27, 1948

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