US3478247A - Microwave tuner having a rapid tuning rate - Google Patents

Description

United States Patent O vs. Cl. sis-39.55 8 Claims ABSTRACT OF THE DISCLOSURE A tunable microwave cavity is provided having a movable wall for changing the resonant frequency of a cavity. The microwave cavity includes a dielectric window which is sealed to portions of the cavity to form a chamber, while the movable cavity wall is positioned outside of this chamber confronting the dielectric window. Input and or output couplings are provided to introduce or extract microwave energy from the cavity. Moreover, in a preferred embodiment the cavity is designed to support a TE circular electric mode.

This invention relates to a microwave cavity tuner, and more particularly, to a rapid tuner adapted to tune the resonant output cavity of a coaxial magnetron by positioning of a cavity wall.

As is known, piezoelectric devices exhibit a piezoelectric effect. That is, in response to the application of voltage, they expand and contract, and with more exotic constructions, flex in proportion to the magnitude of a voltage applied across the electrodes of the device.

In the patent application, Ser. No. 636,540, filed May 5, 1967, of William H. Perkins and Robert E. Klotz, a piezoelectric actuator is disclosed which positions the movable wall of a microwave cavity to form a microwave tuner. More particularly, a piezoelectric biomorph actuator positions the movable wall of the resonant output cavity of a coaxial magnetron as a function of the magnitude of voltages applied across the electrodes of the actuator.

The piezoelectric device disclosed in that application, and commonly termed a piezoelectric sandwich or biomorph creates mechanical movement; that is, bends or flexes in response to the magnitude of applied voltages across the electrodes of the device. This mechanical movement is coupled to a movable microwave cavity wall to effect tuning of a microwave cavity over a moderate bandwidth of frequencies at relatively rapid rates.

As disclosed in that application, the microwave cavity is included in a magnetron. One portion of a piezoelectric biomorph is fixed to a relatively stationary portion of the magnetron housing, while another portion of the piezoelectric biomorph remains free to flex. A movable thin metallic layer forms a wall of a microwave cavity or resonant output cavity of a coaxial magnetron. This movable wall is mechanically coupled with the piezoelectric biomorph. Flexure or bending of the piezoelectric biomorph which occurs as a function of voltages applied across the biomorph electrodes varies the position of the movable cavity wall; and hence, varies the size of the resonant output cavity. Accordingly, this varies the frequency of resonance of the resonant output cavity, and the frequency of oscillation of the magnetron.

The piezoelectric biomorph actuator is adapted for connection to a source of sweep voltages. This causes periodic flexing of the actuator which reciprocates the movable cavity wall through a plurality of positions. Accordingly, the frequency of resonance of the resonant out put cavity and the frequency of oscillation of the magnetron sweeps through a predetermined bandwidth of q a frequencies.

Ip one embodiment of the invention, disclosed in the aforecited copending application, the piezoelectric biomorph actuator is located outside the evacuated portions of the magnetron housing. Movement of the free end of the biomorph actuator is transmitted by a rod and bellows arrangement to a movable wall located inside the evacuated regions of the magnetron housing. The bellows is constructed of a metallic material which requires a relatively large force to move the rod and contract the bellows. Additionally, because of the inherent rigidity of the bellows and its mechanical inertia, the inerti'a of the rod members therein, the sweep or speed rate at which the movable wall is positioned is limited to a rate below that capable with a piezoelectric biomorph alone as disclosed in other embodiments in that application. To compensate for these defects, a plurality of piezoelectric biomorphs are connected in parallel or ganged in order to provide a sufiicient amount of force necessary to move the rod and bellows structure.

In other embodiments disclosed in said copending application, the piezoelectric device is mounted within the evacuated or vacuum portions of the magnetron housing, which avoids the response speed limitations imposed by the bellows and rod arrangement. However, mounting the piezoelectric biomorph within the evacuated magnetron housing involves other complications in practice. First, should the piezoelectric actuator and supports require adjustment, the magnetron must be disassembled, reassembled, and re-evacuated; an obviously expensive proposition. Second, the elements of the piezoelectric actuator are of necessity subjected to a high vacuum. Since the relatively inexpensive commercially available piezoelectric biomorph devices are not intended for high vacuum applications, the elements used in the commercial construction do not incorporate the required low vapor pressure materials. Consequently, the material, such as epoxy, used in the construction of commercial piezoelectric biomorph devices, could over a period of time, vaporize, which may spoil the vacuum, and some of this vapor may condense on other elements with the magnetron housing, which may change the operating characteristics of the magnetron. Accordingly, a more expensive piezoelectric device is custom constructed of low vapor pressure material, and preferably utilized in the various tuners disclosed in that application. Third, in some available procedures of magnetron construction, the magnetron is heated to high temperatures which are above the Curie temperature of the piezoelectric material, a temperature at which the material loses its piezoelectric properties. That construction procedure necessitates the additional step of applying a polarizing current to the piezoelectric device in order to restore the destroyed piezoelectric properties. Moreover, high temperature heating of a piezoelectric device of the commercially available. variety causes some decomposition of elements, such as the aforementioned epoxy, and emission of an undesirable carbonizing vapor. As previously mentioned, a vapor spoils the vacuum and creates undesirable carbon deposits upon the internal elements of the magnetron.

Therefore, it is an object of the invention to provide a piezoelectric tuner that is not exposed to the high vacuum evacuated region of microwave cavity.

It is a further object of the invention to provide a relatively inexpensive piezoelectric tuner for a microwave cavity that is easily accessible for adjustment.

It is another object of the invention to provide a piezoelectric tuner for a magnetron that is assembled subsequent to assembly and evacuation of the magnetron.

It is another object of the invention to provide an external piezoelectric tuner for a coaxial magnetron with out a bellows and rod arrangement.

It is a further object of the invention to provide a piezoelectric tuner for a magnetron that consists in part of ordinary, inexpensive, commercially available piezoelectric biomorph devices of ordinary construction.

In accomplishing the foregoing objects, a tuner struc' ture is presented which is used to advantage with any type of tuner drive, slow or rapid, electrical or mechanical, pieelectric or otherwise that relies upon the positioning of a movable cavity wall to tune the frequency of resonance of a microwave cavity. Heretofore, and as disclosed in the aforecited copending application Ser. No. 636,540, a given frequency change or bandwidth of tuning required a certain positional movement of the movable cavity wall. Desirably with the structure of the present invention, only about one-fourth as much wall movement is required to tune a microwave cavity in a coaxial magnetron over the same bandwidth. As is apparent, this feature provides first, a larger bandwidth of tuning for any fixed amount of cavity wall movement in any prior art tuner; and, second, the sweep rate increases since for a given bandwidth of tuning and a given speed of tuning or movement of the movable wall, the required bandwidth is tuned or swept over in a shorter period of time.

Therefore, it is another object of the invention to increase the tuning rate of a tunable microwave cavity;

It is a further object of the invention to reduce the amount of movement of a movable cavity wall necessary in order to tune a microwave cavity over a given bandwidth of frequencies.

The invention is characterized by a microwave cavity which supports a TE circular electric mode of oscillation, and which contains a movable wall. A chamber is formed within the microwave cavity, and a dielectric window permeable to microwaves and impermeable to air seals this cavity. The movable wall is positioned so that it confronts the dielectric window and, therefore, effectively reflects microwave energy between its surfaces and the other walls of the cavity to form an effective microwave cavity of a given size. Positioning means are connected to the movable wall to position the wall relative to the dielectric window; and, hence, to correspondingly tune the microwave cavity.

Further, in accordance with the invention, the chamber consists of the inner walls and anode of a coaxial magnetron, and the dielectric window is of a washer shape to conform to the shape of the available space between the pole piece and cylindrical walls of the magnetron.

Moreover, in accordance with the invention, the positioning means includes a piezoelectric device connected with the movable wall and which positions the movable wall in response to the magnitude of voltages applied to the device.

The foregoing and other objects and advantages become apparent from a reading of the following description in view of the figure which illustrates in section a partial view of the tunable coaxial magnetron embodying the invention.

The figure shows in section a partial view of a tunable coaxial magnetron embodying the invention. Typical elements found in a coaxial magnetron, such as the magnets enveloping the magnetron structure, the tube socket, remaining portions of the housings and mountings, and electrical power supplies are omitted because they are conventional and do not clarify the invention. The figure shows a housing which consists of a first cup-like housing portion 1 having an inner washer shaped upper wall 2, and an cylindrical inner wall 3. Grooves 4 are included within the sides of housing portion 1 for allowing installation of well known cooling fins, not illustrated. A portion of a microwave window 6 is sealed within an opening in one portion of the cylindrical inner wall 3. This window 6 consists of a washer shaped dielectric insulator which is brazed to the surrounding portions of the window structure to permit the coupling of microwave energy therethrough without permitting the passage of air or other gases. A cylindrical wall 7 is seated at an end to a ring-like seal or joint 8 which is connected to a second seal or joint 9. Seal 9 is, in turn, sealed to the lower cylindrical wall portion of the first cup-like housing portion 1.

A cylindrical pole piece 10 is mounted by a pole piece flange portion 11 to cylindrical wall 7. A second hollow cylindrical pole piece 12 is mounted within an opening in the top portion of the cup-like housing member 1 and protrudes beyond the surface of wall 2. Each of the pole pieces 10 and 12 are conventionally constructed to ferromagnetic material to provide a path for magnetic flux coupled thereto from a conventional permanent or electromagnet, not illustrated, to establish a magnetic field across the gap between the ends of the pole pieces. Pole pieces 10 contains a cylindrical tubular end portion of reduced diameter and pole piece 12 is tapered to a smaller diameter at the protruding end in order to concentrate the magnetic flux into a smaller region. Extending through the hollow pole piece is a cylindrical cathode 13. As is conventional, a filament, not illustrated, is included within the hollow portion 14 of the cylindrical cathode 12 for heating the cathode material.

Alternatively, it is conventional to utilize a filament winding coated with electron emissive material in lieu of a separate cathode, should such be desirable Concentric with and surrounding the cathode is cylindrical anode 15 fastened and brazed in place, in this instance, to the top wall 2 of the first cup-like housing portion 1. As is conventional, the anode contains a plurality of anode resonators surrounding the cylindrical cathode. In the illustrated magnetron, the anode 15 contains an inner surface or wall 16, and an outer surface or wall 17, and each anode resonator is formed thereon in the space between two adjacent ones of the plurality of vanes. Two nonadjacent vanes 18 and 19 of the plurality are illustrated. The vanes are equally spaced from each other around the cylindrical inner wall 16 of the anode 15, and about cylindrical cathode 13. Each vane extends from the inner wall 16 of anode 15 to within a predetermined distance of the cathode surface. This leaves a gap between the vanes and the cathode 13 commonly termed the interaction region.

A plurality of elongated slots 20 extend through anode 15 to form a passage between alternate ones of the anode resonators to the space surrounding the outer wall 17, the resonant output cavity 27, hereinafter described, for allowing microwave energy to be coupled therebetween.

The foregoing details are descriptive of the conventional coaxial magnetron while the following describe the construction embodying the invention.

A first cylindrical support 21 is seated upon the pole piece flange portion 11. A second cylindrical support member 22 is seated around the pole piece 10, and to the outer rim of a washer shaped support disk 23 mounted about and seated upon the end cylindrical pole piece 10 about its reduced diameter cylindrical end portion. The second cylindrical support member 22 supports a third cylindrical support member 24. A washer-shaped dielectric window 25, constructed for example, of alumina, is seated along its outer rim to a groove along the outer edge of the first cylindrical support member 21, and is seated at its inner rim along a groove at the outer end of the second cylindrical support member 24. Each of the support members 21, 22, 23, and 24 and window 25 are brazed or sealed in place so as to form a vacuumtight connection partitioning portions of the magnetron housing within body portion 1. Thus, an air tight evacuated chember is formed between walls 2 and 3, sup ports 21, 22, 23, and 24 and window 25.

A thin lightweight washer-shaped movably mounted or movable conductive wall 26 confronts microwave window 25 on the outside of the formed evacuated chamber. However, because microwave energy penetrates through the window, the conductive wall 26 forms an outer boundary for microwave energy emanating or contained within the formed chamber and forms one wall of a microwave cavity 27, which is the resonant output cavity of the coaxial magnetron.

This resonant output cavity 27 is formed essentially between the top wall 2 of the housing member, a portion of the inner cylindrical wall 3 of the housing portion 1, the outer wall 17 of anode 15, and the movable or positionable wall 26, and is somewhat donut-like in shape. This resonant output cavity, as is conventional, supports a TE circular electric mode of oscillation at the frequencies to which the magnetron is tunable.

As is apparent, there is a small clearance between cylindrical housing wall 3 and the cylindrical support member 21. This clearance opens into the larger regions of the resonant output cavity 27, but is not effectively a part of that cavity. The clearance is very small relative to the wavelength of the dominant mode of microwave generated in the resonant output cavity 27; and, the clearance effectively appears as a very high impedance to such microwaves. Hence, the region in the clearance does not perform any substantial hinderance to the operation of the magnetron.

As is apparent, various types of actuators can position the movable wall 26 to tune the resonant output cavity 27. In the coaxial magnetron illustrated the movable wall is shown to be positioned under the control of both a piezoelectric biornorph actuator 28, and a mechanical or electromechanical positioner 50.

The piezoelectric biornorph actuator 28 is a multilayered sandwich or biornorph which contains a first layer of piezoelectric material 29, a second layer of piezoelectric material 30', a middle conductive layer of electrode 31 between the two piezoelectric layers and two outer conductive layers or electrodes 32 and 33, one on the outer surface of each piezoelectric layer. In the commercial construction the middle electrode is a thin brass shim fastened to the piezoelectric layers by epoxy, and the two outer electrodes are formed with a very thin metalized layer fired on to the piezoelectric layers. In this manner the foregoing elements form a physically integral thin flexible element, exaggerated in dimension in the figure for clarity. Moreover, the particular shape of the piezoelectric biornorph actuator is of a washer shaped geometry, since, like the movable wall 26 and dielectric window 25, it must conform to the available space between the cylindrical pole piece and anode and the larger diameter cylindricalwalls surrounding the pole piece and anode.

Moreover, each piezoelectric layer of the piezoelectric biornorph is oppositely electrically poled. That is, one layer is poled in a direction from the middle to one outer electrode, and the other layer is poled in a direction from the outer electrode to the middle electrode, which is accomplished, in each instance, by applying a large polarizing voltage between an outer electrode and the middle electrode. Subsequent application of the same sweep voltage across both layers cause one layer to expand and the other to contract. Since the layers are integrally joined, a flexing or warping action occurs much like the operation of a bi-metallic thermostat. This construction magnifies the movement available from the expansion and contraction of a homogenous mass of piezoelectric material.

An annular strip, lip, or coupling member 34, as variously termed, is connected to piezoelectric biornorph actuator 28 along the outer rim and to the movable wall 26 along the outer rim thereof to enable movement of the actuator to be coupled to the movable wall. In the figure, lip 34 is formed integrally with movable wall 26 by bending over a portion or annular strip portion. A groove 35 is formed along the juncture of the bend in order to decrease the rigidity of such connection, reducing the stress on the connection of lip 34 and the piezoelectric biomorph actuator 28, especially during flexure of the latter.

The piezoelectric biornorph actuator 28 is fixedly clamped along its inner rim by two ring shaped clamping members 36 and 37. The first ring shaped clamping member 36 is connected to a tubular support sleeve 38, which is in turn mounted along a tubular side to a second tubular support sleeve 39, which surrounds a portion of cylindrical sleeve 24. The second clamping member 37 is connected to the second tubular support sleeve 39. The sleeve-like construction of the second tubular support 39, since it carries both the piezoelectric actuator 28 and movable wall 26, allows another positioner to position the movable wall within resonant output cavity 27 independently of the positioning effected by the piezoelectric actuator 28. Accordingly, the second tubular support sleeve 39 is connected to a linkage 40, and the linkage is in turn connected to a second linkage 41 by a rod 42 that extends through an opening in the pole piece flange portion 10. The second linkage 41 is connected to the mechanical positioner 42 by a rod 43. This linkage member rides on a guide 44 extending through an opening in the linkage and fastened to a portion of the pole piece 10. Since the elements connecting the positioner 50 with the tubular support 39 are rigid relative to the lightweight and thin movable wall 26, any fiexure of the biornorph actuator 28 is communicated from the outer rim portion of the piezoelectric actuator to the movable wall as if the inner rim of the piezoelectric actuator was absolutely fixed in position.

One electrical lead 45 establishing an electrical path to the electrodes of the actuator is soldered to the middle electrode 31. A portion of piezoelectric layer 30 and electrode 32 is cut away to provide access to the middle electrode. Lead 45 extends through an insulator sleeve member 46 mounted within a passage through the pole piece flange 10 to the lower portion of the magnetron housing. An insulator 47 is supported within a passage through the cylindrical wall 7. An enlarged diameter portion of electrical lead 45 extends through this insulator between the interior and exterior of the magnetron housing. Included in series with electrical lead 45 is a conductive helical spring 49 which allows flexure of the piezoelectric biornorph with minimal restraint or pull from electrical lead 45. A like electrical lead, not illustrated, is connected in identical fashion to outer electrode 32. This lead is located directly behind electrical lead 45, and in the sectional view of the figure is not visible. Such structure contains a like helical spring and insulating members within the magnetron housing to provide a second electrical connection or path between a biornorph electrode and the exterior of the magnetron housing. A third electrical connection is made to the other outer electrode 33 through the metallic housing walls of the magnetron or through the positioning mechanism 42. The second clamping member 37 and the second tubular support sleeve 39 are electrically conductive and provide an electrical path between electrode 33 to the linkages 40 and 41 and rods 42 and 43, pole piece 10 to wall 7.

Since the piezoelectrical biornorph 28 is restrained or fixed in position along its inner rim, it flexes along its outer rim as a function of the magnitude of voltages applied across its electrodes. This positions the movable wall 26 to various positions in dependence upon those applied voltages. Moreover, since the piezoelectric biomorph actuator is fixed along its inner rim to a sleeve like member which is free to reciprocate or vary its position along the pole piece, as a function of the movement coupled thereto from the mechanical positioner 50, through rod 43, linkage 41, rod 42, the second linkage 40 carrying the actuator and supports, the wall 26 because it is carried by actuator 28 is also positioned relative to the washer shaped dielectric window 25 by positioner 50.

The magnetron chamber containing the anode and cathode is evacuated in any conventional manner.

As is conventional, the cathode 14 and the filament are connected to a conventional electrical connector assembly, not illustrated, which is mounted to the top of the cup like housing portion 1. Likewise, magnets and cooling fins are assembled to the illustrated structure in the conventional manner. Such conventional elements and mounting brackets are illustrated in US. Patent 3,034,014 to I. DreXler.

As is conventional, a source of high voltage, not illustrated, is adapted to be connected to the appropriate terminals of the electrical connector to establish an electric field between the cathode 13 and anode 15, while the magnets, previously discussed, with pole pieces 10 and 12 establish a magnetic field in the interaction region or gap perpendicular to the direction of such electric field. A conventional source of filament voltage, not illustrated, is adapted to be connected to the filament through the electrical connector in order to stimulate the emission of electrons from the cathode 13.

The operation of a coaxial magnetron is conventional and is described in the literature. In essence, under the interaction of the crossed electric and magnetic fields, e.g., the electric field extending between the cathode 13 and the surrounding anode 15 across the interaction region, and the magnetic field extending axially between ends of pole pieces 10 and 12 within this same interaction region, potential energy is coupled from electrons emitted by the cathode to an electromagnetic wave which appears to travel around the anode at a certain phase velocity. A TE circular electric mode of oscillation is set up within the microwave or resonant output cavity 27. This TE mode has a fixed positioned phase magnetic field extending around the outer wall 17 of anode 15. The coupling slots 20 couple this microwave energy from cavity 27 to alternate anode resonators, which are thus placed in the same electrical phase. Adjacent anode resonators not coupled to the output cavity have voltages induced from the electromagnetic Waves introduced within the output cavity coupled anode resonators which are 180 degrees out of phase with that in the cavity coupled resonators. Thus, between any two adjacent anode resonators there is a forced 180 degree shift in electrical phase. This is the commonly termed 1r mode of oscillation. Since a magnetron is capable of operating in many different modes, it is necessary to select and attempt to maintain operation in only a single mode, and desirably the 1r mode. The resonant output cavity through the alternate anode resonator coupling tends to lock the magnetron in the 1r mode. This microwave energy generated by the magnetron is transmitted from the resonant output cavity 27 through the microwave window 6 to an electrical load or other equipment.

The frequency of oscillation of the coaxial magnetron is determined primarily by the size of the resonant output cavity; and hence, the resonant output cavity is effectively tuned by adjusting the position of movable wall 26. Because the resonant output cavity is so much larger than any of the individual anode resonators, it stores a larger proportion of microwave energy and therefore has a much larger frequency determining effect on the magnetron.

The piezoelectric biomorph 28 is connected by the electrical lead 45, the unillustrated lead connected to electrode 32, and the grounded housing connected to electrode 33 to a source of sweep voltage, not illustrated, which applies a voltage between the electrodes 31, 32, and 33. In accordance with the well known principles of operation of a piezoelectric sandwich, the outer rim of the biomorph is positioned as a function of magnitude of the applied voltages. Since movement of the actuator 28 is coupled with movable wall 26, this likewise positions movable wall 26.

Positional changes or movement of the movable wall effect changes in the frequency of resonance of cavity 27 over a bandwidth of perhaps five percent. However, the biomorph actuator is capable of responding or moving at a very fast rate from DC up to 1 megacycle of applied sweep voltages; and hence, tunes cavity 27 over this bandwidth at a very high sweep rate.

The conventional mechanical or electromechanical positioner 50 provides a driving force through rods 42 and 43, and links 41 and 40, the latter of which carries the piezoelectric biomorph actuator 28 and the movable wall 26, and which in the customary manner produces a periodic reciprocating motion. This periodically reciprocates the position of movable Wall 26 relative to the dielectric window 25 to cause a change in tuning over a relatively wide band-width; however, due to obvious mechanical limitations, this tuning change or sweep occurs at a smaller rate than that available with the piezoelectric actuator. Sweep tuning with both the positioner 50 and actuator 28 is performed simultaneously, or in the alternative, the mechanical tuner is utilized merely to set the initial position of movable wall 26, and frequency sweeping is accomplished in the foregoing manner solely through the action of the biomorph actuator 28.

It is noted that the biomorph sandwich 28 is not directly exposed to microwave energy from the resonant output cavity, but is in fact, isolated therefrom by the movable wall 26. Thus, the piezoelectric material and the epoxy used to commercially form the biomorph sandwich construction which is relatively lossy at microwave frequencies is not heated by the microwave energy appearing in cavity 27. Likewise, since the piezoelectric biomorph actuator 28 is not located within the evacuted regions of the magnetron no special constructions, such as the use of low vapor presure materials in the sandwich is preferred or required, and in fact, any ordinary biomorph construction, as is commercially available, can be utilized. Moreover, in some manufacturing procedures in which the magnetron is heated to very high temperatures above the Curie temperature of the piezoelectric material utilized during the evacuation proces, the piezoelectric biomorph need not be directly exposed to such temperatures, and can in fact, be subsequentially assembled to the magetron housing. In like manner, any piezoelectric biomorph found defective in service, since it is located outside the evacuated region of the magnetron, is more easily removed and another piezoelectric biomorph substituted in its place without destroying the vacuum within the evacuated portions of the magnetron housing.

Not so apparent is the increased rate of tuning obtainable by having the movable wall portion of the tuner confronting and movable relative to a dielectric window contained within the microwave cavity. Experimental results verified by subsequent calculation showed that only about one-fourth as much displacement of movable Wall 26 is necessary to effect the same predetermined change in tuning of the microwave or resonant output cavity 27 as is required without such a window.

The mechanism for this achievement is believed to operate as follows: Normally the TE circular electric mode of oscillation has a distribution or envelope of the magnitude of the electric field component along the anode wall between the top wall 2 and movable wall 26 that approximates a half-wave sinusoid. Where movable wall 26 is in a position very close to the dielectric window 25, the lowest intensity of this half-wave sinusoid distribution is incident upon the dielectric window. The dielectric window 25 appears as a shunt capacitance to the microwave energy and the magnitude of the capacitance has an effect upon the frequency of resonant of the microwave cavity 27 just as the size of the cavity affects the tuning thereof. Unlike a lumped tuned circuit, common at low radio frequencies, however, positioning of a dielectric element within a cavity having a distributed electric field has a definite effect. The larger the magnitude of the electric field incident upon the dielectric element, then the greater is the capacitive eflYect.

Thus, as the movable wall 26 is positioned more distant from the dielectric window 26, to enlarge the size of microwave cavity 27 and lower the frequency of resonance of the cavity, the TE mode at this new frequency would result in a spreading out of the halfwave sinusoid distribution between the movable wall 26 and the top wall 2. However, since the position of dielectric window 25 is fixed, it is exposed to the electric field at a point of higher intensity along the half-wave sinusoidal field; and thus more electric field is shunted by the dielectric window, which results in a larger effective capacitance. As is apparent, increasing the capacitance of the cavity serves to further lower the frequency of resonance of that cavity.

The opposite effect occurs as the movable wall progresses from a remote position to a close position relative to the dielectric window 25.

Thus, there is a cumulative effect due to the positioning of movable wall 26 that results in a greater amount of tuning of about quadruple that previously available for a given displacement of the movable wall. A necessary consequence of this improvement is that for a given amount of displacement of the movable wall, the bandwidth of frequencies over which the resonant cavity is tuned is greater. In the alternative, for a given bandwidth of tuning the rate at which the cavity is tuned is quadrupled since the wall 26 need be moved only one-fourth the distance at a given speed relative to previous movable wall type tuners to eifect a sweep over the desired bandwidth.

Of course it is understood that this invention is not restricted to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. The foregoing embodiment, it is understood, is presented solely for purposes of illustration and are not intended to limit the invention as defined by the breadth and scope of the appended claims.

What is claimed is:

1. A tunable coaxial magnetron comprising:

(A) a cylindrical cathode (B) a coaxial cylindrical anode surrounding said cathode and containing a plurality of anode resonators spaced about and facing said cathode,

(C) said cathode and said anode resonators spaced apart across an interaction gap within an evacuated housing;

(D) a coaxial chamber within said evacuated housing,

surrounding said anode, including:

(A) a plurality of conductive walls, and;

(B) an annular shaped dielectric window pervious to microwave energy and impervious to air to maintain the vacuum in said chamber:

(E) an annular shaped movable conductive wall outside said evacuated housing confronting said dielec tric window defining with said chamber a microwave cavity;

(F) microwave passage means connected between alternate ones of said plurality of anode resonators and said coaxial chamber for introducing microwave energy from said cavity to said alternate resonators.

(G) means for establishing a crossed electric and magnetic field within said interaction gap between said cathode and said anode resonators.

(H) out-put coupling means for passing microwave energy out of said chamber including a second dielectric window; and,

(I) positioning means connected to said movable wall for adjustably positioning said movable wall toward or away relative to said dielectric window to vary the effective size and resonant frequency of said microwave cavity.

2. The invention as defined in claim 1 wherein said annular shaped dielectric window and said annular shaped movable wall are washer-like in shape.

3. The invention as defined in claim 1 wherein said positioning means comprises:

(A) a fixed support;

(B) piezoelectric means connected between said movable wall and said fixed support and responsive to applied voltages for varying the position of said movable wall; and,

(C) electrical lead means for connecting said piezoelectric means to a source of control voltage.

4. The invention as defined in claim 3 wherein said piezoelectric means comprises a piezoelectric bimorph.

5. In a tunable microwave generating tube which contains means for generating radio frequency energy in the microwave region, and at least one resonant cavity having a portion maintained in vacuum for determining the frequency of operation of said means, the improvement wherein said resonant cavity comprises;

(A) an evacuated air impervious metallically walled chamber having at least one dielectric window for preventing the passage of air therethrough into said chamber and permitting the passage therethrough of microwave energy;

(B) at least one outer conductive wall movably mounted proximate to and confronting said dielectric window for reflecting incident microwave energy back through said window and defining a boundary surface to said resonant cavity, and

(C) positioning means for adjusting the position of said movable wall toward or away relative to said dielectric window to vary the effective size and resonant frequency of said resonant cavity.

6. The invention as defined in claim 5 wherein said positioning means comprises:

(A) a fixed support means;

(B) a piezoelectric means;

(C) means connecting said piezoelectric means between said fixed support means and said movable wall, said piezoelectric means responsive to applied voltages for positioning said movable wall,

(D) electrical circuit means; and

(E) means for connecting said first electrical circuit means across said piezoelectric means for applying voltages from a control voltage source to said piezoelectric means.

7. The invention as defined in claim 6 wherein said piezoelectric means comprises a piezoelectric bimorph.

8. A tunable coaxial magnetron, comprising: a cylindrical cathode and a cylindrical anode, said anode having a plurality of vanes contained thereon facing said cathode across an interaction region and spaced about said cathode to form a plurality of anode resonators; means for establishing a crossed electric and magnetic field within said interaction region; a chamber maintained in vacuum surrounding the outer side of said anode; a plurality of slots contained in said anode for connecting said chamber with alternate ones of said plurality of anode resonators; a Washer-like shaped dielectric window bordering said chamber, said washer-like shaped dielectric window being pervious to microwave energy and impervious to air for maintaining a substantial portion of said chamber in vacuum; a movable washer-like shaped wall of conductive material located outside said chamber and confronting said dielectric window; said movable washer shaped wall together with said chamber defining a resonant output cavity; washer shaped piezoelectric means connected to said movable wall and responsive to electrical control voltages for positioning said wall as a function of said control voltages toward References Cited UNITED STATES PATENTS Bach 331155 X Kroger 315-39.55 X Drexler 315-39.77

McLeod SIS-39.61 X Truax 315-39.77 Buck 315-3977 X Plumridge 315-39.55

5 HERMAN KARL SAALBACH, JR., Primary Examiner s. CHATMON, JR., Assistant Examiner US. Cl. X.R.

Linder 332 5 X 310-8;31539.61,39.77;33190, 15s

g ggy UNITED STATES PATENT OFFlCE CERTIFICATE OF CORRECTION Patent No. 3, 147 2 x7 Dated Novemt er 1 I 1969 Inventor(s) Joseph F. Hull It is certified that error appears in the above-identified patent and that. said Letters Patent are hereby corrected as shown below:

In Column 5, bridging lines b3 and M the phrase "conductive layer of electrode" should read --conductive layer or electrode--; in Column 5, line 6b, the phrase "the outer electrode" should read --the other outer electrode--; in Column 7, line 2 3, the word 'positioned" should read --positional--.

SIGNED AND SEALED Attest:

Edward M. Fletcher, Ir. Wml'flf E. SGHUYL m, Attesting Officer I 551011 of Patents

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