US2816248A

US2816248A - Tunable interdigital magnetrons

Info

Publication number: US2816248A
Application number: US147717A
Authority: US
Inventors: Marshall C Pease
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1950-03-04
Filing date: 1950-03-04
Publication date: 1957-12-10
Anticipated expiration: 1974-12-10

Description

DeIi- 1957 M. c. PEASE 2,816,248

' TUNABLE INTERDIGfTAL MAGNETRONS 4 Sheets-Sheet 1 Filed March 4. 1950 INVENTOR MARSHALL C. PEASE 56 ATTORNEY Dec. 10, 1957 3 M. c. PEASE 2,816,243

TUNABLE INTERDIGITAL MAGNETRONS Filed March 4, 1950 4 Sheets-Sheet 2 INVENTOR MARSHALL C. PEASE ATTORNEY Dec. 10, 1957 M, c, PE SE 2,816,248

TUNABLE INTERDIGITAL MAGNETRONS Fil ed March 4, 1950 4 sheets-sheet s INVENT I MARSHALL C. ASE

' ATTORNEY Dec. 10, 1957 v M.'C. PEASE 2,815,248

- TUNABLE INTERDIGITAL MAGNETRONS Filed March 4. 1950 4 Sheets-Sheet "4 244 I a 11 234 i 24;

v v Z40 INVENTOR I MARSHALL C. PEASE ATTORNEY United tates Patent TUNABLE INTERDIGITAL MAGNETRONS Marshall C. Pease, Needham, Mass, assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application March 4, 1950, Serial No. 147,717

' 17 Claims. (Cl. 31539.55)

This invention relates to magnetrons, a class of highfrequency electronic oscillators, and in particular to the interdigital type of magnetron. In this type of magnetron, a cathode is surrounded by an anode assembly having two sets of fingers that are interdigitated with respect to each other. The fingers are disposed in cylindrical array, each finger being spaced arcuately from the next. The fingers of each set are interconnected, and in operation are connected to an annular cavity surrounding the anode assembly, with the respective sets of fingers connected to the opposite ends of the cavity. The anode and cathode assembly are in some instances enclosed in a vitreous evacuated envelope so as to be separable from the resomater, and such specialized enclosed anode-and-cathode assembly is designated a magnetron herein. The magnetron plus the resonator (and the usual direct-current supply and devices for establishing and directing an axial magnetic field) are termed a magnetron oscillator. An object of the present invention is to provide a novel physical form of interdigital magnetron and magnetron oscillator that is adjustable to change its operating frequency.

The operating frequency of an interdigital magnetron can be mechanically adjusted by changing the extent to which each set of fingers projects axially between the fingers of the other set. This type of tuning control is generally discussed in a paper of Crawford and Hare entitled, A tunable squirrel cage magnetron-Donutron, in April, 1947 issue of the Proceedings of the I. R. E., pages 361-369. The mechanical tuning range of the magnetron is increased, according to two embodiments to be described in detail below, by providing a ring connected to one set of fingers and positioned concentrically with respect to the free ends of the fingers of the other set of fingers.

Mechanical adjustment of the operating frequency re-- quires physical access to the adjustable part in the installation. I It is often desirable to change the operating; frequency from a remote point by application of a control potential, or so that the tuning can be changed rapidly as in frequency modulation or so as to correct for freoperating frequency, and only secondarily affects the.

amplitude of the oscillation. In such devices as that of Dahlenbach (Fig. 7), patent #2,l28,237, issued August. 30, 1938, the primary effect is of changed amplitude of oscillation.

Because the electronic tuning is capable of being swept: over only a finite range, it is desirable that mechanical frequency adjustment be provided additionally, to shift the range of electrical tuning to the desired region. Moreover, when the electrical tuning is used for frequency modulation the mechanical tuning serves to adjust the carrier frequency.

The nature of the present invention will be better appreciated from the following detailed disclosure of several illustrative devices embodying various aspects thereof which are shown in the accompanying drawings. In the drawings:

Fig.1 is a lateral external view, somewhat reduced, of an interdigital magnetron oscillator embodying features of the present invention.

Fig. 2 is an enlarged fragmentary sectional view of the magnetron oscillator of Fig. l, the section being taken through the axis and parallel to the plane of that figure.

Fig. 3 is a modified form of a support for the tuning electrode in Fig. 2.

Figs. 4 and 5 are schematic views of the interdigital magnetron oscillator in Figs. 1 and 2 showing the tuning electrode in relation to the remainder of the oscillator, and the resonant cavity, Fig. 4 being a perspective view and Fig. 5 being a plan view.

Fig. 6 is an enlarged longitudinal cross-sectional view of the anode assembly in Fig. 2 on an enlarged scale showing the tuning electrode, and Fig. 7 is a crosssectional view along the line 77 in Fig. 6.

Fig. 8 is a longitudinal cross-sectional View of a modified anode assembly including a tuning electrode and, Fig. 9 is a cross-sectional view along the line 99 of Fig. 8.

Figs. 10 and 11 are views of a modified mechanically and electrically tunable interdigital magnetron oscillator comparable to Figs. 4 and 2, respectively.

An interdigital magnetron oscillator is shown in Figs. 1 and 2, including one form of frequency controlling electrode and having mechanical frequency control means. Thermionic cathode 10 having an electron-emissive coating contains heater 12 and extends through a bore 14 in pole piece 16. Glass seal 18 closes off the space between cathode lead 24) and pole piece 16; and the space between cathode lead 20 (which also serves as a heater lead) and the other heater lead 22 is closed off by a further glass seal 23.

An anode assembly coaxially surrounding cathode 10 includes two

members

24 and 26 supported respectively by the upper wall 28 and the lower wall 30 of a cylindrical resonant cavity 32. As seen in Figs. 6 and 7 these members embody

circular connectors

24a and 26a which interconnect and support the respective sets of fingers 34 and 36. These fingers are disposed at regular arcuate intervals about the anode assembly in cylindrical array, defining an inner cylindrical surface that is the outer limit of the cathode-anode electron-discharge space. In Fig. 7, fingers 34 appear in cross-section, whereas fingers 36 appear in projection. In the present instance the resonator constitutes the tube envelope and is evacuated, although it is readily possible to provide a separate resonator connected to and containing the anode and cathode assembly enclosed in an evacuated glass envelope.

The operating frequency of the magnetron can be adjusted by manipulating thumb screw 38 which, by compound threads, etc. (not shown), shift shaft 49 axially Within pole piece 42 and thus displaces anode member 26 axially in relation to anode member 24. The conventional magnet (not shown) is provided to engage

pole pieces

16 and 42 laterally and provide an axial magnetic field in the radial electron-discharge space between cath-' ode 10 and

anode assembly

24, 26. Output coupling loop 44 in cavity 32 has outer fitting 46 and inner conductor 48 sealed together hermetically by glass seal 50, providing for coaxial connection to an external load,

amazes A '-=sta-ndingwave of one or more wavelengths is produced to which fingers 36 project between fingers 34, a frequency control electrode is provided. An axially extending blade 52-(see also Figs. 6 7) is interposed in a neutralplane midway between one tooth 34a and a secondtoothfioa.

of the anode assembly. The innermost surface of the blade is disposed in the cylin rical anode surface constituting the outer limit of the cathode-anode electron discharge space. This electrode includes a lead portion 54. that extends to terminal 56, emerging. from resonator32 substantially midway along its axial length. Since the potential of finger 34a is always equal but opposite to the in-. stantaneous potential of finger 36a, the slender blade 52;of fl1e-tuning electrode that is interposed betweenthe

adjacent fingers

34a and 36a of the anode assembly will remain at zero potential in respect to the high frequencies. Lead 54, in emerging radially through the cavity midway along its axial length, also remains at virtually stable potential insofar as the high frequency potentials are con cerned.

In operation the interdigital magnetron is thought to form a cloud of electrons in the discharge space between the cathode and the anode assembly. The electrons concentrate into spokes, and the spokes speed around the cathode and interact with the anode fingers to develop the oscillatory energy. A direct current potential is, of course, provided between the anode assembly and the cathode, both members of the anode assembly being of thesame direct current potential by virtue of their interconnection by the resonant cavity 32. The magnetron current is not appreciably affected by a control or modulating'potential applied to the tuning electrode in relation to the anode assembly because blade 52 does not extend into the electron-discharge space. However, when a potential is applied to portion 52 diifering from the anode potential, the rotational speed of the spoked electron cloud is modified, thus changing the operating frequency. If an adjustable potential is applied to blade or finger portion 52 of the tuning electrode, the operating frequency is controlled; or if the voltage is a rapidly varying one, the magnetron oscillation frequency is modulated.

Only one blade 52 is shown between one pair of fingers 34a to 36a. If warranted, additional axially extending frequency-control blades may be added in the neutral planes between other pairs of fingers of opposite anode members. The multiplication of tuning elements increases the tuning effect but complicates the construction disproportionately.

It is desirable that the anode fingers be alike, generally. The equality of capacitance, of each finger to the finger next adjacent it, is disturbed where a tuning blade 52 is interposed, and for this reason the side faces of

fingers

34a and 36a are relieved as shown in Fig. 7.

The interdigital magnetron is capable of several modes of operation. These are commonly designated the can modes and the line or pheripheral modes. These can exist separately or together in any proportions. In the can mode, each anode member is instantaneously at virtually the same potential as every other portion of that anode member, at all phases of operation of the magnetron; but the anode members are of opposite polarity with respect to the average voltage, their potential difference varying sinusoidally with time at the oscillation frequency. Axially midway along the outer wall of the cavity a current maximum is developed in this mode of operation, and at this point a voltage node is found. This mode of operation is subject to the difficulty that a high-frequency potential difference is normally established between the two ends of the cathode that tends to couple to the exterior, unless the cathode leads are constructed to embody a choke.

The interdigital magnetron can be made to operate more stably and at better efficiency by so controlling it as to produce the line type of operation, wherein a about the periphery of each anode member. Where a standing wave of one wavelength is developed about the anode, peripherally, there are two points on member 24a and on member 26:; Where the R.-F. potential is zero, and midway between these points the high frequency variation is a maximum. The potential diminishes sinusoidally toward these node points from the maxima. Where there :is a standing wave of one wavelength established about the anode periphery, the magnetron is said to operate in the ri l mode. higher modes are also possible, where n=2' or 3 or more.

Without special precaution, the introduction of a coupling loop into the resonant cavity tends to shift the minima to such positions as to minimize the output. It is desirable to stabilize the position of the R.-F. field in the cavity and this can be accomplished by providing phase reversal fingers in the anode assembly, described in the foregoing paper by Crawford and Hare, and disclosed and claimed in application Serial No. 719,414, filed December 31, 1946, by Robert M. Bowie, now Patent No. 2,679,615; and by other features of circumferential, or peripheral, asymmetry as disclosed and claimed in copending application Serial No. 619,289, filed September 29, 1945, by Donald L. Benedict, now Patent No. 2,639,405. The phase reversal fingers are described as buck teeth in the Bowie application, and in Fig. 7 a split buck tooth 34b is shown having two portions each of which is like a single finger 34. The buck tooth occupies twice the arcuate interval of the other fingers 34. Similarly, fingers 36b are provided in the other anode member 26. These

fingers

34b and 36b might be removed entirely while preserving the same relative spacing of the fingers shown, the provision of these double fingers serving primarily to phase the remaining fingers, which are alike in form and m arcuate pitch, in proper relation to the spoked electron cloud to establish and stabilize the peripheral mode. Fingers 34b are seen to be peripherally enlarged fingers, in effect, as are fingers 36b. The space between fingers 34b and finger 36b has been left empty as a manufacturmg convenience. The halves of the split buck teeth are connected to the same anode member and operate at virtually the same potential, being quite close to the current maximum of the circumferential standing wave.

Tuning finger 52 is seen to be positioned midway between effectively

enlarged fingers

34b and 36b, of the anode assembly. The enlarged fingers are 180 apart in, the anode assembly, and the tuning finger is away from each of them. These are electrical degrees, but are the same for the n=1 mode as physical degrees. As shown in Fig. 4, the tuning finger 52 is disposed diametrlcally opposite output coupling loop 44, lead 54 emerging radially as near as possible to the midpoint axially.

between the

ends

28 and 30 of cavity resonator 32 to avoid high-frequency coupling to the modulating potential supply. A dotted line plane A appears in Fig, 4'to represent the potential node and fingers 34b and 36?; are seen to lie on opposite sides of this plane. As seen in Fig. 5, the field within the cavity 32 is shown diagrammatically, the anode fingers being replaced by small zeros or xs to indicate their connection to

anode member

24 or 26. Blade 52 of the frequency control electrode and output coupling loop 44 (shown diagrammatically rotated through 90 in this View) are seen to be diamet rically opposite each other, both in a plane 90 away from plane A and the effectively enlarged

teeth

34b and 36b. The plane common to the control electrode and the output loop intersects with the resonant cavity. 32 in a line B. The potential difference between

fingers

34a and 36a is greater than between any other pair of fingers.

34 and 36 (except those diametrically opposite 34a and- 36a) and by locating the tuning element at this point of maximum potential difference, the greatest effect of a single tuning element 52 on the electron cloud. can be realized. Phrased otherwise, a tuning electrodehaving only a single control blade 52 is most effective in the configuration shown for the 11:1 mode and is most effective with the higher modes between the fingers adjacent the voltage maximum.

For mechanical tuning the end wall 30 of the resonant cavity is adjusted and as a result lead 54 is normally not perfectly centered axially of the resonator. For this reason it may be desirable to modify the construction of the tuning electrode lead according to Fig. 3. A choke is there provided, of an odd number of quarter-wavelengths, so as to prevent leakage of power. Lead 54 is seen to be supported by a terminal element 56' that constitutes the inner conductor of a coaxial choke having an outer conductor 56a. These are separated by a film of insulating material 5611 so that a potential difference can be maintained between terminal 56' and the resonator for electronic tuning.

Certain novel features of construction of the anode assembly are useful in achieving a wide range of mechanical tuning for the magnetron oscillator, useful in electrically tunable magnetrons and in magnetrons tunable only mechanically. The usual range of mechanical tuning is increased by arranging for an overlapped relationship between the free ends of each set of anode fingers and the circular connector of the opposite set. Thus, in Figs. 6 and 7 fingers 34 extend inside portion 26b of connector 26a and fingers 36 extend inside portion 24b of connector 24a. As the diaphragm supporting anode member 26 is moved, the extent of interdigitation of the anode fingers is changed; but the frequency range is enhanced by the capacitance between the finger ends and the encircling ring portions of the anode connectors. This physical arrangement affords a wide range of capacitance and frequency change, varying almost linearly with diaphragm adjustment.

Further increase in mechanical tuning range is achieved through use of the embodiment in Figs. 8 and 9. In that embodiment the anode members have axially extending

fingers

134 and 136 interconnected in two sets by

circular connectors

124a and 126a respectively. These circular connectors include a ring portion 124]) and 12612 respectively encircling the free ends of

fingers

136 and 134 respectively. The circular connectors also include annular portions 124a and 3126c axially opposite the free ends of the fingers of the other anode member. A rapidly increasing capacitance is introduced as the free ends of the fingers of each anode member approach the annular portion 124C or 1260 of the other. The overlap capacitance of the fingers to the cylindrical extensions 1241), 126b is increased in Figs. 8 and 9 by making the fingers T-shape in cross-section, providing a cross-bar 1340 on each finger 134. This increases the capacitance between fingers 134 and extension 126b, without unduly increasing the virtually fixed capacitance between the two sets of

fingers

134 and 136. Fingers 136 have a T-shaped crosssection in the region of cylindrical extension 1241; in a symmetrical arrangement and for the same purpose. Each anode member is formed, conveniently, in a cold-pressing operation with splined dies that make the fingers of T-shape cross-section from end to end. The fingers may be left this way, or they may be stripped of the cross-bar of the T between rings 12412 and 12612 as shown. In any event, the fingers flanking blade 152 are required to be relieved to receive that blade.

Fingers 134i) and 1361; which stabilize the 11:1 mode in the anode assembly in Figs. 11 and 12 are seen to be of the solid buck tooth construction and are truly arcuately enlarged relative to the other fingers, not merely effectively enlarged.

Fingers

134a and 136a flanking the tuning blade 152 are relieved at their side faces to compensate for the otherwise increased capacitance due to the introduction of the tuning electrode.

The anode assembly of Figs. 8 and 9 imparts increased mechanical tuning range over that of Figs. 6 and 7 by virtue of the T-shape finger ends in the overlap region,

opposite encircling rings 1240, and also by virtue of the areas 124C and 1260 opposite the finger ends. The latter form tends to make the frequency change abrupt toward the end and may be omitted in the anode shown or it may be included in the anode members of Figs. 6 and 7, as desired, the overlap arrangement being retained wherever wide range of mechanical tuning is desired.

The anodes of Figs. 8 and 9, moreover, are to be considered related to the modified electrically tunable magnetron oscillator of Figs. 10 and 11 as to that of Fig. 2.

Allusion is made in the Crawford and Hare paper referred to above to finger nails or rings to extend the frequency range, and to finger tabs (on the ends of fingers) to increase the tuning range. But the arrangement in Figs. 6 and 7 wherein the axial end of the interdigitated fingers are unobstructed and do not provide any capacitance that increases abruptly near the end of the tuning range is not mentioned, nor is the enhanced overlap construction of Figs. 8 and 9 described.

Another form of tuning electrode comparable to blade 52 and its supporting lead is shown in Figs. 10 and ll. The modified magnetron oscillator there shown is in all respects like that in Figs. 1, 2 and 4, differing in the form of the tuning electrode. The parts are designated with numerals similar to Fig. 2, but of the 200 series. In Fig. 6 the buck teeth, constituted by fingers 23611 and 23411 are positioned away from the plane B containing output coupling loop 244. The tuning electrode that has two finger portions 252 in the cylindrical anode array of this embodiment replaces a pair of fingers 34, 36 and blade 52 of Figs. 2 and 4. Fingers 252 are positioned between a pair of fingers 234, 236 of the opposed anode members, and the tuning electrode is positioned midway between the buck teeth. In contrast to the embodiment of Figs. 2 and 4, the tuning electrode portions 252 are designed to have a maximum of coupling to the cavity field. Lead 254 is formed for this purpose as a loop that has a maximum of coupling to the cavity. In this way the finger portions 252 substitute, in a sense, for the. fingers that they replace on the

anode members

224 and 226. Terminal lead 256, like lead 56, emerges radially from the resonant cavity, midway along the axial length of the cavity. By applying a modulating potential between terminal lead 256 and the anode-and-resonator assembly, the rotational speed of the spoked electron wheel can be modified to accomplish frequency control or frequency modulation.

My copending application S. N. 73,345, filed January 28, 1949, now abandoned, and the continuation application thereof, S. N. 442,506, filed July 12, 1954, are concerned with the control of the operating frequency of magnetrons generally, but these applications specifically disclose other forms of magnetrons than the interdigital type here involved. Copending application S. N. 132,004, filed December 9, 1949, by Henry J. McCarthy, now Patent No. 2,578,569, is for another form of electrical control of frequency of an interdigital magnetron, that depends upon a different principle of operation.

In view of the latitude of detailed modifications of the foregoing that will occur to those skilled in the art, the appended claims should be accorded a broad scope of interpretation, consistent with the spirit of the invention.

What I claim is:

1. A tunable interdigital magnetron oscillator having an annular cavity resonator, an interdigital anode assembly including two interdigitated sets of fingers having their inner surfaces disposed in a substantially cylindrical surface within the resonator, said sets of fingers being connected respectively to the opposite ends of the cavity resonator, a cathode within said anode assembly, and a tuning electrode having its innermost surface disposed in said cylindrical anode surface between adjacent ones of said fingers, said tuning electrode being separated by electrical insulation from said anode assembly.

2. A tunable interdigital magnetron oscillator having an annular resonator, an interdigital anode assembly including two interdigitated sets of fingers having their inner surfaces disposed in a cylindrical surface substantially coaxial with the resonator, said sets of fingers being connected respectively to the opposite ends of the resonator, a cathode within said anode assembly, a coupling device projecting into said resonator, and a tuning electrode having its innermost surface disposed in said cylindrical anode surface between adjacent ones of said fingers and being substantially diametrically opposite said coupling device, said tuning electrode being separated by electrical insulation from said anode assembly.

3. A tunable interdigital magnetron oscillator having a cathode, a cylindrical anode assembly surrounding said cathode, said anode assembly including a series of like axially extending fingers disposed in a cylindrical surface and interconnected into two separate sets interdigitated with respect to each other, a cavity resonator connected to and enclosin said anode assembly, and a tuning electrode interposed between adjacent fingers of the series of fingers, having a conductive support extending radially through the interior of said cavity and being separated therefrom by insulation, and having an external terminal.

4. A tunable interdigital magnetron oscillator having a cathode, a cylindrical anode assembly surrounding said cathode, said anode assembly including a series of like axially ex .nding fingers disposed in a cylindrical surface and interconnected into two separate sets interdigitated with respect to each other and including a pair of diametrically opposite effectively enlarged fingers, a cavity resonator connected to and enclosing said anode assembly, and a tuning electrode interposed between and electrically insulated from adjacent fingers of the series substantially midway between said enlarged teeth, extending radially through the interior of said cavity, and having an external terminal.

5. A tunable magnetron oscillator including a cathode, an interdigital anode assembly including two interdigitated sets of fingers in substantially cylindrical array surrounding said cathode, an annular resonator connected to and enclosing said sets of fingers, and a tuning electrode having two finger-like portions disposed in said cylindrical array of fingers between adjacent fingers of said anode assembly and having a lead embodying a coupling loop extending radially through said resonator for maximum coupling of said cavity to finger-like portions of said timing electrode and having an external terminal midway along the axial length of said resonator for a minimum coupling of said resonator to said external terminal.

6. A tunable magnetron oscillator including a cathode, an interdigital anode assembly including tWo interdigitated sets of fingers in substantially cylindrical array surrounding said cathode, an annular cavity resonator connected to and enclosing said sets of fingers, and a unitary tuning electrode interposed in said cylindrical array midway between two fingers of said sets of fingers and having a lead extending radially outward through said resonator and effectively midway of the axial length thereof for a minimum of coupling to the field within said resonator.

7. A tunable magnetron including a cathode, an interdigital anode assembly including two interdigitated sets of fingers in substantially cylindrical array surrounding said cathode, said fingers being located at regular pitch intervals about the cathode in a cylindrical array, a pair of adjacent fingers being connected together in one of said sets, and a tuning electrode interposedbetween adjacent fingers of the cylindrical array substantially ninety electrical degrees away from said pair of connected fingers.

8. A tunable magnetron including a cathode, an interdigital anode assembly including two interdigitated setsof fingers in substantially cylindrical array surrounding said cathode, adjacent teeth of said cylindrical array being connected together in one of said sets to establish a nodal point in a peripheral mode of oscillation, two

fingers of said set being electrically insulated from the remainder of the anode assembly but interconnected by a radially extending loop positioned substantially ninety degrees from said pair of connected teeth.

9. A tunable magnetron oscillator including a cathode, an interdigital anode assembly having a cylindrical array of axially extending fingers displaced from each other at regular pitch intervals and surrounding said cathode, saidfingers being divided into two interdigitated sets, the fingers of each set being interconnected, an annular cavity resonator connected to and enclosing said anode assembly, and a slender finger interposed midway between two adjacent fingers, one of each of said sets, said slender tuning finger having a lead extending radially outward through said resonator and efiectively midway of the axial length thereof so as to have a minimum of coupling to said field within said cavity.

10. A tunable magnetron oscillator including a cathode, an interdigital anode assembly including a cylindrical array of axially extending fingers, said fingers being divided into two mutually interdigitated sets of fingers, the fingers of each set being interconnected, said anode assembly embodying means fixing the nodes of oscillation having minima and maxima at different points about the periphery of the anode assembly, and a tuning electrode interposed between adjacent fingers of said assembly and located substantially ninety electrical degrees from one of the nodal points as established by said stabilizing means.

11. A tunable interdigital magnetron having a cylindrical interdigital anode assembly including two interdigitated sets of fingers having their inner surfaces disposed in a cylindrical surface, the fingers of each set being interconnected, said fingers being of uniform size except for a pair of eifectively enlarged fingers, mutually disposed electrical degrees apart, a cathode within said anode assembly, and a tuning electrode in the cylindrical surface substantially midway between said enlarged teeth.

12. A tunable interdigital magnetron having a cylindrical interdigital anode assembly including two interdigitated sets of axially extending fingers having their inner surfaces disposed in a cylindrical surface, the fingers of each set being interconnected, a cathode within said anode assembly, and a tuning electrode in the cylindrical surface and electrically insulated from said anode assembly.

13. A tunable magne ron having a cathode, and an anode assembly surrounding said cathode, said anode assembly including two members each having a circular connector and a set of fingers projecting axially therefrom, the fingers of each set having free ends disposed adjacent the connector of the other set, each connector including an annular axially extending portion overlapping the free ends of the opposite set of fingers, said portion having an inner cylindrical surface closely adjacent and radially outward from the free ends of said opposite set of fingers, one of said anode members having axially adjustablesupporting means for varying the operating frequency of the magnetron.

14. A tunable interdigital magnetron having a cathode and an anode assembly surrounding said cathode, said anode assembly including two members, each member having a circular connector axially spaced from the circular connector of the other member and each men1- her having axially extending fingers interdigitated with the fingers of the other anode member, the free ends of the fingers of each anode member extending to a position adjacent the connector of the other anode member, each circular connector embodying an axially extending portion encircling the free ends of the fingers of the other anode member, said portion having an inner cylindrical surface closely adjacent and radially outward from the free ends of said other anode member and means supporting one of said anode members for axial adjustment relative to the other anode member.

15. A tunable interdigital magnetron having a cathode and an anode assembly surrounding said cathode, said anode assembly including two members, each member having a circular connector axially spaced from the circu-lar connector of the other member and each member having axially extending fingers interdigitated with the fingers of the other anode member, the free ends of each anode member extending to a position adjacent the connector of the other anode member, each circular connector embodying a portion surrounding the free ends of the fingers of the other anode member, said portion having an inner cylindrical surface closely adjacent and radially outward from the free ends of said other anode member each finger being of T-shaped cross-section at its free end with the bar of the T disposed radially outward in proximity to said surrounding connector portion, and means supporting one of said anode members for axial adjustment relative to the other anode member.

16. A tunable interdigital magnetron having an anode assembly including two anode members each having a set of fingers in cylindrical array, the set of fingers of each anode member being interdigitated with the set of fingers of the other anode member, and a tuning electrode between an adjacent pair of interdigitated fingers and electrically insulated from said anode assembly and having an external terminal, said anode assembly incorporating a pair of circular connectors respectively interconnecting said sets of fingers and with the fingers of each set projecting axially from its circular connector, the fingers of each set having free ends disposed adjacent 19 the connector of the other set, each connector having a portion concentric with and surrounding the free ends of the opposite fingers and radially separated therefrom, one of said anode members having an axially adjustable support.

17. A tunable interdigital magnetron having a cathode, a cylindrical interdigital anode assembly coaxial with said cathode and including two interdigitated sets of axially extending fingers having their surfaces exposed to said cathode disposed in a cylindrical surface, and a tuning electrode electrically insulated from said anode assembly and interposed between adjacent fingers in said anode assembly and having a surface lying in the cylindrical surface, each anode member having a circular connector axially spaced from the circular connector of the other anode member and each connector having a portion encircling the free ends of the fingers of the other anode member, one of said two anode members having an axially adjustable support.

References Cited in the file of this patent UNITED STATES PATENTS 2,444,435 Fisk July 6, 1948 2,445,282 Slater July 13, 1948 2,462,137 Smith Feb. 22, 1949 2,466,765 Hartman Apr. 12, 1949 2,468,243 Spencer Apr. 26, 1949 2,504,970 Engelmann Apr. 25, 1950 2,505,529 Crawford et a1. Apr. 25, 1950 2,578,569 McCarthy Dec. 11, 1