US2658165A

US2658165A - Magnetron tube with cavity resonator

Info

Publication number: US2658165A
Application number: US651300A
Authority: US
Inventors: John E Evans; Robert C Fletcher; Foster F Rieke
Original assignee: Individual
Current assignee: Individual
Priority date: 1946-03-01
Filing date: 1946-03-01
Publication date: 1953-11-03
Anticipated expiration: 1970-11-03

Description

Nov. 3, 1953 J. E. EVANS ETAL U MAGNEITRON TUBE WITH CAVITY RESONATOR Filed March 1, 1946 INVENTORS JOHN E. EVANS ROBERT c. FLETCHER FOSTER F. RIEKE ATTORNEY Patented Nov. 3, 1953 MAGNETRON TUBE WITH CAVITY RESONATOR John E. Evans, Houston, Tex., Robert C. Fletcher,

- Cambridge, and Foster F. Ricks, Belmont, Mass., assignors, by mesne assignments, to the United States of America as represented by the Secretary of War Application March 1, 1946, Serial No. 651,300

This invention relates in general to cavity type 3 Claims. (Cl. 315-39) magnetrons, and more particularly to means for coupling radio frequency energy from such devices.

As an oscillator for the generation of radio frequencies having associated wavelengths in the centimeter range, the cavity magnetron is very satisfactory. One form of this thermionic tube has a linear cathode, and an anode structure consisting of a number of cavity resonators positioned symmetrically about the cathode. This structure may take any one of a number of different shapes, the various forms usually being the result of the number of designs of the cavity resonators that are possible. These designs are often dictated by the desirable ease of manufacture of the tube. One such structure is incorporated in the so-called vane type magnetron, in which the cavity resonators become the volume between flat vanes which are positioned so as to flare out radially from the cathode-anode space.

A cavity magnetron is quite sensitive to loading conditions, and a small change in the amount of loading will usually cause the tube to oscillate at a slightly different frequency. This phenomenon of frequency variation with loading is termed pulling. The tighter the coupling between magnetron and load, the greater the frequency variation.

Some types of cavity magnetrons utilize a loading arrangement coupled to the output line associated therewith as a means of frequency pulling. As a result of some undesirable features of this arrangement, the double-output" cavity magnetron was developed. This tube has a tuning structure tightly coupled to one cavity thereof, and the regular output transmission line is loosely coupled to another cavity, and the variation in the loading conditions of the regular output thus has a relatively small effect on the tube operating characteristics. The above-mentioned tuning apparatus usually takes the form of a tunable cavity resonator of some sort coupled to the magnetron. If a large frequency range is desired, not only must the cavity have the necessary tuning range but the coupling between the tuning cavity and the magnetron must be able to pass frequencies over the entire band at nearly the same level; that is, the coupling must have a low eifective Q. An eifective Q less than about may be considered to be a low Q.

Accordingly, among the objects of the present invention are:

i. To provide a double output cavity magne= tron;

2. To provide a means of coupling to such a magnetron whereby a relatively large range of frequencies may be obtained; and

8. To obtain such a frequency range by providing a coupling means having a relatively low effective Q.

This invention will best be understood by reference to the drawing, in which Fig. l is a sectional view of a portion of a double-output vane type magnetron employing one embodiment of the principles of the present invention, and Fig. 2 shows a sectional view taken along the line 2-2 of Fig. 1. It will be understood that details not necessary to the understanding of the present invention are omitted from the drawing for purposes of simplicity.

Referring now to a description of the apparatus and to the drawing, there is represented a cathode l0, positioned symmetrically about which are radially extending anode vanes H, l2, l3, l4, I5, [6, i1 and I8. Surrounding the anode vanes and firmly attached to them is a shell IS. The view shown in Fig. 1 of the drawing is one looking in a direction parallel to the long straight cathode of the tube, and therefore the plane of the sheet of drawing is normal to the axis of the cathode. The vanes, cathode and shell extend in a direction normal to the plane of the paper by substantially equal amounts, the shell overlapping the other elements by a small amount in this dimension. The dimensions of the vanes, cathode, and shell in planes parallel to the plane of the drawing are constant for a short distance in a direction normal to the plane of the drawing as is well known to the art. To complete the tubes main structure, end plates 40, 4| (Fig. 2) are fitted on both open ends of the tube. These plates would be parallel to the plane of Fig. 1 of the drawing. Thus the vanes and the shell

form cavity resonators

20, 2!, 22, 23, 24, 25, 26 and 2'6 which are dispersed symmetrically about the cathode.

The tuning structure consists of a coaxial cavity resonator 28 having an outer conductor 28, an inner conductor, and a movable plunger 3|. The 11 arrows show the directions of motion of the plunger. In order to couple the coaxial cavity to the magnetron, the inner conductor 30 is extended into the magnetron cavity 2! and formed into the partial loop 32. Member 32 is brought into the cavity 2i parallel to, and very close to, one wall oi vane H. When the neighborhood of the inner end of the vane is reached,

member 32 bends over and is soldered to the end of vane 03.

The output power is coupled from the magnetron by means of coaxial line 34, comprising inner conductor 35 and outer conductor 38. The inner conductor is brought into magnetron cavity 25, bent into the form of a loop 31, and soldered to the back wall the cavity, which isa portion of the shell l9.

Referring now to a description of the operation or the apparatus and to the drawing, it may be seen that the combination or member 32 and vane It forms an effective loop which is linked by the magnetic field within the cavity. The large area of this eflective loop provides a relatively high coupling from the magnetron to the coaxial tuning cavity 2|. because the loop is made to embrace a relatively large amount of flux within the cavity II.

The magnetron is affected by the coaxial cavity 28 being strongly coupled to it only when the magnetron is generating frequencies in the neighborhood of the frequency to which the cavity 2! is tuned. The resonant ireuuency of cavity 28 then may be varied by changing the position oi plunger 3! and therefore the volume of the cavity into which oscillating energy is coupled. Elements such as used in this type oi. loop coupling are inherently irequency sensitive in the centimeter range, because their dimensions are of the order of a wavelength at the operating frequency. The effective Q of a device such as this loop depends to a large extent on the closeness of member 32 to the wall of vane i4. Thus it is seen that an effective two-conductor transmission line is formed over a short distance, with the straight portion of member 32 being one conductor and the flat vane ill being the other conductor. From transmission line theory it may be said that the spacing of these two conductors determines the characteristic impedance of this line. It is experimentally found that this spacing also affects the Q of the coupling arrangement.

Using a relatively low voltage magnetron oi. the

"2J39" type which has been manufactured by the Raytheon Manufacturing Company, a 0.062" wire as used for member 32 gave a loaded Q oi 3.5 for a spacing of 0.005" between member 321 and vane ll, and a loaded Q of 9.6 for a spacing of 0.010. Tuning ranges of about iii% of the center frequency have been obtained using this type of loop.

The oscillating energy is coupled out in the normal manner by magnetic coupling ioop ti.

While there been iiiescrlhed "What is present considered the preierred embodiment oi the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing ironi the invention and it is, thereiore, ainicd in the appended claims to cover all such changes and modifications as ialLu/ithin the true spirit and scope of the invention.

What is claimed is:

1. A double-output vane-type cavity magnetron, a coaxial cavity resonator including an inner conductor and an outer conductor, means coupling said coaxial cavity resonator to a first cavity of said magnetron comprising an extension 01 said inner conductor into said first cavity, a portion or the length of said extension being positioned substantially parallel and very close to a first vane of said magnetron, and the end of said inner conductor being fastened to a second vane 01 said magnetron at the end near the center of said magnetron or said second vane. said first vane being positioned adjacent to said second vane, and output means coupled to a second cavity of said magnetron.

2. The combination of a double-output vanetype cavity magnetron, a coaxial cavity resonator i'or tuning said magnetron including an inner conductor and an outer conductor, said cavity resonator having a conductive closure at one end, means coupling said coaxial cavity resonator at its other end to a first cavity of said magnetron comprising an extension of. said inner conductor, a portion of the length or said extension being positioned close and substantially parallel to a first vane of said magnetromthe end of said irmer conductor being fastened to a second vane of said magnetron at a point near the inner end of said second vane, said second vane being adjacent to said first vane, and output means coupled to a second cavity of said magnetron.

3. A double-output vane-type cavity magnetron, and a cavity resonator having a low effective Q comprising a coaxial line for tuning said magnetron including an inner conductor, an outer conductor, and a conductive plunger in said coaxial line for effectively short-circuiting said coaxial line, said inner conductor being extended into one of the cavities of said cavity magnetron, a portion of the length of the extension being positioned substantially parallel to and very close to a first vane of said magnetron, the end oi said inner conductor being fastened to a second vane of said magnetron, said first vane being positioned adjacent to said second vane, and the position of fastening on said second vane being close to the end near the center or said magnetron of said second vane.

JOHN E. EVANS. ROBERT C. FLETCHER. FOSTER F. RIEKE.

ltciicrcnces Cited in the file of this patent UNITED STATES PATENTS Number I Name Date ii,4ll9,ti4ll Moles Oct. 22, 1946 2,417,?89 dpencer M Mar. 18,. 1947 2,48l,l5i Powers Sept. 6, 1949 2,520,955 Ell-tress Sept. 5, 1950 2,530,172 Oirress Nov. 14, 1950