Wide range tunable magnetrons

July 30, 1957 p. _L. spencer 2,801,367

WIDE RANGE TUNABLE MAGNETRONS

Filed Jan. 26, 1955 2 Sheets-Sheet 1

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Inventor

Percy l. Spencer

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Attorney

July 30, 1957

2,801,367

P. L. SPENCER

WIDE RANGE TUNABLE MAGNETRONS

Filed Jan. 26, 1955

Sheets-Sheet 2

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Fig. 3

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United States Patent Office

2,801,367

Patented July 30, 1957

2

2,801,367

WIDE RANGE TUNABLE MAGNETRONS

Percy L. Spencer, Waban, Mass., assignor to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware

Application January 26, 1955, Serial No. 484,142

Claims. (CI. 315—39.61)

This invention relates to magnetrons that may be tuned over a wide range of frequencies, and more particularly to tuning devices for such tubes in which concentric slotted tubes, one fixed and the other movable, are inserted in at least one cavity of the anode to vary both the capacity and inductance simultaneously.

This structure provides for flexible control of the tuning of a magnetron and specifically provides means for tuning a magnetron over a wider frequency range than magnetrons of the prior art without inteference with extraneous tuner resonances inherent in prior tunable magnetrons and for achieving a faster tuning rate than realized with the ordinary inductive tuner.

Another feature of the invention is the introduction of capacitive tuning in a portion of the cavity resonators where radio frequency voltage is relatively low, thereby permitting satisfactory operation in high power tubes of the pulsed type. Finally, the tuning structure is so positioned within the cavity resonators that mechanical interference between the tuner and the straps on the anode structure is avoided.

As is well known in the magnetron art, a magnetron anode structure consists of several more or less radially disposed anode segments extending from a generally cylindrical wall portion. Each pair of adjacent anode segments, and the portion of the anode structure lying between, form a cavity resonator whose resonant frequency is determined by the geometry of the elements bounding the cavity. Each cavity resonator has a distributed capacity and inductance, but for practical purposes the portion of each cavity adjacent the cathode on the front part of the cavity is predominantly capacitive, while the inductance is more or less concentrated in that portion of the cavity furthest removed from the cathode on the rear part of the cavity. With a hole-and-slot type of anode the capacitive region is considered to exist almost entirely within the slot between the parallel disposed portions of adjacent anode segments, while the inductive region is concentrated largely within the hole or cylindrically shaped portion of the resonator remote from the cathode. The electric field is greater in the capacitive or forward portion of each resonator, while the magnetic field is greater in the inductive or rear portion, making the latter part a high current region. This is also true of other types of anode structure.

The standard inductive magnetron tuning system of the prior art comprises a plurality of tuning fingers of conductive material, each of which extends into the rear or inductive portion of the corresponding resonator. When such tuning fingers are inserted within a cavity resonator, the surface-to-volume ratio in the high current region of the oscillating resonator is altered and the current flowing about the periphery of the resonator induces a current in the tuning finger which is largely concentrated near the surface of the finger. Thus the inductance of the magnetron radio frequency circuit is decreased as the fingers penetrate further into each resona10 tor, consequently increasing the frequency of oscillation of the magnetron.

In the present invention a slotted tube is mounted within the back portion of at least one cavity and attached to the back of the cavity with the opening towards the front, and a removable slotted tube concentrically mounted about the fixed tube with the slot to the rear. With the movable sleeve completely withdrawn, the fixed sleeve adds inductance to lower the frequency. As the movable sleeve is inserted, more capacity is added. The result is a rapid change in operating frequency, considerably more rapid than that obtained by purely inductive tuning.

The foregoing and other advantages, objects and features of the invention will be better understood from the following description taken in conjunction with the accompanying drawings wherein:

Fig. I is a vertical section of a magnetron incorporating this invention;

Fig. 2 is an enlarged view, partly in section, of the tuner and anode block of the magnetron shown in Fig. 1;

Fig. 3 is a horizontal section through a portion of the anode structure with a tuner element inserted in one cavity;

Fig. 4 is a schematic diagram of a cavity with the movable sleeve completely withdrawn;

Fig. 5 is a schematic diagram Of the equivalent circuit of- the arrangement of Fig; 4;

Fig. 6 is a schematic diagram of the cavity with the movable sleeve completely inserted; and

Fig. 7 is a schematic diagram of the equivalent circuit of the arrangement of Fig. 6.

In Fig. 1 the reference numeral 10 designates the cylindrical anode having a lower cover plate 11. A pole piece 12 associated with a magnet (not shown) is inserted in an opening in the plate 11. A cathode sleeve 13 is inserted through an opening in the pole piece 12 and sealed in one of the well-known ways. This cathode sleeve encloses a heater filament 14. The anode 10 is formed with a plurality of cylindrical openings 15, each connected to the anode-cathode space by a slot 16. A sleeve 17 having an axial slot 18 is formed on the side facing the slot 16 and attached by an extension 19 to the opposite wall of the cylindrical opening 15. A second slotted sleeve 20 is formed to fit over the sleeve 17. The slot 21 receives the extension 19. The sleeves 20 are attached to a support 22 fitted with a rod 22α by which it is moved up and down to move the sleeves 20 into and out of the cavities 15. The support 22 is in turn supported on a ring 23 attached to the anode block 10 by a second ring 24 and a bellows 25 to permit vertical movement while preserving the vacuum within the anode cavity. While the invention is most readily applied to magnetrons having key-hole shaped cavities in the anode block, it may be applied to magnetrons having any shape of cavity, provided these cavities have a definite inductive region.

The operation of the device can best be understood by reference to Figs. 4 through 7. In Fig. 4 the cavity is seen with the movable sleeve 20 completely withdrawn, leaving the fixed sleeve 17 within the cavity 15. It can readily be seen that each half of the sleeve 17 forms a transmission line with the wall of the cavity shorted by the extension 19. When the cavity is formed in accordance with the usual design considerations for such tubes, the length of these transmission lines, represented by the two pairs of straight horizontal lines 30, 31, 32 and 33 in Fig. 5, are less than a quarter wavelength long at the operating frequency. If these transmission lines are considered as having the terminals 16A and 18A for the first and 16B and 18B for the second, they will appear as

0,801,367 inductances across the terminals 16A and 18A and 16B and 18B as indicated by the dotted coils 34 and 35. These inductances are in series with the capacity 36 across terminals 18A and 18B at the slot 18 and the inductance of the interior of the sleeve 17 represented by the coil 37. The whole circuit is shunted across the parallel resonant circuit of the original cavity 15 and slot 16.

With the sleeve 20 fully inserted, as shown schematically in Fig. 6, the outer sleeve 20 forms, with the inner wall of the cavity 15, the extension 19 and the inner sleeve 17, two folded open-ended transmission lines as indicated by the lines 38, 40 and 41, 42. Again these lines are less than a quarter wavelength long at the operating frequency in a tube constructed according to usual design procedures for such tubes. Thus they appear as capacitors across the terminals 16A, 20A and 16B, 20B, respectively, as indicated by the dotted capacitors 43 and 44. The inner sleeve 17 appears as two inductances between the terminals 18A, 19A and 18B, 19B, respectively, one in series with each transmission line 38, 40 or 41, 42 as indicated by the coils 43 and 44. These inductances are very small relative to the capacity added by the transmission lines. The equivalent circuit of Fig. 7 is shunted across the parallel resonant circuit of the cavities 15 and 16 so that the insertion of the movable sleeve makes the circuit more capacitive. The result is a rapid change in operating frequency as the movable sleeve is moved in and out of the cavity 15. This change may be so rapid that the diaphragm type of seal for the anode cavity, such as that shown in applicant’s copending application Serial No. 456,513, filed September 16, 1954, may be used instead of the much more expensive bellows seal shown in Fig. 1 at a considerable reduction in the cost of the tube.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.