US2474485A - Magnetron oscillator - Google Patents

Description

June 28, 1949.

w. P. MASON MAGNETRON OSCILLATOR Filed Sept. 14, 1944.

IN [/5 N TOR W P. MASON WM ATTORNEY I Patented June 28, 1949 PATENT #G FFI- CE '2,474,485 FMAGNETRON OSCILLATOR WarrenP. Mason,"West Orange, 'N. J., assign'or to -BellTelephoneiiabcratories,'-Incorporated,New

Application.SeptemberlM, 1944,-.Seria.l--No. 554,111

-'6 Claims.

This invention relates to oscillators and more particularly to those'empl ying 'a multiple-resonant structure.

The invention is directed; toiseparating :the several resonant frequenciespfthemultiple resonant system so that oscillations at asinjgl'e definite frequency readily may "be :secured without interference from adjacent resonant frequencies.

In accordance with the invention, a coaxial cyiindrical'type .of resonator is employed and is excited in the'kindrof oscillatory mode characterized by circumferential currents in the cylindrical surfaces. Means are provided 'foribreaking the circumference up into a plurality of antinodal regions'separated by*nodes,'this'being .done by means of'afrequency-selective impedance adjusting device at one point'imthe circumference. The resonating "structure may comprise .two spaced substantially smoothparallel cylindrical conductive "surfaces in opposing relation,'f0rming a continuous. and substantially endless J transmis- 5101110019 or ring, except for the introduction of the impedance adjusting device at one point, which device may be combined with an output connection or load circuitifor the oscillator.

In the drawings,

Fig. '1 shows alongitudinal sectional view of a magnetron oscillator embodying the invention, and

Fig.21shows a cross-sectionalview of the. same.

Referringtoithe drawings, I Uis an anode structure comprising "a perforatedcylindrical .block of conductive material, His a cathode of cylindrical form concentrically mounted within 'the anode structure, and I 2 is a. re action .space between the anode and cathodesurfaces.

To facilitatethe mounting of the cathode and the'making of electrical connections thereto for heating purposesjthe anode Nil maybe'fashioned to provide end spaces and is at either end. The spaces l'5ian'd l6 arepreferably closed'by end plates "i9 and'ZU which may, if desired,be of suitabletferomagnetic materiahtofacilitatethe application of a strong steady magnetic field to the reactionspace l2 by means "of an external magnet or magnetsinwell-known manner.

The cathode I H preferably has a coating of thermionicallyhighly emissive material and encloses a heating winding. The cathode assembly be suppor'ted'by apair df'heater leads "32 and 33. Each heater-lead is .in .turnsupported'by .a tubular glass seal3A.and abushingj35. The bushing has a taperedflaring edge13'6'tothefinner surface of Whichtheg'la'ss may readily be sealed. Anoutput coupling arrangement'is p'rovidedby means of an inductive "loop 31 inserted into a. ra'dial'boresii '(Fig. 2). "Ihei'two ends of the'loop 31 are connected respectively 'to the "inner and outer conductors of a coaxialoutputfitting. 'The outer "conductor of the 'coaxial 'fittingpomprises a tubular member "410 sealed into the hole "3'8 while theiinner conductor 4| m'aybe simply an extensionofthe conductorIforming'the'loop"3l. "A seal 41 may be suitably formed as a bead aroundthe inner conductor.

For convenience in:pumpin g out'the structure to the desired degree ofivacuum, a small hole "48 (Fig. 1) may be formed .:as "by drilling'into the anode "and a bushing 49, 'similar to .the "bushings 35:may"be fitted aroundthehole '48. The bushing 49, .preferably'includesaflared "and tapered portion v5i] to whichmay besealed a glass ltubeffor use in pumping. The glass tube may be sealed offafter evacuation in theusual manner'tofform a tip .51 as "shown'in. Fig. '1.

The magnetron disclosed is adaptable .to being built on any suitable scaleaccordingto'the frequenc at whichitis desired to operate. In an embodiment suitable 'for operation in the neighborhood of 3,000 .megacycles (10 centimeters wavelength) the anode'block' l0 may" have anoverall length between the end plates ll) vand' z'll of some'2.'6 inches.

'Thereaction space [2 comprises the spacebetween two cylindricalcoaxial.conductive surfaces,

namely, the inner surface I3 of the block .l'fl and the outer surface [4 .of the cathode H. Except for the output chamber '38, there are no side chambers or resonating cavities such as are usually found in the anodeblockof the conventional magnetron, and the cylindrical surfaces are preferably smooth. The coaxial output transmission line 248, 4| is preferably provided with a side branch, the outerconductor of which .may be integral with or conductively attachedto-the outer conductor 40 and .the inner conductor 43 of which maybe conductivelyconnected to the inner conductor 4|. A slidable short-.circuitingbridge 44 may be provided betweenitheconductors 42..and 43 in the side branch. The bridge 44 .may-beadjusted in position-by meansof a-hollow rod .45-and a knob 46. The operation ofthe system of the invention is .in a. general way similar to that of the conventional magnetron witha plurality of resonant side chambers, except that in the absenceof theside chambers, the standing .wave pattern .exists exclusively within the reaction space :I' 2 and to some .degree.in .the output chamber .38. The

typeiof standing waveipattern which mayadvantageously be utilized is one formed as by a pair of waves traveling in opposite directions around the circumference of the reaction space I2, using the surfaces [3 and Ill as the conductors in what is a continuous and substantially an endless parallel cylindrical plate transmission line. Except for the influence of the output chamber 38 as hereinafter described, there is nothing to disturb the radial symmetr of the system and consequently nothing inherent in the structure to define the angular position of the nodes and antinodes of the standing wave pattern around the circumference of the reaction space nor to determine how many nodes and antinodes there shall be.

It is well known that a node may be established at a particular position along a transmission line by locating a suitably designed impedance controlling device such as a side chamber at the position where the node is desired. The output chamber 33 serves in the present invention as such a side chamber and is adjustable in its wave transmission or impedance characteristics by means of the associated branch transmission line 42, 43. By adjusting the position of the bridge M, a node, indicated diagrammatically by a dot-dash line 60 may be located at or near the entrance to the output chmber as shown in Fig. 2. It is possible to set up such a node 653 to be efiective selectively at any desired frequency over a consid" erable frequency band. To obtain a stable standing Wave pattern in the reaction space [2, however, the operating frequency must be such that the circumference of the reaction space [2 may break up into an integral number of subdivisions separated by nodes. The location of the nodes in an illustrative example is shown in Fig. 2, with eight nodes including the node 60 and additional nodes as at 6| and 62. To secure any desired pattern, an adjustment of the bridge 44 is required whereby the node 50 is established opposite the output chamber 38 with reference to an operating frequency Which is consistent with the standing wave configuration wanted. Other adjustments of the bridge 44 than that illustrated may be found in which the node 60 appears opposite the opening of the output chamber 38, with a different number of nodes formed around the reaction space I2.

In the conventional magnetron of the type having a plurality of side chambers communicating with a central reaction space, the side chambers operate as individual resonant circuits coupled together through the reaction space. The system as a whole then shows the general operating characteristics of a band-pass filter. Accordingly, multiple resonances appear which are generall as numerous as the side chambers. The multiply resonant frequencies are all included Within the transmission band of the structure considered as a filter. In general this means that the resonant frequencies are relativel close together. Trouble often arises due to uncertainty as to Which of the several resonant frequencies will be supported in sustained oscillation when the magnetron is operated. Also, it may occur that the magnetron will pass suddenly and unaccountably from one frequency of oscillation to another. The endless transmission line, loop, or resonant ring which forms the resonant system in the present invention, does not have a band-pass filter type of characteristic but instead its resonant frequencies are harmonically related depending upon the number of subdivisions into which the standing wave is divided. Such harmonic frequencies are in general much more widely separated in frequency than the resosaid resonator,

4 nances in the band filter structure. The actual resonant frequency may be determined :by the setting of the bridge 44 which, as above described gives a frequency-selective control whereby the node 60 is located opposite the output chamber 38 for one particular operating frequency only. Therefore, the operating frequency to be expected may be predetermined and there is substantially no tendency for the system to jump from one operating frequency to another. The system of the invention has the further advantage that, unlike the magnetron with a fixed number of side chambers, permanently incorporated in the anode, the number of nodes may be controlled by means of the bridge 44 and accordingly, the operating frequency may be selected at will from a considerable number of harmonically related resonant frequencies.

Except for the physical configuration of the resonator, the devic disclosed is similar to currently preferred types of magnetrons and it operates on substantially the same principle so that it is thought unnecessary to describe the operation in any further detail, the same being already familiar to those skilled in the art.

What is claimed is:

1. An oscillator comprising a pair of opposed substantially smooth coaxial conductors forming a substantially uniform continuous transmission line resonator in the space between said conductors, a side branch transmission line opening out from said resonator and forming a sole principal node-determining irregularity in the said resonator, and a side branch transmission line of adjustable length opening out from said first-mentioned side branch transmission line at a distance from the said resonator of the order of magnitude of a wavelength of a harmonic oscillation in said resonator.

2. An oscillator comprising a pair of opposed substantially smooth coaxial conductors forming a substantially uniform continuous transmission line resonator in the space between said conductors, and a side branch transmission line opening out from said resonator and forming the predominant irregularity in the symmetry of the said resonator, said side branch transmission line presenting a predesigned impedance to the said resonator at a desired harmonic resonant frequency of said resonator, said impedance determining a node in the oscillations of said resonator at the opening of said side branch transmission line.

3. An oscillator comprising a pair of opposed substantially smooth coaxial conductors forming a substantially uniform continuous transmission line resonator in the space between said conductors, and a side branch transmission line opening out from said resonator and forming a sole predominant irregularity in the symmetry of the said side branch transmission line having a portion of critical length for determining the impedance presented to the said resonator by the said side branch transmission line, said impedance having a value necessary to produce a node in the oscillations of the said resonator at the opening of said side branch transmission line for a harmonic oscillation of said resonator.

4. A harmonic oscillator comprising a pair of opposed substantially smooth coaxial cylindrical conductors forming a substantially continuous transmission line resonator, said conductors comprising an anode and a cathode, a side branch transmission line opening out from the space between said coaxial conductors and forming the main node-determining irregularity in the symmetry of the said resonator, said side branch transmission line being tuned to resonate a harmonic oscillation of said resonator.

5. A harmonic oscillator comprising a pair of opposed substantially smooth coaxial cylindrical conductors, additional conductors attached to the outer of said coaxial conductors to form a cavity resonator including the space between said coaxial conductors, said cavity resonator having an aperture in the outer coaxial cylindrical conductor portion thereof, said aperture constituting a sole prominent node-determining irregularity in the symmetry of the said resonator, and said side branch transmission line being tuned to a harmonic oscillation of said resonator.

6. A magnetron for generating harmonic oscillations comprising a pair of opposed substantially smooth coaxial cylindrical conductors forming a substantially uniform continuous transmission line resonator in the space between said conductors, one of said conductors constituting a cathode and the other of said conductors constituting an anode, means for maintaining a steady magnetic field with lines of magnetic force 6 parallel to the axis of said coaxial conductors, a side branch transmission line opening out from said resonator and forming a sole principal nodedetermining irregularity in the symmetry of the said resonator, and means in said side branch transmission line for adjusting the length of said side branch transmission line to tune said resonator to produce oscillations in said magnetron at a harmonic resonant frequency of said resonator.

WARREN P. MASON.

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

UNITED STATES PATENTS Number Name Date 2,009,368 Usselman July 23, 1935 2,030,178 Potter Feb. 11, 1936 2,108,830 Skellett Feb. 22, 1938 2,108,900 Peterson Feb. 22, 1938 2,129,713 Southworth Sept. 13, 1938 2,241,976 Blewett et al May 13, 1941 2,410,840 Samuel Nov. 12, 1946

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