US3028522A - Magnetrons - Google Patents

Description

Unite This invention relates to magnetrons and more particularly to modulation of segmental type magnetrons. This application is a continuation of my application Serial No. 73,345, filed January 28, 1949, now abandoned.

As is known to those skilled in the art, a magnetron is a diode vacuum tube electronic discharge device which may be used, for instance, as an oscillator to produce radio frequency energy for transmission, for example, from an aerial at very high frequencies which may be of the order of 20,000 megacycles per second or higher. By way of comparison, normal broadcast frequencies are in the neighborhood of l megacycle, and present day frequency modulation broadcast is at about 100 megacycles. The magnetron comprises a cathode along an axis and connected to the negative side of a direct current power source and an anode structure disposed around it in spaced relation and concentric to its axis and connected to the direct current pos-itive side. A cylindrical vacuum envelope, part of which may be the anode, surrounds the cathode and anode structures. The magnetron operates in the field of a magnet between the poles of which the vacuum envelope is placed so that the magnetic field is parallel to the axis of the cathode. The electrons which are discharged from the cathode toward the anode by thermionic emission are affected by the fields of the magnet and of the direct current voltage so that a cloud of electrons spins around the cathode. individual electrons in this cloud migrate either to the anode or back to the cathode. Under proper conditions, if radio frequency voltage of a suitably high frequency is present in the space between the anode and cathode, known as the interaction space, these electrons which migrate to the anode deliver energy to the radio frequency field from the direct current field. These electrons which would otherwise withdraw energy from the radio 'frequency field are returned to the cathode and thus removed from the interaction space. Thus the tube acts as a generator of radio frequency energy by converting direct current to radio frequency energy. Alternating currents or oscillations of very high radio frequency are set up in the elements of the anode structure which may then be drawn olf and transmitted or used as radio wave output for any use desired.

There are many forms of anode structure in magnetrons which distinguished one type from another and this invention is particularly applicable to segmental type magnetrons such as the so-called split anode, slot and hole, vane, rising sun, interdigital and other similar types of magnetrons classified according to anode structures.

The anode structure is that portion of the magnetron whose purpose is to provide the necessary mechanism of interaction between the means Aof storage of radio-frequency energy and of radio frequency generation. in some types of anode structures-eg. the hole-andslot, the vane, and the rising sun types-the means of storage of radio-frequency energy is incorporated in the anode structure itself. In others-eg. the split anode and the interdigital types part of the energy storage means may be provided externally to what is commonly designated as the anode structure. All these structures have in common the feature of presenting to the electron cloud a number of surfaces each disposed over a narrow arc about the axis of the magnetron and extending longitudinally along an imaginary cylinder' of limited axial length fic which provide the mechanism of interaction between the energy storage means and the electron cloud. These surfaces are also used to provide the necessary means of application of the radial direct-current electric field. All of these surfaces receive electrons; and it is on all of these portions on which the magnetron operation deponds, which are here collectively included in the anode structure. Thus in the split anode and the interdigital types all the separated electron-receiving anode elements which are collectively necessary for the operation of the magnetron as an oscillator are collectively included in the anode structure.

In many applications in the use of magnetrons it is desirable to control the radio frequency oscillation output of the magnetron oscillator to hold said output automatically at any predetermined frequency to narrow tolerances, and it is also desirable in many applications to change .or vary the frequency of the output in such a manner that it will carry intelligence, for instance, a signal or voice transmission either in the manner known as frequency modulation or in the manner known as phase modulation.

Various structures and methods have been devised for controlling the frequency of a magnetron. In general, these may be classified into two groups. In one group, one or more elements of the tube is physically moved to cause the change in frequency. This method is limited by the difficulty of making the part move with a high rate ,of acceleration. Practically, it has been found diiiicult to use any of the methods of this group for modulation at greater than audio frequencies, for instance, up to ten kilocycles per second.

The other group employs some method of electrical control. While this is theoretically much to be preferred, the methods of this group heretofore known have not been satisfactory. It has been found difficult to vary the frequency over a wide range without using a large amount of modulating power or grossly affecting the amplitude of the output.

' While my invention falls into the second group in that control or modulation is achieved by purely electrical means, it overcomes the difficulties mentioned. Under suitable conditions, the frequency can be widely varied, yet the modulating power is small since the current is negligible. Within limits, the amplitude is not altered greatly.

Accordingly the objects of my invention are to provide a method and device for controlling the frequency of a magnetron by which its output may automatically be held at a predetermined frequency within close tolerances or by which the frequency of the output may be varied in a predetermined manner to carry intelligence and the like, or both. A further object is to provide a magnetron which accomplishes the above objects at relatively high efliciencies and without varying the amplitude of the output frequency substantially. These and other ob-jects will be apparent from the detailed description.

I have discovered that by introducing a new alternating current or direct current control voltage into the resonator space and on the edge of the interaction space between the cathode and anode elements by means of an element which is isolated from the other alternating current or direct current voltages already present in the magnetron while it is operating, I can control or tune, and also vary the frequency of the radio frequency output of the magnetron without appreciable variation in the amplitude of the output.

Essentially, therefore, my invention comprises mechanism and method for modulating a magnetron electrically by frequency or phase modulation.

There are many possible modifications and uses of the mechanism land method of the invention, and the particular forms described and illustrated herein are some illustrative examples thereof which in no way limit the scope of the invention.

In the drawings:

FIG. l is a partial longitudinal sectional view through a vane or rising sun type magnetron including the mechanism of the invention;

FIG. 2 is a sectional view taken along the lines 2 2 of FIG. l; and

FIG. 3 is a transverse sectional view similar to FIG. 2 of a modification of the invention in the same type magnetron.

For purposes of illustration, my invention is herein particularly described and shown as it is adapted to a segmental type magnetron having the so-called rising sun anode structure, which has become fairly well known in the art, one of the basic types of which is illustrated in the drawings and indicated at 10. It is to be understood that I do not claim any of the basic types of magnetrons as my invention. For purposes of simplicity the magnetron is shown without the usual magnet, cathode heater, output window, exhaust tubulation, the cathode seal. The magnetron 10 has a cylindrical oxide coated cathode 12 on an taxis 13 and an anode structure disposed about the axis 13 in such relation to the cathode 12 that there is a space 17, herein called the interaction space, between them all around the cathode 12 across which thermionic emission takes place and in which electrons from the cathode 12 tend to seek the anode 15 during operation. The cathode 12 and anode structure 15 `are enclosed in a cylindrical hermetically sealed vacuum envelope from which all gases have been substantially voided for effective operation. The cathode 12 is electrically connected to the negative side of a direct current electric current source in the usual manner, not shown. The anode structure 15 is likewise electrically connected to the positive side of the direct current source. When the complete magnetron 10 is placed in a magnetic eld, the lines of force of which are parallel to the axis 13 and pass through the space 17, thermionic emission takes place and forms a cloud of electrons which travels around the cathode 12 in the space 17.

The anode 15 has vanes 35 radially disposed from it into the space 17 toward the axis 13 which form alternately deep and shallow radial resonator spaces or slots 36 and 37 between them leading into the circular space 17, an arrangement which has been found to give high eliiciency. When the magnetron is operating the cloud of electrons which spins around in the space 17 generates radio frequency energy which is stored by the radial slots 36 and 37 from which it may be drawn off as radio frequency output through the slot 42 through the envelope 20 and leading into a wave guide 41.

I have discovered that by inserting an electrode S0, hereinafter called the frequency modulation electrode, up to the outer boundary 61 of the interaction space 17, the trajectory of the electrons in the cloud is affected so that the frequency of the radio frequency output may be altered or controlled in a manner to be further described, by which I accomplish the objects of my invention.

The frequency modulation electrode 50 may be in serted between adjacent vanes and is separated from the normal anode structure 15 by a narrow gap 55 which forms a space or capacitative coupling by which the electrode 50 is isolated from the anode 15 for direct current and alternating current control voltages but coupled to it for radio frequency voltages. The radio frequency currents of the anode 15 can, therefore, pass across the coupling space 55 to complete their circuits, but the potential of the electrode 50 may be controlled independently of all other voltages since it is insulated from the rest of the anode structure 15. The frequency modulation electrode 50 may, therefore, be operated at a voltage which is different from that of the rest of the covered that the radio frequency output may be moda-- lated thereby without materially varying the amphtude of the output radio frequency.

I have discovered, also, that the same result may be' accomplished by isolating one or more sectors of the' anode to permit them to operate at a voltage which is different from that of the rest of the anode. In this case' also, provision must be made so that this change of structure does not appreciably interfere with the operation of the anode as a structure that is resonant for the desired radio frequency. FIG. 3 shows such an alternative arrangement. The sector 51 of the anode structure 15 which includes lthe adjacent vanes 35a and 35b is isolated from the rest of the normal anode structure by the narrow gap 52 to form a frequency modulation electrode. The gap 52 allows close capacitative coupling for the radio frequency but does not materially interfere with the application of the modulating voltage on the frequency modulation electrode 51.

By introducing control voltages through the frequency modulation electrode 50 or 51 at frequencies produced by voice sound waves or other intelligence, I then impose the intelligence from the magnetron 1G on a carrier wave at the ultra high radio frequency of the radio frequency output by means of my frequency modulation device.

If I introduce a suitable direct current voltage to the member 50 or 51 and vary the voltages thereon, I can vary and control the frequency of the radio frequency output. By feeding back a small amount of the output into a frequency sensitive device which responds to variations of output frequency and which in turn varies the control voltage on the frequency modulation electrode 50 to alter the output frequency of the magnetron 10, I can maintain it at a preselected frequency within very close tolerances. This can be done by any suitable automatic frequency control system such as will be familiar to anyone skilled in the art, when the control circuit is connected to the frequency modulation electrode 50 or 51.

In order to introduce the and at the same time control voltages or current preserve the proper functioning of the magnetron, the frequency modulation electrode 50 n the examples illustrated, FIGS. 1 and 2, is an electro-conductive plate having a dimension parallel to the axis 13 not more than that of the vanes 35 with suficient thickness for rigidity tapering toward the axis 13 to a knife edge which is spaced from the cathode equal ly with the vanes 35 and supported by a conducting rod 60 leading through the back of the anode and the shell 20, but insulated therefrom.

In the operation of the magnetron 10, it is usual to hold the direct current voltage on the anode 1S at ground level and to supply a large negative voltage to the cathode 12. I have found that frequency modulation of the radio frequency output is best accomplished by driving the control voltages on the frequency modulation electrode 50 or 51 down to or approaching the voltage on the cathode 12, although frequency modulation according to my invention can be accomplished at voltages anywhere between the potentials of the cathode 12 and the anode 15, and, possibly, in a greater range.

Sensitivity of the device for a given control voltage or change of control voltages may be increased for certain purposes by introducing two or more such electrodes 50 or 51 connected to the control voltage in parallel.

Although I do not intend to be held to an explanation of the theory behind the phenomena which occur as a result of my invention, a brief explanation is that as the electrons on the outer perimeter of the cloud travelling around in the interaction space 17 approach the frequency modulation electrode 59 which has a negative voltage, they are deiiected from their normal trajectory inwardly toward the axis 13. Their angular velocity then becomes greater and, at `some later time they arrive back at the anode further advanced than would have been the case had they not been deflected. This has the effect of introducing a voltage wave slightly in advance of normal phase. It will, therefore, have a component in quadrature to the normal wave and, hence, will change its frequency. Since the component in phase with the normal wave is nearly the same amplitude as it would be without deflection, the amplitude is not greatly affected. There are many factors which affect the trajectory of the electrons, and this simplified explanation is merely to help clarify the invention.

The invention is adapted to any segmental type magnetron and is useful for any application in which it is desired to frequency modulate or control the frequency of the radio frequency output of such a magnetron. It opens up many new uses and adaptations for the magnetron, and provides a solution to the very important problem of efficiently utilizing the very high frequency part of the radio spectrum. For example, if it is desired to relay television intelligence, a very high frequency carrier is desirable to avoid distortion. The bandwidth necessary to carry the television intelligence will otherwise be an appreciable fraction of the carrier frequency, o

and, hence, difficult to handle without distortion. On the other hand, no practical means has been found to use amplitude modulation at very high frequencies without very serious trouble from fading Fading is the phenomenon observed that the signal strength at the receiver varies Widely and rapidly with time due to changing conditions, atmospheric and otherwise, between the transmitter and receiver. Hence, frequency modulation is the best answer to the problem. However, prior to my invention, it had not proven possible to frequency modulate any but low power oscillators in the ultrahigh frequency range with suicient deviation and rapidity for fidelity.

Likewise, it is obvious that the transmission of speech or other intelligence requiring only a narrow frequency band cannot eiciently use the high frequency spectrum unless many messages are simultaneously handled or other provision made so that separate transmitters do not need to be operated at too nearly the same frequency to eliminate unused portions of the sepctrum. The problem then becomes identical to that for television intelligence.

Numerous other applications will be obvious to those skilled in the art.

What I claim is:

l. A multi-cavity magnetron including a cylindrical cathode, an anode structure mounted coaxially about said cathode, said anode structure having a multiplicity of segments extending toward said cathode and terminating in electron receiving surfaces arcuately spaced from each other and lying in an imaginary substantially cylindrical surface, said anode segments being everywhere spaced radially from said cathode and providing a free annular electron path encircling said cathode, and frequency control electrode means insulated from said anode structure including an external terminal for application of control potential and including an axially elongated element fixed interposed between an adjoining pair of said anode segments and having a surface element lying in said imaginary cylindrical surface.

2. A multi-cavity magnetron including a cylindrical cathode, an anode structure having a multiplicity of segments extending toward said cathode and having Iaxially elongated arcuately spaced surfaces lying in lan imaginary substantially cylindrical surface coaxially `about said cath- "ing at regular spaced ode, said anode structure having means including said segments constituting a multiplicity of resonators extendintervals all yaround said cathode, and frequency control electrode means including -a fixed blade having an axially extending edge lying substantially in said cylindrical surface `midway between an adjacent pair of said segments and insulated therefrom, said control electrode means further including an external terminal for application of control potential.

3. A 'multi-cavity magnetron including a cylindrical cathode, an anode structure mounted coaxially about said cathode, said anode 'structure having a multiplicity of anode segments extending toward such cathode and having electron receiving surfaces lying in yan imaginary substantially cylindrical surface spaced radially from said cathode and providing a continuous free'annular electron path, and frequency control electrode means insulated from said anode structure and incorporating a resonator having axially elongated side walls having edges fixed in said imaginary cylindrical surface, said frequency control electrode means having a narrow gap from said anode structure so as to be tightly coupled at the oscillating frequencies to adjacent portions of the `anode structure.

4. A magnetron oscillator, having a cathode, a segmental anode structure Vhaving an axis in common with said cathode `and including all those anode segments required for magnetron operation, said segments being interconnected by metallic resonator structure, said resonator structure extending all around said common axis, the anode segments having surfaces exposed to said cathode and defining the anode limit of the cathode-to-anode electron discharge space for establishing a radial electric field and said anode structure being radially spaced from said cathode and providing a continuous free rannular passage with said cathode, and frequency control electrode means having at least one surface element fixed between an adjoining pair of anode segments at the anode limit of the cathode-to-anode electron discharge space, said freqency control electrode ymeans being insulated from said anode structure and having an external terminal for Vapplication of control potential.

5 A magnetron oscillator in accordance with claim 4 wherein said frequency control electrode means includes two surfaces disposed 4at the `anode limit of the electron discharge space, wherein said two surfaces `are interconnected by a resonator, and wherein said anode structure embodies va multiplicity of resonators interconnecting said segments.

6. A magnetron oscillator, having :a cathode, a segmental anode structure having an axis in common with said cathode `and including all those anode segments required for magnetron operation, said segments being interconnected by metallic resonator structure, said resonator structure extending all around said common axis, the anode segments having surfaces lying in an imaginary cylinder defining the anode limit of the cathode-to-anode electron discharge space, said `anode structure being radially spaced from said cathode and providing a free annular electron path all around said axis, and frequency control electrode means wholly outside the said electron path and having la surface element fixed in said imaginary cylinder between an adjacent pair of anode segments and insulated therefrom andhaving `an externa-l terminal for application of control potential thereto.

7. A magnetron oscillator, having a cathode, |a segmental anode structure surrounding said cathode and including all of those anode segments required for magnetron operation, said segments being interconnected by metallic resonator structure, said resonator structure extending all around said cathode, said anode segments having electron receiving surfaces substantially equally spaced radially from said cathode and providing a free annular electron path all around said cathode, and a frequency control electrode insulated from said anode structure and directly exposed to said cathode rand occupying only a narrow angle as measured from said cathode, said frequency control electrode being fixed between an adjacent pair of said anode segments `and immediately adjacent the electron-receiving surfaces thereof but outside said free annular electron path, said electrode having an external terminal for -application of control potential and being effective to introduce a tangential electric field upon application of a potential difference between said frequency control electrode and said anode structure.

8. A magnetron oscillator, having a cathode, a segmental anode structure including all those anode segments required for normal magnetron operation, said segments being interconnected by metallic resonator structure and having electron receiving surfaces disposed in an imaginary cylinder having a common axis with said cathode, whereby a substantially radial electric field may be established in the cathode-to-anode space during operation by a difference of potential between said cathode and said anode structure, said anode segments and said cathode being radially spaced and providing an annular electron path all around said common axis, said resonator structure extending all around said axis, and a frequency control electrode of narrow arcuate extent yhaving a surface element disposed in said imaginary cylinder, said frequency control electrode being fixed outside said annular electron path between an `adjacent pair of anode segments and being insulated from said anode structure and having an external terminal, whereby a potential difference can be established in operation between said frequency control electrode and the adjoining pair of said anode segments for introducing a tangential electric eld at the anode limit of the cathode-to-anode space.

9. A magnetron oscillator, having a cathode, a segmental anode structure comprising `a multiplicity of segments having electron receiving surfaces disposed longitudinally in an imaginary cylinder which cylinder has an axis in common with said cathode, said surfaces being everywhere spaced radially from said cathode and providing a free annular electron path all around said axis, and said segments being arcuately spaced from each other, said segments being interconnected by metallic resonant cavity structure, said resonant cavity structure extending all around said axis, `and a frequency control electrode having an elongated surface element fixed longitudinally in said imaginary cylinder and interposed between and spaced arcuately tfrom an adjoining pair of said anode segments, said frequency control electrode having electrical insulation separating it from said cathode and said anode structure and having an external terminal at which a frequency control potential may be applied.

10. A magnetron oscillator according to claim 9, wherein said frequency control electrode embodies two elongated surfaces interconnected by a resonator, and wherein said frequency control electrode is closely coupled capacitatively to said anode structure.

l1. A magnetron oscillator according to claim 9, wherein said frequency control electrode is in the form of a blade extending axially and midway between a pair of adjacent anode segments.

12. A symmetrically arrayed multi-cavity device having a cylindrical cathode, a cavity-dening anode structure formed coaxially of and radially spaced from said cathode, and a fixed input frequency control electrode extending from a point externally of said anode structure through said anode structure and terminating at a point spaced equally with said anode structure from the cathode.

13. A symmetrically arrayed multi-cavity device having a cylindrical cathode, a cavity-defining anode structure formed coaxially of and radially spaced from said cathode and defining an interaction space therebetween, and a lxed input frequency control electrode extending from a point externally of said anode structure through said anode structure and terminating at the outer edge of said interaction space, said electrode being insulated from said anode structure.

14. A symmetrically arrayed multi-cavity device having a cylindrical cathode, a cavity-defining anode structure formed coaxially of and radially spaced from said cathode and defining an interaction space therebetween, said anode structure having a radial opening therein, and a fixed input frequency control electrode extending through said opening from a point externally of said anode structure to the outer edge of said interaction space, said electrode being insulated from said anode structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,152,035 Fritz et al. Mar. 28, 1939 2,463,512 Brown Mar. 8, 1949 2,498,763 McNall Feb. 28, 1950 2,504,187 Derby Apr. 18, 1950 2,582,185 Willshaw Jan. 8, 1952 2,822,504 Litton Feb. 4, 1958

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