US2742570A - Magnetron oscillators - Google Patents

Description

April 17, 1956 w. E. wlLLsHAw MAGNETRON OSCILLATORS 5 Sheets-Sheet l Filed Feb. 26, 1952 FIG. 3.

INVENTOR h/l/ L/qM ERNEST M//U-SHYV TTORNEY April 17 1956 w. E. wlLLsHAw 2,742,570

l MAGNETRON OSCILLATORS Filed Feb. 26, 1952 5 Sl'leecs-Sheeil 2 3 ll l 2 FIG 2 I6 INVENTOR M//LLm/vf ERNEST l'V/L/.s/qw

HTTORNEY April 17, 1955 w. E. wlLLsHAw 2,742,570

MAGNETRON OSCILLATORS Filed Feb. 25, 1952 5 SheetS-She. 5

l -c l 1 B M 1:/

FITTORNEY UnitedStates Patent C MAGNETRON oscrrLArronsl f William Ernest: Willshaw,.Kenton,v England, assigner to The M70 Valve. Company Limite.dLondon, Englandv Application February-26, 1952, Serial No. 273,430

Claims, priority application. Great Britain February 28, 1951' 8- Claims. (Cl, Z50-36) This.. invention relates to magnetron oscillator devices of the multiple resonant cavity type. comprisingametal block anode formedwith a cylindrical electron orbit hole withinwhich lies, a therrnionic cathode and having a p1urality of resonant cavities recessed intoy the metal block, the cavities being spaced round the cathode and opening on to.; the electronorbit hole. A

Such` magnetron devices are adapted to.. co-operate. with magnetic'elds.` forv the generation, of electrical oscillations and are. especially.l suitable, for the generation of oscillationsv of; very high frequency. ln some cases it is desirable toV arrange that the frequency ofthe oscillations generatedcanbe varied during operation ofthe oscillator, and mechanical tuning means. for effecting such frequency changes are well know-n; meansy for effecting the changes electronically have also been usedv or proposed, such means in general comprising means for producing a stream or cloud of electrons. with which the electromag; netic. teldof theresonant system of the magnetron oscillator can interact, the interaction resulting in a change in the operating frequency of the oscillator.

The object of the invention is tov provide any improved form of magnetron oscillator device of the type specified provided with means for effecting an electronic control of the operating frequency of the oscillator.

According to the invention a multiple resonant cavity an auxiliary thermionic cathode located within one of the said resonant. cavities and is provided with leads suitable for energizing andl supplying operatingl current to the auxiliary magnetron diode formed by the co-operation of the metal block anodeand said auxiliarycathode.

We have found that with the construction in accord ance with the invention, when the magnetron oscillator is operated under the normal conditions with the auxiliary diode not energised, the normal operating frequency is obtained and there need be little loss of output power occasioned by the presence of the diode; but if the auxiliary diode is then energised with anode-cathode voltage difference below the cut-off value for .such a diode, the operating frequency ofthe oscillator is decreased and the frequency shift produced can be varied by increasing or decreasing the current taken by the diode. We believe that. this frequency dependence is due to the formation of an electron cloud aroundl the auxiliary cathode ofthe diode, which cloudy acts as a rellector to electromagnetic waves imp/inging: on its surface and modifies the .electrostatic capacity of the resonator system; variation of the diode current results in a change in the cloud size and hence in the electrostatic capacity of the resonator system, withresulting change in the frequency of operation of the oscillator.

The change in `operating frequency .of the lmagnetron oscillator is accompanied by some loss .of Eoutput power.,

f but we have found that-by-careful design the lossl of power can be limited to relatively small values, .for example vless than forminirnisingvloss of power itis `,de-

l magnetron oscillator device of the type specified includes` Y `and 2,742,570 Patentedv Apr. 17, l9 5 6 ICC sirableI for the uniformity ot the magnetic lieldi with which the magnetron'. oscillator device co-operatesA to extendi not only across, the electron orbit hole but also? to the cavity. includingY the aumliary. cathode, and.` for this reason: magnetic pole-pieces of somewhaty greater area thanv usual` are desirable withv the arrangement in accord.- ance with the invention; it is alsodesirable that the auxili- Viary cathode shoulda' bey accurately centr-.odin the cavity Vin Y which itis. situated' and` that itshould= be slottedy to mini.-

anode with their axesfparallel to. thev axis of: the electron orbit hole, and the auxiliary thermionic cathode provided in accordance with the invention is then preferably. also cylindrical and arrangedf with its axisv parallel tothe axis off the electron orbit hole. Where the axiliary cathode lieawithin a part of a-v cavity which isf substantially of circular cross-section, the auxiliary cathode isl preferably also of. substantially circular cross-section and coaxial therewith.

The manner of operation of the auxiliary diode in. a magnetron oscillator device in accordance with the invenf tion will. depend on the useto which it is to be put. Thus, for example, iiY a simple shift from one value to another in the operating frequency of the Voscillator is required, the auxiliary cathode may be merely arrangedl to be connected to theanodethrough a switch, closingv of thel switch'in. operationv of the-*oscillator connecting the auxiliary cathode directlyA tof the anode, .thus rendering; the auxiliary magnetron diode operative and reducing the operating frequency of thevoscillator; for obtaining'an increased frequency drop it may be arranged that the diode current is'. increased by underloading the magnetron oscillator and/ or by increasing the anode-cathode voltage of the diode, but this will also further decrease the output power. v Y

It will be appreciated` that the frequency changes obtained with a magnetronV oscillator device in accordance with the invention are related to variations in the other parameters of the oscillator and to the operating conditions of the diode and that all such factors must be taken into account and may be taken advantageof in applying the magnetron oscillator device to any particular use.

The invention will be further described with reference to one particular embodiment, ywhich is given by-'wayof example and is illustratedy in the accompanying drawing iny which Y 'Y Figure l shows a perspective view of the magnetronv oscillator device, partly cut away to reveal the internal details, Y

Figure 2 shows an end View of thedevicewith oneend plate removed, f f

Figure 2(11) shows a' detail of the reverse side of Figure. 2, Figure 3 shows a section'in the plane A-A of Figure 2,

Figures: 4, 5. and 6 show schematically different circuit arrangements for the v.operation .of the magnetron oscillator. device shown in Figures l to 3.

-Referringniow .to Figures 1 to 3, the magnetron oscillator device shown ycomprises a cylindrical copper block 1 recessed from .each end face 2 to leave a .central part-i tion ,3. which .forms the'anode proper, this partition being perforated by .a cylindrical hole 4 which forms 'the electron .orbit hole and .by .eight resonantwcayities (5, .6) which .open into the Velectron orbit holeand are spaced symmetr ally round its. axis.; .each .cavity is in .the form 'of a cylindrical hole. -5; haras its axis parallel to, the axis 3 of the electron orbit hole and communicating with that hole through a narrow radial slot 6.

A cylindrical indirectly heated thermionic cathode 7, provided with circular end discs 8 in the usual manner, is supported within and coaxial with the electron orbit hole by two stout conductors 9 lying one on each side of the central partition and insulatingly sealed through the curved wall of the block through glass-to-metal thimble sidearm seals 10 in the usual way. The cathode is insulated from one of the discs 8 for enabling current to be passed through the cathode heater from the conductors 9, in the usual way. Also one of the resonant cavity holes is ttted with an output coupling loop 11, the lead 12 to which passes along a radial slot extending from the said cavity hole to the exterior of the block and enters into a glass-to-metal thimble sidearrn 13 closing the slot, the end 14 of the lead being sealed through the glass thimble closure 15 of the sidearm in known manner.

Alternate anode segments between the resonant cavities are strapped together on each side of the anode par tition 3 by wire coupling straps 16 in a known system of strapping utilising two straps on one side of the block, as shown in Figure 2, and four straps on the other side of the block, as indicated in Figure 2a, in which only the details required to indicate the strapping have been shown. The ends of the metal block are closed by circular copper end plates 17 attached to the block by gold washer seals in known manner (one of the plates being omitted in Figures l and 2), and metal cooling tins are soldered to the exterior of the block in the completed device; these ns have been omitted from the drawings in order not to obscure the other details of structure.

The construction so far described does not differ from a known construction for a magnetron oscillator device adapted to operate at a wavelength of say, 10 cms. In accordance with the invention, however, one of the resonant cavities is provided with an auxiliary thermionic cathode, and for convenience this cavity is chosen as the next-but-one to that containing the coupling loop. A cylindrical indirectly heated auxiliary thermionic cathode 18, also provided with end discs 19, is supported coaxially within the circular hole part of the said cavity by two stout conductors 20 lying one on either side of the central anode partition 3 and insulatingly sealed through the adjacent curved wall of the block through glass-to-metal thimble sidearm seals 21 similar to, and lying at right angles to, the sidearm seals 10 for the main cathode, the conductors 20 providing the cathode and heater leads for the auxiliary cathode in the same way as the conductors 9 provide the cathode and heater leads for the main cathode.

The auxiliary thermionic cathode consists of a hollovl tube 22 within which lies the heater 23, the outer surface of the tube being coated with electron emissive material (not indicated) in the usual way, and the tube is cut by a narrow slot 24 (see Figure l) extending the whole length of the cathode tube parallel to its axis. The tube 22 is insulated from one end disc 19 for enabling current to be passed through the heater 23 from the conductor 20.

In one form of this construction, designed for normal operation at a wavelength of about 10 centimetres, the magnetron block had the following dimensions:

Mms.

(a) Diameter of electron orbit space 16 (b) Axial length of electron orbit space 20 (c) Diameter of main cathode 6 (d) Radial depth of the parallel-sided slot part of the resonant cavities 3.3

(e) Width of said slot part (d) 2.2

(f) Diameter of the circular hole part of said resonant cavities 10 (g) Diameter of auxiliary cathode 5 When this magnetron oscillator device was pulse operated in a magnetic field of 1735 oersted, produced between polepieces of 3 inches diameter, the anode to main cathode voltage being 17,000 volts, the anode current being 25 amps, the pulse repetition rate being 500 per second and the pulse duration being l microsecond, it was found that with the auxiliary cathode not energised and left oatingf the nominal output frequency was obtained and the efficiency was about 63%. When the auxiliary cathode was connected directly to the anode the output 'frequency decreased by about 2 mc./s. and the efciency fell by about 5%.

When the frequency shift was increased to 5 mc./s. by increasing the diode current by applying a biassing voltage between its anode and cathode, the efficiency fell by about a further 11%.

This use of the magnetron oscillator device is illustrated schematically in Figure 4 of the accompanying drawings, in which the device is represented only by the anode block 1, and in which for simplicity the heating circuits for the main and auxiliary cathodes, and the means for applying the magnetic eld, have all been omitted.

The main operating circuit for the magnetron device is represented by the modulator M connected between the anode 1 and cathode 7 for the application of negative voltage pulses to the cathode 7, and by the waveguide load W connected to the output coupling loop 11. The auxiliary thermionic cathode 18 is connected to the anode through a switch S, and a source of bias voltage B, which is omitted if a simple switching action only is required.

Alternative ways of energising the auxiliary diode are illustrated in Figures 5 and 6, in which parts corresponding to those in Figure 4 are indicated by the same reference numerals as in that gure.

In the arrangement of Figure 5, the auxiliary thermionic cathode 18 is connected to the negative terminal of the modulator M through the switch S and a resistor R, which may be for example of about 14,500 ohms resistance for a magnetron device having the dimensions and operating conditions quoted.

In the arrangement of Figure 6, the auxiliary cathode 18 is connected through the switch S to the junction of a resistor (TiTz) connected between the output terminals of the modulator M; for a magnetron device having the dimensions and operating conditions quoted, the values of T1 and T2 may be, for example, respectively 6950 ohms and 6000 ohms.

In some circuit arrangements it may be desirable to maintain the auxiliary magnetron diode introduced in accordance with the invention permanently operative. Thus with a magnetron device in accordance with the invention as shown in Figures l to 3, if the oscillator anode current is varied, it is found that over a wide range of at least the higher values of oscillator anode current the frequency shift resulting from the presence of the diode decreases as the anode current increases. In pulsed operation this effect may be obtained with either of the arrangements shown `in Figures 5 and 6 by shorting out the switch 5 and varying the modulator output voltage, the frequency shifts, and the changes in the frequency shifts, being different in the two cases.

Such variations of the frequency shift can be arranged to reduce to a considerable extent the frequency pushing experienced with magnetron oscillator devices of the type specified in operation, that is to say the change in operating frequency which occurs when the oscillator anode current is varied from a given value, as is sometimes required. Thus if the operating frequency of a magnetron oscillator device of the type specified not provided with an auxiliary cathode in accordance with the invention is plotted against the oscillator anode current, it is found that the curve exhibits a maximum, the frequency decreasing with variation of the anode current ineither `direction from a particular value ofanode current. But when provided with an auxiliary cathode forminga magnetron diode connected asv shown inFigure or Figure 6 with the switchesl shorted out andthe resistor R,or resistors Tiand T2, of suitable resistance, it can y,

be arranged that for .values of the oscillator anode current above the said particular value, the change in the frequency shift with change in the `oscillator anode current compensates for the frequency pushing elect` to a large extent.

For operation of the magnetron oscillator device in accordance with the invention below the said particular value of oscillator anode current, a biassing control of the diode anode-cathode voltage is required to ensure that the diode current decreases as the oscillator anode current increases; in pulse operation this may be effected Y by applying a reversed polarity pulse to the diode simulthe oscillacathode being disposed within one only of the saidV resonant cavities, and leads to the auxiliary thermionic cathode for energizing and supplying operating current to the auxiliary magnetron diode formed by the cooperation of the said anode and. said auxiliarycathode. said auxiliary cathode being slotted along its length for reducing the induction of circulating currents therein in the operation of the device.

2. A magnetron oscillator device accordingto claim 1 wherein the resonant cavities are vformed by cylindrical slots traversing the metal block anode with their axes parallel tothe electron orbit hole, .and the auxiliary p thermionic cathode is alsocylindrical and arranged with its` axis parallel to the axis of the electron orbit hole.

3. A magnetron oscillatorv device according to claim 2 wherein the auxiliary cathode is of substantially circular cross-section and is arranged coaxially within a part of a resonant cavity which part is of substantially circular cross-section.

4. An electric circuit arrangement comprising a magnetr'on oscillator device in accordance with claim l in combination with means for energising the main magnetron oscillator, and means for energising the auxiliary magnetron diode. at below the cut-oik voltage for the l diode, the last said means includingr switch means for alternatively making and breaking the auxiliary magnetron diode anode-cathode circuit forl varying the operating frequency of the magnetron oscillator.

`5. An electric circuit arrangement according Vto claim 4 comprising a pulse modulator connected between the anode and cathode of the magnetrony oscillator device for the application of high voltage operating pulses thereto, and wherein the said auxiliary thermionic cathode is connected to the output ofY the pulse modulator through the said switch means and through a resistance, for the appli-v cation'of the voltage pulses with reduced amplitude between the anode and cathode of the auxiliary magnetron diode when said switch means is closed.

6. A ymagnetron oscillator device according to claim 1V wherein the auxiliary thermionic cathode is slotted parallel to its longitudinal axis along the whole of its length.

7. A 'multiple resonant cavity magnetron oscillator device comprising a metal block anode having a circular cylindrical electron orbit hole, a main magnetron cathode in said orbit hole, said anode having a plurality of resonant cavities recessed into the block, the cavities being spaced around the main cathode and each consisting of a circular cylindrical slot whose vaxis is parallelvto the axisv of 'the i electron orbit hole and a r'adialslr'ot narrower-,than the diameter ofthe cylindrical slot connecting the latter to the electron vorbitl hole, a single auxiliary thermionic cathode located wholly within a circular cylindrical slotl part ofk only one resonant cavity, and leads to the auxiliary thermionic kcathode' for energizing andsupplying operating current tothe auxiliary magnetron Adiode formed by the cooperation of said anode and and said auxiliary cathode. 8. A magnetron oscillator device according to claim .7 wherein the auxiliary thermionic cathode is circularly cylindrical and coaxial with the circular cylindrical slot part of the resonant cavity in which it is located.

References Cited the le of this patent UNITED S'IA'l-ES PA'IENTSy 2,446,531 ADerby Aug. l0, 1948 2,504,739 Shoupp Apr. 18, 1950 2,617,079 McNall Nov. 4, 1952

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