[1]Patented Apr. 15, 1952 29593,067 UNITED STATES PATENT OFFICE 2,593,067 HIGH-FREQUENCY APPARATUS Percy L. Spencer, West Newton, Mass., assignor to Raytheon Manufacturing Company, Nlewton, Mass., a corporation of Delaware Applica-tion February 13, 1947, Serial No. 728,261 3 Claims. (Cl. 219-17) 2 This invention relates to high-frequency apparatus, and more particularly to a microwave oven. A microwave oven of the type to which this itivention relates consists, generally, of a hollow 5 metallic enclosure or cavity to the interior of wh,'ch a source of radio-frequency energy, having a wavelengtl-i which may lie in the microwave region of the frequency spectrum, is coupled so as to supply radio-frequency or microwave energy 10 thereto for the purpose of heating an object therein. Often it is desired to increase the radio-ftequency power supplied to the oven; this may be convenie ntly accomplished by coupling the out- 15 put of a second radio-frequency oscillator to ,qaid oven. Also, in circuits in which the radiofrequenc y oscillator is connected for A. C. operation in a self-rectifying circuit, for better balancing of, and for more efficient utilization of, 20 the input transformer, it is desirable to connect a second radio-frequency oscillator to said transformer. kowever, it has been found that certain difficultie s arise when tnvo or more closely similar sources of: radio-frequency energy, for example 25 devices of the so-called magnetron oscillator type, are coupled, by means of radio-frequency transmis.gion lines or waveguides, to the same cavity or enclosure. These difficulties arise from the fact th,,it, when such devices have exactly the 30 @ame output frequency, one of the devices tends to act as a low impedance across the output of the other di3vice, due to slight dissimilarities between them, so that one of the magnetrons tends i,o al)sorb the radio-frequency power of the other 15 one, acti-.ig as an addi.tional load across soid other, irstead of supplying radio-frequency power to the common load in the oven as desired. In o't,her words, one of the magnetrons presents a rather low impedance to the other. Such an un-, 40 favorabl e condition is most noticeable, of course, ,,vhen the two rr@agnetrons are bein.- operated on direct current, in which case they are both energ. @zed continuously; however, this conditio-i tends t,o exist even durin,@ alternating current opera- 45 tion of the magnetrons, when they are energized only on successive half-cycles of the energizing source. since in this situation each magnetron may teiid to supply radio-frequency energy to the other during the half-cycle in which the first 50 is energized r,.tther thaii to the load in the ovep. An object. of this inver-tion, therefore, is to devise a radio-frequency enclosure suppiied by a plurality of separate sources of radio-frequeiley energy in al'i efficient mai-i-vier, 55 Another object is to devise a method for coupling a pair of magnetrons to a sin,,Ip. radiofrequency cavity in such a way that there is no appreciable inloerflow of radio-f,@- equency ener.,y between them. A further object is to devise an eff ective inea@ns for supplying high radio-frequency power to i microwave heating apparatus. The foregoing and other objects of the invention w-A'll be best understood from the fol'Lowing description of an exemplification iliereof, reference being had to the Pccompanyiilg drawin@-,, Nvherein: Fig. 1 is a vertleal sect-Ion throu.-h an oven accordip,@ to niy invention, together wil.-h a diagrammatic representation of a system for fee,,Iin,_2@ radio-frequency energy to the same; l@lig. 2 is a partial horizontal sect-'-on take-,'.I along line 2-2 of Fig. I a,.id looking i,-l the direct-ion of the arrows; and Fig. 3 -is a partial vert4cal section of the oven token alon.- line 3-3 of Fi-g. 1. Now referring to the drawing, a hollow rectangular prismoidal enclosure or cavity I is mad-. of a suitable metal .tnd has rather thin xvalls as shown; enclosure I is adapted to serve as the oven of the heating or coc)king apptratus. A cortainer 2 of food or any other materi,,tl to b--@ heated may b,- placed inside oven 1, said cotita4@ner restinl- on the bottom wall of the oven or being placed in any other suitable positior therein while the food is being cooked. A inetal door 3 is hingedly mounted on the front wall of the ove.-i, this door bein.- adapted to close an .),ccess led in the front metalaperture or opening provl." lie wall of the oven. When door 3 is closed, the --nclosure I is entirely closed, except for a pair of similar openings 4 and 5 in the opposite sidf-, walls of said enclosure; these openiiigs are flrther described hereinafter. Numeral 0 generally designates -an electrondischar.-e device of the magnetron type, ivhich includes, for example, an evacuated envelope such made of highly conductive niateriall, as co-oper, and provided with a plurality ofinwardly-directed, radial@ly-disposed anode vanes B. The arrangement is such that each pair of adjscent anode vanes forrns, together vjith that portion of the envelope lyin.@ therebetiveen, ,i cavity resonator whose natural resonant fi,equeiicy is, as is well known to those sk-illed -@n the art, a function of th-, &,;e6metry of the physical elements making up the same. For the purposes of the present iiivention it is desirable t at the dimensions of each slich cavit@-Y resonator be
[2]2,593,067 3 such that the wavelength of the electrical oscillations adapted to be generated therein, or the output frequency of t.@ie magnetron, has a predet.,rmined value. For example, said wavelengtli ri,iay lie in the microwave region of the frequency 5 spectr-am, which region includes ivavelen.-ths less than about thirty centimeters. . Centrally located in envelope 7 is a highly eleetron-emissive cathode member 9, for example of the well-known alkaline-eartli metal-oyide typ e,. 1,0 said cathode member being provided with conventional means (not shown) for raising the temperature thereof to a level sufficiert for ihermionic emission. The electron discharge device 6 is completed by 15 magnetic means (not shown) for establishing a magnetic field in a direction transversely of the electron path between the cathode and anode . members thereof. Magnetron 6 is energized from a suitable 20 source, in a manner to be describ,,d hereinafter, and when so epergized delivers radio-frequency electromagnetic energy of a predetermined wavelength to a hollow rectangular waveguide . fO through a coaxial transmission line I I which 25 is coupled to magnetron oscillator 6 by loop 12. The central conductor 13 of line I I extends into the interior of waveguide IC near one end thereof, through an opening provided in the center of the Ionger side of said guide, to serve as an ex- 30 citing rod or exciting probe for said guide. The end of guide 10 nearest this excitin@ rod is closed, whili3 the opposite end of said guide ,is fastened to the left s-ide wall of enclosure I by fastenip.- means 1,1 and is open. The in- 35 terior of guide 10 is ulaced in energy-transmitting relationship with the interior of the cavity I by means of a rectangular aperture 4 provided in the left side wall of said cavity, this aperture being of the same size and configura40 tion as the interior of guide IO and being aligned with s-aid guide to place the interior of said guide in commtinication with tle interio I r of cavity 1. Aperture 4 is located in a central position with respect to the front all-d rear walls of oven 1, as shown in Fig. 2, and is preferably 'located above the . horizontal midplane' of the oven. By means of the above-described structure, the radio-frequency output of source or oscil- 0 lator 6 is coupled to the interior of enclosure 1. The exciting rod of guide IO is arranged t6 set up TEoli mode waves therein, as vjill be apparent to those skilled in the art. These waves propagate down the waveguide and 'emanate from the open end thereo'L into the cavity 1. As stated a-bove, for the purposes of increasing the radio-frequency power supplied to oven 1, a second radio-frequency oscillator 16 may be utilized, the radio-frequency output of this second oscillator also being coupl,-d to oven 1. Oscillator I 6 is of the same type as oscillator 6 alid i-iicludes an outer envelope 17, anode vanes IS, and a cathode member 19. The cathodes 9 and 19 of the electron-dis- 65 charge devices 6 and I 6 are conriected, respectively, by conductors 15 and 25, to the opposite terininals of the secondary winding 28 of an input transformer 29, tlie primary winding 30 of said transformer bei-ng connected to a source 70 of raw A. C., for elxample, the conventional 60- cycle power lines. The conductive envelopes 7 and 17 of sa-id electron-discharge devices 6 and 16 are tied together by conductors 31 and 26, which are, in turn, connected, by a conductor 15 4 2T, to a center tap on the secondary iviliding 28 of the transformer 29. The radio-frequency output bf oscillator 16, which has a predetermined wavelength, is coupled to enclosure y means o a coa a ansmission line 21 and a hollow rectangu ar waveguide 20, line 21 being coupled, to magnetron oscil a r 6 loop 2 . Ce a con or of line 21 extends into the interior of waveguide 20 to serve as an exciting rod or exciting probe therefor. The open end of guide 2 0 is fastened to the right side wall of overi I by fastening means 24, and the interior of said guide is placed in, energy-transmi-Iting relationship with the interior of cavity I by means of a rectangular aperture 5 aligned with said guid,e, aperture 5 being similar to aperture 4 and being located in the right side wall oil enclosure I directly opposite aperture 4. With the two magnetrons connected in a circuit as abovedescribed, and with the rbdiofrequency output of both sources c6upled to the interior of the enclosure, the electron-discharge devices 6 and 16 are connected for push-pull A. C. operation in a selfrectifying circuit, whereby said devices alternately del-iver hyper' frequency energy or microwave energy to the common enclosure or 6avity 1. I have found that, if the predetermined natural resonant frequencies of oscillators 6 and 16 are exactly the same, or in other words, if the two oscillators have exactly the same output fr6quency, one of the devices tends to act as a low impedance across the output of the other, so that one of the devices tends to absorb iadiofrequency energy from the other, which absorption is made possible by the radio-frequenc@ coupling of both to the same enclosure. However, I have discovered that if thepredetermined radio-frequency output wavelengths of the two devices 6 and 16 differ appreciably, this tendency of one o'L the magnetrons to ab@6rb radio - frequency energy or radio - frequenc'y power from the other may be reduced to a level that is inappreciable. As a result, an inappreciable amount of such power is shunted away from the load. I have found that when the predetermined wavelengths of the two mag@netron oscillators are different from each othpri as above explained, each magnetron will @rbsent to the other an impedance which is sufflciently high to prevent any appreciable absorption of radio-frequency energy by either oscillator from the other, or to preven'u any appreciable transfer of radiofrequency power from one magnetron to the other. Expressed in :another way, wh6n the output freaueheies of the two magnetron oscillators a,@e siifficiently different from each other, the impedances of the tw6 oscillators are also sufficieritly different from each other to prevent any appreciable transf6r of radio-frequency power from one to the bthet. As an example of tne output frequeiicy difference above alluded to, I have found that at fiequencies on the :ord6r of 2450 megacycle@ (frequencies o,.i tl-iis order lie within the microwave band), there should be ja frequer@ey difference o-n the order of 25 to 50 megacycles betweeh the two magnetrons, in order for tv,,o -Lnagnetrons or radiofrequency oscillators to oderate effldiently, according to this invention, when cou@led to the same metallic -cavity or ericlosu@e. It will be hoted froni all@ of the foregoing , tli'a't I have devised a siniple yet 'effective means . foicoupling a pluialit@ of sources of radio-fr6@uericy
[3]2,593,007 5 energy, such as magnetron osciUat46rs, to a single radio-frequency cavity, in such a way that there is no appreciable interflow of radio-frequency energy between them. of course, it is to be understood that this invention is not lirnited to the particular details as described iabove, as many equivalents will suggest themselves to those skibed in the art. For example, when the two magnetrons are being operated on alternating current, it is possible to use a single or common waveguide for coupling the radio-frequency output of the two magnetrons to the oven. Also, although in Fig. 1 alternating current operation of the magnetrons is disclosed, my invention is equallyapplicable to direct current operation of the magnetrons. In fact, as stated above, the difficulties existing, when two equalfrequency magnetrons are operating on direct current, are even greater than when the magnetrons are operating on alternating current. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art. What is
