US2480682A - Microwave heating apparatus using circularly polarized horn - Google Patents

Description

Aug. 30, 194l K. J. STIEFEL MICROWAVE HEATING APPARATUS USING CIRCULARLY POLARIZED HORN Filed Sept. 2l, 1946 2 Sheets-Sheet l Aug. 30, 1949. K. .l. STIEFEL MICROWAVE HEATING APPARATUS USING CIRCULARLY POLARIZED HORN' 2 Sheets-Sheet 2 Filed Sept. 2l, 1946 Patented Ang. 3o, 1949 2,480,682

menage v.,Kaujl J. Sxtiefel, Waltham, Mae-rs1, assignnorjto Rayfrom which it f onovie ehe 156e) 5) S bstituiing for cos u and sin u in (4),

enogeoffenefoinfinie? if 'one 'fwo u ereine'noeryigeofnetfy inwnion *one noffoffeqierrernnmoofe, offif ineffrn' shown, said device consisting of a hollow rectangular metallic wave guide I open at one end and having a horn coupled to said open end, said horn comprising a hollow metallic transition member 2, open at both ends, and a hollow substantially square metallic wave guide 3, also open at both ends. The guide 3 is rotated substantially 45 with respect to guide I, so that the walls of guide 3 lie at an angle of substantially 45 to the respective walls of guide I, the center lines of said guides being collinear (or said two guides being joined axially to each other) and the transition member 2 joining the open end of guide I rigidly to the contiguous end of guide 3; guides I and 3 are of constant cross-section throughout their lengths while member 2 has a constantly varying cross-section throughout its length, in accordance with its transition function. The opening at one end of transition member 2 has the same area as the open end of guide l and matches said open end, while the opening at the other end of said transition member has the same area as the contiguous open end of guide 3 and matches said open end. One crosssectional dimension ci guide I may, for example, be twice the other; for example, guide I may be a standard one-and-a-half inch by three-inch guide.

A coaxial line 4, consisting of an inner conductor 5 supported coaxially in an outer hollow conductor 6 by dielectric spacing means such as 7, has its outer conductor 6 attached to the exterior centralportion of one of the wider side walls of guide I' near the closed end of said guide, the coaxial line extending at right angles to said side wall. Inner conductor 5 projects through an opening in said side wall into the interior of the guide I for a substantial distance, for example, approximately two-thirds of the distance to the opposite side wall. The free end of coaxial line Il is adapted to be connected to the output pipe or lead of a source of high-frequency energy, for example, a magnetron.

The end of inner conductor 5 projecting inside the guide, which may for convenience be termed .f

the exciting rod, is spaced a distance of a quarter wavelength (in the guide) from the closed end of said guide. When a high-frequency continuous wave, having a wavelength on the order of l0 centimeters for example, is supplied to guide I by means of line 4, a wave having a TEM mode is set up in said guide. With such a wave, the electric vector E, at the output end A of guide I, is perpendicular to the longer side walls, or the wider faces, of said guide, so that the wave produced at A is linearly polarized; this linearlypolarized wave is applied to the input end A of transition member 2. Due to the substantially 45 rotation of guide 3 with respect to guide I, at the output end B of transition member 2 (or the input end B of guide 3) vector E, which at A was perpendicular to the wide faces of rectangular guide I, now appears across diagonal corners of the substantially square guide 3. Of course,

because guide 3 is not exactly square, variation walls of said guide and the other, Ey, perpendicular to the other pair of opposite walls. Therefore, the iirst step in converting a linearly-polarized wave into a circularly-polarized wave, which is to split the linearly-polarized wave into two equal components at to one another, has been accomplished when the wave reaches B.

Two vectors of equal amplitude 90 apart in space exist at the output end B of the transition member or wave splitter 2; however, vectors Ex and Ey are still in time phase and the resultant is a linearly-polarized wave, since Equation 7 degenerates into that of a straight line when 6:0. The only other condition to satisfy for circular polarization is to make =90 or 270, either value reducing (7) to the equation of a circle.

In the discussion above, the nearly square guide 3 has been assumed square for all practical purposes in saying that the wave splits into two equal components. Actually, the magnitude of the two components will be slightly different, but not enough to be more than a second-order` effect; the reason for making guide 3 not quite square will become apparent.

The wave guide 3 as seen by the component Ex is alittle different in width from that seen by Ey. Therefore, their velocities of propagation are somewhat different, and in the passage from B down the nearly square guide 3 the phase angle between EX and Ey progressively departs from zero and the wave becomes elliptically polarized; at some point C, the phase diierence reaches 90 and the resultant wave is substantially circularly polarized. The substantially square wave guide 3 is terminated at point C whereby the radiated wave is substantially circularly polarized.

Of course, it is impossible, as a practical matter, to produce exact circular polarization of the wave at point C. This can be seen from the fact that, in order for the component vectorsV Ex and Ey to be of equal amplitude, wave guide 3 must be of exactly square cross-section; however, if guide 3 is exactly square, the phase angle between Ex and Ey can never be changed from zero and therefore can never be brought to the 90 phase angle necessary for circular polarization. However, by proper dimensioning and positioning of the guides, it is possible to obtain substantially circular polarization of the waves, and, as will be pointed out hereinafter, the advantages of this invention are obtainable with waves having merely a substantial degree of elliptical polarization (as contrasted to linearly-polarized waves), as well as with waves having true circular polarization. Y

If desired, the device may be terminated by another hollow, horn-like section (not shown) which converts from a nearly square throat (at C) to a circular aperture. This accomplishes an impedance match to space from the substantially` square guide, but is not essential to the generation of a circularly-polarized wave.

It is obvious, from the above discussion, that true circular polarization occurs at only one frequency, and that the wave becomes elliptically polarized at either side of this frequency. I-Iowever, with proper design the variation from true circular polarization with frequency may be made sufficiently low to permit operation over a rather wide frequency range, since the advantages of the invention are obtainable with waves having a substantial degree of elliptical polarization.

In the device as used for dielectric heating, as shown in Figs. 3 and 4, the dielectric material 8 to be heated or defrosted, such as food for example, is supported, byasuitable dielectric supporting means 9; adjacent the free'end oi wave guide er horn 3. A metallic cover l0, which acts asia ree'ctorfof-electromagnetic waves, is placed adjacent material 8, on the side thereof `opposite from guide 3. The material 8, together with cover l0, acts as a partially absorbing and partially reflecting medium. Irhave rfound that, with a substantial degree of elliptical polarization, as well as with circular polarization, of the wave, the standing wave ratio in the ,energy feed line is 'less than with linear polarization, when the energy is used for heating a partially absorbing anda partially reflecting medium. When the standing wave ra- Atia in a high-frequency feed line is low, a maxi muni amount of power may be obtained from a high-frequency source such, as a magnetron,. and, with a reduced standing wave ratio, thel efficiency ofthe. .source is increased. Also, a high standing wave ratioV may cause rapidv deterioration of the Ahighfrequency source.

Let 11sA delinethe. electric field of the wave at Bixby-the equations I. Y ai; cos (tiene, (a)

and,

rene, and lfrom 30 to 45 per cent of acronitrile. `Lotus further assume that the length and dimensions ofy the phasing section of guide 3y are such that the componentEk suffers a progressive phaser change ofv 2-1r `from B to C, and that @2v=%'1- Therefore at C, thewavehas the-components and EtF cos (wt-gir) (172)A whichV yield a circularlyepolarized wave since the components are 90 apart in time and space; assuxningv the components to be equal 'inaniplitude Now let ius assume that the wave impinges upon an absorbing means which partially absorbs-andpartially reflects the incident energy. The reflected wave-components Will be -altered'in` tixnegphase` in some manner dependent on the properties ofv the absorbingA medium although the spacerelationship of the components will stay the same. We can merely say for our purpose here that the time phase of each com'- ponentis shifted on reflection by an arbitrary angle c. Then; the reflected wave `may be written as follows where a=per cent reflection of the in- By .thev time the reflected wave has arrived at Bt; Aitq'hasV again been retarded as before (Ex retarded; by-21r .and llily by w), so at B, the reflected wave has the components-r which may be written Eff/ tlg cos (ctie-'+o e 5% corretta) us) The minus sign for Ey may be looked upon'- as' a space reversal of the vector Ey. This means that-.the resultant of these twoy vectors (they are now shown to be back' in time phase) is at right angles to the initial vector E.

If the wave guide' l: has a cross-section of 1% inches by 3 inches,A the critical wavelength fora mi wave whose electric vector is perpendicular to the 3inch side is 2X3;v or 6 inches, which approximately i5 centimeters; the critical wavelength for a TEM wave whose electric vector is perpendicular to the 11/2 inch side isv 2 X11/2', or 3 inches, which is approximately 'I1/2 centimeters; is truebecause, as is? well-known to those skilledY in the art to which this invention relates; the critical wavelength for a: TEo'i (or Hei) wave being transmitted through a rectangular wave guideis equal'to twice the dimension of the guide to which the electric vector of theA wave isip'erpendicular. In other words,l the longest wave that caribe transmitted in thej waveguide t has a. half-wavelength in air equaltoor lessgtliariy the dimension of the guide normal .to the direction of the electric vector ofthe wave. Therefore, s wave whose wavelength'is, greater than 15 cnil will not be propagated in' guide. l at all; one whose wavelength; lies between 'l1/ a'nd 15 cm: will b e propagated in said-- guide when the electricvcctor thereof isy perpendicular tothe wider face. ofl the vguide but. not: when `the electric Vrector perpendicular tothe. narrower face o! the guide; one Whose-wavelengthfis lessfthan '111/2 cm. Will be propagated insaid'euide when-its; el tric vVector isxpeipendlcttlnr to either thewide or the narrow facesoftheguide l.v As long-asthe dimension a normal tothe directionl ofthe electric vec-tor is equal to or'greater thanthe halfwavelength-aln. the characteristic impedance ofthe guide is real and positive, a necessary eqndition for the propagation of wave energy. This iseexpressed by the equation for the characteristic impedance Z for the TEni mode:

fi "e irreal and ip osinweI then z is' rear andfposmve'as long as m '2a,' v Y p If a guidefcapable of supporting the 'IEgi inode is joined-toa guide whose dimension' a normal to 'thefdirection of' the electric vector is'tco smalll to support propagation of 'energy at the wave` length being .u-sed', or-in which Ao 2a' then the wave energy incident 'upon this junction'will be reflected because the waveguide impedance b e-'l yond the junction isno longer Yreal and positive, but'"is-imaginary-i` This'y is true'becaus'e the term in Equation f'laibec'on'ies greater 'than' one, and entire'i expression Vbecomes imaginary, In other words the Wave strikes, an'extreme `impedarice1 discontinuity and is* reilected;

As stated above, a Wave having a wavelength lo of ten centimeters is used in this invention, which means that M 2awhen the electric vector is perpendicular to the shorter or 11/2-inch side of the rectangular guide I. Therefore, this wave is 'capable of being propagated in guide I only when the electric vector thereof is perpendicular to the wider face of said guide, as is vector E in Fig. 1; if said vector is at right angles to E in Fig. g1, the wave is not capable of propagation in guide I, but is reected at the junction A between said guide and a guide 2 whichis capable of propagating such a wave mode, as discussed above in connection with Equation 18a.

As described above, with reference to Equation 18, the resultant of the two vectors Ex and Ey, after reflection, is at right angles to the initial vector E. Therefore the reflected wave cannot propagate along guide I; the wave is therefore reflected back again, suffers attenuation from another passage through the absorbing material, and on the second reflection from the absorber is free to propagate along rectangular guide I and return to the source, since the minussign of Equation 18 becomes positive on the second reflection so that the resultant at this time is in the original direction E of Fig. 1.

The above action can be more clearly explained' by reference to Fig. 5, which represents, atB and C, respectively, the space relationships of the electric component vectors at points B and C. At point B, as described above, the component vectors Ex and Ey ofthe wave, as supplied by the source and as split by the wave splitter 2, are 90 apart inspace as4 shown and their resultant has theVv direction of E at this time; the space relationship of rthese vectors is not changed by passage through horn or guide 3, so that at point C, the mouth of the horn,'the components Ex and Ey are also 90 apart in space as shown.

It is 'assumed that there is a 180 phase'shift in the wave during the passage ofv said wave from point C, 'through load 8 to reflector I0 and back through load 8 to point C, so thatq =1r in Equations 17 and 18. Therefore, from said equations, at point B the reected wave has the components:

The wave represented by components EX and Ey is applied to the load 8, being attenuated thereby, from whence it passes to the reflector I0, which reflects it back through the load toward the source, in the same direction, as Ex and Ey. When the reflected wave reaches point B, as shown above by Equations 19 and 20, the corn- -ponent Ex is reversed in space with respect to its original position Ex, as is indicated by the minus sign for Ex in Equation 19; also, when two vectors are 180 apart-in time, they reach their maxima-simultaneously, but in opposite directions, so that, if said vectors are originally 90 apart in space, this 180 phase difference is equivalent to a space reversal of one of them. Therefore, Ex has the direction shown at B in Fig. 5. This means that the resultant E' of Ex' and Ey' at the bottom B of the horn 3 is turned 90 from its original position E when it entered phasing section or horn 3; therefore this reflected wave E cannot enter therectangular guide I toward the source since its wavelength is greater than the critical wavelength for propagation in this direc- 8 tion down guide I; the reflected wave E' is therefore refiected from end Aoi the source feed line I back toward load or body 8 as components Ey" and Ex", assuming no space shift of these vectors at this reection.

This reflected wave reaches point C with th component directions Ey" and Ex", and again passes through load or body 8 to reflector I0, from which the remaining unattenuated portion of the wave is reflected back toward the source, through point C, as components Ex" and Ey", which are in the same direction as Ex" and Ey", respectively. When this reflected wave, after its second roundtrip through body 8, again reaches point B, the vector EX" is again reversed in space with respect to its former position Ex", since the minus sign of Equation 19 becomes positive on the second reflection. Therefore Ex" has the position shown at B.

The resultant E" of Ex'" and Ey" at the bottom B of the horn 3 is in the same direction as the original resultant E, so that this remaining unattenuated portion of the original wave can propagate down guide I, proceeding to and beyond point A of guide I to the source itself, setting up standing waves in the feed line I.

It is thus apparent that, by the use of circularly-polarized waves, no Wave energy will be reflected back to the source until the original wave has made two complete roundtrips through the body or load 8; during these two roundtrips the high-frequency energy of the source is highly attenuated by said body, giving substantially uniform heating of said body due to the two roundtrips of the wave therethrough. In a test of a device using circularly-polarized waves, measurements showed that the food 8 absorbed about 65 per cent of the high-frequency power one Way; however, after two reflections ofthe wave, only 1.5 per cent of the total power remained unabsorbed. It is therefore apparent that, with this invention, a maximum power output may be obtained from a high-frequency source of any given capacity; therefore the efliciency of the system is increased.

Although, in the above discussion, particular reference has been made to circular polarization, this has been done only for purposes of illustration; the advantages of the invention are also obtainable with waves having a substantial degree of elliptical polarization, as contrasted to linearly-polarized waves. For example, if the two components EX and Ey of the wave are unequal in amplitude, the wave is elliptically polarized. In this case, the greater of the two vectors may be considered to be made up of two collinear vectors, one havingl a magnitude equal to that of the other orthogonally-related vector; and the second having a magnitude which is that of the total vector minus said one vector. Said one vector and its orthogonally-related mate give a wave which is circularly polarized, and-which makes two roundtrips through the load, as described above, before being reflected back to the source; the remainder of the larger of the two orthogonally-related vectors acts as a linearly-polarized or plane-polarized wave, travelling back Vto the source after its first roundtrip through the load, thereby creating standing waves in the highfrequency feed line. Y

Therefore, with waves having a substantial degree of elliptical polarization the advantages of this invention are obtained, since with such waves a substantial portion of the reflected Waves, and therefore also a substantial portion of the stand- -plane polarization due to ingwaves, are eliminated or attenuated bythe load. It is apparent, from the above discussion, that the maximum reduction of standing waves in the feed line is obtained with waves having true circular polarization; somewhat less of a reduction of the standing wave ratio is obtained with elliptically-polarized waves, this reduction getting less the closer we get to linear or plane polarization, becoming zero if a linearly-polarized or plane-polarized wave is used-that is, relatively high standing waves are produced with the single roundtrip of such waves through the load, as contrasted with the relatively low standing waves produced when waves having a, substantial degree of elliptical polarization are used, due to the two roundtrips of such waves (or a portion there-of) through the load.

' In the preceding discussion, it has been assumed that there is a phase shift of 180 in the load, material, or Ibody S. 1t will be apparent that, if this load phase shift diiers from 180, the wave may be reilected back and forth through the load 8 three, four, or even more times before the components of the reflected wave will have such space relationship that they can be combined to give a resultant vector which is in the direction of E in Fig. 1.

By the use of this invention, using waves having a substantial degree of elliptical polarization, the standing wave ratio is much smaller than if a linearly-polarized system were used. The

standing Wave voltage ratio, for linear polarization, is

For circular polarization, this becomes l-l-crz S. W. V. Il az (22) invention the reflected waves have a minimum v amplitude as a result of their high attenuation during two roundtrips through the absorbing medium, it is apparent that minimum standing waves are set up in the high-frequency system feed line.

Figs. 6-8 show another embodiment of the invention; this embodiment also is a high-frequency `heating device which converts a linearly-polarized wave into a wave having a substantial degree of elliptical polarization, and in which said ellipti cally-polarized wave is passed through the dielectric body to be heated, the attenuation oi said wave by said body serving to heat said body. A hollow rectangular metallic wave guide II, open at one end, has its open end directly and rig-idly joined to a horn I2, said horn comprising a hollow substantially square metallic wave guide open at one end and having an opening at its opposite end of the same area as the open end of guide II, said opening matching said open end. The

of the horn guide or horn .I2 is rotated substantially 45 with respect to guide II, so that the walls of guide I2 lie at an angle of substantially 45 to the respective walls of guide II, the center lines of said guides being collinear (or said two guides being joined axially to each other); guides II and I2 are of constant cross-section throughout their lengths. One cross-sectional dimension of guide II may, for example, be twice the other; for example, guide II may be a standard one-and-ahalf-inch by three-inch guide.

A coaxial line 4, consisting of an inner conductor 5 supported coaxially in an outer hollow conductor 6 by dielectric spacers such as 1, has its outer conductor 6 attached to the exterior central portion of one of the wider side walls of guide II near the closed end of said guide, the coaxial line extending at right angles to said side wall. Inner conductor 5 projects through an opening in said side wall into the interior of the guide II for a substantial distance, for example, approximately two-thirds of the distance to the'opposite side wall. The free end of coaxial line 4 is adapted to be connected to the output pipe or leadl of a source of high-frequency energy, for example, va magnetron.

The end of inner conductor 5 projecting inside the guide II, as in Fig. 1, is spaced a distance oi' a quarter wavelength (in the guide) from'the closed end of said guide, so that when a high-frequency wave whose wavelength is on the order of 10 cm., for example, is supplied to guide II by means of line 4, a linearly-polarized wave having a TEM (or H01) mode is set up in said guide. As described above, in connection with Fig. '1, in order to convert a linearly-polarized wave into a substantially circularly-polarized Wave, it is necessary to resolve the vector representing the .original wave into two components 90 apart in space and 90 apart in time. In the embodiment of Figs. 6-8, the wave splitter, or transition member between the rectangular guide and the substantially square guide, is omitted, as will be seen. In this second embodiment, the original vector is split, or resolved into two components at 90 to each other, at the plane of the junction between the two guides I I and I2, which plane would correspond to point B in Fig. 1. This splitting is caused by the substantially 45 rotation of guide I2 with respect to guide I I, together with the substantially square cross-section of guide I 2, so that the original electric vector lies along the diagonal of guide I2. Then, as the wave travels to the end or phasing section I2, the two orthogonally-related component vectors are caused to have a 90 phase relation to each other, so that at the end of guide I2 remote from guide I I, a, circularly-polarized wave appears. This phase shift along guide I2 is in accordance with the principles stated above for the phasing section or substantially square wave guide 3 of Fig. 1.

Here, as in Fig. 1, the length and the cross-sectional dimensions of the substantially square wave guide are made such that the phase change between the two components EX and Ey (from end to end of the guide) is 90 or 270, and such that the two components EX and Ey are substantially equal in magnitude.

In the device as used for dielectric heating, as shown in Figs. 7-8, the dielectric material 8 to be heated or defrosted, such as food for example, is supported, by a suitable dielectric supporting means 9, adjacent the free end of wave guide or horn I2. A metallic cover I0, which acts as a. reflector of electromagnetic Waves, is placed adaggasese ressenti materialironr vdslfierfsideitherenfe,opposite iii-ront guide :l2

z'lzfhe ioperation aofi thisr .embodimentg lss exactly milar to-that: described lindet'ailzabovef witlr refecrenceto iligs: .115 abyrutilizingsubstantially-cirf.culariy@polarized".vvavesV fonzthefenergyiapplied to thei bodystoffbe?heatedtheiincidentiswaves pass 'rtl'rmugl-ritheimateriali 81 atleast Vtwicefbefore. they -marrntenthe'rectangulanwayeguide H--that'is,

the-Mares makea at least ltwoi fcompletefroundtrips through material K8ebeforei2the5uiget'baclrl tothe aaource. uringiheseieomplete roundtips-,ra large nproportioniofrahe energy-.contentmfflthe original wave isI abstnuzted,t :fromiits: soi that@`l al1-maximum eheatingefsbodyr: 8 :isaaccomplished togethexmth eax-reduction of zthea amplitude auf: thezirstanding awaresset uprin"tthethighefrequency eedzline be- .zicausezof thefreductiomonamplitudeiofathe .wave rree'ctedrhackrlntcrthe source.

sTheroperatlomofsahefmodicatiorrlofeFigsf6a8 ffsfexactlyg'similan innallgspects tlthatznfffigs. ila-4e except ichatfrin'.thecrnodification rofiligsxse aactrarrsitionimemberzisfnotiusedbetweenthe-recetangirlar wavezguideandfthe 'substantially square aware rgude; both modifications :lare edielectric heating devices in which a linearlyepolarlzed aware sis:convertedA iosa; hircularlyf-polarizedrwave, eolttozoneihavingsarsubstantialf degree off'. elliptical apblarizationL-beforfbeing applied `to thezdieleictric cmaterial, the aware r azfter its @conversion xbeing i apassed throughithezmaterial toiheat thesamee and gsaid Wweunakingiat least'. two` roundtrips. through .isaz'idmateriakhefore'fsaidfmvave `getsubacky -tof the nfeediiline from thethighafrequencyzz-source.

, Althoughnuseaof @the eapparatus for fonlyrthe cheating:sendedefrostingrmfrfoodsnhascbeenfmen- 'fitionedeitewillrbappreciatedxthat thezapparatus aisapllicabieatd otheriuses,v fonffexampleitztheranpeuties.

,.edthough.the:structuresiillustrated are prefer- .ablyr-madefromisheetzcopper; theeinside `surfaces ...-'Qfsitheizldertieesrmayebesilyeraplated rifflde'siredso that athe idoissessrnay: besreduced; silverifrlossles esmaller than; copper loss.

` f courseesitsrisito bezzunderstoodzthatdsh-isrlnrelention iisinotgiim'rted: to .the'particularidetailsz as @described abormasfanany. equivalents. willrsuggest ethemselreszatothose .skilled in the art. iItisf-'aceoordinglyidesiredtthatstheaappended cclaimsrbe gitrcma :broad interpretation :commensurate-.With Lathe .scope :ifzifthis inventionsmthinftheiart.

What :isz'claimedels gprotdi'ngxat said mneeendoffisaid vvavelfguide sa plinearlyepolarized high-frequency :vv-ave, lsaid #Jorn being constructed-fand arranged7 to-fvconve'rt Ssaidirlniearhepolari-zedi wave intoiawave'havi-ng nasaubstantialidegreeef elliptical :polarizationfia bdd f 'dielectricaimateriatrcoupledftothe'output @n said thorn -to `rreceive `risaid 2 ellipticallyfplaleized suave, aand :an :electromagnetioewavesreiietting rsuraceson fthe-z side#ofssaidifbddyecpnpnsitesrom .saidnorn,f'evvlflerebyrelectromagnetic s-reiiecte'd hackfthroughsaidlbdy. 22. FA -high-frequency' heating=2system vcomprising, in combination, a hollow'nwavefguideaehorn @coupled-to onmendsofasaidwave guide; meansfor ssupplying ra ehighefrequency QWave tto said *wave gguidnsaidihorinbeingeadaptedtofproducefrcurlafrfpolarizationf:nisaid-rwave; a bo'dy of; dielectric rmatenialecoupledttoi thereoutputeendaof saidihorn stmrecivesaidfcircularlyepolaifized Wave, fandean eeiectnomagneticsivaveeeefiecting :fsurface `on the i i12 s'fsidef.iff'said,fbodyf4 opposite :fromssaid horn;.:where :by selectromagnetic :energy ris e'reflected 'iback fsthrouglrfsaibody. Y

3. A1: 4high-ffrequency heating: system-r comprising, inztcombination; a .hollowfrectangular -Wave guidata,A substantially square lWave guidejcoupled to one. endof said-rectangular guide, the lWalls Vofisaid ysubstantially Asquareguide jlyingxat/:an aangleof substantially 45to the. respective Walls .ofasaid rectangulargguide and: thesicentervlines .ff ofsisaidl` guides being. collineanp` :means:V for supplyifing a high-frequency WaveV to said rectangular .f fguide, la-,ihody of dielectric -material-coupledlto the free-end of -said substantially squaretgulde `toreceive` Wave energy. therefrom, and :ans-elece..tromagnetic-Wave-reecting: surface on the-'side 1 .of ,-said body opposite efrom said` substantially square guide, said reflecting surface beingfsubstantially ,-.perpendicular to the sides Tof *said squaregzguide 4..A dielectric heatingsystem comprisingfin combination, ra hollow rectangular -iwaveguide gmeans, for-.settinguupa transverse electric-mode .wave cfa :predetermined energy content1 in said guide :with the lelectric vector thereof `extending atright .angles tothe longer-sideV of said guide, .the..vvavelength ofsaid-Wave being suchthat said Wave is incapable of propagation-in said.guide when the electric vector-thereof extendsat other` thana right angle .Ito-said longerfside, .means for producing va substantial. degree. of ellipticatpo- ,larization of said wave,v a -dielectricvbody-ito be heated coupled to saidlast-namedmeans toreceive said elliptically-polarized Wave andato` attenuate the same as it passes through said body, ,and means forreflecting the .,unattenuated enrergy of said wave back toward said waveguide. 5. A dielectric heating system comprisingin combination, 44a hollovv rectangular waveguide, ,m means for setting up a transverse electric. mode vWave of a predeterrriined energy content in said guide with the electric vector thereof extending uat rightanglestotthe longer. side of.said.guide, rrthe wavelength of saidwave being-suchr that said I wave .is incapablecf propagation-in. said guide 4J when'the electricvectorthereof extends 4at other than aright angle to said longer side, means. for producing a substantial. degree of. elliptical polarizationof said wave,..a dielectric Abody -to be c heatedcoupled to. said last-named. meansrtoy re- 0 ceive `said `elliptically-.polarized wave and f .to r`attenuate the same as it passesthrough said body, Vand meansfor reflecting the-.unattenuateden- `ergyiif said Wave back towardsad waveguide, theelectric vectorof said. reflected .Wave extend- 5'Ting vat other than a right angle to said longer side.

T6. `A dielectric ,heating system comprisingfin "combination, .a 'hollowrectangular waveguide, vmeans for setting up a transverse electric mode GO Wave `0f a, predetermined energy .content `irl-said guide with` the electric vector` thereof extending :atri'ght angles to fthe. longer side of4 saidguide, '1 thewavelength of said Wave .being suchthat said ""Wave is incapable of propagation in. said; guide t5 when the'electric vector thereof. extendsat .other .thana right angle to saidlonger 'side,.means.for 'producingfa substantial degree ofellipticaLpoilarization of said Wai/aia Adieleci'lriclbody tolbe "heated coupled to said last-named meansto re- '70 Aceive said` lliptically-polarized Wave I.andi to fattenuatethe samegasi it passes throughsaid. body, and, vmeans 'for reflecting -the unattenuated. enuergy of lsaid wave back toward said waveguide, the-electric vector of' said reecte'dwaveeextend- =ing at'o'ther'than Va right angleto saidlonger 13 side so that it cannot enter said waveguide but is reflected therefrom back toward said body.

7. A dielectric heating system comprising, in combination, a hollow rectangular waveguide, means for setting up a transverse electric mode wave of a predetermined energy content in said guide with the electric vector thereof extending at right angles to the longer side of said guide, the wavelength of said wave being such that said wave is incapable of propagation in said guide when the electric vector thereof extends at other than a right angle to said longer side, means for producing a substantial degree of elliptical polarization of said wave, a dielectric body to be heated coupled to said last-named means to receive said elliptically-polarized wave and to attenuate the same as it passes through said body, and means for reil'ecting the unattenuated energy of said wave back toward said waveguide, the electric vector of said reflected wave extend- 8. A dielectric heating system comprising, in combination, a hollow 14 other than a right angle to said longer side, means for producing a substantial degree of elliptical polarization of said wave, a dielectric body to be heated coupled to said last-named guide but is reflected therefrom back toward said body to again pass therethrough, said reflecting wave being absorbed by said body in the two roundtrip passages of said wave through said body, said energy serving to heat said body.

KARL J. STIEFEL.

REFERENCES CITED The following references are of le of this patent:

UNITED STATES PATENTS record in the OTHER REFERENCES The Welding Engineer, December 1945, page 90.

Certificate of Correction Patent N o. 2,480,682 August 30,1949

KARL J. STIEFEL It is hereby 'certified that errors appear in the printed speciiication of the above numbered patent requiring correction as follows:

Column 1, line 88, after the Word invention strike out the period and insert instead a semicolon; column 5, line 31, strike out "rene, and from 30 to 45 per cent of acronitrile.; column 6, Equation 18a, for that portion reading read l) and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oce.

Signed and sealed this 17th day of January, A. D. 1950.

THOMAS F. MURPHY,

Assistant Gommz'm'oner of Patents.

Certificate of Correction Patent No. 2,480,682 August 30,1949

KARL J. STIEFEL It is hereby certified that errors appear in the printed specication of the above numbered patent requiring correction as follows:

Column 1, line 38, after the Word invention strike out the period and insert instead a semicolon; column 5, line 31, strike out rene, and from 30 to 45 per cent of acronitri1e.; column 6, Equation 18a, for that portion reading read and that the said Letters Patent should be read with these corrections therein that the saine may conform to the record of the case in the Patent Oce.

Signed and sealed this 17th day of January, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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