US2586754A - Radio-frequency system - Google Patents

Description

Feb. 19, 1952 N. R. WILD RADIO-FREQUENCY SYSTEM Filed Nov. 16, 1946 3 Sheets-Sheet `l FIG.

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Nm A fw 0 /NVENTOR NORMAN R' WILD BY MM5@ AT O/NEV Feb. 19, 1952 N, R, w|| D 2,586,754

RADIO-FREQUENCY SYSTEM Filed NOV. 16, 1946 3 Sheets-Sheet 5 o /9 ISM- l y/ MAGNETRON OSCILLATOR /N vsNro/z NORMAN f?. WILD atentedY Feb. 19, 1952 RADIO -FREQUENCY SYSTEM Norman R. Wild, East Natick, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application November 16, 1946, Serial No. 710,339

2 claims. 1

This invention relates to a radio-frequency system, and more particularly to a microwave transmission system useful for the heating or cooking of foods.

An object of this invention is to devise a microwave cooker which will uniformly heat or cook food.

Another object is to devise a microwave transmission system by the use of which the source of microwave energy, which may for example be an oscillator of the so-called magnetron type, is maintained at a point of favorable phase in the standing wave system of the transmission line, throughout rather large variations in the standing wave ratio of said line.

A further object is to devise a transmission system by the use of which a rather large area of food may be uniformly heated from a single source of microwave energy.

A still further object is to prevent leakage of microwave energy over the edge of the mouth of the horn, which horn is used to transmit energy from the source to the food to be cooked, thereby eliminating the possibility of the feedover of energy from one horn to another, when two horns, each fed by a separate source of energy, are running side by side.

An additional object is to reduce the standing wave ratio in a microwave transmission and radiation system.

Still another object is to maintain the magnetron at a point of favorable phase in the standing Wave system of the feed line, irrespective of wide variations in the characteristics of the object being heated.

The foregoing and other objects of the invention will be best understood from the following description of exemplications thereof, reference being had to the-accompanying drawings, where- Fig. l is a central longitudinal cross-section through a microwave energy transmission system according to my invention;

Fig. 2 is a view taken on line'2-2 of Fig. 1;

Figs. 3 and 3A are a plot of the electric field intensity in the horn of Fig. 1;

Fig. 4 is a central longitudinal cross-section through a modified transmission system;

Fig. 5 is a View taken on line 5--5 of Fig. 4; and

Figs. 6 and 6A'are a plot of the electric field intensity in the horn of Fig.4.

Referring, now, to the drawings, and more particularly to Figs. 1 and 2 thereof, a magnetron oscillator I is adapted tomsupply microwave energy, by means of a radio-frequency transmission system 3, to a hollow waveguide or horn 4 which is substantially closed at its upper end and open at its lower end; the energy is radiated from the open end of said horn onto the food 6 or other substance to be heated, which is positioned below horn 4 by means of a suitable container "I, to -be later described.

Magnetron I is conventional and is schematically represented as consisting of a cathode 8 surrounded -by a hollow anode structure 9, which includes a plurality of radially-disposedanode vanes I0 which divide the interior of the anode structure into a plurality of cavities or chambers.

As is wel1 known, such a device can be constructed and operated to produce a rather large amount of radio-frequency power in the microwave region of the frequency spectrum, and may be operated to produce continuous oscillations if desired. The output frequency of such a device depends mainly on the geometrical configuration thereof.

As an example, this device I may be operated to produce an energy output having a frequency on the order of 3000 megacycles, said output being coupled to system 3 by means of a coupling loop Il formed on one end of the inner conductor I2 of a coaxial line I3; the loop II is positioned in one of the cavities provided between adjacent vanes I0.

One end of loop II is connected to the inner conductor I2 of coaxial line I3, while the other end thereof is connected to the outer conductor I4 of said line. Conductor I2 extends straight from point A (at the coupling loop) for a suitable distance, determined as set forth hereinafter, to point B. In order to support the inner metallic conductor I2 in outer metallic conductor I4, said inner conductor is extended a distance of an odd number of quarter-wavelengths (at the operating frequency of magnetron I) beyond point B, to point C, and is at said latter point rmly connected, mechanically and electrically, to a solid disk which is integral with outer conductor I4. In other words, the quarter-Wavelength stub B-C supports inner conductor I2 of the coaxial line.

I In order to supply energy to the horn or waveguide 4, conductor I2 makes a 90 turn at point B and extends to point D, where said conductor ends. As is more clearly shown in Fig. 2, hollow elongated metallic waveguide 4 is substantially square in cross section; the dimensions of this waveguide will be set out more in detail hereinafter. Horn 4 is a square hollow prism which is entirely open at one end and is substantially closed at its opposite end, as at 4a. A circular aperture, of sufficient size for the mounting therein of conductor I4, is centered at the point of intersection of the diagonals of the square closed end face of horn 4, and point E of conductor I2, which is in the same horizontal plane as is the inner face of the closed end of horn 4, is spaced a distance of approximately A/4 from point D; as a result, portion DE of conductor I2 serves as a quarter-wavelength probe or exciting rod for guide 4. The section AC of the line may bev termed the main transmission line, the section BD being termed a branch transmission line.

I have found that excitation of the waveguide 4 in the transverse magnetic (TMWW) mode produces a very even voltage gradient over thebottom or mouth of the horn 4. Element 4 may be termed either a waveguide or a horn, since it propagates a eld therealong as does a waveguide, and also radiates energy from its open end, as does a horn. lf an even voltage gradient is produced over the mouth of the horn, a plate of frozen food 6 placed adjacent thereto will. heat evenly over the entire surface of the food. As a general rule, foods which` are over M8 in length are heated, by microwave energy, partly by induction heating (caused by the H or magnetic lines) and partly by dielectric heating (caused by the E or electric lines). Therefore, in order to heat the food evenly, both the E and H lines should be substantially uniformover the mouth of the horn.

Now referring to Figs. 3 and 3A, a plot of the electric l'ieid intensity for this type of wave, with m: l and n=l (known as a TMi,i wave) is shown. Fig. 3 represents a transverse cross-section through the horn, while Fig. 3A represents a side sectional View (a section parallel to the side of the horn and passing through the center of the horn). As represented in Fig. 3, excitation-in this mode produces a radial pattern of E lines substantially uniformly over the mouth of the horn. Although the H lines are not shown in these gures, it will be remembered that these lines are everywhere at right angles toA the E lines; it should therefore be apparent that the H lines will consist ofa plurality of substantially equallyspaced concentric curves. It will therefore be seen that very uniform heating of the food will be produced ifthe TMm, mode is utilized. This is contrasted with the conventional transverse electric modes (for example, those designated as TEo,i and TEM) which produce concentrated E lines in the center of the horn, making alternate hot and cold spots in the food being heated by them.

It may be seen, from Figs. 3 and 3A, that there is a small hole present in the center of the horn with the TMi,i mode. In the drawing, the hole is greatly exaggerated in size for purposes of clarity. This hole must be reduced in size as much as possible, so as not to produce a cold spot in the food. By dimensioning the cross-section of the waveguide 4 to makev its cutoff angular frequency as close to the operating frequency of the magnetron I as possible, the two verticallyspaced fields shown in Fig. 3A will be squeezed together, thereby narrowing down theV hole. It has been found that, for the TMi,i mode, a horn 3.25 square will give a cutoff wavelength of 11.57 cm. Such a horn will squeeze the hole downr provide a reasonable safety factor and also, byl

virtue of being square, will produce an even radial E pattern as shown in Fig. 3.

Excitation of the guide 4 by exciting rod DE, which is substantially M4 in length and whose center line is collinear with the longitudinal center line of said guide, will produce therein waves of the desired TM11 type.

Container 'I is made entirely of metal and is constructed to have a wave-reflecting bottom surface 'Ia on which the food 6 rests, spaced a distance of M2 (measured in the guide) from the mouth of thehorn 4. The side walls lb of the container are arranged to enclose substantially the same cross-sectional area as that of horn 4. These wallsenclose the food 6 substantially completely, so that no microwave energy can escape at the sides thereof;l therefore substantially all of the power has to dissipate itself in the food, as will be more fully explained hereinafter.

Surrounding all four sides of the horn 4, at the mouth thereof, is a channel I5 which opens downwardly (that is, it opens toward the mouth of the horn), this channel being provided, for example, by a metallic member I6, of invertedl L-shape, which has its shorter leg firmly bonded to the outer surface of horn 4. The channel i5 is substantially M4 deep (measured in the guide), for a reason which will appear hereinafter. The width of the channel I5is somewhat exaggerated in the drawing, for purposes of clarity.

The length of part BE of the branch portion BD of conductor I2 is designedl to make the distance from point E to point C (via point B) an integral number of half -wavelengths Since the coaxial line I2 is short-circuited at point C, there is an extreme impedance mismatch at this point, so that waves are reflected at said point. The short-circuit at point C acts as a very low impedance, so that a voltage node (an Emin in the standing wave system) exists at this point. At point B, which is an odd number of quarterwavelengths away from point C, this Emin reflects a yoltage loop (an Emax inthe standing wave system), since in any system of standing waves, loops are spaced \/44v from nodes. Therefore, at point B, due to reflections from point C, we have an Emax point.

At this point we will consider what happens in the transmission system when the horn 4 is designed to have a length (in the guide) of an integral number of half-wavelengths (an even number of quarter-wavelengths) with food- 6 placed between the reflecting surface 'Ia and the horn 4; it will be recalled that the surface 'Ia is spaced a distance of M2 from the mouth of the horny 4.

Wave energy which propagates down the horn (this energy can be propagated because its frequency is above the cutoff angular frequency of the horn) suffers a very small reflection at the mouth of the horn,vdue to the slight discontinuity thereat, equivalent to aA small impedance mismatch; this produces essentially an. Emin atthis point due tothe discontinuity or space between the horn and the upper end of container 'I, which space extends entirely aroundthe horn; Energy which impinges upon the' upper surface of the food E experiences a very slight reflection, also, due to the small impedance mismatch at this point; this energy is in phase or only very slightly out of phase with the energy refiected at the horn mouth because of the close positioning of this. upper surface tothe mouth of the horn. The great bulk of the energyy passes through the food` passes through body 6 a second time and is again A partially attenuated thereby, the remaining unattenuated energy proceeding toward the end 4a of the horn and, together with the energy reflected at the mouth of the horn, setting up a system of standing waves in the horn '4. Since the mouth of the horn is spaced a distance of M2 from surface la, the energy reflected from said surface will be in phase withvthat reflected from the mouth of the horn.

The end 4a of the horn looks, to the reflected wave, like a short-circuiting plate with a small hole in its center, thus in effect looking like a low impedance to the reflected Wave but like a high impedance to any component of the reflected Wave trying to enter the coaxial feed line; the smaller the hole is, the higher such impedance will be. As a result, most of the reflected wave energy does not go down the coaxial feed line, but is reflected by the short-circuiting plate 4a toward the food 6, to be further attenuated thereby, resulting in further heating of the food.

It is, therefore, desirable to make the hole in the upper end 4a of the horn as small as possible, in order to reduce the amount of reflected energy which couples with the coaxial feed line, thereby both reducing the standing wave ratio in the feed line and also utilizing the output energy of the magnetron more eiliciently to heat the food. The space between inner conductor I2 and outer conductor I4 of the coaxial line should therefore be made as small as possible, consistent with other considerations which may limit the minimum size of the outer conductor I4.

The closed end surface 4a of the horn 4 is, as stated above, a short-circuiting plate, which has a loW impedance. Therefore an Emi will be established at this surface, since a short circuit is equivalent to an Emin for a standing wave system; this Emin is consistent with the fact that the horn is an integral number of half-wavelengths long, making an integral number of halfwavelengths from the Emin at reflecting surface 'la to the end rla of the horn. The loW impedance of the short-circuiting plate 4a, which is the closed end of the horn, is reflected at the hole in the center thereof, so that, for waves reflected from horn end 4a which enter the coaxial feed line through said hole, a voltage node or Emin is also established at point El, Which is in the same horizontal plane as the inner surface of end 4a of the horn.

Since the distance from point E to point C (via point B) is an integral number of halfwavelengths, and since the distance from point C to point B is preferably an odd number of quarter-wavelengths, the distance from point E to point B must necessarily be an odd number of quarter-wavelengths. This means that, if an Emm is established at point E, the reflected waves travelling along the coaxial line will establish an Emu at point B. It has previously been established that the reflected waves from point C establish an Emax at point B. l

It will therefore be seen that the reflected waves from the three locations 1a, 4a, and C are all in phase at point B, since all the reflected Waves establish Emaxs or voltage nodes at said point. Since all of these reflections are in phase, a rather high standing Wave ratio is produced in the feed line. This means that the feed line becomes a very highly reactive or very high-Q circuit.

There is a complete short-circuit at point C, and there is substantially a complete short-circuit for reflected Waves at horn end 4a, both lof these short-circuits being exposed and unimpeded in effectiveness by any energy-absorbing object. As will be seen, the reflecting surface 1a, although it is a short-circuit also, is masked in effectiveness by the presence of the load 6. As a result of these characteristics, there is a rather high standing wave ratio in the line I3 substantially independently of reflections from surface la, so that the line I3 is a high-Q circuit even in the absence of such reflections; in effect, therefore, the reflected energy from the two locations C and 4a, is what determines and holds fixed the phase at point B. Therefore, the phase in the coaxial line is substantially independent of what goes on at the lower end of the horn; wide variations in the characteristics of the food being heated do not noticeably interfere with the phase of the standing waves in the feed line. Stated in another way, this system has very great phase stability, the phase in the coaxial line remaining substantially constant throughout 'wide variations in the standing wave ratio.

If the voltage standing wave ratio be plotted against frequency for the above-described design, in which the horn has a length of an integral number of half-wave lengths, we obtain a curve in which the standing wave ratio is rather low over a very wide frequency range, being within reasonable limits over as broad a range as 60 mcs. on each side of a nominal operating frequency of 3000 mc. If the system is designedv so that the reflections from the three locations 1a, 4a, and C are not quite in phase, there will be some cancellation, thereby reducing the standing wave ratio and broadening the hollow in the curve of voltage standing wave ratio vs. frequency. The voltage standing wave ratio is reduced in the region of the nominal operating frequency because, at this frequency, the horn Il is the proper length to provide a very efficient reflecting surface or very low impedance at the bottom plate 4a. In this region the standing wave ratio may reach a value on the order vof 1.811.

It is known that a magnetron will operate most favorably, that is, its operation will be more stable over a wide range of standing wave ratios, when the phase of the feed line at the tube output has a certain optimum value, which value will be different for different types of tubes. vBy estab-v lishing a known and constant value of phase at point B of the feed line (which value may be an Emax in my invention, as defined above), itis possible to match the tube I to the line, so that the phase of the tube with respect to the line may be put at a value which is favorable for the tube. To accomplish this matching, it is only necessary, the phase at point B being known, to makethe distance AB such as to place point B, and therefore also the tube I, at the desired phase angle. Since the phase at point B remains sub stantially fixed during operation of the system, the phase at vpoint A will remain substantially at the optimum value...

value on the order of 1.2:1. The standing wave ratio in a system of this kind is low over a rather wide frequency range, being within reasonable limits over as broad a range as 45 mc. on each side of a nominal operating frequency of 3000 mc.

Since the standing wave ratio in the feed line with a horn of this length is not as high as with a horn which is an even number of quarterwavelengths long, the Q of the coaxial feed line is not as high as in the first-described horn design. This can also be seen from the fact that refiections from surface Ia are substantially eliminated from the feed line; therefore refiected wave energies from only the two locations E and C are the energies which are in phase at point B in the feed line I3, and the energies which determine the phase at point A in said feed line. However, the Q in the coaxial feed line is still quite high, due to the presence of the excellent shortcircuits at points C and E, and the reiiected energy from these two points is what determines and holds fixed the phase at point B. Therefore, the advantages of phase stability in the feed line, irrespective of different foods E, are obtainable with this latter horn design as well as with the first-described horn design.

To recapitulate, the designer may make a choice between a low standing wave ratio over a very broad frequency range and a very high-Q circuit (obtainable with a horn an even number of quarter-wavelengths, or an integral number of half-wavelengths, long), and a lower standing wave ratio over a somewhat narrower frequency range and a somewhat lower-Q circuit (obtainable with a horn an odd number of quarter-wavelengths long). In both designs, the advantages of maintaining the magnetron at the point of most favorable phase, and the frequency-pulling effect of a high-Q feed line circuit, are obtained, as well as the uniform heating of the food (because of the uniform neld pattern) and the efficient utilization of the energy in heating of the food (due to the very small percentage of the reflected waves which find th'eir way back to the feed line).

By properly dimensioning the length of the horn, it is possible to have the same horn operate as an 'even quarter-wavelength horn (as described in the first horn design) at one limit of the range of frequencies over which different magnetrons vary, and as an odd quarter-wavelength horn (as described in the second horn design) at the other limit of said frequency range.

It will be recalled that the cross-sectional dimensions of the horn 4 are such that the cutoff frequency thereof is as close to the magnetron operating frequency as possible, in order to narrow down the hole in the Vcenter of the Fig. 3 E pattern. The said cross-sectional dimensions of the horn are varied to give a characteristic impedance for the horn which matches the characteristic impedance of the coaxial line. A complete impedance match cannot be obtained in this manner because of the limits imposed on the cross-sectional dimensions of the horn by considerations of horn cutoff frequency; however, as close a match as possible is obtained in this manner and the impedance mismatch then yet remaining is eliminated by varying slightlyT the length DE of the probe or exciting rod, from its nominal M4 length.

If it is desired to use a single magnetron and one horn rather than two magnetrons and two horns, the structure shown in Figs. 4-5 may be y, utilized. Referring, now, to Figs. 4-5, inwhich elements the lsame as those of Figs. 1-2 are denoted by the same reference numerals, a magnetron oscillator I is adapted to supply microwave energy, byv means of a radio-frequency transmission system I'I, to a hollow waveguide or horn I8 which is substantially closed at its upper end and open at its lower end; the energy is radiated from the open end of said horn onto the food lI5 or other substance to be heated, which is positioned below horn I8 by means of container 1.

The microwave energy output of magnetron I is coupled to system I'I by means of a coupling loop II formed on one end of a first section I9a of the inner conductor I9 of a coaxial feed line 20; the loop II is positioned in one of the cavities provided between adjacent vanes IU of the magnetron.

One end of loop II is connected to the inner conductor I9 of coaxial line 29, as stated, while the other end thereof is connected to the outer conductor 2I of said line. Section yI9afof the conductor I9 extends straight from point F, at coupling loop I I, for a `suitable distance to point G. At point G the conductor I9 makes a right angle or turn, away from horn I8, extending a suitable distance in this new direction to point H, at which point another 90 turn is made, away from magnetron I, to provide another'section I9b of inner conductor I9. At the opposite end J of section I9b from point H, in order to support the inner metallic conductor I9 in outer metallic conductor 2|, said inner conductor is firmly connected, mechanically and electrically, to a .solid disk which is integral with outer con'- ductor 2 I.

The linear portion GH of inner conductor I 9 is extended beyond point Gy toward and into horn I8 through a circular aperture provided -therein which is of suitable size to accommodate the outer conductor 2I of the coaxial line 20, to point K, the outer conductor 2| being terminated flush with the inner surface of the closed end I 8a of horn I8. The distance from point L, which is in the same horizontal plane as the inner surface of horn end I8a, to point Kpis approximately M4, being the wavelength of the output energy of magnetron I, the length KL of conductor I9 providing a quarter-wavelength exciting rod whose axis extends parallel with the longitudinal center line of horn I8.

Measuring back, along line JH, a distance of an odd number of quarter-wavelengths from point J, there is established point M. A branch section I9c of conductor I9 extends, at righ-t angles to section I9b and therefore parallel to section HK, toward horn or waveguide I8. A second circular aperture, of sufficient size for the mounting therein of conductor 2|, is provided in horn end I8a, said second aperture be'- ing aligned with section I9c of the conductor I9. Section I9c extends, for a distance MN which is equal to the distance HK, toward and into horn I8 through said second aperture which accommodates the outer conductor 2l of the coaxial line 20, the outer conductor 2I being again ter.- minated flush with the inner surface of the closed end I8a of horn I8. Point O is in the same horizontal plane as the inner surface of horn end I8a, and the distance NO, like the distance KL,

is approximately M4; portion NO provides a sec- Y ond exciting rod whose axis is parallel to the longitudinal center line of horn I8. Point P is located, along section I9c, in the same horizontal plane as point G and section I9a of the feed line. The sections FG and HMJ ofthe line together may l be i termed the-main transmission line sections,` PON and GLK being'termed branch transmission lines and sections MP and HG being -i-s'rectangular in cross-section, and may have,

orexample, -twicethe cross-sectional area of the -guide 4 of Fig. 1 for the same magnetron'operating ffrequency. The two spaced apertures in end -Wall I8a are-aligned with each other along 'the flonger'dimensions of the rectangle-and are fboth located centrally of the shorter dimension `of saidrectangle.l The center of each aperture A4is preferably spaced a distance of one-fourth 'thelonger dimension lfrom its adjacent (short) side of the-rectangle, thus spacing the two aperture center lines apart a distance of one-half the longer dimension-of the rectangle.

The lportion GHMP of conductor I 9 is made anoddnumber ofi-half-wavelengthslong. Therevc qualfto `the jlength fPN, the vincident energy `apfpearing'at-probe tip `Kywill be 180" 1out of phase with Vthat Y'appearing at `probe vtip N. iSince the patterns radiated by the `two -probes are 180 apart, `and since" the probes are parallel tothe longitudinal axis ofthe guide I8, -waves of the TMm type are set upin said guide.

Nowreferring'to FigsxandA, aplot of the electric 'field intensity for the TM1,2 wave is shown. yFign `represents a transverse cross-sectionfthrough vthe horn, while Fig. 6A represents aside sectional view 'Ca section `parallel to the side of'the horn-and passing'through the center of 4thehorn). As representedin Fig. 6, excitation inthis mode produces adouble radial pattern ofE lines which'is substantially uniform or even over the mouth of ,the'horn. AAlthough the H eldlines.are not shown lin thesegures, it will "belapparent that vthegpattern of such lines is also substantially uniform over themouth `of the horn. As 4aresult,.very uniform heating ofthe `food 6 will be produced if this T Migmode is utilized.

V`It will'bejseenihat, with the TM1,2 system. 'the samejpattern 'will beproduced and the same .areawillbe heated as intheTM1,1 system using two magnetrons and two horns, but the 'I'Mrz system requires onlyasingle magnetron and a single horn.

It may be seen, from Figs. 6 and 6A, that there are-two small holes present in the horn with the'TMLz Inode. In thedrawing, theholes are `greatlyexagsrated in size forf purposes of clarity. "Bydirnensioning'the cross-section of the horn I8 to makeits cutoi angular frequency as close to vthe operating frequency of the magnetron I as possible, .the flines shown in Fig. 6A will be squeezed together, thereby narrowing down the holes.V It has been found that, for the TML ,modena rectangular horn havinga cross-section of ,6.5 x 3.25 will give a cutoff wavelength of v,11.5'7 .cm., .which .will ,narrow the holes pretty well andat the same .time provide lafreasonable .safety factorif the'nominal operating wavelength `oi'- the source is -10 cms.

fAs infthe Figs. 12`embodiment, the cross-sem tional dimensions -of Vthe `horn I8 are varied to .give va characteristic ,impedance for the horn -which Vmatches 4the :characteristic impedance of the Acoaxial line. This variation can be made only within'llimits imposed Vby considerations of horn cutoff frequency which arise as explained above; howeven'theimpedance mismatch which cannot A'be remedied by such variation s'e1im- `12 nated by varying slightly the lengths'of theexciting rods KL and NO.

As explained above, the distance GHMP is made equal to an odd number of half-wavelengths, so that the patterns vrradiated by the two exciting rods are apart in phase. The distance JMPO is made an integral number of half-wavelengths, and the distance JMHGL is also made an integral number of "half-wavelengths. AIt is apparent that, for physical reasons, distance distance JMHGL must be greater than distance JMHGL must be greater than distance JMPO. For reasons that will appear hereinafter, the distance LG is made an. odd number of quar ter-wavelengths. It has 'been lfound that, if the distance LG is to `be made an odd number of quarter-wavelengths, if the distance PMI-IG is to be an odd number of half-wavelengths, and if the distance JMHGL is greater than the distance JMPO, then when OPMJ is an odd number of half-wavelengths, JMHGL must be an even number of vhalf-wavelengths, and when OPMJisan even number of half-wavelengths, JMHGLmust be an odd number of half-wavelengths.

With the distances PMHG, JMPO, JMHGL, and JM (which is a quarter-wavelength or an odd number of quarter-wavelengths) being known ordesigned in accordance with the'above enumeration, the remaining distances, suchas MH, GH,'GL, etc., may be readily calculated.

In this embodiment, as in the embodiment -of Figs. 1-2, the food 6 is placed in container "I adjacent the mouth of the horn I8, the'container having a metallic reflecting surface la at ythe bottom thereof.

Also, surrounding all four `sides of the horn IIS. at the mouth thereof, is a channel I5 which opens toward the mouth of the horn, this channel being provided by the `metallic member I6 of L-shape, having its shorter leg firmly attached to the outer surface of horn I8. Channel I5', like channel I5, is substantially 7 \/4 deep, and functions in exactly the same way to prevent leakage of energy over the lip of the horn. The width of channel I5 is exaggerated in the drawing, forpurposes ciclarity.

`As in the Fig. 1 embodiment, the horn I8 may have a length which is either an odd number of quarter-wavelengths or an even number of quarter-wavelengths (an integra-1 number of halfwavelengths), both of these lengths being determined in accordance with the wavelength in the guide or horn, which wavelength is somewhat different from the wavelength in free space.

In operation, the phasing, with the exception of the excitation of the two exciting rods 180 out ofkphase, works out exactly the same as with the TML; horn previously described. First assuming a horn which is an even number of quarterwavelengths, or an integral number of halfwavelengths, long, a small amount of the incident wave energy propagating down guide I 8 reilected from the upper surface of the food, and a small amount is also reflected from the mouth of the horn, due to the discontinuity at this point.

The wave energy which is unattenuated by food 6 after passing therethrough impinges upon 'Horn `end Ia looks to the reflected wave like a short-circuiting plate with two small holes therein, thus in effect looking like a low impedance to the reflected wave but like a high impedance to any component of the reflected wave trying to enter either of the two holes for the coaxial feed line; the smaller such holes are, the higher such impedance will be. As a result, most of the reflected wave energy does not go down the coaxial feed line, but is reflected by horn end plate i8a toward the food 6, to be further attenuated thereby.

Although there are two holes in plate 18a, as contrasted to only one hole in plate 4a, plate [8a is much larger in total area than is plate 4a, so

that the amount of reflected energy which couples f f with feed line 20 is quite small, since the area of the holes in plate |8a is still small as compared to the total area of lsaid plate.

An Emin is established at short-circuiting plate |8a, this being consistent with the fact that there is an Emis at surface 'la and with the distance from surface la to plate l8a, which distance is an integral numberof half-wavelengths. In the same manneras before, voltage nodes or Emins are also set up at points O and L, which are in the same horizontal plane as the inner surface of horn end I8a.

As before, due to the short-circuit at point J, an Emir, is established at said point, and, since point M is spaced a distance of an odd number of quarter-wavelengths from point J, the waves reflected from point J establish an Emax at point M. Since the distance J MPO is an integral number of half-wavelengths, and since the distance JM is an odd number of quarter-wavelengths, the distance OPM is an odd number of quarter- Wavelengths. Therefore, the waves reflected from point O also tend to establish an Emax at point M, and such reflections are therefore in phase at point M lwith those from point J.

The distance MHG is an integral number of half-wavelengths, as may readily be seen by computation. Therefore, the reflections from points J and O will tend to establish an Emx at point G. Since the distance LG is an odd number of quarter-wavelengths and since an Emin is established at point L, as described above, the reflections from point L will also tend to establish an Emax at point G.

It will therefore been seen that the reflections from points O, J, and L are all in phase at point G, establishing an Emax at point G. Due to the complete short-circuit at point J and to the substantially complete short-circuit for reflected waves at horn end lBa, the reflected energy from these two locations is in effect what determines and holds fixed the phase in the feed line 20 at point G substantially independently of what goes on in the lower (food) end of the horn. This system has great phase stability, due to the rather high standing wave ratio in the feed line (points O, J, L, and la being tied together in phase) and the consequent high-Q of the line.

By establishing a known and constant value of phase at point G of the feed line (which value may, for example, be an Emax), the distance GF may be made such as to put the magnetron I at whatever phase angle is desired for most favorable operation of the magnetron.

As in the Fig. 1 embodiment, the advantages of a rather low standing wave ratio over a broad frequency range and a fixed-phase, high-Q feed line are obtained, but in the Fig. 4 embodiment a rather large cooking area is obtained with a single magnetron and horn.

If horn I8 is an odd number of quarter-wavelengths long, the phasing and operation are similar to that explained in detail above in connection -with Fig. l, the phasing being as above WithyEmin at points O, L, and J. Inthis case an Emx is established and fixed at point G due to the length dimensions of the parts of the feed line and the consequent inphase reflections. from the points O, J, and L at point G, although in this case, of course, the standing wave ratio and the Q of the line are both somewhat lower than in the case of the multiple-half-wavelength horn, due to the substantially infinite impedance presented to reflected waves by horn wall I8a as a consequence of the length of the horn.

Here, also, the advantages of a very low standing wave ratio over a rather broad frequency range and a fixed-phase, rather high-Q feed line are also obtained.

Of course, it is to be understood that this invention is not limited to the vparticular details vas described above, as many equivalents will suggest themselves to those skilled in the art. 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 claimed is: 1. A microwave energy transmission system, comprising a source of microwave energy, a transmission line of U-shape with two extra arms extending substantially parallel to the base of the U, one of said extra arms ending at and intersecting one of the legs of the U intermediate the ends thereof and the center line of said one extra arm lying in a horizontal plane which divides each of the legs of the U into a stub line and a branch line, the stub line of each leg being adjacent to and intersecting the base of the U, the free end of said one extra arm being connected to said source, the other of said extra arms ending at and intersecting the base of the U and being short-circuited at its free end, the lengths of the two stub lines plus the length of the base of the U being an odd number of half-wavelengths at the frequency of said microwave energy, the length of said other extra arm being an odd number of quarter-wavelengths, and a hollow waveguide having a longitudinal axis, said branches extending into said guide at one end thereof parallel to said axis to serve as exciting rods for said guide, the lengths of the portions of each branch between the stub ends of the same and said one end of said guide being an odd number of quarter-wavelengths long at said frequency.

2. A microwave energy transmission system, comprising a source of microwave energy, a transmission line of U-shape with two extra arms extending substantially parallel to the base of the U, one of said extra arms ending at and intersecting one of the legs of the U intermediate the ends thereof and the center line of said one extra arm lying in a horizontal plane which divides each of the legs of the U into a stub line and a branch line, the stub line of each leg being adjacent to and intersecting the base 0f the U, the free end of said one extra arm being connected to said source, the other of said extra arms ending at and intersecting the base of the U and being short-circuited at its free end, the lengths of the two stub lines plus the length of the base of the U being an odd number of half-wavelengths at the frequency of said microwave energy, the length of said other extra arm being an odd number of quarter-wavelengths,

yand a hollow waveguide having a closed end kand aiongitudinal axis,.saicl branches extending into said ,guide through the closed `.end thereof parallel to said axis to serve as 'exciting rods for said guide, the lengths of the portions of each ysaid'other extra arm, being an integral multiple of anali-wavelength, and the distance along the 4other leg of the "U and along the base of the U, from a point lying in the plane o'f the closed -end of 4said guide, to the vshort-circuited end of said other fextra arm, being an integral multiple of ahalf-'Wavelength long at said frequency.

NORMAN .R. WILD.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Name Date Mouromtsef 'Oct 14, l1941 Number Number Name Date 2,281,550 Barrow May 5, 1942 2,283,935 King May 26,1942 2,297,512 Baeyer Sept. 29, 1942 2,373,233 Dow Apr. 10, 1945 2,392,511 Thompson et a1 Jan. 8, 1946 2,398,606 Wang Apr. 16, 1946 2,404,797 Hansen July 30, 1946 2,407,318 Mieher Sept. 10, 1946 2,407,690 Southworth Sept. 17, 1946 2,408,032 Beck Sept. 24, 1946 2,427,094 Evans Sept. 9, 1947 2,497,670 Hanson et al Feb. 14, 1950 2,500,752 Hanson et al Mar. 14, 1950 2,513,205 lRoberts June 27, 1950 2,537,182 Bertrand Jan. 9, 1951 OTHER REFERENCES fPractical Analysis of Ultra .High Frequency im Transmission Lines, Resonant Sections, Resonant Cavities, Wave Guides, by J. R. Meagher and H J. Markley, August 1943, R. C. A. Service Company, Inc., Camden, N. J.

Steel, vol. 117, No. 20, November 12, 1945, page

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