US3478188A

US3478188A - Multimode cavity resonator with two coupling holes at wall corners

Info

Publication number: US3478188A
Application number: US675172A
Authority: US
Inventors: Jerome R White
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-10-13
Filing date: 1967-10-13
Publication date: 1969-11-11
Anticipated expiration: 1986-11-11
Also published as: DE1802741A1; GB1233030A

Description

United States Patent O 3,478188 MULTIMODE CAVITY RESONATOR WITH TWO COUPLING HOLES AT WALL CORNERS Jerome R. White. San Carlos. Calif. assignor to Varian Associates, Palo Alte, Calif. a corporation of California Filed Oct. 13, 1967, Sex. N0. 675,172 Int. Cl. H05b 9/06, 5/00; H01 1/00 U.S. Cl. 21910.55 10 Claims ABSTRACT OF THE DISCLOSURE A rectangular 'multimode cavity resonator is excited by microwave energy introduced into the resonator through two coupling holes located at different corners of one of the boundary walls forming the rectangular cavity resonator. Each of two dominant mode TE waveguides couple the cavity resonator at its two coupling holes to a microwave' energy source. The waveguides are oriented relative to each other so that the electric field component of the electromagnetic field propagated by one of the waveguides is in a direction transverse to that of the field propagated by the other waveguide in the plane of the wall defining the coupling holes. Mechanical mode stirrers are mounted within the cavity resonator and coupled to a suitable drive motor located exteriorly of the cavity resonator. Supporting structure is provided within the cavity resonator to support the material being heated at a distance of about 2 /2)\ from the nearest boundary wall.

BACKGROUND OF INVENTION Heating materials with microwave energy has become common in a number of industrial applications. Generally one of the m0st important objects in designing microwave heating systems is to construct the system so that uniform heating is produced in the material independent of its size and shape. In certain applications, the energy of the electromagnetic field should be distributed in a particular non-uniform manner in the zone provided for heating, while in others, the energy should be distributed uniformly in the heating zone. An improper microwave energy distribution creates localized regions of maximum and minimum heating in the materials. In most cases, such non-uniform heating is undesirable and often is harmful to the materials. Unfortunately, the inability to control the microwave energy distribution conveniently and properly has limited the extent of use of microwave energy in industrial heating applications. v

Multimode microwave cavity and waveguide resonators are susceptible to being excited in a plurality of electromagnetic field mode patterns, each occurring at a different frequency. All of the mode patterns of a particular microwaVe resonator may be classified into three distinct subsets of resonator modes. The resonator modes of each subset are distinguishable from those of the others by the orientations of their electromagnetic fields and associated wa1l currents. F01 a given microwave source frequency and microwave coupling means, a multimode resonator may be excited in a mode pattern whose frequency is close to the source frequency. The bandwidth of the mode determines how close 'the frequency of the source must be to the mode frequency to excite the resonator in the particular mode. Heretofore, mode stirrers have been employed in multimode microwave resonators in an attempt to obtain a particular, usually time-averaged uniform, electromagnetic field, hence, microwave energy distribution throughout the zone provided for heating materials. Operation of mode stirrers causes the electrical space as seen by the electromagnetic field in the resonator t0 change. This change in the electrical space shifts the frequencies at which the various mode patterns occur. By shifting the mode pattern frequencies so that they sequentially and cyclically coincide with the frequencv of the microwave energy source, the distribution of the electromagnetic field, hence, microwave energy in the resonator is caused to change cyclically.

The time-averaged distribution of the energy depends upon the number of ditferent mode patterns coupled to within the resonator and the amount of energy delivered to each of the diiferent mode patterns excited in the resonator. For example, to obtain a time-averaged uniform distribution of microwave energy throughout a beating zone having dimensions 011 the order of multiples of the free-space wavelength of the applied energy, it Es preferred to deliver energy uniformly to a large number of the possible resonator modes. However, the prior art techniques commonly employed in delivering microwave energy to the resonator provide coupling to only a few of the possible resonator modes. As a consequence of coupling to only a few of the possible resonator modes, either the uniformity of the time-averaged energy distribution is enhanced only slightly or the undesirable nonuniformity is intensified due to reenforcing mode patterns being excited Within the resonator. Furthermore, when only a few of the possible resonator modes are excited in the resonator, the ability to establish particular nonuniform time-averaged microwave energy distributions is curtailed.

By cyclically energizing a number of resonator modes belonging to all of the resonator mode subsets, the localized field regions throughout the resonator can be time-averaged to provide a desired net electromagnetic field, hence, microwave energy distribution therein. T0 couple to a large number of resonator modes belonging to all three subsets, microwave energy must be delivered to the resonator in the form of electromagnetic fields which have components of electric field orientated in the directions of the wall currents which would be characteristic of each of the possible Sets of resonator modes at the point at which energy is introduced into the resonator. In an article by Paul W. Crapuchetts, Microwaves on the Production Line, Electronics, March 7, 1966, pp. 123-130 it is suggested that a time-averaged uniform electromagnetic field distribution for heating materials can be realized by exciting a multimode cavity resonator with microwaves coupled thereto by a coupling loop located in the cavity at the junction cf three intersection cavity boundary Walls. I-Iowever, coupling loops have certain limitations and disadvantages associated therewith that render them less attractive than other microwave guide structures such as single conductor waveguides. Specifically, microwave tran smission systems employing coupling loops have limited ower handling capabilities. Furthermore, very cornplicated structures are required if the coupling loop system must be cooled. In addition coupling loops tend to spark and become dirty thereby requiring cleaning which often is difiiicult to accomplish unless the system is disassembled.

SUMMARY OF THE INVENTION The present invention relates to microwave heating systems and, more particularly, to a microwave heating system which provides a selected time-averaged electromagnetic field distribution for heating materials introduced therein. In accordance with the present invention, a desired time-averaged electromagnetic field distribution is obtained by providing a resonant chamber for heating materials comprised of at least three conductive boundary walls which intersect to define a junction. A corner of one of the boundary Walls adjacent a junction of three intersection boundary walls is provided with a first coupling hole for introducing into the resonator through a wall plane thereof microwave energy transmitted by a first single conductor waveguide structure joining the resonator to a microwave energy source. When delivering energy to a resonator at a Wall corner, energy Will be coupled to resonator modes belonging to at least first and second resonator mode subsets regardless of the mode of propagation in the Waveguide. To achieve selected coupling to any of the subsets of resonator modes, at least a second coupling hole is provided at another corner of a boundary wall adjacent a junction of three intersecting boundary Walls for delivering to the resonator through a Wall plane thereof microwave energy transmitted by a second single conductor waveguide structure joining the resonator to a microwave energy source. The second waveguide is constructed to propagate energy in a mode pattern having an electromagnetic field distribution including an electric field having components orientated in directions parallel to components of wall current which would be characteristic cf at least the third and one of the other subsets of resonator modes at the location of the second coupling hole if a boundary Wall portion existed in its place.

Energizing the various modes is accomplished by also providing means for shifting the frequencies at which the various modes occur so that the mode pattern frequencies sequentially and cyclically coincide with the source frequency. When the frequency of a mode pattern belong ing to either the first 01 the second resonator mode subset coincides with the source frequency, energy will be delivered into that mode from at least the Waveguide associated With the first coupling hole. Similarly, when-the frequency of a mode pattern belonging to the third resonatot mode subset coincides With the source frequency, energy will be delivered into that mode from at least the waveguide associated with the second coupling hole.

Accordingly, it is an object of this invention to provide a microwave heating system for heating materials.

More particularly, it is an object of this invention to provide a microwave heating system which presents a time-averaged electromagnetic field distribution for uniformly heating materials.

Another object cf this invention is to provide a multimode resonator type of microwave heating system wherein the resonator is excitable in resonator modes belonging to all of the subsets of resonator modes.

Still another object of this invention is to provide a microwave heating system which presents a time-averaged uniform electromagnetic field distribution for heating materials.

Yet another object of this invention is to provide a multimode resonator type of microwave heating system wherein the electromagnetic field mode pattern 1's varied cyclically through resonator modes belonging to all of the resonator mode subsets.

It is a further object of this invention to provide a multimode resonator type of microwave heating system wherein a nurnber of resonator modes belonging to all of the subsets of resonator modes are uniformly excited.

It is yet another object of the present invention to provide a microwave heating system which presents a desired time-averaged electromagnetic field distribution for heating materials which does not have the limitations and disadvantages associated with such systems employing coupling loops to deliver the microwave energy to the apparatus in which the materials are heated.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and advantages of the present invention will become more apparent from the following detailed description and appended claims considered together With the accompanying drawing in which:

FIGURE 1 is a perspective view of one embodiment of a microwave heating system in accordance with the present invention,

FIGURE 2 is an end sectional view of the microwave heating system taken along line 22 of FIGURE 1.

FIGURES 3a-c schematically illustrate the orientations of the conduction currents in the boundary walls 01 a resonator at a junction of three intersecting boundary Walls at an instant for the three resonator mode subsets.

FIGURE 4 is a fragmentary top view of the microwave resonator of the heating system taken along line 4-4 of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIGURES 1 and 2, a conveyorized multimode rectangular cavity resonator embodiment of the microwave heating system 11 of the present invention is illustrated. The microwave heating system 11 includes a rectangular type multimode cavity resonator 12 of a size suflicient to support a plurality of modes of electromagnetic waves at the operating frequency. The cavity resonator 12 is constructed of conductive material, such as aluminum, and has a plurality of planar boundary Walls including

end Walls

13 and 14,

side walls

16 and 17, and top and

bottom walls

18 and 19 respectively secured together, as by welding, to define enclosure 21. The height, length and width dimensions of the enclosure 21 are made large compared to the free space wavelength cf the applied energy, for example, approximately 7 17k, and 81 respectively.

Products to be heated by microwave energy delivered to the cavity resonator 12 are transported through the resonator by a moving conveyor belt 22 constructed of microwave transparent material, such as cotton cloth, Teflon or polypropylene. T0 facilitate the circulation of air around the products, the conveyor belt 22 is perforated to define air passages 23 therethrough. The conveyor belt 22 enters the cavity resonator 12 from its feed end 24 through a first microwave absorbing end trap 25 located at the end Wall 13. The conveyor belt 22 leaves the cavity resonator 12 at its discharge end 26 through a second microwave absorbing end trap 27 located at the end wall 14. The end traps serve to prevent the escape of hazardous microwave energy from the cavity resonator 12 while allowing access thereto during operation.

Bach of the absorbing end traps 25 and 27 includes a rectangular aluminum box 28 surrounding an annularlike container in the form of a tubular member 29 for holding lossy material, preferably such as water, ethylene glycol, glycerol or any 10W molecular weight monohydric alcohol. The tubular member 29 defines a product tunnel 31 of rectangular cross section through which the conveyor halt 22 passes to transport produets through the enclosure 21. The end traps 25 and 27 are mounted t0 the

end walls

13 and 14 respectively, for example, by welding, With their respective product tunnels 31 aligned with passageways 32 defined in eac'h of the

end walls

13 and 14.

For non-hazardous operation, the amount of microwave energy escaping to the surroundings of the microwave heating device should be maintained below the accepted standard of 10 milliwatts/(centimeter) -(mw./ cm). With 5 kilowatts (kw.) of power at 2450 mc. applied to the cavity resonator 12, the microwave energy escaping to the sunoundings will be maintained substantially below the level of 10 mw./cm. by constructing the end traps 25 and 27 to have dimensions of at least 6x long, 5 wide and 5) high with the tubular member 29 having a wall th.ickness of 31/2.

T0 provide ease of access to enclosure 21 in Order t0, for example, facilitate the erformance of rnaintenance, the side Wall 17 is provided with two conductive access doors 33 longitudinally spaced in the direction of the conveyor belt travel. Bach door 33 is hinged at 34 and is provided with a pivotally mounted bar handle 36 for opening and closing the door. A compressible conductive V-shaped rib structure 37 is mounted circumadjacent about each door opening 38 to prevent hazardous mlcro- WaVe energy firom escaping between the doors 33 and the side wall 17. Bach door 33 is forceably held against the rib structure 37 by the engagv:ment of the bar handle 36 in the bar receiving member 39.

T0 enhance the microwave heating operations, a aluminum air plenum 41 communicating With an air compressor (not shown) delivers an air flow to the enclosure 21 through aluminum open-ended air ducts 42 spaced longitudinally in the direction of the conveyor belt travel. The air ducts 42 are secured by welding one of their ends to the air Plenum 41. The air plenum 41 and ducts 42 are secured at side wall 16 by welding the opposite ends of the air ducts 42 thereto at locations above the couveyor belt 22. T0 convey away the circulated air, an aluminum ex'haust plenum 43 receives the circulated air from the enclosure 21 through aluminum open-ended exhaust ducts 44. The exhaust ducts 44 are secured by welding one of thei1' ends to side wall 16 at locations below the conveyor =belt 22 longitudinally spaced in the direction of conveyor belt travel. The exhaust lenum 43 is fastened by welding to the other ends of the exhaust ducts 44.

T0 prevent the escape of microwave energy through the air and

exhaust ducts

42 and 44, the ducts are coustructed to have a length of at least three times their diameter and cross sectional dimensions so that the free space cutolf wavelength relative to the highest frequency significant energy mode excited in the cavity resonator 12 is significantly less than the free space wavelength 0f that mode. With the dimensions of the

ducts

42 and 44 adjusted in accordance With these limitations, the

ducts

42 and 44 will not Support the free transmission of electromagnetic fields at hazardous or undesirable micro wave energy levels.

Pursuant to the present invention, microwave energy is delivered to excite the cavity resonator 12 through at least two particularly located coupling holes 51 and 52. The coupling holes 51 and 52 are located in the same or different, resonator boundary walls at corners thereof adjacent any of the junctions defined by three intersecting resonator l oundary walls. Microwave energy is introduced into the cavity resonator 12 from single conductor waveguide structures, for example,

rectangular waveguides

53 and 54 through the coupling holes 51 and 52 respectively coupled in energy receiving relationship to the waveguides. The

waveguides

53 and 54 are comstructed and coupled to the coupling holes 51 and 52 so that resonator modes belonging to all three resonator mode subsets are susceptible to being excited in the resonator 12 at selected energy levels. Coupling to all three resonator mode subsets is accomplished by coustructing 0ne of the waveguides, such as waveguide 53, and orientating it relative to the resonator wall defim'ng the coupling hole 51 so that the electric fieldof the electromagnetic field propagated by the waveguide 53 has components orientated in directions parallel to components of Wall current which would be characteristic of resonator modes belonging to at least two of the resonator mode subsets at the location of the coupling hole 51 if: a resonator wall portion existed in its place. Coupling to the resonator modes belonging to the third resonatormode subset is effected -by constructing waveguide 54 and orientating it relative to the resonator wall defining the coupling hole 52 so that the electric field of the electromagnetic field propagated by the waveguide 54 has components orientated in directions parallel to components of wall current Which would lae characteristic of resonator modes belonging to at least the third resonator mode subset at the location of the coupling hole 52 if a resonator boundary wall portion existed in its place.

T0 more fully appreciate the importance of locating the coupling holes 51 and 52 at the resonator boundary wall corners and of orientating the

waveguide structures

53 and 54 relative to the resonator boundary walls in the manner described, attention is directed to FIGURES 3a-c. Bach of the FIGURES 3a-c schematically depict the wall current pattern of a resonator mode of one of the three resonator mode subsets at a particular instant in the vicinity of the junctions formed by three intersecting resonator boundary walls, such as, junction 56 formed by the intersecting end wall 13, side wall 17, and top wall 18 of the resonator 12 illustrated in FIGURES 1 and 2. The wall current pattern for each of the possible resonator modes will be the same as one of those represented by arrow-

s

57, 58 and 59 in one of the FIGURES 3a-c. At a given source frequen-cy, the wall currents of any resonator mode which at any instant intersect the edges -of the boundary walls, e.g., 13, 17 and 18, converging to the

corners

61, 62 and 63 will be of the same sense along the edge lengths hetween each of the corners and points 64 located at distances M2 therefrom. Furthermore, the sense of the instantaneous wall currents will reverse at positions 66 located at perpen'clicular distances of /4 or greater from the edges of the boundary walls. addition, the wall currents will be zero at the corners of the boundary walls for any of the possible resonator modes. Consequently, instantaneous wall currents of opposite senses will never simultaneously intersect the edges of the walls along the lengths thereof up to 7\/2 from their corners, and will never exist simultaneously in the portions of the boundary walls up to 4 from the boundary wall edges.

Therefore, since coupling will exist between a waveguide structure and the resonator modes of a subset when electrornagnetic fields propagated by the waveguide includes components of electric field which are parallel to the wall currents characteristic of the subset at the location at which the waveguide is coupled to the resonator, resonator modes belonging to all three of the resonator mode subsets can be excited in the resonator 12 by coupling such waveguides thereto through coupling holes located in the corners of the resonator boundary walls at the junctions. By confining the coupling holes to L-shaped zones of the boundary wall defined by M4 wide and 2 long extremities intersecting at the corners of the wall, the guide structures can 'be joined to the resonator so that the instantaneous field in the guide will not simultaneously include components of electric field which are parallel and antiparallel to the instantaneous wall currents which would be characteristic of the possible resonator modes at the location of the coupling holes if boundary wall portions existed in their place. This facilitatcs uniform energy coupling to the resonator modes. However, some degree of simultaneous parallelism and antiparallelisrn might be desirable, for example, when a particular neu-uniform energy distribution is wanted. Equal amounts of simultaneous parallelism and antiparallelism is to be avoided when energy transfer between the waveguide and resonator is desired, because under such conditions, very little if any energy is transferred between the resonator and waveguide. T0 insure coupling to a number of resonator modes belonging to all three subsets of resonator modes, the coupling holes should not be farther than from the wall corners formed by the intersecting edges of the boundary walls at which the coupling holes are located, should extend to points less than along the boundary wall edges and to points less than M2 away from the edges, and should have greater area portions in L-shaped zones of the boundary Wall defined by /4 wide and 2 loug extremities intersecting at the corners than outside the zones.

Selective coupling to the three resonator mode subsets can be achieved With Waveguides propagating either TE or TM waves. A waveguide propagating TE waves and coupled to the resonator 12 through, for example, a corner of the top wall 18 With the electric field of the wave propagated in the guide having components orientated with a high degree of parallelism in the direction of arrows 57 of FIGURE 3a will couple to resonator modes belonging to two subsets, thse depicted by FIGURES 3a and 3c. T0 couple the resonator modes belonging to the subset depicted in FIGURE 3b, energy is introduced into the resonator at another corner of one of the boundary walls from a waveguide propagating electromagnetic fields in either TE or TM modes. Since TM mode waveguides propagate electromagnetic fields having components of electric field which are in a direction parallel to the wall currents characteristic of all three subsets of resonator modes, the orientation of the waveguide relative to the resonator 12 is not critical to elfect coupling to the three subsets of resonator modes. I-Iowever, where a second TE mode wavegnide is employed, it would be orientated so that electric field component of the electromagnetic field propagated thereby would be parallel to either

arrows

57 or 58 in FIGURE 3b, or arrows 57 in FIGURE 3c.

Two TM mode waveguides coupled to the resonator 12 at diflerent Wall corners also conld be employed to excite the resonator in modes belonging to all three subsets. Although a single TM mode waveguide will couple to resonator modes belonging to all three subsets, two TM mode Waveguides facilitate the selective coupling to the resonator modes of each of the subsets.

Coupling uniformly to the resonator modes of the three subsets can be achieved by selecting the location 0f the two coupling holes and the associated waveguides from which energy is introduced into the resonator so that the electric field component of the electromagnetic field propagated by one of the waveguides is perpendicular at its associated coupling hole to the direction of the Wall currents that would be induced thereat by energy from the other waveguide, if a boundary wall portion existed in place of the associated coupling hole. In other words, at the location of the coupling holes, the electric field component of the electromagnetic field propagated by one of the waveguides extends in a direction which is transverse to an edge of a Wall of the resonator which is transverse to the edge of the wall which is transversely intersected by the electric field component propagated by the other waveguide. For example, referring to FIG- URES 1 and 3a-c, this could be achieved by exciting the cavity resonator 12 with energy delivered through the sarne or opposite resonator boundary Walls from two

TE mode waveguides

53 and 54 particularly orientated relative to one another. More specifically, energy would be introduced into the cavity resonator 12 from the T E mode waveguide 54 located at, for example, the wall corner 63 of the top Wall 18 so that the electnc field component of the electromagnetic field propagated in the waveguide is orientated in the direction of the Wall current arrows 58 in FIGURE 3b. The second TE mode waveguide 53 would introduce energy into the cavity resonator 12 at a second wall corner, for example, 67 of the top Wall 18 and would -be orientated so that the electric field component of the electromagnetic field propagated thereby extends in the direction of the wall current arrows 57 in FIGURE 3a, or in other words, transversely to the direction in which the electric field component of the electromagnetic field propagated in the first waveguide 54 e'xtends at the location -of the coupling hole 52. When the frequencies of the resonator modes belonging to the subset depicted in FIGURE 3b coincide With the source frequency, energy will be delivered to those resonator modes from the waveguide 54. When the frequencies of the resonator modes belonging to the subset depicted in FIGURE 3a coincide With the source frequency, energy Will be delivered to those resonator modes from the waveguide 53. Energy from both

waveguides

53 and 54 will be delivered to the resonator modes of 8 the subset depicted in FIGURE 3c When their frequencies coincide with the source frequency.

Referring again to FIGURES 1 and 2 one embodiment of the present invention has rectangular shaped coupling holes 51 and 52 at the

wall corners

67 and 63 of the rectangular top Wall 18. The width, w, and the height, h, of each of the coupling holes are M2 and M4 respectively. Although large coupling holes are desired for reasons of eflicient coupling between the

waveguides

53 and 54 and cavity resonator 12, smaller coupling holes of other shapes could be employed. The design of the coupling holes is undertaken in accordance with standard impedance matching techniques, taking into consideration the impedance of the resonator 12, coupling holes 51 and 52,

waveguides

53 and 54, and the microwave energy source. For heavy loads, it is desired to tightly couple to the resonator modes. Hence, large coupling holes 51 and 52 would be employed, preferably, rectangular and each covering an area of M2 along the houndary Wall edge by M4 away from the edge. For light loads, looser coupling is desired to prevent undesirable reflections. In these cases, smaller sized coupling holes would be employed. T0 facilitate impedance matching between the resonator 12 and any microwave energy source coupled thereto, common adjustable impedance matching elements 78 and 79 could be inserted in the waveguide paths coupling the cavity resonator 12 to the energy sources employed.

R ferring to FIGURE 4, in order to assure coupling to modes belonging to all of the subsets of resonator modes, the coupling holes 51 and 52 are located in the top wall 18 so that their

respective sides

68 and 69, and 71 and 72 pr0ximate the

edge portion

73 and 74, and 76 and 77 of the top Wall 18 converging to the

Wall comers

67 and 63 are in the plane of the inside surface of the vertical end and

side Walls

13, 16 and 17. The long side 68 of cou-- pling hole 51 is orientated parallel to the edge portion 73 of the top wall 18 extending longitudinally in the direction of the travel of the conveyor belt 22. The coupling hole 52 is orientated With its long side 71 parallel to the edge portion 76 of the top wall 18 which extends laterally of the direction of the travel of the conveyor belt 22.

Coupling hole 51 is coupled in energy receiving relationship with a rectangular type waveguide 53 excited by a microwave energy source 81 to propagate energy in the dominant TE mode. The waveguide 53 is joined to the cavity resonator 1 2 by fastening it to a mating rectangular waveguicle flange 82 secured to the top Wall 18 as by Welding. The mating flange 52 and waveguide 53 are secured to the cavity resonator 12 so that the electric field of the electrornagnetic field propagated by the dominant mode waveguide is perpendicular to the edge portion 73 of the top Wall 18.

The coupling hole 52 is coupled in energy receiving relationship with a rectangular type waveguide 54 excited by a second microwave energy source 83 to propagate energy in the dominant TE mode. If desired, a single microwave energy source could be employed to excite the cavity resonator 12 at both of the coupling holes 51 and 52. In such circumstances, the

waveguides

53 and 54 would be coupled to the single source by, for example, a suitable T-type waveguicle junction. In any case, the waveguide 54 is joined to the cavity resonator 12 by fastening it to a mating rectangular flange 84 secured to the top Wall 18 by welding. The mating flange 84 and waveguide 54 are secured to the cavity resonator 12 so that the electric field of the electrornagnetic field propagated by the dominant mode waveguide is perpendicular to the edge portion 76 of the top wall 18 and, hence, to the direction of the electric field cornponent at the coupling hole 51 of the electromagnetic field propagated in the waveguide 53.

For uniform coupling to the resonator modes belonging to all three subsets of resonator modes, the

waveguides

53 and 54 are excited and respeetively orientated relative to the cavity resonator 12 so that the instantaneous electric field components of the electromagnetic fields propagated by each at their associated coupling holes 51 and 52 are simultaneously in the direction towards or away from the proxitnate edges of the top wall 18 perpendicularly orientated with respect to the electric field components.

As described hereinbefore, TM mode waveguides also could be employed to couple microwave energy to excite the cavity resonator. When such waveguides are employed, the size of the coupling holes 51 and 52 preferably would be x/4 by 4. As in the case when TE mode waveguides are employed, the coupling holes 51 and 52 could be smaller and of various shapes.

In one embodiment employing 2.5 kW.

energy sources

81 and 83 operating at about 2450 mc., the height, h, and width w, of the coupling holes 51 and 52 are 1.2 inches and 2.4 inches respectively. WR 340

rectangular waveguides

53 and 54 are employed to couple the

sources

81 and 83 to the cavity resonator 12. The guide dimensions of the WR 340 waveguides are a width of 3.4 inches and a height of 1.7 inches. The

mating waveguide flanges

82 and 84 have'dimensions corresponding to the associated

waveguides

53 and 54. Since the guide dimensins' of the WR 340

waveguides

53 and 54 are larger than the dimensions of the associated coupling holes 51 and 52, obstacles 86 and 87 are forrned at each of the coupling holes 51 and 52 by the portions of the top wall 18 which extend into the waveguide paths. These obstacles insnre coupling to the highest frequency modes of the resonator mode subsets that can exist in the cavity resonator 12.

T energize various modes of all of the resonator mode subsets, means 88 are also provided for shifting the frequencies at which the various modes occur so that sequentially and cyclically each mode pattern frequency coincides with the source frequency. In the embodiment illustrated in the figures, three mechanical type means 88 for mode stirring are employed of the type, for exarnple, as described in the United States application, Ser. N0. 624,503 of Rexford E. Black filed Mar. 20, 1967 entitled Disc Mode Stirrer and assigned to the assignee of this application. Bach mode stirring means 88 includes a discshaped member 89 of conductive material having diametncally

opposite chord segments

91 and 92 extending angularly from the plane of the disc member. The three disc-shaped mernbers 89 are rotatably mounted within the cavity resonator 12 to be rotated by a drive motor 93 located exteriorly of the resonator. As the disc-shaped members 89 are rotated, the electrical space of the enclosure 21 as seen by the electromagnetic field within the resonator changes. As the electrical space of the enclosure 21 changes, the frequencies of the resonator modes are caused to shift relative to the source frequency. As the frequencies of the resonator modes shift, diiferent ones will coincide with the frequency of the sources. In one embodiment, stirrers 88 employing discs 89 measuring about eight inches in diameter with identical

chord seg ments

91 and 92 having a chord lengt-h of about seven inches inclined away frorn the plane of the disc at 30" were located at the center of the upper half of the side wall 16, the center of the top wall 18, and the center of the upper quarter section of the end wall 14 proximate the side wall 16. This arrangement of mode stirrers 88 provides a time-averaged uniform electrornagnetic field distribution in the zone about the conveyor belt 22.

The average intensity of the electrornagnetic field is uniform throughout most of the enclosure 21. However, in regions of the enclosure 21 measuring M4 to M2 from the boundary Walls of the cavity resonator 12, the. electric field intensity falls 011 rapidly to be zero at the boundary walls. Therefore, t0 accomplish uniform beating of products, the conveyor belt 22 should be supported at least 4 above the bottom wall 19 of the resonator 12 and, preferably, at least M2. T0 support the conveyor belt 22 as it passes throngh the cavity resonator 12, a plurality of 2 inches by /2 inch slats 94, of polypropylene or other microwave transparent material, are supported by 10 brackets 96 secured to the

end walls

13 and 14 to extend longitudinally in the direction of the conveyor belt travel. The slats 94 are mounted about one foot above the bottom wall 19. T0 allow air to be circulated about the products transported through the cavity resonator 12, the slats 94 are spaced apart about /2 inch.

In many industrial ap1 lications an air flow is needed to rnaintain the preferred range of hurnidity at the products being heated. Since it is only necessary to direct the air flow over the surfaces of the products, in large volume resonators, a panel 97 of polypropylene or other microwave transparent material would be mounted just above the air ducts 42 and conveyor belt 22 by brackets 98 secured to the resonator walls to confine the air flow to the zone of the enclosure 21 through which the products pass. This makes more eflicient use of the air flow system.

In the embodiment of the present invention illustrated in the figures and described in detail hereinabove, energy was coupled into the cavity resonator 12 at two locations. However, energy could be delivered into the resonator 12 at additional wall corners or other locations of the resonator walls if: desired. Delivering energy into the resonator 12 at more than two wall corners will facilitate, for example, introducing more energy into the resonator for heating materials and tailoring the tirne-averaged distribution of the electromagnetic field in the resonator as desired. The use 0f additional coupling holes rn2iy be employed when it is desired t0 generate non-uniform timeaveraged field distributions within the cavity resonator 12.

While the present invention has been described in detail with respect to a particular embodiment, it is apparent therefrom that numerous modifications and variations are possible within the spirit and scope of the invention. Particularly, the waveguides coupling the multimode resonator to the microwave energy sources could be shortened in length so that the sources would be mounted directly to the multimode resonator walls at the coupling holes with the output windows comrnonly employed in the sources located at the coupling holes. Hence, the present invention is not to be limited except by the terrns of the following claims.

What is clairned is:

1. Apparatus for heating materials with microwave energy contained in a selected time-averaged electromagnetic field distribution comprising a multimode microwave cavity resonator which is susceptible to being excited in a plurality of frequency dependent electromagnetic field mode patterns each of which belongs to one of three classes of resonator mode subsets, said resonator having at least three conductive boundary walls which intersect at a junction defining a heating zone for subjecting the materials to microwave energy, a portion of one of the boundary walls having a first coupling hole disposed adjacent a corner thereof defined by intersecting wall edges at a junction of three intersecting walls of said cavity resonator for introducing microwave energy into said resonator through the plane of said wall, one 0f said boundary walls having at least a second coupling hole disposed adjacent a corner thereof defined by intersecting wall edges at a junction of three intersecting walls 0f said cavity for introducing microwave energy into said cavity resonator through a plane of said Wall, at least a portion of each of said coupling holes being located within a boundary wall zone measuring )\/2 frorn the adjacent corner along the edge of said wall and M4 perpendicularly from the edge of said wall, a first single conductor waveguide for deliverying microw-ave energy from a source to said cavity resonator through said first coupling hole to couple t0 every resonator mode belonging to at least two of the resonator mode subsets, a second single conductor waveguide for delivering microwave energy from a source to said resonator through said second coupling hole, said second waveguide constructed and oriented at the second coupling hole relative to said cavity resonator so that the mode pattern in which energy is propagated in said second waveguide has an electrornagnetic field distribution including an electric field having a cornponent oriented in a direction parallel to a wal1 current component which would be characteristic of at least the third subset of resonator modes at the location of said second coupling hole if a boundary wa11 portion existed in its place t couple to every resonator mode cf the third subset.

2. The apparatus according to claim 1 including, means for shifting the frequencies at which the resonator mode patterns occur relative to the source frequency.

3. The apparatus according to claim 1 wherein a larger portion of each of said coupling holes is located within a boundary wa11 zone measuring M2 frorn the adjacent cor- 11er along the edge of said wal1 and 7\/4 perpendicularly from the edge of said wall than is located outside the boundary Wall Zone.

4. The apparatus according to claim 1 wherein said waveguides are rectangular waveguides having long and short sides.

5. The apparatus according to claim 4 wherein the coupling holes are located in the same boundary wa11 of the resonator, and the first waveguide fixed to the wa1l With its long side at a first edge of the wall and the second waveguide fixed to the wall with its long side at an edge perpendicular to the first edge.

-6. The apparatus according to claim 5 wherein said coupling ho1es are rectangular having dimensions of M 2 by M4 and each located at a corner of the wal1 with its long dimension extending in the direction of the long side of the waveguide coupled thereto.

7. The apparatus according to claim 6 further including means for supporting the material within said resonator at least M4 from the boundary walls of said resonator.

8. The apparatus according to claim 4 further including a first microwave energy source coup1ed to said first waveguide to deliver microwave energy at a selected frequency to said resonator, and a second microwave energy source coupled t0 said second waveguide to de1iver microwave energy at a selected frequency to said resonator.

9. The apparatus according to claim 8 further including impedance matching means in circuit connection With said resonator, first and second waveguides, and first and second microwave energy sources.

10. The apparatus according to claim 4 further including means for supporting the material within said resonator at least M4 from the boundary Walls of said resonator.

References Cited UNITED STATES PATENTS 2420,354 5/1947 Carter 33383 2593,067 4/1952 Spencer 219-1055 2,640,760 8/1953 Hall et a1 219-10.55 3365562 1/1968 Jeppson 219-10.55

OTHER REFERENCES Schmidt, Germany application 1081,987 printed May 19, 1960, (K1. 21h36), 3 pages spec., 3 sheets drawing.

Heyne, Germany application 1,134,779 printed Aug. 16, 1962 (K1. 21h36), 3 pages spec. 1 sheet drawing.

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S. C1. X.R.