Microwave Oven

Abstract

A source of microwave energy such as a magnetron feeds the energy via a waveguide to preferably the top of a microwave cavity having access thereto for disposing food therein. A quarter wavelength transformer intercouples the waveguide and the top of the cavity, and the waveguide dimensions and operating frequency are preselected so that preferably only the dominant TE.sub.1,o mode is excited. The transformer properly matches the waveguide with the cavity dimensions and, when the food load is disposed in the oven, the dominant mode is established in the cavity thereby causing predictable and relatively uniform heating over a useable area of the cavity.

Description

FIELD OF THE INVENTION

The present invention relates in general to microwave heating or cooking ovens. More particularly, the present invention is concerned with an improved microwave oven construction wherein the cooking or heating cavity has primarily only the dominant TE.sub.1,o mode established therein.

BACKGROUND OF THE INVENTION

In the prior art U. S. Pat. No. 3,218,429 issued Nov. 16, 1965 to T. Lenart shows a typical microwave oven structure using a magnetron for supplying electromagnetic energy to the oven cavity. In an attempt to provide more uniform heating in the oven cavity a mode stirrer is used in many commercially available ovens. A mode stirrer is shown in the above mentioned patent and is used to produce a homogeneous electromagnetic field by supposedly mixing the modes thereby assuring that a plurality of modes are established. In theory, the mode stirrer presents a varying impedance to the magnetron causing the magnetron to "pull" (oscillate at different frequencies), causing the standing wave pattern to change and excite different modes. However, in practice the frequency variation is generally not sufficient to excite additional modes and therefore, the mode stirrer has not been completely effective in providing more uniform heating patterns in the oven.

In the present invention the cavity is conceptually considered as a second waveguide with the food in the oven functioning as a termination for the waveguide. By providing a matching quarter wave transformer between the waveguide coupling from the magnetron and the cavity waveguide and operating at the appropriate microwave frequency it has been possible to excite primarily only the dominant TE.sub.1,o mode within the cavity. The E field heating pattern established in the cavity when operating in the dominant mode is relatively uniform over a useable area of the cavity specifically about the middle of the cavity. Also, with the structure in accordance with the teachings of the present invention there is no need for providing mode stirrers and it is preferred that they not be used.

OBJECTS OF THE INVENTION

Accordingly, it is one important object of the present invention to provide a microwave oven of improved construction and that is adapted to be operated to establish primarily only the dominant TE.sub.1,o mode within the cavity.

Another object of the present invention is to provide an improved microwave oven construction in accordance with the preceding object wherein the heating pattern is predictable and relatively uniform over a useable area of the oven.

A further object of the present invention is to provide an improved microwave oven construction that is relatively simple to fabricate and is less expensive to manufacture than presently available microwave ovens.

Still another object of the present invention is to provide an improved microwave oven construction wherein existing ovens may be relatively easily modified to incorporate the teachings of the present invention.

SUMMARY OF THE INVENTION

To accomplish the foregoing and other objects the microwave oven of the present invention comprises a conductive box-shaped structure defining a cavity, including means for supporting a load within the cavity, and having an opening preferably at the top thereof for receiving the microwave energy. The oven further comprises a source of microwave energy such as a magnetron and a rectangular waveguide positioned to receive energy from the source for transmission through the waveguide. The frequency of operation of the microwave energy source and the cross-sectional dimensions of the waveguide are selected so as to excite primarily only the dominant TE.sub.1,o mode in the waveguide. A quarter wavelength transformer having one or more steps couples from the other end of the waveguide to the opening into the cavity and is constructed symmetrically so as to enable the establishment of the dominant TE.sub.1,o mode when a load is present in the cavity.

In one disclosed embodiment of the present invention the waveguide has a 1.7 inches .times. 3.4 inches cross-sectional dimension, the cavity has a 12 inches .times. 12 inches cross-section and three, quarter wavelength, steps are used as the matching transformer between the waveguide and the open top of the cavity. In another embodiment the top of the box-shaped structure defining the cavity has a rectangular opening dimensioned to receive the end of the waveguide and an effective transformer is provided by extending a rectangular sleeve downwardly from the inner surface of the top of the structure. This sleeve is disposed intermediate the opening for the waveguide and the outer walls of the structure at the appropriate symmetrical position to provide the proper quarter wavelength match. In still another embodiment of the present invention the sleeve hereinbefore referred to may be annular in configuration, or either of the sleeve configurations may be replaced by spaced posts disposed along the locus of either the rectangular or annular sleeve. In one other embodiment the waveguide enters the structure from the side and a conductive strap extends from the bottom edge of the waveguide curving upwardly to contact the top of the structure or may be spaced from the top thereof to form effectively an inductive or capacitive coupling.

DESCRIPTION OF THE DRAWINGS

Numerous other objects, features and advantages of the present invention will now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a partially cut-away perspective view of a preferred embodiment of the microwave oven of the present invention;

FIG. 2 is a cross-sectional view through the oven of FIG. 1 taken along line 2--2;

FIG. 3 is a cross-sectional view through the oven of FIG. 1 taken along line 3--3;

FIG. 4 is a perspective view of another embodiment of the present invention specifically showing the inner upper oven construction;

FIG. 5 shows another embodiment of the present invention using an annular matching transformer;

FIG. 6 is still another embodiment of the present invention employing a plurality of spaced posts;

FIG. 7 is an embodiment of the invention wherein the power feed is from the side rather than the top of the oven;

FIG. 8 is a cross-sectional view through an oven similar to the one shown in FIG. 7; and

FIG. 9 is a perspective view of another arrangement for the waveguide feed of FIGS. 7 or 8.

DETAILED DESCRIPTION

Referring now to the drawings and in particular to FIGS. 1-3, there is shown a preferred embodiment of the present invention including an oven enclosure 10, step transformer 12 and feeding waveguide 14. FIG. 1 is a partially cut-away perspective view and FIGS. 2 and 3 are orthogonal cross-sections taken along lines 2--2 and 3--3, respectively, of FIG. 1. The enclosure 10 is constructed of a metal such as aluminum or stainless steel and includes side walls 16 and 18, back wall 20, top wall 22, bottom wall 24, and door 26 having handle 28 for enabling access thereto. A food retaining screen or tray 20 may be disposed on bottom wall 24 as depicted in FIG. 2.

The transformer 12 comprises three steps 32, 34 and 36 which may be constructed by extrusion molding in a single piece. In one embodiment the walls of each step are three-eighths inch thick. The step 32 is characterized by the dimensions L.sub.1, H.sub.1 and W.sub.1. The waveguide 14 is of conventional 1.7 .times. 3.4 inch size. Similarly, the steps 34 and 36 are characterized by the dimensions L.sub.2, H.sub.2 and W.sub.2 ; and L.sub.3, H.sub.3 and W.sub.3, respectively. The dimensions W.sub.4 and H.sub.4 of the enclosure 10 are both 12 inches.

The following known equations define the relationship between the H dimension of adjacent steps:

H.sub.1 = .sqroot.H.sub.o H.sub.2

H.sub.2 = .sqroot.H.sub.1 H.sub.3

H.sub.3 = .sqroot.H.sub.2 H.sub.4

Similarly, the following equations define the relationship between the W dimension of adjacent steps:

W.sub.1 = .sqroot.W.sub.o W.sub.2

W.sub.2 = .sqroot.W.sub.1 W.sub.3

W.sub.3 = .sqroot.W.sub.2 W.sub.4

In the above equations the dimensions H.sub.o, W.sub.o, H.sub.4 and W.sub.4 are known and thus these equations can be solved to yield:

H.sub.1 = 2.77 inch,

W.sub.1 = 4.66 inch,

H.sub.2 = 4.52 inch,

W.sub.2 = 6.40 inch

H.sub.3 = 7.36 inch, and

W.sub.3 = 8.76 inch.

The solution of the values of L.sub.1 , L.sub.2 and L.sub.3 involve the following equation:

.lambda. = .lambda..sub.o /.sqroot.1 - (.lambda..sub.o /2W).sup.2

where .lambda..sub.o = c/f, c = velocity of light, f = frequency and thus, .lambda..sub.o = 5.20 inch and .lambda..sub.3 = 5.01 inch. The corresponding values of L.sub.1, L.sub.2 and L.sub.3 are:

L.sub.1 = .lambda..sub.1 /4 = 1.405 inch,

L.sub.2 = .lambda..sub.2 /4 = 1.300 inch,

L.sub.3 = .lambda..sub.3 /4 = 1.253 inch.

The microwave energy is coupled from a typical magnetron tube via waveguide 14 and transformer 12 to the enclosure 10 to thereby establish the fundamental TE.sub.1,o mode in the oven.

FIG. 4 shows another embodiment of the present invention including oven enclosure 40 which may be substantially the same as enclosure 10 of FIG. 1, and waveguide 42 which may be a typical rectangular waveguide of 1.7 .times. 3.4 inch dimensions. The matching means in this embodiment includes a rectangular ridge 44 which depends downwardly from top wall 46 of the enclosure. When using the same dimensions for the enclosure and waveguide, as indicated in conjunction with FIG. 1, the dimensions H and W of the ridge are calculated to be 4.70 inches and 6.56 inches, respectively. The L dimension is on the order of 1.3 inches.

FIG. 5 shows another embodiment of the invention including enclosure 50, waveguide 52 and annular matching ridge 54 which depends downwardly from top wall 56 of the oven. The ridge 54 may have a diameter on the order of 5.5 inches and may extend downwardly about 1.3 inches. In an alternate arrangement the ridge 54 may be elliptical in shape.

In the embodiment of FIG. 6 the annular ridge 54 has been replaced by a series of posts 58 extending downwardly from top wall 59 of the enclosure. These posts are disposed along the locus defined by ridge 54 and in the disclosed embodiment there are eight posts. These posts may extend downwardly about 1.3 inches.

The embodiment shown in FIG. 7 is different from those previously discussed in that the waveguide 60 couples into a side wall 62 of enclosure 64. A relatively rigid strap 66 extends from wall 62 at the bottom edge 68 of the waveguide, and the strap has an upturned end 70 fixed at point 72 to approximately the midpoint of top wall 74. The fixing at point 72 may be provided by a screw extending through a loop in the end 70 and mating in a tapped hole in wall 74. Strap 66 may be replaced by a wire mesh or by a series of spaced rods.

FIG. 8 shows a cross-sectional view through another embodiment of the invention quite similar to the one disclosed in FIG. 7. However, in this embodiment the strap 80 has an upturned end 82 that does not attach to top wall 84 but instead defines with wall 84 a gap 86. The tap 86 may be 0.5 inches wide.

FIG. 9 shows still another embodiment of the invention in a perspective view wherein the strap 90 is adjustable in a vertical direction relative to the waveguide 92. One end 93 of the strap connects to a bar 94 which is moveable in walls 95 and 96 of the waveguide. Preferably, the rod 94 may be fixed in position by the use of detent means, for example.

Tests have been conducted with the oven of the present invention and also with commercially available ovens to determine the heating uniformity within a useable portion of the oven. To conduct these tests a temperature tray such as the one disclosed in copending patent application Ser. No. 214,095 has been employed. The tray includes 25 separate sensing compartments arranged in a 5 .times. 5 matrix. Tables I and II show the temperature pattern measured when using the oven of the present invention and another commercially available oven, respectively. ##SPC1##

The numbers shown in Tables I and II indicate the temperature rise measured in the corresponding compartment of the temperature tray when the tray is filled with a predetermined amount of water and the oven is operated for a predetermined time interval. Usually the initial temperature in all compartments is about 70.degree.. The Table I readings were taken using the embodiment of FIG. 4 of the present invention.

In order to evaluate the results shown in Tables I and II there are three ratios that are defined:

R.sub.1 = Highest Temperature in all Boxes/Lowest Temperature in all Boxes

R.sub.2 = Sum of Temperatures in Center Boxes/Sum of all Temperatures

R.sub.3 = Highest Temperature in Center Boxes/Lowest Temperature in Center Boxes

For the values shown in Tables I and II the following ratios have been calculated:

Table I Table II R.sub.1 =3.75 R.sub.1 =2.82 R.sub.2 =0.52 R.sub.2 =0.27 R.sub.3 =1.87 R.sub.3 =1.82

the ratio R.sub.1 is not that critical and generally any value of 4.0 or less is suitable. The most critical ratio is probably ratio R.sub.2. This ratio indicates that for case I (Table I) 52 percent of the total heat measured in the 25 boxes is concentrated in the center nine boxes where the majority of the food would be located, and where the heat is thus desired. Alternatively, in case II (Table II) only 26 percent of the total heat is concentrated in the center boxes. The ratio R.sub.3 is approximately the same in both cases.

Claims

What is claimed is:

1. A microwave oven comprising;

a source of microwave energy,

a waveguide of predetermined cross-section positioned with one end receiving energy from said source for transmission through said waveguide,

the frequency of operation of said source and the cross-sectional dimension of said waveguide being selected to excite the TE.sub.1,0 mode in said waveguide,

means defining an enclosure including means for providing access to the enclosure and means defining an opening into the enclosure,

and matching means coupling from the other end of the waveguide to the opening in the enclosure for establishing only the TE.sub.1,o mode in the enclosure.

2. The oven of claim 1 wherein said opening is rectangular in shape and said waveguide is a rectangular waveguide, the cross-sectional area of said opening being larger than the cross-sectional area of said waveguide.

3. The oven of claim 2 wherein said matching means includes a quarter wavelength step transformer coupling between said waveguide and opening.

4. The oven of claim 3 wherein said transformer comprises a plurality of quarter wavelength steps.

5. The oven of claim 1 wherein said matching means includes a quarter wavelength step transformer and said enclosure has means for supporting a load provided therein.

6. The oven of claim 1 wherein said matching means includes a continuous ridge depending from the wall of the enclosure defining the opening and disposed intermediate the edge of the opening and another wall defining the enclosure.

7. The oven of claim 6 wherein said ridge is rectangular in shape, said another wall includes oppositely disposed side walls, and said ridge is disposed intermediate the opening and side walls.

8. The oven of claim 7 wherein said waveguide and opening are rectangular and said ridge is disposed symmetrically with relation to said opening and side walls.

9. The oven of claim 6 wherein said ridge comprises a circular sleeve.

10. The oven of claim 6 wherein said ridge comprises an elliptical sleeve.

11. The oven of claim 1 wherein said matching means comprises a plurality of spaced posts depending from the wall of the enclosure defining the opening and disposed along a circular path about said opening.

12. The oven of claim 1 wherein said matching means comprises a plurality of spaced posts depending from the wall of the enclosure defining the opening and disposed along an elliptical path about said opening.

13. The oven of claim 1 wherein said matching means comprises a plurality of spaced posts depending from the wall of the enclosure defining the opening and disposed along a rectangular path about said opening.

14. The oven of claim 1 wherein said means defining an enclosure includes a pair of oppositely disposed walls one of which defines said opening.

15. The oven of claim 14 wherein said pair of walls are side walls of the oven.

16. The oven of claim 14 including strap means coupling from an edge defining said opening to a top wall defining said enclosure.

17. The oven of claim 14 including strap means coupling from an edge defining said opening to adjacent a top wall defining said enclosure, defining a gap between said strap means and top wall.

18. The oven of claim 16 including means disposed adjacent the opening for adjusting in a vertical direction the position of said strap means.

19. The oven of claim 17 including means disposed adjacent the opening for adjusting in a vertical direction the position of said strap means.

20. The oven of claim 1 wherein said matching means comprises impedance matching means.

21. The oven of claim 20 wherein said impedance matching means is designed in accordance with the dimensions of said waveguide and the dimensions of said enclosure.

22. The oven of claim 21 wherein said opening is defined in one wall of said enclosure, said matching means being designed in accordance with the spacing of others of said walls.

23. The oven of claim 22 wherein said opening is in a top wall and said matching means is constructed as a quarter wavelength transformer designed in accordance with the dimensions between the side walls of said enclosure.

24. A microwave oven comprising

a source of microwave energy,

a waveguide of predetermined cross-section positioned with one end receiving energy from said source for transmission through said waveguide,

the frequency of operation of said source and the cross-sectional dimension of said waveguide being selected to excite the TE.sub.1,o mode in siad waveguide,

means defining a cooking enclosure including means for providing access to the enclosure and means defining an opening into the enclosure,

and impedance matching means associated with said enclosure for transferring the mode established in the waveguide and establishing only the TE.sub.1,o mode in the enclosure.