US3430022A - Microwave oven - Google Patents

Description

Feb. 25,l 1969 R. L. COUGOULE MICROWAVE OVEN Filed OCT.. 14, 1966 United States Patent 3,430,022 MICROWAVE OVEN Robert L. Cougoule, San Carlos, Calif., assignor to Litton Precision Products, Inc., Sau Carlos, Calif., a corporation of Delaware Filed Oct. 14, 1966, Ser. No. 586,684

U.S. Cl. 219-10.55 2 Claims Int. Cl. H05b 9/06 ABSTRACT OF THE DISCLOSURE A microwave oven is provided which includes a source of microwaves coupled to the microwave heating cavity by means of a waveguide. The waveguide is short circuited at a position remote from the microwave source and includes a pair of microwave passages or feed means for passing microwave energy from the waveguide into the heating cavity. The first microwave passage means contains a pair of parallel spaced narrow slots elongated in the width direction of the waveguide for favoring the introduction of E-field from said waveguide into the cavity. The second microwave passage means is located between the first microwave passage means and the short circuited position of the waveguide and contains a large iris or opening, substantially larger in area than either of said narrow slots, for favoring the introduction of H-field from the waveguide into the cavity.

The present invention relates to microwave ovens and, more particularly, to an improved microwave oven which provides uniform heating patterns in the oven without requiring any moving or rotating mode mixers.

In known microwave cooking or heating devices, standing waves of high frequency are set up within the oven cavity due, for example, to reflection of the supplied high frequency waves from the cavity walls. Such standing wave comprises locations of high electric field intensities and low electric field intensities at different locations within the oven cavity. This results in relatively hot or cold spots at various locations therein and, with a large piece of food or with several pieces placed about the cavity, causes some portions of food to be heated more than other portions.

In order to reduce these hot and cold spots, it has generally been the practice to increase the number of modes excited within the cavity by the addition of either fixed or moving elements. This is possible since a cavity having its length, width and height different in dimension and large relative to the Wave length of the microwave source is inherently capable of sustaining a plurality of different modes. By adding together the electric fields of many different modes, a substantially more uniform electric field is obtained than is obtainable with a single dominant lInode and the magnitude of the differences between the high and low electric field intensities at different locations within the cavity is considerably reduced.

The usual solution for solving this problem is the mode stirrer, a fan-like unit consisting of many blades each of which refiects incident microwave enenrgy within the cavity and which is rotated therein by a motor. By rotating the stirrer within the cavity, the aforesaid high and low field intensity spots, or hot and cold spots, are sequentially shifted to different locations around the oven and each portion of a large piece of food on the average over a period of time receives approximately the same amount of heating energy as it is sequentially exposed to high and low intensity fields.

Although this method of distributing the heating field within the cavity has proven satisfactory, it possesses the obvious disadvantages of requiring additional moving parts. From the point of View of the manufacturer, this is undesirable since such parts wear and require additional expense.

Various proposals have been made in the prior art for exciting a plurality of modes without any moving parts. One such proposal is disclosed in U.S. Patent No. 2,943,- to G. Guanella. Numerous others appear in the prior art, .but none have proved satisfactory.

Therefore, it is an object of the present invention to provide a novel means for uniformly heating food within a microwave oven without the use of moving elements.

It is an object of the present invention to distribute simultaneously a .substantially uniform microwave heating field to substantially all locations within the cooking chamber.

It is a further object of the present invention to distribute the energy to various locations within the ycooking chamber by a purely static or fixed device which is less expensive to manufacture and which requires no maintenance.

It is a further object of the present invention to provide means for simultaneously exciting as large a number of the modes of high frequency waves as is possible within the given cavity, the cooking chamber, the addition of which avoids hot or cold spots.

In accordance with the present invention, a fixed nonabsorbing, relatively lossless substantially fiat conductive surface or plate is provided and is located within the cavity exposed directly to the food. It is mounted by an insulator `means so that it substantially obstructs a direct path between the food and location at which microwave energy is introduced into the cooking chamber. The plate is positioned with its surface substantially parallel to the food supporting tray. Moreover, in a preferred embodiment, distributed feed passages or irises of different kinds are utilized to introduce microwave energy into the cavity in order to enhance the uniform distribution of the electric field within the oven cavity. One passage may be a susceptively stubbed iris while another may be a simple resonant iris.

The -foregoing and other advantages and objects of the present invention will become apparent from a consideration of the following detailed description and drawings of which:

FIGURE 1 is a side view of a microwave oven embodying the invention in which the side wall has been removed for purposes of illustration.

FIGURE 2 is a cross section of the bottom view taken along line 2-2 of FIGURE 1.

FIGURE 3 is a cross section similar to the cross section of FIGURE 2 but embodying other forms of the invention.

FIGURE 1 shows an oven cavity 1 having a front door Wall 2, a rear wall 3, a bottom wall 4, side wall 5 and top Wall 6. The oven cavity also contains a side wall 7 which, in FIGURE 1, has been removed for purposes of illustration. Within the cavity, there is shown a dielectric tray 8, lpreferably of relatively lossless dielectric material but which may take the form of a metal rack, for receiving food. Tray 8 is supported parallel to the top wall 6 by supports 9. A well-known magnetron oscillator or other suitable source of microwave energy is symbolically sho'wn by the block labeled 10.

In the usual manner, the walls of the cavity are constructed of material impervious to microwave energy so as to prevent leakage from the cavity. The walls are sealed together to further prevent leakages through the boundaries. Moreover, the door wall of the cavity, through which food is introduced into the cavity, contains the usual high frequency seals and electrical interlocks which have been omitted for clarity.

In the preferred form of the invention, the material used for the walls is stainless steel. Not only is stainless steel impervious to microwave energy, but it is relatively lossy or absorptive relative to manv other metals or metallic compositions. Being lossy, a certain electral load is maintained on the magnetron oscillator by the cavity during periods of operation. In so doing, the magnetron is protected to an extent against no-load operation which might otherwise destroy the magnetron. 'Microwave energy is fed from the magnetron oscillator through a coupling means such as the waveguide 11 although other forms of coupling can be utilized.

In the preferred embodiment, microwave energy is transferred from waveguide 11 into cavity 1 through a unique distributed feed arrangement consisting of a susceptively stubbed iris in the form of two traverse slots 12 and a resonant iris in the form of a rectangular passage 13. This feed arrangement is more particularly shown in FIGURE 2, later discussed. These passages or irises may be physically open or, as an alternative, may be closed with any material known in the art that is pervious to microwave energy but which prevents vapors from entering the waveguide. The waveguide 11 also has a closed end 14 which is spaced from the resonant iris 13 by approximately a quarter wave length, or a multiple thereof, at the microwave source frequency.

Further, in accordance with the present invention, a fiat nonabsorbing conducting plate 15 is provided, supported within the cavity, spaced from the top, bottom and side walls of the cavity and maintained parallel to the top wall 6 and dielectric food tray 8 by an insulating means shown as insulator 16 to which it is fastened by a screw 17. The insulator is shown supported by the top wall of vthe oven cavity. The plate 15 substantially obstructs a dilrect :path from at least one of `t'he irises to the lower half of the cavity. Plate or surface 15 may be constructed of cold rolled steel, aluminum or any other metal.

A cross section of the microwave oven taken along line 2--2 in FIGURE 1 is shown in FIGU'RE 2. Sidewall 7, deleted from FIGURE l, has been included in FIGURE 2. This view shows the front door wall 2, side walls 5 and 7, rear wall 3 and top wall 6 previously discussed with respect to FIGURE l. FIGURE 2 discloses in greater detail the preferred form of the substantially lossless oonductive plate 15 and the distributed feed means utilized in accordance with the invention. The plate 15 is supported on insulator 16 and aiiixed to the insulator, for instance, by means of a screw 17. The plate utilized in this embodiment is generally rectangular but has two opposed edges, 18 and 19, which have a curvature extending in a concave manner toward the plate center. The two -remaining edges 20 and 21 are straight and parallel. The .plate in this embodiment has its front edge 21 nearer the front door wall portion 2 of the cavity than its back edge 20 is to the rear wall 3 and exposes a portion of iris 12. This construction of the preferred embodiment has been found by experiment to attain better results.

The edges 18, 19, 20 and 21 of the plate are spaced from the Walls of the cavity. 'Because of the complexity in mathematically determining the type and locations of the modes within a cavity, results are generally found experimentally. The arrangement and construction of the plate disclosed in FIGURE 2 has provided a substantially uniform heating field. In accordance with the invention, other fiat shapes and positions of the plate in a plane parallel to the food tray are possible; and the optimum for each particular oven of a different size or shape and associated feed means utilized may be found experimentally.

FIGURE 2 also discloses in greater detail the arrangement of the irises or passages used to couple microwave energy from the waveguide 11 into the cavity 1. In this embodiment, a distributed type of coupling or feed means has been utilized to introduce microwave energy into the cavity; first, for introducing equal amounts of power at each of two respective locations along the top wall 6 of the cavity and, second, for maximizing the number of rmodes within the cavity, each of which enhances the uniformity of heating therein. Located in the top wall 6 of the cavity 1 is a first pair of slots 12 which comprise a susceptively stubbed iris as is known in the art. Spaced therefrom is a rectangular passage 13 which is known as a resonant iris. The waveguide 11 contains irises identical to those located directly overlaying those in the -top cavity wall 6 in order to form passages for introducing microwave energy from the waveguide into the cavity. However, instead of cutting irises in the top wall of the cavity, a large opening in the top Wall might be cut away to allow the waveguide to fit therein and have its irises and surface exposed directly t-o the cavity. These irises may physically be Open or contain a window of any well known material pervious to microwave energy while preventing the introduction of vapors from the oven into the waveguide. The waveguide also contains a closed end 14 spaced approximately a quarter wave length, or a multiple thereof, from the resonant iris, rectangular opening 13, in order to provide a short circuited termination to the waveguide which maximizes the amount of power introduced 'into the cavity from the waveguide. It is believed that a desirable feature is to have the resonant iris 13 proximate the closed end of the waveguide rather than the iris 12.

The narrow slots 12 forming the susceptively stubbed iris, as is known, favors the passage of electric field or E waves while the resonant iris passage 13, favors the passage of magnetic field or H waves from the waveguide into the cavity. The waveguide 11 coupling the high frequency energy from the oscillator 10 to the cavity 1 is designed to support the propagation therebetween of what is known as a traverse electric wave, that is a propagating wave containing both electric fields E and magnetic fields H in which the magnetic field H has its vector in the direction of propagation and the electric field E has its vectors traverse to the direction of such propagation. Any of the standard texts on waveguides give a mathematical analysis of such phenomena.

Moreover, a cavity such as 1 is capable of sustaining different modes of field distribution therein depending upon whether an E field or an H field is coupled from the waveguide into the cavity. Thus, by introducing an E field at one location and an H field at another, a greater number of modes are excited, enhancing the result desired of -maximizing the number of excited modes for obtaining uniformity of heating, than is generally possible with two couplings each of which couples either E fields or H fields alone.

Further, as is known, a high frequency passage such as the disclosed irises possesses a definite value of resistance, termed radiation resistance, governed in part by the dimensions of the passage. Since it is desired to distribute the high frequency power evenly through the cavity, it is desirable that equal amounts of power enter the cavity through each of the t-wo irises 12 and 13. This would not be possible if the two irises 12 and 13 possessed equal values of radiation resistance because, in that instance, maximum power would be coupled throu-gh the first iris and less through the second iris. Hence, in the present invention, the irises are dimensioned so that these values of radiation resistance are unequal. In this light, it may be noted that resonant iris 13 while favoring the passage of the H field from the waveguide into the cavity allo-ws, in addition, some E field to pass.

FIGUR-E 3 shows another form of the invention which -may be utilized in particular ovens. This figure is a cross sectional view of an oven similar to the view ofthe preceding form of this invention shown in FIGURE 2. In FIG- URE 3, the conductive plate 22 shown is rectangular in shape. This rectangular plate 22 is mounted symmetrically with respect to the front wall 2 and rear wall 3 and also with respect to side walls 5 and 7 of the cavity 1. A single iris is formed in the top wall 6 of the cavity shown as a rectangular opening 23 in top wall 6 in order to introduce the microwave energy into the cavity. Overlying this is a like passage formed in the waveguide 11, the latter being 4affixed by a iiange 24 to the top wall 6 of the cavity 1.

In this embodiment, the plate 23 is mounted, as an example, on two insulators 25 by screws 26. In this embodiment, the insulators 25 are also shown as being attached to the top wall 6 of the cavity. However, as is obvious, the plate 22 may be supported from other walls or by a larger number of insulators while maintained parallel to the food tray. lIn accordance with the teachings of the invention, the plate 22 substantially obstr-ucts the passage of microwave energy directly from the waveguide to the food tray (not shown) and is also mounted parallel to the food tray.

In each of the disclosed embodiments, it will be noted that the mounting of the plate in front of the feeding irises causes microwave energy to be reected back to the magnetron, an unavoidable property resulting in an increased voltage standing wave ratio, VSWR. However, by properly spacing the plate from the top wall, this property is used to advantage. The phase of the reflected power is aligned such that any change in the electrical load within the oven or cavity, such as due to the differences in size and moisture content of heated objects, from heavy to noload, never reiiects a VSWR in a phase near or in the magnetrons critical region, which would ordinarily cause the tube to mode and burn out. This feature, therefore, additionally acts as a protective device for the magnetron.

The cavity modes which are excited in the manner of the disclosed invention possess particular utility for heating food articles of relatively flat shape or having a large length relative to their height. Referring again tol FIG- U-RIES l and 2, by mounting the plate 15 parallel to the food tray .8 as in the disclosed oven, uniformity of heating has been peculiarly successful in the planes parallel to and proximate the plane containing the food tray. Since one of the major functions of the microwave oven is to supplement the ordinary oven for quickly reheatin-g precooked foods, the results assume a greater significance. Mounting the iiat plate 15 parallel to the food tray 8 maximizes the number of modes in the desired manner and more so in the proper direction although results otherwise seem mathematically completely unpredictable and are mathematically too complex to determine.

As is evident from the standard texts and as hereinbefore mentioned, various modes can exist within a cavity each of which is characterized by three integral numbers corresponding to the length, width and height dimensions of a cavity. These numbers indicate the number of half waves of the standing wave pattern for the particular mode along each of the three dimensions of the cavity. The lmounting of the conductive plate 15 as disclosed and the resulting excitation of a uniform field especially within those planes 4proximate to and parallel to the food tray #S indicate that the proper mode excitation has been achieved as a major function of the height dimension or distance of the plane containing the tray from the plane of the conductive plate.

The foregoing embodiments of this invention have been described for purposes of illustration only and are not intended to limit the invention as set forth within the spirit and scope of the appended claims.

What is claimed is:

1. A microwave oven comprising a microwave cavity bounded by conductive walls, a source of microwave energy, waveguide means connected between said source and one of the walls of said cavity for coupling microwave energy therebetween, said waveguide means having a short circuited end remote from said source, tray means within said cavity for supporting objects to be heated by microwave energy, means supporting said tray means parallel to said one of the walls of said cavity, a relatively lossless plate of conductive material having a surface exposed directly to said tray means mounted on insulator means within the cavity spaced from the walls thereof and substantially parallel to said tray means, and feed means for coupling microwave energy from said waveguide means through said one wall and into said cavity, said feed means comprising: iirst microwave passage means having a pair of spaced narrow slots elongated in the width direction of said waveguide, and parallel for favoring the introduction of E-eld from said waveguide into said cavity and second microwave passage means located between said first microwave passage and said short circuited waveguide and comprising a large opening, substantially larger in area than either of said narrow slots, for favoring the introduction of H-iield from Said waveguide into said cavity.

2. A microwave oven comprising a microwave cavity bounded by conductive walls, a source of microwave energy, a/waveguide connected between one wall of said cavity and said source for coupling microwave energy therebetween, said waveguide having a short circuited end remote from said source, and means for feeding microwave energy from said waveguide into said cavity comprising: iirst micro-wave passage means havin-g a pair of spaced narrow slots elongated in the width direction of said waveguide and parallel for favoring the introduction of E-iield from said waveguide into said cavity, and second microwave passage means comprising a large opening, substantially larger in area than either of said narrow slots, for favoring the introduction of H-eld from said waveguide into said cavity.

References `Cited UNITED STATES PATENTS 2,704,802 3/ 1955 Blass et al M9- 10.55 3,263,052 7/ 1966 Jeppson et al 219-10.55 3,300,615 1/1967 Smith 2l9-10.55

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

Images