Microwave oven with premixing of wave energy before delivery to its heating cavity

[57] ABSTRACT

An antenna protrudes into the interior of a semicyclin drical coupling and distribution structure extending from the rear of the heating cavity. Contained in the structure is a rotatable reflector unit composed of a horizontal flat arm and a curved vane depending downwardly therefrom. The end of the arm remote from the vane is mounted for rotation about a vertical axis at a location between the antenna and the heating cavity, the arm having a length such that the vane passes between the antenna and the semicircular rear wall of the structure during each revolution. The vane’s radius of curvature is less than the radius of curvature of the wall so that the distance or spacing of the vane is continually varying with respect to the wall as well as continually varying with respect to the antenna, thereby causing considerable mode alterations or premixing of the microwave energy prior to its delivery into the heating cavity. Additional dispersion of the energy is effected as it enters the cavity by means of an inclined dispersive plate extending into the cavity from the opening or window through which the energy is introduced. In this way, the vertical dimension of the cavity is substantially reduced and at the same time the usable space therein is maximized by reason of the externally located coupling structure, which modulates the wave pattern of the energy entering the cavity.

Claims, 3 Drawing Figures

<img file="US3789179A_D0001.tif" />

/6

PATENTED JAN 2 9:074 3,789,179

SHEET 1 OF 3

<img file="US3789179A_D0002.tif" />

PATENTED JAN 291974

3,789.179

SHEET 2 OF 3

<img file="US3789179A_D0003.tif" />

PATENTED JAN 2 91974

3.789.179

SHEET 3 OF 3

<img file="US3789179A_D0004.tif" />

3,789,179

MICROWAVE OVEN WITH PREMIXING OF WAVE ENERGY BEFORE DELIVERY TO ITS HEATING

CAVITY

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to microwave heating apparatus, and pertains more particularly to a microwave oven having reduced volumetric dimensions consistent with the material to be heated.

2. Description of the Prior Art

Past attempts to improve the distribution of electromagnetic energy within microwave heating compartments have been only partially successful. For example, a mode stirrer with multiple fan-like blades has been installed inside the heating cavity for the purpose of continually changing the concentration of the electrical fields propagated therein. Provided the cavity Q is high in such instances, that is if all the cavity dimensions are large compared to the freespace wavelength of the microwave energy, some degree of uniformity is realized in the heating pattern; however, when this type of stirrer is used with a smaller heating cavity, its effectiveness becomes noticeably impaired. Moreover, since the mode stirrer occupies a portion of the heating cavity, the useful cooking space is diminished. In general, the cavity size in such prior art situations is out of proportion with respect to the items to be heated.

With a realization of these disadvantages, attempts have been made heretofore to reduce the volume of the cooking cavity or compartment, yet still provide an adequate heating pattern therein. A plurality of mode stirrers, revolving food trays, and a host of other devices have been conceived and tried, but all have included ³⁵some drawback as far as their practical application is concerned. In this regard, either the device did not suitably accomplish its intended purpose or the apparatus was so complicated as to render it impractical. Additionally, such devices were designed for use within the heating cavity, thereby reducing the useful cooking space.

SUMMARY OF THE INVENTION

Accordingly, an important object of the present in- ⁴⁵vention is to provide a microwave oven in which an article may be uniformly heated. In this regard, an aim of the invention is to provide an effective R.F. input transition device for dispersion of the wave energy into the ₅θ heating cavity.

Another object is to provide a compact oven, especially with respect to its vertical dimension. In this regard, it is planned that the transition and dispersive device be located externally of the heating cavity, thereby ₅₅providing more usable space within the cavity. Specifically, it is within the purview of the invention to eliminate the need for installing a conventional mode stirrer within the usable oven cavity space.

Also, an object is to provide a microwave oven that ₆θ will be easier to clean inasmuch as there is no internally positioned stirrer which can become spattered with food particles.

Another object is to provide a microwave oven in which not only the vertical dimension of the heating ₆₅cavity is reduced but in which the entire height of the oven is decreased in that the present invention obviates the need for locating a waveguide above the heating cavity. Stated somewhat differently, an aim of the invention is to minimize not only the cavity height but the overall height of the microwave oven.

A further object of the invention is to provide a mi5 crowave oven having a power input structure whereby relatively equal R.F. fields are transmitted longitudinally across the input opening to the oven cavity.

Still further, the invention has for an object the provision of a microwave oven utilizing an R.F. input transit) mission device which transmits the microwave energy into the heating cavity by means of multiple transmission paths having relatively large phase variations determined by an eccentric mechanical and electrical placement of a moving reflective system.

Yet another object of the invention is to provide a microwave oven having a power input transmission system associated therewith which substantially prevents arcing or high voltage stationary standing wave points within or about the microwave oven input structure, thereby enabling the use of mechanically compact R.F.

input components.

A still additional object is to provide a microwave oven utilizing an R.F. input structure presenting a suitable cross sectional area in which a portion of the air used to cool the magnetron is further routed through the coupling structure and the cavity to prevent objectionable vapor condensation within the oven.

While it is contemplated that my coupling device, even though employing movable parts, will be longlasting, nonetheless it is planned that such parts will be readily accessible for whatever maintenance or replacement proves necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view taken from the rear of a microwave oven equipped with my coupling and distribution structure, a portion of the top wall of the structure being broken away in order to reveal certain parts and their relationship with each other;

FIG. 2 is a vertical sectional view taken in the direction of line 2—2 of FIG. 1, and

FIG. 3 is a horizontal sectional view taken in the direction of line 3-3 of FIG. 2 in order to show to better advantage the orbital path taken by the curved reflecting vane with respect to the antenna, the view also illustrating the vane’s radius of curvature with respect to the radius of curvature of the rear wall of the structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It will be of assistance to explain at the outset that the following detailed description of the microwave oven and the coupling structure'associated therewith are all of metal, preferably aluminum, unless otherwise mentioned.

Referring now in detail to the drawings, the microwave oven exemplifying my invention has been designated generally by the reference numeral 10. It includes a cabinet 12 which is typical other than that the dimensions thereof have been reduced as will become evident during the ensuing description. To facilitate the description, it will be observed that the cabinet comprises a front wall 14, a rear wall 16, side walls 18, a top wall 20 and a bottom wall 22. These walls 14-22 form a heating cavity labeled 24 in FIG. 2. The top wall 20 has a panel 21 containing small perforations or holes 23

3,789,179 free end. As best understood from FIG. 3, the arm 62 has an arcuate length such that it will traverse a circular path 66 passing between the antenna 48 and the rear wall 42. Also from FIG. 3 it should be apparent that the 5 radius of curvature of the vertically oriented vane 64 is less than the radius of curvature of the rear wall 42. More specifically, the vane 64 has a radius of curvature corresponding to the radius of the circular path 66, whereas the rear wall 42 has a radius of curvature cor10 responding to approximately half the length of the rectangular opening 36 in the rear wall 16. It will be recognized that a plurality of reflective surfaces are provided by the walls 42, 43 and 44. Additionally, the arm 62 and the vane 64 provide still additional reflective sur15 faces.

PHYSICAL DIMENSIONS

Although perhaps better appreciated from the operational description yet to be presented, nonetheless at 20 this stage it will be helpful to give certain dimensions of the oven 10 that have proved advantageous in actual practice.

The heating cavity 24, it can be explained, has a vertical height of 5.75 inches. From the front of the cavity, 25 that is from the front wall 14, to the rear thereof, that is to the rear wall 16, is 13.5 inches. The cavity has a width of 14.0 inches, that is from side wall 18 to side wall.

Considering now the dispersive plate 38, it can be ex³⁰ plained (as seen from FIG. 2) that this plate has a height of 1.5 inches, the plate being inclined at an angle such that its lower edge is 0.625 inch from the opening 36 in the rear wall 16. The lower edge, owing to the angulation of the plate 38, is perpendicularly spaced 1.25 ³⁵ inches in a vertical direction beneath the top wall 20.

Continuing with the dimensional description, the width of the arm or strip 62 is 0.5 inch and the length thereof 2.5 inches. As far as the vane 64 is concerned, it will be noted from FIG. 3 that it subtends an arc ⁴⁰ which is 4.5 inches in length. The vertical height of the vane 64 is approximately 1.25 inches.

The coupling and distribution structure has an overall height of approximately 2.25 inches. Its diameter is 9.0 inches, thus making its radius 4.5 inches.

⁴⁵ As far as the radiating antenna 48 is concerned, the vertical axis on which it is located is spaced approximately 1.75 inches from the rear wall 42 of the structure 40, being also centrally disposed with respect to this wall 42. The antenna 48 is 2.75 inches from the ⁵⁰ cavity wall 16. The glass dome 50, which encloses the antenna 48 protrudes into the interior of the structure 40 to a height of approximately 1.9 inches.

Continuing further with the dimensional description, the centerline of the shaft 56 is situated at a distance ⁵⁵ of approximately 1.8 inches from the rear wall 16 containing the opening 36. As already mentioned, the width of the arm or strip 62 is 0.5 inch and its length 2.5 inches; however, the arm 62 is attached to the shaft 56 so that the effective radius of rotation of its free end is 2.0 inches. As far as the vane 64 is concerned, which is integral with the free end of the arm 62, it can be explained that it subtends an arc of 4.5 inches, having a vertical height of 1.25 inches.

of size to permit the passage of cooling air but providing a high impedance to the passage of microwave energy. As is conventional with microwave ovens, there is an access opening 26 in the front wall 14 having a door 28 hinged at 30. Within the cavity 24 is a cooking tray 32 having an article or “load” 34 thereon which is to be heated. As already explained, the construction up to this point has dealt with conventional details other than that the dimensions of the cabinet can be reduced, particularly its height, thereby providing a more compact oven than heretofore.

A rectangular coupling opening or window 36 is provided in the rear cavity wall 16, the opening being in the upper half of the rear wall 16. Also, as seen in FIGS. 1 and 2, there is a dispersive plate 38 extending across the upper edge of the opening 36, the inclined plate angling downwardly into the cavity 24. A cover plate 31 of low loss dielectric material, such as polypropylene, is attached to the plate 38 at 33 and to the wall at 35, the plate 31 having perforations or holes 37 to permit passage of air. The cover plate 31 functions as a shield to prevent spattered food particles from passing through the coupling opening 36. Also, it precludes the possibility of any articles (or a portion of a given article) being inserted through the opening 36 into the premixing structure yet to be described. Although the microwave energy delivered to the cavity 24 via the opening 36 is premixed, as will become clearer during the subsequent description, nonetheless the plate 38 performs an additional dispersive function which promotes uniform heating of the article or load 34 within the cavity 24.

A coupling and distribution structure denoted generally by the reference numeral 40 is semicylindrical, being comprised of a semicircular vertical rear wall 42 having its ends extending from locations near the ends of the elongated opening 36. There is also a top wall 43 and a bottom wall 44, the three walls forming a hollow enclosure in which the R.F. energy is premixed before entering the cavity 24. By means of a flange 39 and screws 41, the structure 40 is removably attached to the wall 16.

A source of microwave energy in the form of a magnetron 46 is provided. The bottom wall 44 has a number of perforations or holes 47 through which a portion of the cooling air for the magnetron 46 is forced into the structure 40, the air flowing through this structure, the holes 37 in the cover plate 31, and through the cavity 24; finally exiting via the earlier-mentioned holes 23. There is a conventional antenna 48 associated with the magnetron 46 and there is also the usual enclosing dome 50. The antenna 48 protrudes into the interior of the structure 40 through a circular opening 52 formed in the bottom wall 44. It will be apparent that the antenna 48 is located on a vertical axis that is fixed with respect to the rear wall 16 of the cavity 24. More will be said later on concerning this spacing.

Also located at a fixed distance from the rear wall 16 and at a fixed distance from the antenna 48 is a stub shaft 56 of a low loss dielectric microwave material, such as polypropylene, that projects downwardly through the top wall 43. The stub shaft 56 rotates in a counterclockwise direction as indicated by the arrow

58, being driven by a motor 60.

Attached to the stub shaft 56 by a member 54 is a rotatable metallic reflector unit 61 comprising a strip or flat arm 62 having a depending curved vane 64 at its

OPERATION

It will be assumed that the magnetron 46 is connected to a power source (not shown) and therefore is

3,789,179 able to generate high frequency energy in the form of microwaves. These waves are radiated by the antenna 48 into the interior of the coupling and distribution structure 40. With the drive motor 60 energized, it follows that the unit 61 is rotated in the direction denoted by the arrow 58. Although the vane 64 traverses a predetermined arc as indicated by the reference numeral 66 in FIG. 3, owing to the location of the shaft 56, the ' vane 64 continually changes its position with respect to the fixedly located antenna 48. Consequently, the spacing or distance between the vane 64 and the antenna 48 is continually changing. In other words, the energy waves emitted by the antenna 48, or at least a portion of the radiated energy, is always received and concurrently reflected by the vane 64 and the radial arm 62 to which it is integrally attached. Since the vane 64 is continually changing its relationship with respect to the antenna 48, the amount of wave energy received and reflected continually varies in the direction of reflection.

Additionally, the vane 64 is continually changing as far as its spacing is concerned with respect to the curved wall 42 of the structure 40. Inasmuch as the wall 42 has a semicircular configuration, almost an infinite number of reflective points are presented.

Consequently, the microwave energy is reflected by any of the surfaces provided by the structure 40 and the resulting multiple reflections influence the energy propagation paths into the cavity 24. In other words, a state of wave disparity is produced within the structure 40 which causes the microwave energy to enter the cavity 24 through the opening or window 36 in a diverse order, thereby producing relative uniformity as far as the heating pattern within the cavity 24 is concerned. The continuous change in phase relationship of the various reflective surfaces due to the orbital path traveled by the vane 64, this being derived from the eccentric arc indicated by the numeral 66 in FIG. 3, results in an interesting and novel concept. Also peculiar to the arrangement is the disseminative properties of the structure 40 in which mode mixing occurs before the energy reaches the heating cavity 24. The vane 64 is never in the same phase with respect to the antenna 48 during any one revolution. Additionally, there exists considerable disparity between the vane 64 and the curved wall 42 as the vane 64 revolves inharmoniously about the antenna 48.

Although effectively mixed by the time the wave energy reaches the opening or window 36, nonetheless there is a predominantly vertical polarization of the electric field due to the vertical orientation of the antenna 48. The dispersive plate 38, which functions as a baffle, effectively turns or rotates the electric field so that a portion of the energy enters the cavity 24 with its electric field horizontally polarized, the plate 38, as its name implies, further contributing to the dispersive action that has already been provided by the structure 40 and the interplay of the parts constituting the structure 40 and the components contained therein. In other words, the plate 38 changes the regularity of the cavity 24 and further enhances the distributive characteristics thereof. The member 38 further has the effect of making the opening 36 of an irregular nature. In actual practice, this causes the microwave energy to enter the cavity 24 at different levels relative to the vertical height. Consequently, the plate 38, in combination with the multiple reflections of a discordant nature which have occurred within the structure 40, provides an omni-directional energy feed into the cavity 24, thereby producing a highly uniform energy pattern within the heating cavity which contributes appreciably to the uniform heating of the article or load 34.

REPRESENTATIVE TEST DATA

Although many experiments were conducted under a plurality of operating conditions with the described microwave heating cavity 24 and dispersive coupling system 40, 61, only exemplary results are deemed necessary to realize the advantages to be gained by practicing the teachings of my invention. Accordingly, the amount of power coupled to water loads ranging from 200 to 900cc in a l,000cc dielectric bowl of a conventional nature was measured. The energy source or magnetron 46 used had an average output power into a matched load of approximately 600 watts. The results are listed below:

Test Conditions

a. water load: varied 200-900cc

b. original temperature: 20°-21° C

c. heating time: 70 seconds

Test Results

Water Load Approximate Power to Water

Load

<td> 200cc</td><td> 492</td><td> watts</td>

<td> 300cc</td><td> 414</td><td> watts</td>

<td> 400cc</td><td> 500</td><td> watts</td>

<td> 500cc</td><td> 450</td><td> watts</td>

<td> 700cc</td><td> 525</td><td> watts</td>

<td> 900cc</td><td> 540</td><td> watts</td>

Tests were performed with a relatively flat plastic container (7 X 7 X 1.5 inches) equally divided into 12 thermally insulated sections or cubicles. Placing this container in approximately the center of the heating cavity 24 with about 37cc of water in each section, the following results were obtained:

Test,Nq.l

Test Conditions

a. water load: approximately 450cc/12

b. original temperature: 20°- 21° C

c. heating time: 70 seconds

Test Results power output—446 watts maximum difference in heating pattern — 10° C average temperature increase— 16.5° C

Test No. 2

Test Conditions

a. water load: 720cc/12

b. original temperature: 21° C

c. heating time: 100 seconds

Test Results power output— 470 watts maximum difference in heating pattern— 14° C average temperature increase— 16° C

TestNo.3

Test conditions

a. water load: 700cc — dielectric bowl

b. original temperature: 22° C

c. heating time: 140 seconds

3,789,179

Test Results power output — 473 watts

It will be obvious to those skilled in the microwave heating art that the plastic walls dividing the container 5 into the 12 sections or cubicles provided inherent thermal barriers, owing to their relatively low heat transfer coefficient, against the conduction of any appreciable amount of heat from one section or cubicle to another. This resulted in the 10° C and 14° C temperature differ- ¹⁰ences listed in Tests Nos. 1 and 2, respectively. Such temperatures, it will be appreciated, would not occur when heating a unitary load.

Under the test conditions the magnetron 46 necessar- _Jily operated at a lower rate of efficiency, a phenomenon common to all microwave heating cavities wherein multiple load variations are experienced. However, as the test results clearly illustrate, the new dispersive coupling system comprising the structure 40 and unit 20 61 produced satisfactory heating over the range of loads and varied test conditions.