Beam Stirrer

The present invention relates in general to microwave ovens and more particularly to a beam stirrer used in the microwave oven for providing a more uniform heating pattern.

In order to provide uniform and rapid heating of materials and foods, high frequency energy, generally in the microwave region, has been utilized. Materials or foods to be heated are generally placed in an enclosure which is sealed to prevent the escape of high frequency energy. The energy generated, usually from a magnetron, is inserted either directly into the enclosure or indirectly through the use of a waveguide or other suitable coupling device.

Mode stirrers are well known in the art and are shown, for example, in U.S. Pat. No. 3,106,629. Although these mode stirrers do provide somewhat of an improvement in the uniformity of the heating pattern, there are certain disadvantages associated with these prior art structures. For example, there is generally a matching problem between the cavity and the magnetron which is occasioned by the abrupt discontinuity that occurs each time the blades of the stirrer rotate across the mouth of the waveguide opening.

Accordingly, one object of the present invention is to provide an improved structure for location within a microwave oven for providing a uniform heating pattern.

Another object of the present invention is to provide a beam stirrer for use in a microwave oven and that is in the form of a resonant structure capable of providing good matching with the magnetron.

A further object of the present invention is to provide a beam stirrer in accordance with the preceding object and that is moveable in an eccentric manner to provide beam motion over an area of the oven.

To accomplish the foregoing and other objects of this invention there is provided a beam stirrer for use in a microwave oven having an oven cavity, a source of microwave energy and means coupling the energy from the source to the cavity. The means for coupling the energy is preferably a waveguide structure and the source of microwave energy may be a conventional magentron. The beam stirrer is disposed in the oven cavity adjacent to the end of the waveguide which couples to the cavity and is in a position to receive the coupled energy. The beam stirrer basically comprises an energy coupling structure which in one embodiment is a hollow cylinder, means eccentrically supporting the structure and means for rotating the structure in an eccentric manner to thereby deflect the beam in many directions with the energy being coupled relatively uniformly as the structure rotates. The structure is preferably a quqrter wavelength structure that theoretically couples all of the energy from the waveguide to the cavity. By rotating the structure about an off-center axis energy is thereby effectively spread over a predetermined cooking area of the oven to provide a relatively uniform heating pattern over that area.

Numerous other objects, features and advantages of the 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 cross-sectional view of a microwave oven using the beam stirrer of the present invention;

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

FIG. 3 is a perspective view of the beam stirrer shown in FIGS. 1 and 2; and

FIG. 4 shows an alternate embodiment for the beam stirrer.

FIG. 1 shows a microwave oven 10 the structure of which may be similar to that shown in U.S. Pat. No. 3,764,770. The oven typically includes a rack 12 for holding a plate 14 which may support a foodstuff 16. Energy is coupled to the oven cavity by way of waveguide 18 from magnetron 20. In accordance with the teachings of my U.S. Pat. No. 3,764,770, there may be provided a quarter wavelength step 22 between the output end 19 of the waveguide and the oven cavity 24. This quarter wavelength step is for establishing primarily only the dominant mode within the oven cavity. Although this structure is shown in the illustrative embodiment it should be understood that the mode stirrer of the present invention may also be used with oven structures wherein the cavity has established therein multiple modes.

Referring now to FIGS. 1-3, the beam stirrer comprises a support rod 26, a cylindrical coupling structure 30 and a drive motor 32. The support shaft 26 may be suitably supported at one or both ends and includes a pulley 34 attached to the top end. The pulley 34 is driven by way of a small belt 36 from the motor 32. The motor is suitably supported from a top wall 37 of the waveguide 18.

The coupling structure 30 is of cylindrical shape and is attached to the shaft 26 by means of an L-shaped tab 40 which is fixed at one end to the inner surface of structure 30 and at the other end to the bottom of shaft 26.

FIG. 2 clearly shows the eccentric arrangement of the present arrangement wherein the cylindrical coupling structure sweeps to cover an area larger than the area of the cylindrical structure itself.

FIG. 2 shows the structure 30 also in dotted in two other alternate positions. With this arrangement the energy from the waveguide is coupled to the cavity and the eccentric movement of the coupling structure 30 tends to spread the beam over the cooking area of the oven. It has also been found that it is desirable to provide a slight tilt to the coupling structure 30 relative to its support shaft 26. FIG. 3 shows this relatively small angle a which may be on the order of 5.degree.-10.degree.. This angle a can be provided simply by bending the tab 40 slightly.

The coupling structure 30 is basically a quarter wavelength structure but the dimensions are preferably slightly altered from this quarter wavelength structure (height h) to provide optimum matching between the magnetron and the oven cavity. Previously, the matching required the use of posts or other elements at the output end of the waveguide.

In the disclosed embodiment the height of the coupling structure may be 0.7 inches and the inside diameter may be two inches when using a waveguide of dimensions 1.50 by 3.75 inches.

It is known that the magnetron source should not be operated in the sink region of the Rieke diagram. See M.I.T. Radiation Laboratory Series, Radar System Engineering, Pages 336-340. If the height of the structure is too high the tube will be operating undesirably close to this sink area with too large a VSWR. Alternatively, if the height is made too small the match will not be proper. The height of 0.7 inches provided a proper match wherein the coupling structure represents a relatively low impedance to the magnetron source. By the proper selection of both the diameter and height of the resonant structure a relatively low VSWR can be provided. The coupling structure of this invention can also be used with an existing structure wherein the matching has already been provided. In that case, the structure may be more nearly a quarter wavelength structure.

FIG. 4 shows a perspective view of another embodiment of a stirrer. This embodiment employs a solid disc 42 connected to shaft 26 by means of a strip 44. This arrangement also provides a spreading of the beam coupled from the waveguide structure. In still another embodiment the disc 42 may be in the form of an open ring.

Having described a limited number of embodiments of the present invention it should now be apparent that there are numerous modifications that can be made in this ivention without departing from the scope thereof. For example, in the drawings a cylindrical coupling structure has been shown. However, this structure could also be square, eliptical, rectangular or could take on various other shapes. Also, the structure has been shown as being a continuous structure but could also be a segmented structure. Also, regarding the embodiment shown in FIG. 4 a dielectric material could be coupled to the end of the shaft and function as a coupling structure.