US2814708A - Microwave ovens - Google Patents

Description

J. BLASS MICROWAVE OVENS Nov. 26, 1957 2 Sheets-Sheet 1 Filed Jan. 5, 1952 Amt/W0 Jaw 54/955 a ay ATTORA/[V United States Patent MICROWAVE OVENS Judd Blass, Waltham, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application January 5, 1952, Serial No. 265,134 7 Claims. (Cl. 2l9-10.55)

This invention concerns a heating apparatus and more particularly an oven-type microwave heating apparatus having a relatively uniform distribution of microwave energy therein.

Microwave ovens of the type disclosed in Patent No. 2,500,676 to Hall et al. have been constructed with sides which are mutually perpendicular. With such cavities, regardless of the position of the radiating means, a considerable portion of the energy supplied to the cavity is reflected back towards the energy source. As a result, an excessive amount of energy is fed back into the magnetron or source of microwave energy with undesirable results.

By placing reflecting plates at least one wave length long in a microwave oven, particularly in the corners, and by orienting them properly, the incident waves, in accordance with the quasi-optical theory of electromagnetic radiation, can be made to undergo a maximum number of reflections from the oven walls before returning to the source. Since a dissipation of energy occurs whenever energy is reflected from the walls of the microwave oven, each reflection will result in a loss of energy to said oven, and thus the more reflections the less energy is returned to the magnetron. The same effect may obviously be produced by shaping the oven itself rather than by inserting reflecting plates in the conventional rectangular microwave oven.

By coupling microwave energy from a rectangular wave guide into the microwave oven through a flared H-plane horn having an inclined reflecting plate at the point of entry into the microwave oven, the latter will be substantially uniformly illuminated along a dimension parallel to the length of the horn aperture.

Furthermore, if energy is fed into the microwave oven by means of a magnetron probe instead of by means of a flared horn, the reabsorption of energy by magnetron as a result of reflection from the oven walls can be materially reduced by locating the probe at the intersection of two lines which are parallel to the center lines of the oven and displaced therefrom by an eighth wave length at the operating frequency.

If a probe coupling is used, the combined effects of the slanting oven sides and of the location of the probe offcenter with respect to the geometrical center of the surface of the oven into which the probe is inserted result in a substantial improvement in the standing wave ratio and energy distribution in the microwave oven. If an aperture feed is used, the same result is obtained by virtue of the sloping oven sides and the use of the horn type'transition which produces a broad beam of relatively uniform intensity in the H-plane.

An object of this invention is to provide a means for producing improvement in the standing wave ratio and energy distribution in a microwave oven.

Another object of this invention is to provide a means for modifying the energy distribution in a microwave oven ice 2 and for minimizing the amount of energy reflected from the oven walls back to the microwave source.

A further object of this invention is to provide a means for coupling microwave energy into a microwave oven so as to obtain more nearly uniform energy distribution within said oven.

A further object of this invention is to provide a compact arrangement for coupling energy into a microwave oven.

Other and further objects and advantages of the invention will become apparent as the description thereof progresses, reference being had to the accompanying drawings wherein:

Fig. 1 is a view of a first embodiment of a microwave oven according to the invention;

Fig. 2 is a longitudinal section view of the microwave oven of Fig. 1 taken along the line 2-2 of Fig. 3;

Fig. 3 is a transverse section view of the oven of Fig. 1 taken along line 33 of Fig. 2;

Fig. 4 is a transverse section view illustrating a second embodiment of a microwave oven using a probe type feed;

Fig. 5 is a top plan view of the embodiment shown in Fig. 4;

Fig. 6 is a transverse section of a third embodiment of the invention similar to that of Fig. 4 except that the probe is displaced from the geometrical center of the top surface of the microwave oven; and

Fig. 7 is a top plan view of the embodiment shown in Fig. 6 showing certain pertinent dimensions.

Referring to Figs. 1-3 in the drawings, a hollow cavity or microwave oven 1 made of any suitable metal is shown. The dimensions of the oven are large compared with the Wave length at the operating frequency. An article of food 2 to be cooked or heated is placed in a container 3 on the bottom wall of the microwave oven. In order to allow for insertion or removal of food from the oven, an aperture closable by a pivotally supported metal door 11 is provided in the front wall of the oven. This door is clearly shown in Fig. 2 which is a side view of the oven shown in Fig. 1. 7

Referring again to Fig. l, a magnetron or other source of microwave energy (not shown) delivers microwave energy having a predetermined frequency, such as 2,450 mc., to a wave guide 4 which is designed to propagate only the TE mode and which includes a conventional right angle bend. Wave guide 4 is coupled directly to the small end or throat of an electromagnetic horn 5. Horn 5 is a flared H-plane horn, that is, the flare of the horn from the throat at which it joins wave guide 4 to its mouth or aperture is an H-plane. The flare extends for several wave lengths and the flare angle is a function of the beam width desired. Horn transition 5 is terminated by an inclined plate 6 which reflects the energy directly into the oven through aperture 7, shown in Figs. 2 and 3. The angle of this plate is arbitrary but should be in the vicinity of forty-five degrees for minimum reflection of energy back from the oven into the horn and wave guide. Inclined plate 6 has a flanged portion 8 which admits of connection to the back wall of cavity 1 adjacent to aperture 7 by means of fastening devices, such as screws, or by welding to the back wall. If desired, the mouth of the horn may be bent at an angle of approximately forty-five degrees instead of using an inclined plate 6.

As is well known in the art, an electromagnetic horn radiator may be designed to provide a relatively uniform illumination across the mouth or aperture thereof. A well designated H-plane horn, for example, provides a substantially uniform pattern from side to side. Since the aperture of the horn extends across a substantial portion of the width of the microwave oven, as shown in Figs. 1 and 3, a relatively uniform distribution of energy is attained in a horizontal plane, thereby cooking or heating the food within the microwave oven more evenly than with conventional probe feeds heretofore in use in microwave ovens. As shown clearly in Fig. 2, the hornis mounted flush with the rear surface of the oven while the ninety-degree bend in wave guide 4 allows for the positioning of the magnetron close to the oven. This arrangement of the waveguide and horn results in a sizable reduction in space required for oven and power supply.

To further modify the energy distribution within the oven and to minimize the amount of energy reflected back toward the microwave source, sloping corner surfaces 12- and 13.- are provided in oven l, as clearly shown in Figs. 1 to 3. These sloping sides are preferably one wave length or more long. For example, when the oven is illuminated from a microwave source whose frequency is 2,450 mm, the slanting surfaces of the oven are made at least five inches long, since the wave length at 2,450 me. is approximately five inches. The length of the sloping sides may be greater than one wave length but it has been found in practice that sloping sides one wave length long are satisfactory. Although four corner surfaces are shown in Figs. l3, it has been found that satisfactory operation may also be obtained by the use of but two corner surfaces, either at the front and back of the oven or at the opposite sides, that is to say, either pair of corner surfaces 12 or 13 may be eliminated without seriously affecting the desired results. The angle which the slanting sides make with the horizontal is approximately from thirty degrees to forty-five degrees, although this angle does not necessarily have to be limited to a value within the above-mentioned range.

Energy in the form of electromagnetic Waves is radiated from the aperture of horn 5 in several directions. A large number of these waves leave the aperture along paths such as to impinge upon the sloping corner surfaces 12 and 13. A substantial number of the waves also impinge upon the top surface of the oven and are thence reflected to the corner surfaces. The waves im inging upon the corner surfaces are reflected therefrom and are caused to undergo a large number of reflections from the oven walls, in accordance with quasi-optical theory of electromagnetic wave propagation, before returning toward the source. Since energy is dissipated whenever the waves are reflected from the oven walls, each reflection results in loss of energy to the microwave oven and not only is less energy returned to the microwave source, but also a greater distribution of energy within. the microwave oven is produced, with consequently more even heating r or cooking of the food contained therein. The same effect may be obtained by placing reflectin plates on the walls of the microwave oven, preferably in the corners, and so locating and inclining them as to provide a maximum number of reflections of energy from the oven walls, in the manner previously described.

The advantages inherent in the sloping walls may be utilized when a magnetron probe, of the type described in application Serial No. 163,902, of Argento and Haagensen, is used to couple microwave energy into the oven. in Fig. 4, the magnetron probe 9 is inserted in an aperture 10 in the top of microwave oven 1 which may be of the same configuration as that described in Figs. 1 to 3. The probe is centrally located with respect to the ends or sides of the oven, as clearly shown in Fig. 5. Much of the energy radiated from a probe positioned in a perfectly rectangular cavity is directed along a path normalto the walls and is reflected back along substantially the same path to the source. The results in an unduly large standing wave ratio and causes the production of. positions of maxima and minima of energy distribution in the oven, as well as an increase in heating of the magnetron and possibly an irreparable damage thereto. If the incident waves are caused to strike the sloping walls of the microwave, oven,.then the waves are reflected in, a comparatively where A is the wave length at the operating frequency. The off-center positioning of the probe is shown in Fig. 6, which is an end view similar to Fig. 4. The exact details of the location of the probe is shown in Fig. 7. The best position for the center of the magnetron probe is at the intersection of two lines which are parallel to the center lines of the oven and displaced from them by Where A is the wave length corresponding to the frequency of the magnetron or microwave energy source. For example, if a magnetron having a frequency of 2,450 me. is used, the displacement is approximately 12.25 cm. There are four such positions which are located on a circle whose diameter is A construction necessary to obtain one such location is shown in Fig. 7.

Referring to Fig. 7, the following relationships exist:

where S and S are the distances from the center lines of the probe to the rear and front walls, respectively, and S and 8.; are, the distances from the center line of the probe to the two opposite side walls. The first image (that is, the image formed by the first reflection of the incident wave) on the front wall is located 28 from the probe. The first image on the rear is located 23 from the probe. Since 2(S S is a half wave length, it follows that the contribution from the first images cancels. Similarly, the first images on the two side walls cancel, since 2'(S' S is also a half wave. Furthermore, the odd images, that is, the images resulting from an uneven number of reflections back and forth from opposite walls, also cancel. Since the walls are lossy to some extent, the strength of each image decreases the further it is from the wall. By locating the probe in this manner, the contributions from the side walls to the energy reabsorbed by the magnetron are considerably decreased.

While particular embodiments of this invention have been illustrated and described herein, it is not intended that this invention be limited to such disclosure, and changes and modifications may be made and incorporated within the scope of the claims. For example, the probe need not be inserted in the top wall or surface of the oven. Furthermore, the number of slanting surfaces and the positioning of said slanting surfaces are not necessarily limited to the number and positioning shown and described.

What is claimed is:

1. A microwave oven comprising a substantially rectangular closed cavity having mutually perpendicular top, side and end surfaces and containing an aperture in one of said surfaces, a source of microwave energy of wave length small compared with the dimensions of said cavity, a flared H-plane horn having the small end connected to said source and the flared end positioned in alignment with said aperture, said hornv further including an inclined reflecting plate adjacent said flared end, said horn being positioned in contact with said oven, said cavity having a first pair of oblique surfaces each interconnecting said top surface and said side surfaces, and a second pair of oblique surfaces each interconnecting said top surface and said end surfaces, said corner surfaces being positioned to intercept a substantial portion of the energy radiated from said aperture and being at least one wave length long.

2. A microwave oven comprising a source of micro- Wave energy, a substantially rectangular cavity having mutually perpendicular top, side and end walls, said cavity further having a first set of tapered surfaces interconnecting said top wall and each of said side walls and extending along the entire length of either side wall, said cavity also having a second set of tapered surfaces interconnecting said top wall and each of said end walls and extending along the entire length of either end wall, means connected to said source for propagating energy into said oven, said means for propagating being positioned to effect substantial reflection of energy from said tapered surfaces.

3. A microwave oven comprising a substantially rectangular closed cavity having mutually perpendicular top, side and end surfaces and containing an aperture in one of said surfaces, a source of microwave energy of wave length small compared with the dimensions of said cavity, a flared H-plane horn having the small end connected to said source and the flared end positioned in alignment with said aperture, said horn further including an inclined reflecting plate adjacent said flared end, said cavity having a first pair of oblique surfaces each interconnecting said top surface and said side surfaces, and a second pair of oblique surfaces each interconnecting said top surface and said end surfaces, said corner surfaces being positioned to intercept a substantial portion of the energy radiated from said aperture and being at least one wave length long.

4. A microwave oven comprising a source of microwave energy, a substantially rectangular enclosure having mutually perpendicular top, side and end walls, one of said walls containing an aperture therein, said enclosure further having a first set of tapered surfaces interconnecting said top wall and each of said side walls and extending along the entire length of either side wall, said enclosure also having a second set of tapered surfaces interconnecting said top wall and each of said end walls and extending along the entire length of either end wall, transmission means having one end connected to said source and the other end positioned in alignment with said aperture for propagating energy into said oven, said means for propagating being positioned to effect substantial reflection of energy from said tapered surfaces.

5. A microwave oven comprising a source of microwave energy, a substantially rectangular enclosure having a top wall and side and end walls perpendicular to said top wall, one of said walls containing an aperture therein, said enclosure further including a pair of tapered surfaces extending along opposite sides of said top Wall over substantially the entire length of said top wall, transmission means including a flared H-plane horn having one end connected to said source and the other end positioned in alignment with said aperture for propagating energy into said oven, said means for propagating being positioned to effect substantial reflection of energy from said tapered surfaces.

6. A microwave oven comprising a source of microwave energy, a substantially rectangular enclosure having mutually perpendicular top, side and end walls, one of said walls containing an aperture therein, said enclosure further having a first set of tapered surfaces intercon necting said top wall and each of said side walls and extending along the entire length of either side wall, said enclosure also having a second set of tapered surfaces interconnecting said top wall and each of said end walls and extending along the entire length of either end wall, transmission means including a flared H-plane horn having one end connected to said source and the other end positioned in alignment with said aperture for propagating energy into said oven, said means for propagating being positioned to effect substantial reflection of energy from said tapered surfaces.

7. A microwave oven comprising a source of microwave energy, a substantially rectangular cavity partially bounded by four side walls and having a top wall perpendicular to said side walls, said cavity further including a pair of tapered surfaces each interconnecting said top wall and each of a pair of oppositely disposed side walls, said tapered surfaces extending along the entire length of the interconnected walls, means connected to said source for propagating energy into said oven, said means for propagating being positioned to effect substantial reflection of energy from said tapered surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,500,752 Hanson Mar. 14, 1950 2,518,383 Schelkunofl Aug. 8, 1950 2,549,721 Straus Apr. 7, 1951 2,609,449 Otis Sept. 2, 1952

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