Antenna Construction for Microwave Oven

It has been common practice to generate microwave energy for cooking purposes by a magnetron which is coupled to the cooking cavity by a waveguide either with or without the use of an "antenna" element projecting from the waveguide into the cavity. The energy is generated at an allocated frequency, for example, 915 or 2450 megahertz, and sets up standing waves within the cavity in modes which depends upon the physical dimensions and proportions of the cavity. In a cavity of the usual rectangular shape the modes are relatively simple and well defined, resulting in concentrations of energy at spaced anti-nodal points producing "hot spots" within the foodstuff so that portions of the foodstuff may become overdone or even burned, while adjacent portions are underdone.

In an effort to equalize the energy field it has been common practice to employ a rotating "stirrer" within the cavity having blades which cyclically vary the modes so that the hot spots are continually shifted within the foodstuff to more evenly distribute the heating effect.

However stirring devices are not completely effective and merely alleviate rather than cure the problem of hot spots. Also, stirring devices take up valuable space within or adjacent to the cavity. Efforts have been made to utilize a stirring device within the waveguide itself as a space conservation measure but, again, the effectiveness is limited. While it is possible to change the modes on a cyclical basis by modulating the frequencies of the waves, this introduces circuit complications and is an expensive solution.

In an effort to improve upon mode stirring it has been proposed to rotate an antenna extending from a waveguide into the oven cavity. By way of example, reference is made to Long U.S. Pat. No. 2,961,520 in which a spindle mounted axially within a hollow conductor is provided with a cross member extending sharply at right angles thereto to provide an antenna of unbalanced "T" configuration. A rotated antenna in the form of a dipole is also shown in Fritz U.S. Pat. No. 3,189,722. Such efforts have failed to produce equalized energy distribution, however, and have required resort to added elements such as glass plates, polarizing guides, or the like.

In analyzing devices of this kind it is helpful to consider two components of directly radiated wave energy. The first may be referred to as the "inertial" or unmodified wave which feeds directly into the cavity from the waveguide or hollow conductor. The other directly radiated component, which is superimposed upon the inertial wave, is the "guided" wave which is radiated by the portion of the antenna which is in the cavity and which differs in directivity from the inertial wave. In the absence of an auxiliary stirring device, it will be understood that the mode pattern of the inertial wave remains fixed and hence capable of producing hot spots, and it is only the guided component which is, by rotation of the antenna, continually shifted. Thus it is important to be able to control the directivity of the guided wave as well as to establish an energy ratio between the guided wave and the inertial wave.

However, analysis shows that rotated antennas of "L" or "T" shape, exemplified by the Long patent, produce a relatively low proportion of guided wave energy. It would appear that where the "crossbar" portion of the antenna forms a sharp rectangular joint with the spindle of shank portion, the crossbar simply acts as a discontinuity, impeding the feeding of inertial energy to the cavity and transmitting only a small amount of the total energy to the crossbar for radiation therefrom. Because of the relatively small amount of energy which is in guided form in the Long patent, and thus affected by the rotation, the main component of energy is in stationary, relatively simple modes highly affected by the dimensions and proportions of the cavity in which the antenna is used.

Our studies have shown that there is a still further factor which affects cooking efficiency and that is the extent to which the foodstuff is acted upon by direct radiation, that is, energy proceeding directly from the waveguide or antenna, as compared to the indirect radiation resulting from reflections within the cavity or reflections caused when blocking or altering the inertial wave. Increasing the amount of direct radiation reaching the foodstuff with controlled energy distribution not only improves the quality of cooking but also increases cooking efficiency. Analysis again shows that rotating antennas of sharply bent "L" or "T" configuration, being low in guided energy, are also low in the proportion of the energy reaching the foodstuff via a direct path, except when the foodstuffs are located centrally and in close proximity to the inertial wave.

It has been found that all of the above problems associated with prior art forms of rotating antenna can be avoided, or at least alleviated, and cooking efficiency and performance improved, by forming a rotating antenna into two or more distinct portions. In the most simplified form, the rotating antenna consists of a straight shank portion which extends into the waveguide, and an angled or contoured "transitional" portion extending at an obtuse angle to the shank portion. In a more complex form, the antenna consists of a radial arm portion which projects approximately at right angles to the straight shank portion, and an angled transitional portion interposed between the shank and the arm which extends at an obtuse angle to both of them. In all cases, the transitional portion has a length at least equal to the radius of the hollow conductor through which the antenna projects into the cavity. In the simplified antenna configuration, it is found that the transitional portion, regardless of whether it is straight or curved, provides a direct guided wave component which controls the patterns of radiation within the cavity while minimizing obstruction of the directly radiated inertial wave. In the case of the more complex multiple segment antenna, the guided wave of the transition section is directly radiated in a directionalized manner at the juncture of each subsequent transitional segment, which also controls the patterns of radiation. Upon rotating the antenna about the shank axis the guided directionalized wave components are cyclically swept throughout the cavity to produce improved energy distribution, relative freedom from hot or cold spots, and an increase in the proportion of energy reaching the foodstuff along a direct radiation path.

It has been found that such advantages may be economically obtained by bending a straight rod in two places. The rod is first bent at a point on the base or shank portion which is spaced slightly inwardly from the wall of the cavity to form an obtuse included angle thereby to define a dogleg having an angularly extending portion with a minimum length which is preferably greater than the radius of the hollow conductor. The rod is then bent a second time to form a second obtuse included angle to define a radially extending arm portion extending at approximate right angles to the shank portion. To obtain a minimum projection into the cavity, the sum of the two included angles should be approximately 270.degree. and the total length of the antenna from the shank axis to the end of the radial arm should preferably be in the range of 2.0 to 10.0 times the radius of the hollow conductor. Lengths in this range facilitate impedance matching and alter radiation patterns, while encroaching to minimum degree upon the space within the cavity.

In a simplified form of the invention the rod forming the antenna is bent once at the shank to form an obtuse included angle thereby to define an angular dogleg portion having a length which is at least equal to the radius of the hollow conductor and which is found to produce a directionalized guided wave component, a multiplicity of swept radiation patterns, and a relatively large proportion of energy directly reaching the foodstuff. However, extending of the height of this form of antenna for the purpose of impedance matching or improving radiation patterns results in an antenna which extends further into the cavity, so that its use, while practical, is not preferred to the same degree.

Employing an antenna conductor having a second bend results in a significant amount of energy being radiated at the second bend, accounting for the large proportion of guided, directly radiated energy; but concentrated bends are not essential to the practice of the invention and the transitional, or dogleg, portion of the antenna may include additional partial bends or indeed may be smoothly "faired" or curved to join the shank to the arm, provided that the transition is spread over a minimum length at least equal to the radius of the outer conductor.

While the outer arm has been assumed, in the above discussion, to be relatively straight, this is not essential to the practice of the invention and the arm may be slightly curved in one direction or the other in the plane of the shank and may have substantially any desired curvature in a plane perpendicular to the shank.

To summarize, it is an object of the present invention to provide a rotating antenna which is so formed as to radiate into the cavity a high proportion of the energy in guided, as contrasted with inertial, form, the radiation pattern produced by the guided wave being swept on a cyclical basis so as to overcome the problem of hot spots in the foodstuff more effectively than prior designs of rotating antennas or stirring devices.

It is another object of the invention to provide a design of rotated antenna which includes an angled transitional, or dogleg, portion and which produces a directionized wave for the setting up of a relatively large number of controllable radiation patterns subject to cyclical sweeping.

It is still another object to provide a design of rotated antenna which produces a more equalized distribution of directly radiated energy within the cavity, consisting of both inertial and guided waves and in which a larger proportion of the energy is transmitted directly to the foodstuff via a direct radiation path.

It is a general object of the present invention to provide a design of rotated antenna in which the above features and advantages are obtained largely independently of the dimensions or portions of the cavity in which the antenna is used or which may, at the least, be modified for different cavities with only minor change and at negligible expense, with the result that a power supply and antenna system constructed in accordance with the present invention may be flexibly adapted to a wide range of existing cavities and so that the same power supply and antenna system may be furnished on an O.E.M. basis, to different range manufacturers for inclusion in both new and existing range structures.

It is another general object of the present invention to provide an antenna system which may be employed with particular advantages in a conventional thermal range, for example of the electrical type, for simultaneous application of thermal and microwave energy to the foodstuff and which is capable of efficient operation in a conventional range cavity in the presence of thermal heating elements and the usual conductive supporting shelves which form part of a conventional range.

Accordingly, it is an object to provide an antenna system which is not only highly efficient, and overcomes the problem of hot and cold spots, but which is both simple and economical to manufacture and install, permitting a source of microwave energy to be added, for greater cooking efficiency, at low incremental cost, to conventional designs of ranges, with the only additional precaution being the sealing of the door and cavity openings against escape of the microwave energy.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIG. 1 is a perspective view of a free standing electric range of conventional design but incorporating a source of microwave energy and having provision for use of thermal and microwave energy simultaneously in a common oven cavity.

FIG. 2 is a vertical cross section looking along the line 2--2 in FIG. 1.

FIG. 2a shows the relation between the antenna and heating element looking along the line 2a--2a in FIG. 2.

FIG. 3 is a front view of the oven of FIG. 1 with the door and drawer removed and looking along the line 3--3 in FIG. 2.

FIG. 4 is a perspective view of a microwave module constructed in accordance with the present invention.

FIG. 5 is a vertical section taken through the microwave module, with the front wall removed, and looking along line 5--5 in FIG. 2.

FIG. 6 is a transverse section taken through the module looking along the line 6--6 in FIG. 5.

FIG. 6a is a fragment taken along line 6a--6a in FIG. 6.

FIG. 7 is a cross sectional view through the hollow conductor and antenna assembly looking along the line 7--7 in FIG. 2.

FIG. 7a is a profile view of the antenna in FIG. 7.

FIG. 8 is a top view of the antenna looking along line 8--8 in FIG. 7.

FIG. 9 is a schematic diagram of a control circuit permitting three modes of cookery plus self-cleaning.

FIG. 10 is a profile view of an antenna of modified contour.

FIG. 11 is a profile view of a further modification of antenna.

FIGS. 12 and 13 show profiles of antennas of dogleg shape in straight and curved versions.

While the invention has been described in connection with certain preferred embodiments, it will be understood that we do not intend to be limited to the particular embodiments shown and that we intend, on the contrary to cover the various alternative and equivalent constructions included within the spirit and scope of the appended claims.

Turning to FIGS. 1 and 3 there is shown a typical free standing electric range to which the present invention has been applied. The range has an oven cavity 20 including a top wall 21, a bottom wall 22, side walls 23, 24 and a back wall 25. All of the walls are of "sandwich" construction formed of spaced sheets of metal with insulation, preferably in the form of glass wool, in between. Spaced downwardly a short distance from the top wall 21 is a heating element 27. A second heating element 28 is spaced a short distance above the bottom wall. The oven cavity is enclosed by a hinged door 30 having a gasket 31 which provides continuous and unbroken engagement with a land surface 32 on the range body. The gasket is of the type intended for shielding against escape of thermal and microwave energy, and its effect may be supplemented by use of a continuous choke 33 for which additional reference may be made to prior patents and technical publications. It will be assumed, as the discussion proceeds, that the range has well-known provision for high temperature self-cleaning. Consequently, a latch is used having a latching control 34, with the mode of operation being selectable by a mode switch 35 at the top of the range. Within the oven cavity is a grid-type shelf 36 formed of metallic conductors in spaced parallel relation and extending horizontally over the entire area of the cavity. The shelf preferably has no central crossbar extending perpendicularly to the conductors. Metallic supports are used at the respective ends of the shelf; however, we employ no means to assure positive grounding of the supports. We have found with our feed system that arcing does not occur in the oven in spite of intermittent metal to metal contact, therefore, we neither intentionally ground rack and rack supports in the conventional manner nor intentionally insulate them.

Below the oven cavity 20, and of generally conforming shape, is a storage space having a closure 40 forming a portion of a drawer 41 mounted upon slides 42 and dimensioned to define a space 43 to the rear of the drawer. Extending horizontally at the bottom of the rear space 43 are supporting members 44.

In carrying out the present invention there is provided, at the rear of the storage space, a microwave module including a microwave power supply housing of generally rectangular shape having a magnetron, blower and associated circuitry, with a waveguide of rectangular section secured at its rear end to the top of the housing and having its front end extending forwardly cantilever-fashion under the bottom wall of the cavity. Referring to FIGS. 2-5 the microwave module, indicated generally at 50, is formed of a power supply housing 51 and a forwardly extending waveguide 52, the total width W and height H of the module being such that the module may be both inserted and withdrawn through the drawer opening.

The power supply housing 51 is of rectangular or box shape having top and bottom walls 53, 54, front and back walls 55, 56 and end walls 57. Each of the latter terminates in a mounting flange 58 which is secureable by suitable screw fasteners 59 in a seated position upon the supporting members 44.

Vertically mounted in the power supply housing is a conventional magnetron 70 (FIGS. 5 and 6) having cooling fin structure 71 and a glass or ceramic enclosure 72 for the antenna, which extends upwardly through a waveguide inlet 73 formed in the underside of the waveguide 52. Reference is made to prior patents for typical magnetron power and control circuits.

For the purpose of cooling the magnetron and ventilating the cavity during microwave usage, a blower 80 is provided having an inlet 81 and an outlet 82. The blower has a rotor 83 driven by a motor 84 and which sucks in air through an opening 85 in the rear wall of the housing. Air from the blower is conducted to the underside of the magnetron through a duct or connection 86 passing upwardly through the fins. The slightly warmed air, discharged at the top of the magnetron fin structure, flows into the housing to create a slight internal pressure. The bulk of the air is vented directly from the housing through several large openings includings a set of louvers 87 in the end wall of the housing, an auxiliary air opening 88 in the front wall, to which further reference will be made, and a rear discharge opening 89. The remaining space within the housing, in addition to that occupied by the blower and magnetron, will be understood to be taken up by the transformer and other necessary magnetron circuit components.

Turning attention next to the waveguide 52, it is preferably of rectangular cross section, secured at its rear end 91 to the top of the housing 51 and cantilevered forwardly so that its front end 92 underlies the central portion of the bottom wall 22 of the range cavity. The waveguide has top and bottom surfaces 93, 94, side surfaces 95,96 and end surfaces 97, 98, thereby defining a tube leading from inlet 73 to an outlet 99. The outlet 99 is in axial register with an opening 100 (see especially FIG. 7) formed in the bottom wall 22 of the oven cavity. The opening 100 penetrates first and second bottom plates indicated at 101, 102, the space 103, in between, being filled with glass wool or similar thermal insulation.

For the purpose of coupling the outlet 99 of the waveguide with the cavity opening 100, a connector is provided in the form of a hollow-open-ended conductor or tube 110 which is telescoped into the opening 100 and which has a circular flange 111 at its upper end which seats on the bottom wall 22 of the cavity. More specifically, the flange seats upon an upraised annular land 112 which is formed in the plate 101 and which surrounds the opening 100, the land serving as a dam to prevent leakage of oven spills into the hollow conductor and thence into the waveguide.

For the purpose of sealing the hollow conductor 110 to the top of the waveguide in register with the outlet 99, a socket 115 is provided on the waveguide, the socket receiving the circular lower end 113 of the conductor. Such socket is defined by an annular mount 116 made of pressed metal and which is spot welded or otherwise securely fastened to the top wall 93 of the waveguide. At the root of the socket is a resilient conductive gasket 117 which may, for example, be formed of metal gauze.

For clamping the hollow conductor against the gause in the socket 115, a pair of clamping screws 121 are provided which extend through registering openings in the flange 111, with the threaded tips of the screws being received in tapped openings 122 in the mounting member 116.

The screws 121 are not, however, relied upon to keep the flange 111 tightly seated against the land. Preferably three additional screws 123 project through registering openings in the flange to engage tapped holes 124 in the land.

Spaced above the flange 111 is a mounting ring 125 which is supported upon short pedestals 126. The mounting ring 125 is secured by screws 127 which telescope through the flange and into registering tapped holes in the land.

Thus in assembling the device the flange is seated on the land and the screws 121 are tightened in the threaded holes 122 to seat the conductor 110, at its lower end, firmly against the socket 115 in the waveguide. The screws 123 are then tightened in place to doubly insure that the flange 111 is intimately seated against the land. Finally, the screws 127 are inserted into the mounting ring, and tightened.

Because of the cantilever extension of the waveguide 52 upon the housing 51, the waveguide occupies a nominal position from which it may be resiliently deflected without strain or damage to the module; for example, when the screws 121 are turned to draw the hollow conductor down into contact with the gasket 117, the end of the waveguide is simultaneously drawn upwardly through a small angle, resiliently twisting the module, until opposition to turning of the screws indicates that the hollow conductor 110 is fully bottomed. Seating of the hollow conductor to provide a tight joint at both ends is assured, notwithstanding dimensional variations, under production-line conditions, between the bottom 22 of the cavity and the plane of the supports 44 in the storage space.

In accordance with the present invention (FIG. 7) an antenna is mounted for rotation coaxially in the hollow conductor 110, the antenna being coupled at its lower end to a motor and drive train mounted upon the waveguide. As will be discussed in greater detail, the antenna, indicated at 130 has a straight shank portion 131 which is coaxially arranged in the conductor 110, a "transitional" portion 132 which is upwardly angled and a radial arm portion 133. In the preferred embodiment the antenna is formed by first and second abrupt bends 134, 135. The lower end of the antenna, indicated at 136 is keyed for reception in a rotatable mount 137 journaled in the bottom wall of the waveguide. To serve as a bearing for the upper end of the antenna, and to shield the upper end of the hollow conductor from food spills or other foreign matter, is a ceramic disc 140 supported within the amounting ring 125 on the posts 126 and having a central opening 141. The antenna is held captive in the central opening by a snap ring 142 which is in interfering relation with the underside of the disc.

It is one of the features of the present construction that the entire antenna and hollow conductor assembly may be conveniently removed, when service becomes necessary, from the inside of the oven cavity. By unscrewing the screws 127, the mounting ring 125, disc 140, and the antenna itself may be drawn clear of the conductor. Next, unscrewing the screws 121 frees the lower end of the conductor 110 from its seat on the waveguide, and unscrewing the screws 123 permits separation of the conductor flange 111 from the land 112 on the bottom wall of the oven so that the conductor may be lifted clear. The conductor assembly may be reinstalled by simply reversing the procedure.

For the purpose of rotating the mount 137, a small, clock-type driving motor 150 is provided having internal gear reduction. The motor is mounted on the top side of a motor mounting flange 151 which extends from the side of the waveguide. Pinned to the motor shaft is a first gear 152 located on the underside of the waveguide and which meshes with a second gear 153 which is pinned to a shaft 154 forming a part of the mount 137. The shaft is journaled in a bearing 155 of plastic or the like which occupies an opening 156 in the bottom of the waveguide, the opening being sufficiently small so that negligible leakage occurs. The motor gear reduction, and the ratio of gears 152, 153, is such that the antenna rotates at a low speed which may, for example, be on the order of 5 to 10 revolutions per minute, in any event providing a period of rotation which is less than the shortest cooking cycle.

In accordance with one of the features of the invention air passages are provided between the power supply housing so that a portion of air from the blower 80, and which cools the magnetron, flows through the waveguide and hollow conductor for ventilating the cavity when the blower is on, for final venting from the top of the cavity, with the same air path being utilized to set up flow of convected air during thermal cooking, and during self-cleaning, when the blower is off. Thus a pattern of small air passages 161 is provided in the wall of the waveguide, and between the housing and waveguide, as shown in FIG. 6, and a vent 162 is provided at the top of the cavity as shown in FIGS. 2 and 3. The path of the convected air is indicated by the dotted arrows 176 to distinguish over the forced air path which is indicated by the arrows which are solidly drawn. The passages 161, 162 are preferably sufficiently small, or screened, so that there is no leakage of microwave radiation either from the module or the cavity. Moreover, the passages 161 are so related, in area, to the main air discharge opening 87 and auxiliary air openings 88, 89 that the forced air flow through the cavity is limited to something on the order of three to eight cubic feet per minute. During thermal cooking, when the blower is turned off, the fact that the openings 85, 87 and 88 in the housing 51 are in a "low" position, below the bottom of the cavity, the fact that the waveguide 52 feeds air inwardly at the bottom of the cavity, and the fact that the vent 162 is in a "high" position at the top of the cavity, provides sufficient convection, on the order of 0.5 to 2.0 cfm., depending upon temperature, for both baking and self-cleaning operations. The path of the convected air is, however, sufficiently constricted so that the air flow for self-cleaning is below that which runs risk of explosion. It may be noted that while air flows outwardly of the air discharge openings 87, 88 during the blower mode, air flows inwardly, through the same openings, in the convection mode. The vent 162 may optionally be formed in the back or side walls at the very top of the cavity.

As a further feature combining both ventilation benefits and physical protection, a trough shaped guard or shield is provided at the front of the power supply housing for surrounding and underlying the waveguide for protection of the waveguide and motor driving train against impact from articles stored in the drawer as well as for conducting forced air from the opening 88 in the housing along the waveguide for cooling both waveguide and motor. The guard 170 has a bottom wall 171, side walls 172, and a sloping front wall 173. The latter terminates in a flange 174 which is secured by screws 174a to the underside of the bottom plate 102 of the cavity. A second flange 175 at the rear of the guard is secured by screws 175a to the front wall 55 of the housing. It will be seen that such enclosure not only provides physical protection but defines a path for the pressurized air, the air flowing in the direction of arrow 176 from the opening 88 (see FIG. 6), making a "U" turn at the front end of the waveguide, and flowing backwardly along the sides of the waveguide for exit at the top of the housing. In this way air is usefully employed which would otherwise simply be discharged into the room. Because of the relatively small clearance requirements under the waveguide, the guard 170 does not appreciably subtract from the useful storage volume.

A rudimentary control circuit providing for use of either type of energy, or both, is illustrated in FIG. 9. The mode selector switch 35 is used having a first contact 181 for thermal heat, a second contact 182 which energizes the magnetron, and a third, or dual, contact 183 which turns on both of the energy sources. As indicated in this figure, the blower motor 84 and antenna motor 150 are turned on whenever the magnetron is turned on but are off at all other times. A fourth or "self-clean" position 184 is also provided, with the self-clean temperature being regulated by a thermostat 185. In the case of normal cooking, the temperature is regulated by a conventional thermostat 186 in series with the heating elements. It will be understood that a practical control circuit is much more elaborate than that illustrated in FIG. 9, particularly as regards the safety features which are effective in the self-clean mode.

While the operation and easy replacement of the microwave system will be apparent from the foregoing, the following summary is offered: With the mode switch 35 in the "combined" mode, the heating elements establish a temperature set by the thermostat 186, and the magnetron, blower motor 84 and antenna motor 150 are all energized, with the wave from the antenna being swept throughout the cavity for equalized distribution of microwave energy, as will be described. A portion of the blower air feeding through air passages 161, and thence through the waveguide and hollow conductor, is discharged under the antenna supporting disc 140 into the cavity for eventual escape through the vent 162 at the top of the cavity. The bulk of the pressurized cooling air is discharged directly from the housing through the openings 87, 89 and, via opening 88, into the guard 170 which surrounds the waveguide and antenna motor so that all of the microwave components are kept at a safely low temperature regardless of the thermal temperature achieved in the cavity.

Because of the fact that all of the inlet openings for the convected air are well below the level of the bottom wall of the cavity, the convection pressure differential is sufficiently great to insure upward flow through the system even in the face of pressure developed by "auto ignition" as the soil is decomposed, so that there is no back-up of the decomposition products into the waveguide. Air follows the dotted air flow paths for convected passage of the air through the housing, waveguide and cavity with eventual venting at the top vent 162, without reliance upon the usual gap in the door gasket free of possibility of back-up or explosion.

In the event that servicing of the module is required, the hollow conductor assembly which communicates with the front end of the waveguide is loosened and retracted upwardly as previously described. Unscrewing of the screws 121 frees the front end of the waveguide from the oven structure. Subsequent removal of screws 174a, 175a which secure the trough 170 and screws 59 which hold the housing in seated position, permits the module to be pulled forwardly through the drawer opening. Electrical disconnection of the power supply module may be facilitated by a simple multi-prong plug and socket. In a typical service call a new module is slipped in place by simply reversing the procedure.

The preferred form of antenna 130 disclosed in FIG. 7a, rotated at a relatively slow speed on the order of a few revolutions per minute, has been found to be highly effective in bringing about an equalized distribution of microwave energy throughout the cavity with relative freedom from the hot spots which usually characterize microwave cookery. The antenna is distinguished by a straight shank portion 131 and a radially extending arm portion 133 which is arranged approximately perpendicularly with respect to the shank, with an angled transitional portion 132 in between, the angled portion bearing an obtuse angle with respect to both the shank and the arm and having a length of at least the radius r of the hollow conductor for radiation of a guided wave in the cavity which sets up a plurality of radiation patterns.

In understanding the operation of the antenna it will, first of all, be appreciated that microwave energy is directly propagated from the waveguide into the cavity independently of antenna action, the resulting wave been referred to as the "inertial" wave. In addition, microwave energy which is coupled to the lower end of the antenna is guided along the antenna for radiation therefrom in the form of a "guided" wave. Where an "L" shaped or "T" shaped antenna is employed having an abrupt 90.degree. bend, it is found that the bend constitutes a major discontinuity which actually prevents microwave energy from being converted to a guided wave. However, where the antenna, at point of exit, is bent on a more gradual basis, that is, is bent over a transitional region of at least the radius of the hollow conductor through which it projects into the cavity, the proportion of energy in the guided wave, as compared to the inertial wave, is increased and, in addition, the guided wave controls the pattern of radiation within the cavity. The increased number of radiation patterns, with continuous sweeping, inherently produces fewer hot spots than either the simpler, stationary mode patterns achieved using, for example, an antenna of the conventional monopole type, or a conventional antenna in combination with auxiliary stirring means.

In the preferred form of the invention, illustrated in FIG. 7a, the antenna has two concentrated bends 134, 135 to define its three portions. The radial arm portion 133 has a length which is preferably greater than the transitional portion to improve radiation patterns and/or facilitate impedance matching while maintaining a low profile. In practical cases the total radial length of the antenna may be on the order of 2.0 to 5.0 inches. Preferably the total radial extent of the antenna should be 2 to 10 times the radius r of the hollow conductor.

It is one of the features of the antenna that the bends 134, 135 are so related to one another that the antenna lies in the plane of the lower heating element 28. Although the antenna illustrated in FIG. 7a can be employed with conventionally formed "O" shaped heating elements whose surfaces are remote from the antenna, for those cases where the antenna tip overlies a heater surface, the preferred heater shape, as shown in FIG. 2a, is circumferentially distributed about the antenna path at a substantially constant radius so that the effect of the heating element 28 upon the radiated energy remains substantially constant throughout the rotational cycle.

Moreover, by spacing the radial portion 133 of the antenna, and the heating element 28, at the same relatively close spacing to the adjacent (bottom) wall 22, there is minimum encroachment upon the space within the cavity; in other words, a high degree of compactness is achieved. In a practical case, the spacing of the heating element and antenna from the adjacent wall is on the order of one and one-half inch. By keeping the antenna within the confines of the bottom heating element, and with the latter distributed as shown in FIG. 2a, the antenna itself does not take up any additional space within the cavity, making the utilization of space much more efficient than where a stirring device or conventional type of rotating antenna is employed.

It will be understood, however, that the invention is not limited to the exact profile illustrated in FIG. 7a and, if desired, the slightly higher profile, illustrated in FIG. 10, may be used. In this embodiment, in which corresponding reference numerals are used with addition of subscript a to indicate corresponding parts of the device, the included angle a at the first bend 134a is increased and the included angle at the second bend 135a is decreased. Preferably the angle a should be kept within the range of 105.degree. to 165.degree., while the included angle at the second bend should be approximately (270.degree. - a) to provide a low profile.

While it is preferred to form the antenna with concentrated bends, the antenna may be gradually bent or smoothly "faired" over the length of the transitional portion as indicated in FIG. 11, which corresponds to FIG. 7a, the same numerals being employed with addition of subscript b. Thus it will be noted that the transitional portion 132b has a length at least the radius r of the hollow conductor between the point 134b denoting the end of the shank and the point 135b denoting the beginning of the radial arm 133b.

The invention is not, however, limited to the embodiments illustrated in FIGS. 10 and 11 but includes the dogleg configuration set forth in FIGS. 12 and 13 where corresponding elements are denoted by the same reference numerals with addition of subscripts c and d, respectively. Thus referring to FIG. 12 the antenna has a straight shank portion 131c which is coaxially arranged in the hollow conductor, with a bend 134c which is substantially less than 90.degree. to produce an included angle a which preferably lies within the range of 95.degree. to 165.degree., the angled portion 132c being preferably at least equal in length to the radius of the hollow conductor. The structure of FIG. 13 corresponds to that of FIG. 12 except that the concentrated bend 134c has been replaced by a gradual bend to define an angled portion 132d having the same effective length.

In each of the illustrated embodiments of FIGS. 10- 13 the use of an obtuse angle a to define a transitional portion 132 of dogleg configuration results in efficient guidance of energy from the shank to the radiating portions of the antenna with minimum electrical discontinuity thereby to control the ratio between the energy in the guided wave and that in the directly emitted, or inertial wave, with the guided wave being effectively superimposed upon the inertial wave and swept continuously throughout the cavity for improved energy distribution in the cavity with relative freedom from hot and cold spots. Thus the distribution is to be distinguished from that using a conventional monopole which tends to concentrate energy in a radial ring concentric with the monopole and the various types of "mushrooms" antenna configurations which tend to distribute energy about the periphery while leaving a relatively large "hollow" energy region at the center of the cavity. It is found that using a rotated antenna of the above-described design the usual metal grid type of shelf employed in conventional electric ranges is effectively penetrated. It is one of the observed benefits of the invention, that arcing between metallic conductors placed within the oven cavity does not occur. Special precautions such as proper microwave grounding or insulated standoffs are not needed for metal grid shelves, shelve supports, or heating elements. This, while not fully understood, is attributed to the uniform energy distribution within the cavity.

It is one of the observed benefits of the present invention that a larger percentage of energy reaches the foodstuff along a direct radiation path than in conventional microwave ovens. Thus, using an electrically transparent vessel to contain the foodstuff, with the vessel supported in a generally central position, microwave energy received from the waveguide and from the rotating antenna creates even internal heat within the foodstuff, free of hot or cold spots as described. Such energy transfer is more uniform than that which occurs by reason of uncontrolled multiple reflection against the walls of the cavity, thereby reducing the time it takes to completely cook the foodstuff.

While it is preferred to use electrically transparent cooking vessels, another of the observed benefits of the described antenna is the fact that the cooking pattern does not seriously degrade when cooking with metallic utensils, nor do metallic cooking utensils cause adverse operating conditions harmful to magnetron operating life. Evidently the secondary reflections which occur also produce a more uniform energy distribution than that which occurs from conventional monopole or stirrer type microwave energy distribution systems.

Conventional sources of waveguide energy are highly sensitive to the dimensions and proportions of the cavity in which the energy is utilized. Thus, starting with a cavity of a certain dimension it has been necessary in the past to "tailor" the antenna and waveguide to the cavity and to make other adjustments including the position of point of feed in order to create a desired modal pattern. It is found, by contrast, that the present design of module and its associated antenna is largely independent of the size and dimensional proportions of the cavity so that the module and antenna assembly may be utilized by the manufacturers of different types, sizes and designs of oven cavities, free standing, built-in, and portable as well, on an O.E.M. basis, and with the only modification of the basic range structure being the provision of seating for the housing at a proper level and the use of a hollow conductor and clamping screws of appropriate length accommodated to the thickness of the lower insulated wall of the cavity. As a result almost any design of thermal oven may be modified to include microwave energy at a relatively small increment in cost, greatly expanding the market for microwave devices and indeed expanding the market for ranges in general.

While the above discussion has emphasized conversion of an electric range to microwave usage, it will be understood that the present microwave module is also applicable to conversion of gas ranges, although the latter will, of course, require a greater degree of modification. Consequently, the term "thermal element" will be understood as not being, necessarily, limited to an electric heating element. The term "storage space" has been used for convenience to apply to any available space adjacent the regular oven cavity.

Although it is preferred to seat the microwave power supply housing in a remote position in the storage space, it is conceivable that in a range design providing a wide oven having a limited front-to-back dimension, it may be more convenient to mount the microwave housing along the side of the storage space with some sacrifice in the width, but not length, of the drawer. Consequently, the term "back" used in connection with storage space shall be considered to be a relative term including both of the positions. The term "hollow conductor" which has been applied to the hollow tube 110 is also a general term applying to means defining any opening or passage between a waveguide and an oven cavity and the length of which depends upon the thickness of any insulating layer which may be provided. The term "waveguide" shall be considered to cover any conductor of wave energy from the magnetron for discharging microwave energy into, or which energizes an antenna in a shielded oven cavity. Thus the term "Waveguide" as used herein includes a coaxial transmission line. The term "magnetron" refers to any source of high intensity microwave energy.

It is found that the structure described above produces a field which is so uniform and a result which is so predictable, that "formula cookery", based upon conventional cookbook instructions, is possible for the first time. Specifically, in the case of pastry and similar baked goods, the foodstuff, in normal quantity, is subjected to microwaves for a period of five minutes, after which baking is completed using thermal energy at the temperature, and in just one-half of the time, given in the conventional cookbook. In the case of other foods, both types of energy are applied from the beginning of the cycle and the total cooking time is reduced to one-quarter of that given in the cookbook.