US3651300A

US3651300A - Microwave heating apparatus with radiation control and monitoring

Info

Publication number: US3651300A
Application number: US109303A
Authority: US
Inventors: Duane B Haagensen
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1971-01-25
Filing date: 1971-01-25
Publication date: 1972-03-21
Anticipated expiration: 1989-03-21
Also published as: NL7117393A; FR2124646A5; GB1322857A; JPS5148576B1; DE2160958A1; NL162286C; DE2160958C3; DE2160958B2; NL162286B

Abstract

A radiation safety device located about the entrance to a microwave oven is comprised of a wave trough-type of transmission line within which a printed circuit cross-polarized antenna is disposed to permit induction of radiation of either transverse electric or transverse magnetic waves into propagation circuits containing both dissipative and conductive elements for reducing environmental radiation leakage.

Description

United States Patent Haagensen [54] MICROWAVE HEATING APPARATUS WITH RADIATION CONTROL AND MONITORING [72] lnventor:

[73] Assignee:

Duane B. l-lalgensen, Edina, Minn.

Osaka, Japan [22] Filed: Jan. 25, 1971 [21] Appl. No.: 109,303

[52] U.S.Cl

.....2l9/l0.55 51 lm. Cl. .l-l05b 5/00 [58] Field ofSearch ..219/10.55

[56] References Cited UNITED STATES PATENTS Mntsuhita Electric Industrial Co., Ltd.,

g ,9ss,7s4 11/1950 Hahn 21 g p. ss

[451 Mar. 21, 1972 3,511,959 5/1970 White ..219/10.55 3,525,841 8/1970 Hangensenetal ..219/l0.55

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-Dugger, Peterson, Johnson & Westman [5 7] ABSTRACT A radiation safety device located about the entrance to a microwave oven is comprised of a wave trough-type of transmission line within which a printed circuit cross-polarized antenna is disposed to permit induction of radiation of either transverse electric or transverse magnetic waves into propagation circuits containing both dissipative and conductive elements for reducing environmental radiation leakage.

20 Claims, 7 Drawing Figures I PATENTEDMARZI I972 SHEET 3 OF 3 CONTROL SWITCH UNIT SWITCH UNIT CONTROL TOP SIDE SWITCH UNIT RIGHT SIDE SWITCH UNIT LEFT SIDE SWITCH UNIT INVENTOR.

SWITCH UNIT BOTTOM SIDE OVEN FRONT i [/0 At/Zrneys MICROWAVE HEATING APPARATUS WITI-I RADIATION CONTROL AND MONITORING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electronic heating apparatus and more particularly to a safety device for controlling environmental microwave radiation.

2. Description of the Prior Art In the past various attempts have been made to contain or reduce the amount of radiation leakage from the cavity of microwave ovens. The United States Radiation Control Act of 1968 has emphasized the importance of maintaining this leakage below an absolute level of 5 milliwatts per square centimeter at a distance of -5 centimeters from the oven. Accordingly, various types of door seals have been used in an attempt to meet this requirement. However, recent tests conducted by the Department of Health, Education, and Welfare have shown that these door seals are not sufficiently complying in all instances with the regulations set forth by the Radiation Control Act. During extended use of the oven, these seals begin to wear and deteriorate and thus the radiation leakage is allowed to exceed the permissible limit. Therefore, merely providing an effective radiation seal for a new product is not always adequate to maintain the safe level of 5 milliwatts throughout the useful life of the product.

Also, in the past an alarm or radiation detection means consisting of a loop of wires around the door of the oven has been used. However, studies of this particular detection arrangement have shown certain defects. The detector wires rely on a single polarization of the leaking energy, which is not always the case when a seal becomes ineffective due to wear or an abnormal operating condition. A faulty seal can cause an abnormal condition which allows energy of a different polarization to leak past the detector and into the environment. In addition, this particular alarm system provides no diversion or reduction of leaking energy. In the instance that the alarm system should ever fail, no protection is afforded the person using the oven. Thus, simply an alarm system dependent on a single polarization of the radiation has not proven capable of satisfactory environmental control of radiation.

The use of oven door interlocks for compliance with the requirements of the Radiation Control Act has also met with limited success. Since these interlocks are only able to insure the door being in a mechanically closed position and in many instances are not fail-safe, no protection is afforded when a door seal becomes worn or defective.

SUMMARY OF THE INVENTION A general object of the present invention is to provide a means of controlling and monitoring environmental radiation from microwave heating apparatus.

A more specific object of the invention is to provide a microwave oven with an integral dissipative, conductive and detecting system which uses multiple reduction principles to insure a fail-safe control of environmental radiation.

A further object of the invention is to control environmental radiation from a microwave oven in which the control is sensitive to cross-polarized electric fields. A more specific aim of the invention is to provide a resistive cross-polarized wave trough antenna element which reacts in parallel with dissipative dielectric material contained within the wave trough, thereby furnishing a greater amount of attenuation and radiation protection.

Another object is to provide an environmental protective system for microwave ovens which is intrusion or tamperproof as far as the protective elements are concerned.

Another object is to provide a transmission circuit for the entrance of a microwave oven that is capable of absorbing excessively large amounts of microwave radiation.

Still another object is to provide additional attenuation of microwave energy escaping through the access opening of a microwave oven by serially or successively connecting together the individual wave troughs situated at the sides of the opening. More specifically, my invention recognizes that excessive leakage may occur at only one side of the oven's access door, and by feeding the radiated microwave power picked up by the particular wave trough at that side through the wave trough located where little or no leakage is taking place, a more effective attenuation is realized.

While it is planned that the various wave troughs be connected in series, as mentioned above, to obtain greater at tenuation, it is within the purview of the invention to provide detection equipment for each trough, thereby enabling the user of the oven to determine whether the primary door seal at the right, left, top or bottom of the access opening to the cooking cavity is defective.

In a preferred embodiment of the present invention a dielectric material with a sufficient dielectric constant and loss factor is used to attenuate the flow of microwave power across and past a wave trough transmission line surrounding the periphery of an entrance to a microwave heating cavity, while simultaneously acting as a microwave phasing element for the integral microwave antenna circuit. The dielectric material is placed within a metallic trough which acts as a reflector element in conjunction with a microwave detector propagation circuit which may be slightly embedded or covered with appropriate insulation. The center conductor of the wave trough transmission line is orientated within thestructure in such a manner as to intercept microwave energy having multiple electric field of propagation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a microwave oven having an exemplary form of my control and monitoring device installed thereon;

FIG. 2 is an enlarged fragmentary perspective view of the lower right hand comer of the oven illustrated in FIG. 1 for the purpose of showing to better advantage certain details not clearly set forth in FIG. 1;

FIG. 3 is a top plan view of a portion of one of the printed circuit boards depicted in FIGS. 1 and 2, the board carrying a cross-polarized radiation antenna that plays an important role in the practicing of my invention;

FIG. 4 is a schematic diagram showing a plurality of units comprising my radiation monitoring device which units are connected in series with alarm circuitry shown at one end for warning the user when an excessive amount of leakage occurs, the antenna array having a high frequency short at its other end;

FIG. 5 is a diagrammatic view corresponding to FIG. 4 but illustrating an additional termination at the end to which the warning circuitry is connected, the additional termination absorbing any excess power that may be radiated from the oven into my device;

FIG. 6 is still another diagram showing how my invention may be employed, the four units at the sides of the access opening being individually connected to an alarm circuit so as to signify more precisely where the fault exists, and

FIG. 7 is a sectional view taken in the direction of line 7-7 in FIGS. 1 and 2 but illustrating a somewhat modified oven construction which embodiment has been used during actual tests of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It will be helpful to preface the following detailed description of apparatus exemplifying my invention by stating that all parts hereinafter mentioned are of metal, preferably aluminum, unless otherwise stated.

Referring now to FIG. 1, a typical microwave oven has been denoted generally by the reference numeral 10. The oven 10 includes a cabinet 12 containing a cooking cavity 14 therein.

A microwave generator 16 appears at the rear and excites the cavity 14 through a wave guide 18. The cabinet 12 has a front wall 20 with an access opening 22 therein. A door 24 provides access to the cooking cavity 14 through the opening 22. More specifically, the door 24 includes an overlapping or overlying marginal portion or border 26, an offset panel portion 28 and a peripheral wall 30 that extends between the

portions

26 and 28. A hinge 31 permits the door 24 to be swung open in order to permit an article that is to be heated or cooked to be inserted through the opening 22 into the cavity 14.

As explained above, the panel portion 28 is offset with respect to the overlapping or overlying marginal portion or border 26. Actually, the panel portion 28 functions in cooperation with parts presently to be described to provide a primary radiation seal. In this regard, it will be perceived that a spring seal 32 extends around the access opening 22 and that the offset panel portion 28 bears thereagainst when the door 24 is closed. Inasmuch as the access opening 22 is rectangular, there are sections of the seal 32 at each side of the opening 22. Accordingly, it is believed of assistance to assign specific distinguishing reference numerals to the portions of the spring seal 32 at each side of the access opening 22. In this regard, it will be noted that the spring seal at the bottom of the access opening 22 has been given the reference numeral 328, the portion at the right the numeral 32R, the portion at the top the numeral 32T and finally the portion at the left the reference numeral 32L. The spring seal 32 composed of the

portions

32B, 32R, 32T and 32L is reversely folded or bent to form a mounting strip 34 and a diverging resilient or flexible sealing strip 36. It is actually the strip 36, there being a strip associated with each

portion

328, 32R, 32T and 32L, that is contacted or engaged by the door 24 when closed, more specifically the panel 28 thereof. A stiffener 33 is interposed between the mounting strip 34 and the diverging strip 36. Any preferred number of mounting holes 38 (see FIG. 7) can be provided in the mounting strip 34, attaching screws 40 extending through these holes, through aligned holes in the stiffener 33 into tapped holes in the front wall 20 of the cabinet 12; one such screw 40 can be seen in FIG. 2.

The primary radiation seal that involves the door 24, more specifically the panel portion 28 thereof, and the spring seal 32 is not per se novel. The difficulty experienced with such a seal, however, has entailed certain drawbacks and the present invention seeks to obviate the shortcomings of such an arrangement standing alone. In this regard, it will be appreciated that the spring contact method has resulted in leakage where the spring parts are not kept clean, thereby permitting an objectionable amount of radiation leakage to take place. Furthermore, when there is any appreciable discontinuity with respect to the engaging surfaces of a metal seal, such as that alluded to above, arcing has taken place with a concomitant deterioration of the seal itself. However, by combining the sealing construction described above (which is only typical of several primary radiation seals) with what will now be referred to results in a highly effective and safe sealing arrangement.

Playing a very important role in the practicing of my invention is a radiation or transmission line device designated generally by the reference numeral 42. Actually, the radiation device 42 is composed of four individual units that may be arrayed in two different ways, as will become apparent as the description progresses. Once again, inasmuch as the access opening is rectangular and therefore has four sides, the individual units constituting the radiation device 42 have been labeled 42B, 42R, 42T and 42L (all four units appearing in FIG. 4, as well as in FIGS. and 6), the letter designations corresponding to those employed in conjunction with the spring seal 32. Each

unit

42B, 42R, 42T and 42L includes a channelshaped trough member 44 having spaced

side walls

46, 48 and a connecting wall 50. The trough member 44 thus forms a longitudinal recess or groove 52 having an entrance slot 54 lying in the plane of the edges of the

side walls

46, 48. The foregoing construction is pictorially represented in FIG. 2 and reference thereto will provide a ready understanding of the detailsinvolved. In this regard, it is to be observed that any preferred number of screws 56 passing through a corresponding number of holes 58 in the front wall 20 serve to anchor the radiation device 42 in place, the screws 56 being threadedly engaged with holes 60 in the side wall 46 of each trough member 44. Actually, the radiation device 42 can be succinctly described as resembling a rectangular picture frame, the trough members 44 constituting the molding thereof.

It has already been stated that unless otherwise designated the materials used in constructing the various parts of not only the oven 10 but the additional parts comprising my invention are of metal. Consequently, the trough member 44 is of metal, preferably extruded aluminum, but the material contained therein, which has been indicated by the reference numeral 62, is a dielectric material. Various synthetic resins may be used as the dielectric material 62. For example, a number of synthetic resins are satisfactory, such as epoxy (Araldite), urea, phenolic, polyester or if desired a ceramic material may be employed. The specific type of dielectric material utilized during the testing procedures will be described hereinafter when referring to other data concerned with the actual testing of my radiation device 42.

At this time, attention is directed to the level of the dielectric material 62 within the longitudinal recess or groove 52 of the channel-shaped trough members 44. The dielectric material 62 does not completely fill the recess 52 to the level of the entrance slot 54, but instead space is left for the emplacement of a printed circuit board 64. The printed circuit vboard 64 includes a plastic substrate or base strip 65, such as fiber glass epoxy, and has evaporated thereon or otherwise applied thereto a copper antenna 66 in the form of a flat ribbon. Inasmuch as the antenna 66 is a cross-polarized antenna, the specific configuration or pattern thereof is important and in this regard it will be discerned from FIG. 3 that the antenna 66 is composed of spaced longitudinal portions 660 residing near the access opening 22 and alternately spaced portions 66b spaced farther from the access opening 22 there being transverse portions 660 connecting the various portions 66a and 66b together so as to provide a continuous transmission line. The actual dimensions of the antenna 66 will be given when describing the specific test embodiment. At this time, though, it should be recognized that the antenna 66 resides in a plane perpendicular to the plane of the access opening 22. The plane of the antenna 66 also is perpendicular to the general plane of the door 24 when closed. It will be appreciated that the overlapping or overlying marginal portion or border 26 of the door 24 confronts the side wall 48 of each trough member 44 when the door 24 is closed. There is a small gap between the overlying portion 26 and the side wall 48; therefore, the side wall 48, that is the face thereof toward the overlying portion 26, is coated with a suitable gasket type of material (not shown), which may be an epoxy resin. The use of gasket material on the side wall 48 is not necessary to the invention nor is it a novel idea. For instance, it could very well be added for more attenuation or prevention of intrusionary objects into the mentioned gap. To protect the cross-polarized radiation antenna 66 from wear, a somewhat similar covering or coating should be applied thereover; this coating has not been shown in the drawings but it should be a protective covering material that does not interfere with transmission circuits.

Quite obviously, the antenna 66 changes planes at the four comers of the access opening 22'. One such corner in both FIGS. 1 and 2 has been indicated by the reference numeral 68 and this same comer appears in the several schematic diagrams set forth in FIGS. 4-6. However, a different mode of connection is resorted to in FIGS. 4 and 5 than appearing in FIG. 6. It will be recalled that the radiation device 42 is composed of

individual units

42B, 42R, 42T and 42L.

Referring to FIG. 4, the various antennas 66 are connected at three of the corners 68 so as to provide a series-connected antenna array. In other words, the configuration or pattern shown in FIG. 4 results in a relatively long antenna composed of the various individual antennas 66 and to simplify the ensuing description one end of the antenna array has been given the reference numeral 70 and the opposite end thereof the reference numeral 72. It will be noted from FIG. 4 that the end 70 is terminated by a capacitor 74 having one side or plate thereof connected to the end 70 and the other side or plate connected to the trough member 44 at 76. The various trough members 44 are grounded at 78. It will be recognized that the capacitor 74 provides a low impedance, actually a short circuit as far as microwave frequencies are concerned, to the metallic trough member 44. On the other hand, the capacitor 74 provides an isolated or open circuit as far as direct current is concerned.

The detection means for monitoring or sensing the amount of microwave energy picked up by the radiation device 42 will now be described. Accordingly, the other end 72 of the antenna array appearing in FIG. 4 is connected to the cathode of a diode 82, the anode thereof being connected to the trough 44 at 84. Also connected to the end 72 of the antenna array appearing in FIG. 4 is a high reactance microwave inductance labeled 86. The inductance 86 is connected to a signaling circuit denoted in its entirety by the reference numeral 88. The circuit 88 includes a potentiometer 90, the wiper arm thereof being connected to the inductance 86. The potentiometer 90 is in circuit with the gate of a silicon controlled switch 92, such as a General Electric type 3N58 series, the gate being identified by the numeral 94. It will be perceived from FIG. 4 that the anode of the switch 92 is connected to a relay coil labeled 96 having normally open contacts 98. In circuit with the normally open contacts 98 is a warning lamp 100 that is supplied from a source of power, such as the battery 102. Hence, when the relay coil 96 is energized, the contacts 98 close so as to energize the lamp 100, thereby providing the warning that an excessive amount of radiation is taking place.

As far as FIG. 5 is concerned, in the monitoring or sensing of the microwave energy the end 70 of the serially connected antenna array is connected in the same fashion as in FIG. 4, and therefore the capacitor 74 appears again in this figure. Likewise, the end 72 is connected through the same diode 82 to the trough member 44. The circuitry of FIG. 5 differs from that constituting FIG. 4 by virtue of an additional termination assigned the reference numeral 104. The additional termination 104 is in the form of an absorbing medium such as ferrite or graphite material, the additional termination being grounded at 105. In the embodiment of FIG. 5, the additional termination 104 assures the absorption of excessive amounts of power delivered to any of the antennas 66.

With reference to FIG. 6, this embodiment depicts an independent monitoring connection of the various wave troughs 44. More specifically, it will be observed that none of the corners 68 are electrically connected together and to distinguish them from the connected corners they have been labeled 68a. The purpose of the embodiment set forth in FIG. 6 is to specifically locate any fault that might occur, the signaling arrangement apprising the user as to which side of the access opening 22 the fault is located. In the carrying out of this objective, four capacitors corresponding to the capacitor 74 are utilized, there being one such capacitor for each antenna 66. Likewise, the end of each antenna 66 opposite or remote from the end to which the capacitor 74 is attached is connected directly to an individual signaling circuit 88. Inasmuch as the trough members 44 are electrically isolated from each other, a separate ground is provided at the locations 106B, 106R, 106T and I06L. Consequently, each circuit 88 has its own warning lamp 100. To conform with the plan adopted with respect to the several sides of the access opening 22, the various circuits 88 have been distinguished from each other by the letter suffix that has been used heretofore. Consequently, the circuit 88 for the bottom side of the access opening 22 has been assigned the reference numeral 888, the circuit for the right side 88R, the circuit for the top the numeral 88T and the circuit for the left side the numeral 88L.

Operation With the door 24 closed so that the access opening 22 is blocked, it will be appreciated that the onset panel portion 28 engages the spring seal 32, more specifically the resilient or flexible sealing strip 36. Consequently, the microwave energy provided by the generator 16 is, for the most part, confined to the cooking cavity 14. However, as pointed out when discussing the prior art, door seals of this type do not always sufficiently comply with the regulations specified under the Radiation Control Act.

Accordingly, any leakage escaping between the door 24 and the spring seal 32 is directed to the radiation device 42. The trough members 44, being metallic, reflect any leaking energy not first intercepted by the antenna 66. Owing to the somewhat lossy nature of the dielectric material 62, some of the microwave power is attenuated as it is reflected by the interior of the trough members 44. Due to the orientation of the antenna 66, being cross-polarized, the microwave energy that escapes past the primary seal is not only intercepted but is propagated along the antenna in a transmission line fashion. The serial connection depicted in FIG. 4 causes the attenuation to be accentuated, for whether the escape route is at the bottom, sides or top of the access opening 22, the energy is picked up and the entire device 42 serves to dissipate the energy picked up by the wave trough-type transmission line constituting the radiation device 42. This is true of either transverse electric or transverse magnetic waves, and the energy or power not dissipated is propagated through the entire antenna array to the capacitor 74. As previously pointed out, the capacitor 74 serves as a virtual short circuit for any energy reaching the end 70 of the antenna array depicted in FIG. 4. If the escaping radiation exceeds a predetermined level, this level being selectable through the agency of the potentiometer 90, then a triggering signal is applied to the gate 94 of the silicon controlled switch 92 which results in the energizing of the relay coil 76 and the simultaneous closing of the normally open contact 98. This causes the warning lamp to be illuminated and the user is thereby apprised that the oven 10 is not functioning properly. The contacts 98 can also be used to disable the microwave generator 16 so as to completely inactivate the oven 10 and prevent further use thereof until the leakage situation has been corrected.

As an added precaution, the load referred to as the additional termination 104 will be effective in absorbing any microwave power that would not be dissipated when utilizing the circuit arrangement of FIG. 4. Otherwise, the embodiment of FIG. 5 functions in the same manner as the embodiment of FIG. 4.

As far as FIG. 6 is concerned, each

radiation unit

42B, 42R, 42T and 42L is responsible for dissipating any leakage radiation that is received by the particular unit. The distinct advantage when utilizing the teachings of FIG. 6 is that the particular lamp 100, that is the lamp 100 associated with the

circuit

88B, 88R, 88T or 88L, localizes the fault and advises the user of the particular location so that the fault in the primary sealing arrangement can be promptly corrected.

It will be appreciated, though, that no objectionable levels of radiation can take place into the ambient surroundings. This is true irrespective of both the magnitude and the polarization of the microwave power.

DESCRIPTION OF TEST EMBODIMENT Although closely resembling the construction of the oven 10, the test embodiment differs in certain respects and therefore the modified oven of FIG. 7 bears the reference numeral 110. The oven includes a cabinet 112 containing a cooking cavity 114. A front wall provides the access opening 122. The flat surface labeled f on the front of the oven 110, this being the forwardly facing surface of the front wall 120, equals about 2 inches. In other words, the border is approximately 2 inches wide that extends about the access opening 122.

The door identified by the numeral 124 in FIG. 7 differs somewhat from the door 24. The door 124 includes a'marginal portion or border 126, an offset panel portion 128 and a peripheral connecting wall 130.

The spring seal 132 includes a mounting strip 134 and the diverging sealing strip corresponding to the previously mentioned strip 36 is composed of two sections 136a and l36b. The section 136a actually diverges away from the strip 134, whereas the section 136b is parallel to the strip 134. The width of the section l36b has been indicated by the letter k and equals 0.7 inch on the test oven 110. A stiffener 133 is interposed between the mounting strip 134 and the sections 136a, 136b. The overlapping or overlying marginal portion or border 126 has a width of approximately 1 inch.

With respect to the radiation device 42, this device as shown in FIG. 7 corresponds closely to what has been illustrated in FIGS. 1 and 2. it will perhaps be well, though, to list the dimensions of the trough member 44. The inside dimensions of the trough member 44, that is of the recess or groove 52, have been identified by the letter h and the width by the letter w. In the test embodiment, h 7% inch and w A inch.

As far as the dielectric material 62 is concerned, the actual material used possessed a dielectric constant of 11.2 and a dielectric loss tangent of 0.04 as measured by the Massachusetts Institute of Technology when subjected to a frequency of 3,000 mHz. The material was a polyester resin marketed by Ashland Chemical Company, a division of Ashland Oil & Refining Company, and modifications of this material are aptly described in their technical bulletin l 148 of June 1968. Such materials are highly desirable in that they contain moisture on the order of 50 percent on a weight basis. For an operating frequency of 2,450 i- 20 ml-lz., the antenna 66, which is detailed in FIG. 3, had the following dimensions:

a 0.268 inch b 0.268 inch c 0.300 inch d 0.032 inch e 0.437 inch It will be appreciated that e in the above listing is the width of the printed circuit board 64. Missing from the above tabulation is the thickness of the copper circuitry constituting the antenna 66; in practice it was 0.0028 inch. The thickness of the substrate 65 was 0.023 inch.

The silicon controlled switch 92 has already been identified as a General Electric type 3N58. However, the diode 82 has not been identified. This diode constituted a Hewlett-Packard series 5082 rectifier of the epitaxial, planar, passivated diode type whose construction utilizes a combination of both a conventional PN junction and a Schottby barrier. This rectifier, quite desirably, has the high breakdown and temperature characteristics of silicon, the turn on voltage of germanium, and the speed of a Schottby barrier, majority carrier device.

As far as the cooking cavity 114 is concerned, the dimensions of this cavity were 30 cm. (depth) by 39 cm. (width) by 24 cm. (height). The cavity power supplied by the generator 16 was 700 watts. The frequency, as indicated above, was 2,450 i 20 mHz.

Radiation Test Results Although extensive tests were made under different conditions, only a summary thereof is believed necessary in order to appreciate the benefits to be derived from a practicing of my invention. Accordingly, the summarized radiation survey and sequence of operation included the following modes of operation:

l. 700 watts into 275 cc. load of water Normal Door Closure 2. 700 watts into empty oven Normal Door Closure 3. 700 watts into 275 cc. load of water Abnormal Door Closures against 0.050 paper shim (2.0 inches by 1.25 inches) placed at right, left, top and bottom positions 4. 700 watts into empty oven Abnormal Door Closures against 0.050" paper shim 2.0 inches by 1.25 inches) placed at right, left, top and bottom positions Without the use of this invention, conditions (3) and (4) in which the door seals were faulted gave radiation levels in excess of 20 milliwatts per square centimeter at a distance of 5 centimeters from the oven. With the installation of the wave trough transmission system all tests for each mode of operation were below the maximum radiation level prescribed by Federal Radiation Standards. Additionally, provision was made for disabling the apparatus if objectionably high voltages appeared, but the sensed voltages simply did not reach environmental hazard levels due to the effectiveness of my method.

I claim:

1. Microwave heating apparatus comprising a cabinet having a cooking cavity therein, means for supplying microwave power to said cavity, and an access opening in one wall thereof through which an article to be heated is inserted, a door overlying the marginal portions of said cabinet wall that reside adjacent said access opening, and a radiation sealing device extending along at least one side of said opening which includes metallic trough means providing an elongated recess open at the side thereof nearer said access opening to form a slot having a width extending in a transverse direction between said cabinet wall and door, said radiation sealing device further including antenna means disposed in said recess to receive radiation leakage.

2. Microwave heating apparatus in accordance with claim 1 in which said radiation device is disposed at a location between said wall and a confronting portion of said door.

3. Microwave heating apparatus in accordance with claim 2 in which said radiation device includes a dielectric material contained in said recess for supporting said antenna means.

4. Microwave heating apparatus in accordance with claim 3 in which said antenna means constitutes an element having a first series of spaced portions thereof located nearer said cavity, and a second series of alternate spaced portions thereof located nearer said door and a third series of connecting portions extending between said first and second portions.

5. Microwave apparatus in accordance with claim 1 in I which said first and second spaced portions are parallel to each other and said third portions are perpendicular thereto.

6. Microwave heating apparatus in accordance with claim 5 in which said antenna element is in the form of a flat ribbon residing in a plane perpendicular to the plane of the access opening.

7. Microwave heating apparatus in accordance with claim 4 in which said radiation device includes termination means at one end thereof providing a relatively low impedance path between said antenna means and said trough means.

8. Microwave heating apparatus in accordance with claim 7 in which said termination means includes a capacitor for shorting high frequency power from said antenna means to said trough means.

9. Microwave heating apparatus in accordance with claim 7 including detection means connected to the other end of said radiation device for monitoring the amount of microwave energy picked up by said radiation device.

10. Microwave heating apparatus in accordance with claim 9 in which said detection means includes a control switch and means for rectifying a portion of the microwave power picked up by said radiation device to supply direct current to said control switch and cause operation thereof when the direct current supplied thereto reaches a predetermined level.

11. Microwave heating apparatus in accordance with claim 8 in which said radiation device includes a plurality of antenna elements connected in series and said capacitor is connected at one end of said serially connected antenna elements.

12. Microwave heating apparatus in accordance with claim 1 1 including an additional termination means at the other end of said serially connected antenna elements, said additional termination means being capable of absorbing excessive amounts of power delivered to said plurality of antenna elements.

l3. Microwave heating apparatus comprising a cabinet having a cooking cavity therein and an access opening in one wall thereof through which an article to be heated is inserted, a

door overlying the marginal portions of said cabinet wall adjacent said access opening, primary radiation sealing means attached to the marginal portions of said cabinet wall in a proximal relationship to said access opening and engageable by said door, and radiation means attached to the marginal portions of said cabinet wall outwardly of said primary sealing means for dissipating microwave power leaking past said primary sealing means when said door is closed, said radiation means further including a cross-polarized antenna element disposed generally in the plane of the open side of the trough member with which it is associated.

14. Microwave heating apparatus in accordance with claim 13 in which said trough members are joined to form a serially connected trough configuration extending along the sides of said access opening and in which said individual antenna elements are also serially connected to form a unitized construction, and means connected between one end of the thus formed antenna array and one of said interconnected trough members for dissipating at least some of the microwave power radiated to said antenna array which power has leaked past said primary sealing means.

15. Microwave heating apparatus in accordance with claim 14 including means connected to the antenna array for sensing the amount of microwave power radiated to said antenna array.

16. Microwave heating apparatus in accordance with claim 15 in which said sensing means is connected to the other end of said antenna array.

17. Microwave heating apparatus in accordance with claim 15 including means connected to said antenna array for absorbing excessive amounts of power radiated to said antenna array.

18. Microwave heating apparatus in accordance with claim 17 in which said absorbing means is connected to the other end of said antenna array.

19. Microwave heating apparatus in accordance with claim 13 including respective means connected between one end of each antenna element and the trough member with which that antenna element is associated for dissipating at least some of the microwave power radiated to said antenna elements.

20. Microwave heating apparatus in accordance with claim 19 including respective means connected to the other end of said antenna elements for sensing the amount of microwave power radiated to each antenna element.

Claims

5. Microwave apparatus in accordance with claim 1 in which said first and second spaced portions are parallel to each other and said third portions are perpendicular thereto.

12. Microwave heating apparatus in accordance with claim 11 including an additional termination means at the other end of said serially connected antenna elements, said additional termination means being capable of absorbing excessive amounts of power delivered to said plurality of antenna elements.

13. Microwave heating apparatus comprising a cabinet having a cooking cavity therein and an access opening in one wall thereof through which an article to be heated is inserted, a door overlying the marginal portions of said cabinet wall adjacent said access opening, primary radiation sealing means attached to the marginal portions of said cabinet wall in a proximal relationship to said access opening and engageable by said door, and radiation means attached to the marginal portions of said cabinet wall outwardly of said primary sealing means for dissipating microwave power leaking past said primary sealing means when said door is closed, said radiation means further including a cross-polarized antenna element disposed generally in the plane of the open side of the trough member with which it is associated.