US3629537A - Microwave oven door seal having dual cavities fed by a biplanar transmission line - Google Patents

Abstract

A microwave oven door seal is established when an access opening of a heating cavity is closed by a door. The seal includes a biplanar transmission line which extends in a first direction from within the heating cavity to a point outside the heating cavity. At such point, the biplanar transmission line turns and extends in a second direction away from the access opening. A first electromagnetic wave filter is fed by the first portion of the biplanar transmission line and a second electromagnetic wave filter is fed by the second portion of the biplanar transmission line for reducing the amount of electromagnetic wave energy which leaks from the heating cavity. The filters are cavities which are located along the biplanar transmission line and are designed to occupy a minimum of space to provide room for an observation window in the door. To improve the effectiveness of the seal on one side of the heating cavity in the event the door is pivotally mounted to an opposite side of the heating cavity the door is mounted so that it extends from the opposite side toward the one side at an obtuse angle relative to a wall at the one side of the heating cavity. As a result, both the width and the length of the first portion of the biplanar transmission line on such one side decrease so that the sealing characteristics thereof remain relatively constant during an initial opening movement of the door.

Description

United States Patent vI'll 3,629,537

{72] Inventor Duane Buford Haagensen Attorney-Bugger, Peterson, Johnson & Westman Edina, Mlnn. [2|] Apple No. 70,641 [22] Filed Sept. 9, 1970 ABSTRACT: A microwave oven door seal is established when [45] Patented Dec. 21, 1971 an access opening of a heating cavity is closed by a door. The [73] A'ssignee m m mu-1 m-l m 1 seal includes a biplanar transmission line which extends in a K'doma. O k Japan first direction from'within the heating cavity to a point outside the heating cavity. At such point, the biplanar transmission line turns and extends in a second direction away from the acl l MICROWAVE VEN DOOR SEAL HAVING DUAL cess opening. A first electromagnetic wave filter is fed by the CAV ITIES FED BY A BIPLANAR TRAN MI first portion of the biplanar transmission line and a second electromagnetic wave filter is fed by the second portion of the 26 (,lalms, lDrawlng Flgs. biplanar transmission line for reducing the amount of elecs21 u.s. Cl 219/1055 mmasnflic wave energy Whieh leak wing Wilylsll lnLCl "05b 9/06 The filters are cavities which are located along the biplanar Field of Search 219/1055 transmissim and are design OccuPY minimum of space to provide room for an observation window in the door. [56] References Cited To improve the effectiveness of the seal on one side of the UMTED STATES PATENTS heating cavity in the event the door is pivotally mounted to an 2,958,754 ll I960 H h 9 the is 3 182 164 511965 z g it extends from the opposite side toward the one side at an ob- 3197600 7/1965 Muller 2|9/l0'55 tuse angle relative to a wall at the one side of the heating cavi- 324973l 5/1966 i 219/1655 ty. As a result, both the width and the length of the first portion of the biplanar transmission line on such one side Pnr nary ExamineP-J. V. Truhe decrease so that the sealing characteristics thereof remain Asszstanl Examiner-Hugh D. .laegen relatively constant during an initial opening movement of the door.

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llrvplfy a a I I A r4 I I V a a a A l //VVE/VTO/? DUANE a. HAAGENSEN A Home MICROWAVE OVEN DOOR SEAL HAVING DUAL CAVITIES FED BY A BIPLANAR TRANSMISSION LINE BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to a heating cavity for subjecting products to electromagnetic wave energy. More particularly, it relates to a microwave oven door seal having dual electromagnetic wave energy seals fed by a biplanar transmission line which extends around an access opening of the heating cavity to limit the leakage of such energy from the heating cavity.

In the past, a substantial number of microwave ovens have been manufactured for commercial and domestic use. These ovens have been provided with a variety of seals which were intended to prevent microwave energy from leaking around a closed door and out of the oven during the operation of the oven. Also, a number of such ovens have been provided with safety switches which are actuated in response to varying amounts of opening movement of the door or in response to the unlatching of a door handle. These ovens have been sub ject to the present safety standard which has been generally accepted within the electronic industry. The present safety standard permits a maximum safe level of microwave radiation of milliwatts per square centimeter measured at a specific distance from the oven. Notwithstanding the existence of the present radiation standard, it is significant to note the conclusion stated in connection with the presentation of a Report dated Dec. 1969 entitled Microwave Oven Surveys prepared by the U.S. Department of Health, Education and Welfare, Public Health Service, Consumer Protection and Environmental Health Service, Environmental Control Administration.

Although the Report was not based on a statistically valid sample of microwave ovens, the numbers and types of such ovens surveyed and reported was considered sufficient to warrant the conclusion that a significant number of all microwave ovens in use are leaking radiation and justify the initiation of corrective measures. Further, in the presentation it was stated that the data contained in the Report demonstrate that control of radiation leakage has been lost in an unpredictable manner for many of the microwave ovens surveyed.

Subsequent to the Report, under the provisions of the Radiation Act of 1968 (Public Law 9602), the Department of Health, Education and Welfare (HEW) proposed the establishment of comprehensive regulations which would apply to the emission of radiation from microwave ovens.

Under the proposed regulations, the maximum permissible radiation from a new microwave oven would be 1.0 milliwatt per square centimeter and that from any microwave oven (independently of the duration of use thereof) would be 5.0 milliwatts per square centimeter.

The data in the Report and the proposal of such regulations by HEW indicate not only the importance of the problem of radiation leakage from microwave ovens but that many types of prior art microwave oven door seals appear to be inadequate for their intended purpose.

2. Description of the Prior Art The many seals which have been used in the past to limit the amount of radiation emitted from microwave ovens include the metal-to-metal contact seal. This seal depends upon electrical contact between two metal surfaces. Thus, to maintain this seal in efiect the contacting metal surfaces must be cleaned frequently. If the surfaces are not clean, arcing may occur which deteriorates the surfaces and renders the seal ineffective. I

To avoid these problems, compression seal plates have been used. However, these seals require excessive pressure across the two surfaces to be sealed and the pressure spring used therein may wear or break, rendering the seal ineffective. Further, because the compression seal plates are generally thin and fragile, they are easily broken by cooking utensils.

In an attempt to overcome the problems attendant electrical contact type seals, a gap has been provided between prior oven doors and the door frame. In an effort to provide a very low impedance at the origin of the gap, single, one-quarter wavelength, closed-ended chokes have been provided in either the door or the door frame at a distance of one-fourth wavelength from the origin. While such chokes may have been effective when measured by the present radiation standard, it is not certain whether such a single choke would satisfy the proposed standard under all of the various operating conditions. For example, when no load is in the oven, the amount of radiation transmitted through the gap may exceed the sealing capability of such a single choke. Further, in one type of design, as the door is opened, the one-fourth wavelength spacing of the opening of the choke from the origin may decrease.

In the event the size of such single choke must be increased to satisfy the proposed radiation standards, problems may be encountered in keeping such commercially advantageous features as observation windows at a maximum, functional size.

Other attempts to provide microwave oven door seals have resulted in the development of a single planar transmission line having a series of sections of alternately high and low impedance. From the standpoint of the size of the observation window, for example, such single planar seals are of limited applicability because the transmission line is relatively long and extends for its full length across the front of the oven. Thus, the size of the window would be reduced considerably.

SUMMARY OF THE INVENTION Research has been conducted in an endeavor to provide microwave oven door seals which are commercially practical and which will comply with the proposed radiation standards at the time of manufacture and after indefinite periods of service. Such research indicates that an effective microwave oven door seal may be provided between noncontacting, opposed surfaces of a heating cavity access opening and a microwave oven door. The seal is provided by having a biplanar transmission line extending between such surfaces and feeding a pair of microwave energy filter cavities. By providing the transmission line in a plurality of planes, it is unlikely that wear which may occur during service will significantly reduce the sealing effect of both of the filter cavities. Further, even though a pair of filter cavities are used, by spacing the openings to such cavities in a selected manner relative to the origin of the transmission line, and by providing one of the filter cavities with a serpentine electrical transmission path, the size of the seal is compatible with an objective of maximizing the area of an observation window in the door. Moreover, because a high impedance appears at a given location along the serpentine path within the one filter cavity, a device to absorb microwave energy during no-load operation of the oven may be located at the high impedance location so that the volume of the heating cavity need not be reduced to provide no-load protection.

In addition, to improve the effectiveness of the seal on one side of the heating cavity in the event the door is pivotally mounted to an opposite side of the heating cavity, the door in a second embodiment of the present invention is mounted so that it extends from the opposite side toward the one side at an obtuse angle relative to a wall at the one side of the heating cavity. As a result, as the door is initially opened, both the width and length of a first planar portion of the transmission line on the one side decrease so that the sealing characteristics of the seal remain relatively constant.

Accordingly, an object of the present invention is to provide a new and improved microwave oven door seal.

Another object of the present invention is to limit the amount of electromagnetic wave energy which leaks from a microwave oven when the oven door is closed and during an initial portion of the opening movement of the door.

A further object of this invention resides in the provision of a microwave energy door seal which is effective to limit the amount of microwave energy which leaks from a microwave oven and which occupies a minimum amount of space so that other features, such as an observation window for viewing a product in the oven, may have an increased and more functional size.

Still another object of the present invention is to provide a transmission line which commences within and extends out of a microwave oven cavity in more than one plane for feeding microwave energy to a pair of compact microwave energy filters, wherein the transmission line and the filters are effective to limit the amount of such microwave energy that leaks from the microwave oven.

An additional object of the present invention resides in the provision of an obtuse angular relationship between a first wall of a microwave heating cavity and the closed position of an inner wall of a microwave oven door, wherein the angular relationship positions the elements of a door seal for predetermined movement during the initial opening of the door so that the effectiveness of the seal remains relatively constant during such opening of the door.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the present invention may be appreciated upon reference to the following description of the preferred embodiments when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a first embodiment of a microwave oven which may include a door seal embodying the principles of the present invention to limit the amount of radiation which is transmitted past a door and is emitted from a heating'cavity of the oven;

FIG. 2 is another perspective view of the oven depicted in FIG. 1 showing the door in an open position to expose the heating cavity;

FIG. 3 is vertical sectional view of the microwave oven shown in FIG. 1 illustrating the door seal including a biplanar transmission line and first and second filter cavities fed by the transmission line;

FIG. 4 is an enlarged view of a portion of FIG. 3 showing the details of one of the door seals;

FIG. 5 is a top partial sectional view of a modified heating cavity structure in conjunction with a door provided with a door seal;

FIG. 6 is a partial vertical sectional view illustrating a first filter cavity provided in the wall of an outer housing rather than in the door;

FIG. 7 is a partial vertical sectional view of the first filter cavity containing a block of material for attenuating microwave energy during no-load operation of the microwave oven;

FIG. 8 is a perspective view of a second embodiment of the microwave oven of the present invention showing a slanted cavity frame and a door in the open position;

FIG. 9 is a vertical cross-sectional view of the oven of FIG. 8 showing the door in a closed position wherein an inner wall thereof extends at an obtuse angle relative to a given wall of the heating cavity; and

FIG. 10 is a partial vertical sectional view of the oven shown in FIG. 9 illustrating successive positions of the door as the door is opened, wherein the length and the width of a first portion of the transmission line decrease as the door is opened.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring in general to FIG. 1 of the drawings, amicrowave oven 10 embodying the principles of the present invention is shown including an enclosure orouter housing 12 containing a heating chamber orcavity 14 and apower source 16 connected to awaveguide 18 for supplying electromagnetic wave energy to theheating cavity 14 for heating a product (not shown) received in theheating cavity 14. Adoor 20 is shown provided with ahandle 22 for opening the heating cavity to receive the product. When thedoor 20 is closed, an electromagnetic waveenergy door seal 24 is effective to limit the amount of electromagnetic wave energy which leaks from theheating cavity 14 as the product is heated. The door is also shown having afrontal face 26 which borders a window orscreen 28 which is perforated to minimize the radiation of electromagnetic energy while permitting the product to be observed during heating.

In accordance with an object of this invention, thewidth 30 of thefrontal face 26 is minimized so that theobservation screen 28 may have a maximum area for a givenoverall height 32 andwidth 34 of thedoor 20. Further, according to another object, thedoor seal 24 is provided while retaining a maximum amount of theheating cavity depth 36.

With these and other objectives in mind, reference is made in general to FIG. 2 in which a first embodiment of the present invention is shown including theouter housing 12 and theheating cavity 14 received therein. Thedoor 20 is shown in an open position to expose anopening 38 at one side of theouter housing 12. The opening 38 permits access to theheating cavity 14 for placing products (not shown) in theheating cavity 14.

As shown in FIGS. 2 and 3, thedoor 20 is provided with aninner wall 40 which is positioned in the heating cavity when the door is closed (FIG. 3). Theperimeter 42 of theinner wall 40 is spaced from thesidewalls 44 of theenclosure 12 by a selectedgap 46. The electromagneticwave energy seal 24 is provided for limiting the amount of energy which leaks through thegap 46 and out of theoven 10 when thedoor 20 is closed and the electromagnetic wave energy is supplied to theheating cavity 14. Theseal 24 includes afirst portion 48 of abiplanar transmission line 50 which has anorigin 52 adjacent theperimeter 42 of theinner wall 46. Thefirst portion 48 extends out of the heating cavity and feeds asecond portion 54 of thetransmission line 50. Thesecond portion 54 extends along a cavity frame orborder member 56 of the enclosure in a plane other than that of thefirst portion 48 so that thetransmission line 50 is biplanar.

Theseal 24 also includes dual, first and second electromagnetic wave filters 58 and 60, respectively which are located along thebiplanar transmission line 50 such that a low electrical impedance appears at theorigin 52 of thebiplanar transmission line 50. More particularly, afirst opening 62 is provided in thefirst portion 48 of thetransmission line 50 at afirst distance 64 of less than one-quarter wavelength from theorigin 52; whereas asecond opening 66 is provided in thesecond portion 54 of thetransmission line 50 at asecond distance 68 of about an odd number of one-quarter wavelengths from the origin. Apartition 70 is provided in thefirst filter 58 to establish therein a selectedelectrical length 72 extending from thefirst opening 62 to a terminatingsurface 74 of thefirst filter 58 such that the sum of thefirst distance 64 from theorigin 52 to thefirst opening 62 plus the selectedelectrical length 72 equals about an even multiple of onequarter wavelength. In a preferred embodiment of the present invention, thesecond distance 68 from theorigin 52 to thesecond opening 66 equals about one-quarter wavelength and the total distance from theorigin 52 to a closed end or terminatingsurface 74 of thefirst filter 58 equals about one wavelength.

Referring now in detail to FIGS. 2-4, it may be understood that theenclosure 12 contains an inner,rear wall 76 and four of theinner sidewalls 44 which extend forwardly from therear wall 76. For convenience, the upper sidewall may be referred to as thetop wall 78 and the lower sidewall may be referred to as thebottom wall 80. As shown in FIG. 2, thesidewalls 44 and therear wall 76 mutually intersect to define the heating cavity.

Each of thesidewalls 44 extends forwardly from therear wall 76 to aterminus 82 which fonns aperimeter 84 of theaccess opening 38. A dashedline 86 is shown in FIG. 2 to define anouter portion 88 of each of thesidewalls 44. Theouter portion 88 extends rearwardly from eachterminus 82 for a selected distance 90.

Although theenclosure 12 has been described as having planar walls, it is to be understood that the present invention may also be provided on an enclosure formed from one or more arcuate walls. In addition, the present invention may be used in conjunction with anenclosure 92 having a single wall structure as shown in FIG. 5. There, arear wall 94 andsidewalls 96 form both a housing and define aheating cavity 98. Also, forward ends 100 of thesidewalls 96 have a border section orcavity frame 102 provided with areturn lip 104 which extends back into theheating cavity 98. Aninner surface 106 of thelip 104 defines the perimeter of anaccess opening 108 and also corresponds to theouter portion 88 of the sidewalls 44 shown in FIG. 2.

Referring again to FIGS. 2-4, it may be appreciated that thecavity frame 56 has an inner perimeter which is coextensive with thetermini 82 of thesidewalls 44. Further, thecavity frame 56 extends from the inner perimeter thereof in a plane other than that of thesidewalls 44. As shown in FIG. 2, thecavity frame 56 surrounds and provides an outer border for theaccess opening 38.

Thedoor 20 is mounted on a hinge 1 which extends along a lower portion 1 12 of theenclosure 12. If it is desired to move the door relative to theenclosure 12 in a different manner, thehinge 110 may be provided on the left or right side of the enclosure, for example. Alternatively, thedoor 20 may be secured to theenclosure 12 in such a manner that it moves in a rectilinear, rather than arcuate, path away from thecavity frame 56.

After a product has been inserted into theheating cavity 14, thedoor 20 may be rotated counterclockwise on thehinge 110 into the closed position shown in FIGS. 3 and 4. As shown in detail in FIG. 4, when thedoor 20 is closed, theinner wall 40 is received in theheating cavity 14 and extends across theaccess opening 38. As shown in perspective in FIG. 2 and in cross section in FIG. 4, the central area of theinner wall 40 of thedoor 20 is provided in a well-known manner with many small diametercylindrical apertures 114 which form thescreen 28 to permit visual observation of the product in theheating cavity 14 without permitting more than a minute amount of electromagnetic energy to be transmitted from theheating cavity 14.

Considering FIG. 4 in detail, it may be appreciated that when thedoor 20 is closed, theinner wall 40 extends across and closes the access opening 38 except for thegap 46. Because theseal 24 causes a low electrical impedance to appear at theorigin 52 of thetransmission line 50, the amount of electromagnetic wave radiation which is transmitted through thetransmission line 50 and out of theoven 10 is less than the maximum value of 1.0 milliwatt per square centimeter permitted under the proposed radiation standards.

Thebiplanar transmission line 50 includes a firsttransmission line element 116 extending from each side of theperimeter 42 of theinner wall 40 of thedoor 20. Each of the first elements 1 16 extends generally parallel to theouter portion 88 of thesidewall 44 opposite thereto and is spaced therefrom by the width of thegap 46. Thefirst elements 116 extend out of theheating cavity 14 and past theplane 118 of thecavity frame 56 by an amount equal to the width of thegap 46 whereupon each intersects a secondtransmission line element 120. Each of thesecond elements 120 extends in parallel, overlapping relationship with thecavity frame 56.

The firsttransmission line elements 116 and theportions 88 of thesidewall 44 opposite thereto form thefirst portion 48 of thetransmission line 50 having a width equal to the width of thegap 46. Thefirst portion 48 extends from each side of theinner wall 40 of thedoor 20 and has a length which is less than an odd multiple of one-quarter wavelength as measured from theorigin 52 of thetransmission line 50.

Each secondtransmission line element 120 and the section of thecavity frame 56 which is overlapped thereby form thesecond portion 54 of thetransmission line 50. As shown in FIG. 4, the first andsecond portions 48 and 54, respectively, of thetransmission line 50 extend in different planes. As will become clear, the biplanar arrangement permits thewidth 122 and theheight 124 of the access opening 38 to be maximized for a givenheight 32 andwidth 34 of theenclosure 12 without significantly reducing theusable depth 36 of theheating cavity 14. Moreover, the biplanar arrangement uses a minimum amount of thewidth 30 of thefrontal face 26 of thedoor 20 so that the area of theobservation screen 28 is maximized for a given width and height of thedoor 20.

Still referring to FIG. 4, thefirst opening 62 is in the form of a slot which extends through each firsttransmission line element 116 at thefirst distance 64 from theorigin 52 so that electromagnetic wave energy is fed from thetransmission line 50 into thefirst filter 58. Thefirst filter 58 is provided with acavity 126 defined byrespective surfaces 128 and 130 of theinner wall 40 and thefirst element 116. Further, acavity wall 132 mounted opposite to thefirst opening 62 is provided with asurface 134 which extends from theinner wall surface 128 to asurface 136 of thefrontal face 26 of thedoor 20. In view of the low electrical impedance which appears at theorigin 52, the electrical length from theorigin 52 to the terminatingsurface 74 of thefirst cavity 126 must be equal to about an even multiple of onequarter wavelength so that a low electrical impedance appears at the terminatingsurface 74 of thefirst filter cavity 126. For this purpose, thepartition 70 is mounted to theclosed end 74 of thefirst cavity 126. Thepartition 70 extends generally parallel to thecavity wall 132 and defines a serpentine or tortuouselectrical path 138 within thefirst cavity 126. Thepath 138 defined by thepartition 70 has the selectedelectrical length 72 from thefirst opening 62 to the terminatingsurface 74 so that the selectedelectrical length 72 plus thefirst distance 64 are equal to about an even multiple of onequarter wavelength. As a result, at least one high impedance appears along theelectrical path 138 inside thefirst cavity 126 between thefirst opening 62 and the terminatingsurface 74.

Thesecond opening 66 is in the form of a slot which extends through each secondtransmission line element 120 at thedistance 68 from theorigin 52 so that electromagnetic wave energy is fed from thetransmission line 50 into thesecond filter 60. Thesecond filter 60 is provided with asecond cavity 140 defined by asurface 142 of thesecond element 120 and by asurface 144 of acavity wall 146 which is opposite to thesurface 142. Thefrontal face 26 extends around a corner toward thecavity frame 56 to provide a terminatingsurface 148 for closing thesecond filter cavity 140. With a low electrical impedance appearing at theorigin 52, thesecond distance 68 is about an odd multiple of one-quarter wavelength. Because thesecond cavity 60 is closed ended, the distance from thesecond opening 66 to the terminatingsurface 148 is selected such that a low electrical impedance also appears at the terminatingsurface 148.

The present invention may be more fully appreciated by reference to the following example, in which the integers 1,2,3,4... refer to units which are multiples of one-quarter wavelength. Initially, it has been established herein that the distance 68 (the sum ofdistances 64 and 150) is an odd multiple of one-fourth wavelength and that the sum of thedistance 64 and thelength 72 is about an even multiple of one-fourth wavelength. To minimize the amount ofcavity depth 36 used, thedistance 64 may be 0.5 units, for example, and thedistance 68 may be 1 unit. Further, if the total distance (distance 64 length 72) is selected as 4 units, for example, thepartition 70 will be effective to reduce the door depth and thewidth 30 of thefrontal face 26 required for theseal 24. With these distances selected, it may be understood that the following relationships result:

1.Distance 64+distance 150= 1 unit, thus:

0.5 unit+0.5 unit= 1 unit 2.Distance 64length 72= 4 units, thus:

0.5length 72= 4units length 72= 3.5 units Further, because thepath 138 from theopening 62 to the terminatingsurface 74 is tortuous, the physical length of 3.5 units is less than 3.5 times 1.25 inches, which is the numerical value of one-fourth wavelength in air at 2,450 megacycles per second. As a result, the actual physical length of thepath 138 from theopening 62 to the terminatingsurface 74 is 10 cm., for example. Accordingly the door depth can be as short as 4 cm. plus the thickness of the sheet metal used to fabricate the door.

Referring to FIG. 6, thedual filters 58 and 60 of the first embodiment are shown in a different arrangement which permits an even smallerdoor face width 30 and a resulting greater area for theobservation screen 28. Theheating cavity 14 is shown defined by theinner door wall 40 and theside wall 44. Theorigin 52 of thebiplanar transmission line 50 is adjacent the intersection of theinner door wall 40 and the first section 1 16. Thetransmission line 50 extends from theorigin 52 along thefirst portion 48 and thesecond portion 54. However, a first opening 62' extends through thewall 44 to permit electromagnetic wave energy to enter a cavity 126' of thefirst filter 58 which is provided in aspace 151 between theinner walls 44 andouter walls 154. Thefirst cavity 126 is defined by asurface 156 of awall 158 and asurface 160 of awall 162 opposite to the first opening 62'. A terminating surface 74' is provided in the first cavity 126' and supports a partition 70' for defining theelectrical path 138. It may be understood that because the first cavity 126' is provided in thespace 151, thewidth 30 of thefrontal face 26 is decreased so that the area of theobservation screen 28 may be increased.

Thesecond filter 60 is similar to the second filter described above in reference to FIGS. 3 and 4. Also, the spacing of the first andsecond openings 62 and 66, respectively, as well as the dimensions of thecavities 126' and 140 are similar to those described above.

Referring now to FIG. 7, a view similar to FIG. 4 but reduced in size illustrates thedoor seal 24 provided withfacilities 166 to protect theoven 10 in the event thepower source 16 supplies electromagnetic wave energy to theheating cavity 14 when no product or load is in the heating cavity to absorb such energy. More particularly, the door is shown in the closed position with thebiplanar transmission line 50 and thedual filters 58 and 60 positioned to limit the amount of radiation emitted from theheating cavity 14. Normally, a product or other lossy material is placed in theheating cavity 14 and absorbs a major portion of the electromagnetic wave energy which is supplied to the heating cavity. As a result, only a minute amount of input energy leaks into thetransmission line 50 past the low impedance which appears at theorigin 52. However, when only a small amount of energy is absorbed by a product in the heating cavity or when there is no lossy material in theheating cavity 14, themicrowave oven 10 is said to be operating at no-load." Under such no-load conditions, a substantial amount of electromagnetic wave energy may leak past theorigin 52 and into thetransmission line 50. To render the seal effective in reducing the amount of electromagnetic wave energy which leaks completely out of theoven 10, the high impedance which exists in thecavity 126 of the first filter $8 is used to advantage. In particular, it may be recalled that because of the location of thefirst opening 62 relative to theorigin 52, one or more high impedances appear within thefirst cavity 126. Ablock 168 of material having selected properties is mounted within thefirst cavity 126 at the location of one of such high impedances. The material selected resists deterioration at high temperatures such as 1,200 F. for example, and is electrically lossy. As an example, the following materials may be used: high-temperature resistant silicone carbide, water extended polyethylene and silicone rubber graphite. These materials may be used in their commercially available forms.

Theblock 168 mounted at the location of one of the high impedances is effective to absorb the electromagnetic wave energy which leaks into thebiplanar transmission line 50. As a result, a substantial portion of the electromagnetic wave energy supplied to theheating cavity 14 is absorbed so that only a limited amount of such energy is emitted from theoven 10.

Turning now to FIGS. 8-10, there is shown a second embodiment of the present invention. The advantages of the second embodiment are particularly significant in the event the microwave oven is not equipped with a safety lock (not shown) for the door, for example. On ovens having such locks, thedoor 20 may be held tightly closed by the lock and the lock must be secured before thepower source 16 can be conditioned for operation. Instead of using such locks, some manufacturers provide microwave ovens with a device (not shown) which interrupts or prevents the operation of thepower source 16 in the event thedoor 20 is open more than a very slight amount. After a period of usage, the part of such devices which senses the position of thedoor 20 may wear or otherwise require adjustment. As a result, the device may permit thepower source 16 to continue to operate even though thedoor 20 is open to such an extent that conventional energy seals around the door are no longer effective and allow as much as 200 milliwatts per square centimeter, for example, of energy to leak from themicrowave oven 10. The second embodiment overcomes this disadvantage by providing a door seal which is effective over a greater range of door movement than conventional door seals.

Referring now to FIGS. 8-10, the second embodiment of the present invention is shown provided on amicrowave oven 180. Theoven 180 is similar in design to theoven 10 in that it is provided with anouter housing 182 which defines aheating cavity 184. Also, electromagnetic wave energy is supplied from thesource 16 to theheating cavity 184 through thewaveguide 18. In combination with thedual filters 58 and 60 and thetransmission line 50 of the first embodiment, theoven 180 is provided with aheating cavity frame 186 and adoor 188 which are mounted in a new relationship with respect to thewalls 44 of theoven 180. This new relationship enhances the effectiveness of the radiation seal provided by atop section 190 of thefirst portion 48 of thebiplanar transmission line 50 and thefirst filter 58 as thedoor 188 is opened. More particularly, as shown in FIG. 9, in its closed position thedoor 188 slants or slopes toward therear wall 76 of theoven 180 and is positioned at an acute angle relative to avertical line 192. Thedoor 188 also extends toward thetop wall 78 at an obtuse angle therewith and extends away from the bottom wall at an acute angle.

As thedoor 188 is moved from the closed position (FIG. 9) to the open position (FIG. 8), the length 194 (FIG. 10) of thetop section 190 of thefirst portion 48 of thetransmission line 50 decreases. However, because of the initial positional relationship between thedoor 188 and thecavity frame 186, there is at the same time a decrease in the width of thegap 46 between theouter portion 88 of thetop wall 78 and the firsttransmission line element 116. Thefirst portion 48 of thetransmission line 50 consists of inductive and capacitive elements, thus, the capacitance thereof increases as the width of thegap 46 decreases. The increase in the capacitance decreases thelength 194 of thetop section 190 which is required to produce a given resonant frequency for the top section of the transmission line. Therefore, with thedoor 188 mounted to the bottom of theenclosure 182 at the obtuse angle relative to theouter portion 88, it may be appreciated that as thedoor 188 is initially opened, the decrease of the width of thegap 46 across thetop section 190 is counteracted by the decrease in thelength 194 of thetop section 190 so that the resonant frequency of the top section of thetransmission line 50 remains relatively constant during the initial open ing movement of thedoor 188. As a result, even though thedoor 188 is rotated clockwise from the closed position through a small initial opening angle, thetop section 190 of thefirst portion 48 of thetransmission line 50 and thefirst filter 58 will continue to limit the amount of electromagnetic wave energy which leaks out of theheating cavity 184. The provision of such improved radiation seal along thetop section 190 of thetransmission line 50 decreases the likelihood that excessive radiation will be emitted in the event the door is opened before a door actuated safety switch (not shown) interrupts operation of thepower source 16.

Considering FIGS. 810 in greater detail, theouter housing 182 is shown containing therear wall 76 and thesidewalls 44 which extend vertically between the top andbottom walls 78 and 80, respectively. In the second embodiment, thehousing 182 is truncated so that theinner wall 40 of thedoor 188 will be positioned at an acute angle with respect to thetop wall 78. In addition, when therear wall 76 is perpendicular to thesidewalls 44, it may be observed that thetop wall 78 extends forwardly from the rear wall 76 a shorter distance than thebottom wall 80 extends forwardly from the same wall. As a result, aterminus 196 of thebottom wall 80 is spaced from therear wall 76 by a greater distance than the spacing of aterminus 198 of thetop wall 78 from therear wall 76. Thetermini 196 and 198 cooperate withtermini 200 of thesidewalls 44 to form the perimeter of a tiltedaccess opening 202. Theouter portion 88 of each of the top, bottom and sidewalls 78, 80 and 44, respectively extends from the perimeter of access opening 202 rearwardly to the dashedline 86 as shown in FIG. 8.

Referring to FIGS. 8 and 9, it may be appreciated that thecavity frame 186 has aninner perimeter 204 which is coextensive with the perimeter of the tiltedaccess opening 202. Thecavity frame 186 is positioned at an acute angle relative to thevertical line 192 and extends from theinner perimeter 204 to theouter housing 182. Further, thecavity frame 186 extends at an acute angle relative to theouter portion 88 and at an obtuse angle relative to thebottom wall 80.

Thehinge 110 is shown mounting thedoor 188 for rotary movement from the closed position shown in FIG. 9 to the open position shown in FIG. 8. Thehinge 110 is designed to stop the rotation of the door when it is in closed position with theinner wall 40 thereof at the acute angle relative to thevertical line 190.

Alternatively, other well-known devices, such as spacers or the like, may be provided to limit the inward movement of either of thedoors 20 or 188 so that thegap 46 exists between the respective cavity frames 56 and 186 and the secondtransmission line elements 120.

When thedoor 188 is closed, theinner wall 40 closes the access opening 202 except for thegap 46 which exists between theperimeter 42 of theinner wall 40 and the top, bottom and sidewalls 78, 80 and 44, respectively.

As in the first embodiment, theoven 188 is provided with thebiplanar transmission line 50 which includes thefirst portion 48 and thesecond portion 54 extending around the four sides of thedoor 188. Thefirst portion 48 includes one of the firsttransmission line elements 116 extending from each side of theperimeter 42 of theinner wall 40. Because theinner wall 40 is not perpendicular to thetop wall 78, a top transmission line element 206 intersects theinner wall 40 at an acute angle. Thetop element 206 extends generally parallel to theouter portion 88 of thetop wall 78, which is opposed thereto and is spaced therefrom by the width of thegap 46 when thedoor 188 is in the closed position (FIG. 9'). The first transmission line element 206 extends out of theheating cavity 184 and intersects the respective secondtransmission line element 120. As shown in FIG. 9, in the closed position of thedoor 188, the second position of thedoor 188, the secondtransmission line element 120 is spaced from thecavity frame 186 by the width of thegap 46 and extends at an acute angle relative to thevertical line 192 in parallel relationship with thecavity frame 186, which is overlapped thereby form thesecond portion 54 of thebiplanar transmission line 50.

As in the first embodiment, the first andsecond filters 58 and 60, respectively, are mounted in thedoor 188 and cooperate with thetransmission line 50 so that a low impedance appears at theorigin 52 of thegap 46 when thedoor 188 is closed.

The significance of the positioning of theclosed door 188 relative to the slopedcavity frame 186 and thetop wall 78 may be more fully appreciated by referring to FIG. 10 where the initial opening movement of thedoor 188 is shown in successive steps. There, thesloped cavity frame 186 and theouter portion 88 are shown in cross section, whereas (for the closed l 0 position of the door) the first and secondtransmission line elements 206 and 120, respectively, are shown in solid lines. Thegap 62 and thelength 194 of thefirst portion 48 of thetransmission line 50 are also shown for thedoor 188 in the closed position.

As thedoor 188 is initially opened, the toptransmission line element 206 and the other structure of the door assume the dashed line position in which the width of thegap 46 has decreased to asmaller gap 220 and thelength 194 of thefirst portion 48 of thetransmission line 50 has shortened to a length 222.

Because the width ofthev gap 46 and thelength 194 of thefirst portion 48 of thetransmission line 50 decrease simultaneously, the resonant frequency of thetop section 190 of thetransmission line 50 remains relatively constant during the de picted initial opening movement of thedoor 188. As a result, thetop section 190 of thefirst portion 48 of thetransmission line 50 and thefirst filter 58 fed thereby will continue to limit the amount of electromagnetic wave energy which leaks through thetop section 190 out of theheating cavity 184.

In the practice of the present invention, amicrowave oven door 20 having abiplanar transmission line 50 extending across the upper section of thecavity frame 56 was found to limit the leakage of electromagnetic wave radiation to less than 1.0 milliwatt per square centimeter measured at the appropriate distance from thedoor 20 for the electromagnetic wave frequency (2,450 megacycles per second) used with thedoor 20 in its normal closed position.

The door constructed had acavity frame 56 having a width of 1.156 inches and a length of 17.0 inches. When thedoor 20 was in the closed position, the gap was 0.125 inches between thecavity frame 56 and the second element and between theouter portion 88 of thetop wall 78 and thefirst element 116. Theinner wall 40 extended 1.875 inches into theheating cavity 14 past theaccess opening 38.

Thecavities 126 and 140 were constructed from metal having a thickness of 0. inch. Thefirst distance 64 was 0.875 inch and the size of thefirst opening 62 was 0.25 inch by the width of thecavity frame 56.

Thedoor 20 was constructed to illustrate the radiation sealing effect without minimizing the size of the seal, thus thesecond distance 68 was selected to be about three-fourths of l wavelength in air. However, to shorten the physical length 212 of the second cavity and permit use of a standard test door, thesecond cavity 140 was filled with polypropylene which decreased thedistance 68 to 3,563 inches. However, the data taken are still representative of the improved sealing characteristics of theoven 10 shown in FIG. 4.

The size of thesecond opening 66 was 0.1875 inch and the distance 212 was 0.938 inch, and the internal depth of thesecond cavity 140 was 0.406 inch.

The first cavity had an internal depth of 0.75 inch, and a length of 1.75 inches. The terminatingsurface 74 was located 0.625 inch from thesurface 136 and thepartition 70 had a length of 1.0 inch. The width 214 of theelectrical path 138 was 0.406 inch and the width 216 ofsuch path 138 was 0.250 inch. The length of thepartition 70 was 1.0 inch.

With this construction, thepower source 16 provided electromagnetic wave energy at the frequency of 2,450 megacycles per second, thedoor 20 was in its closed position and the amount of radiation was measured with a sensitive radiation detector. The constructeddoor 20 was found to effectively prevent the electromagnetic wave energy from being emitted from theheating cavity 14 through thegap 46 and out of theoven 10.

It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of this invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. In a microwave oven comprising:

a heating cavity having an access opening, said heating cavity being defined by at least one wall;

a member secured to said wall in a plane other than that of said wall;

a door movable into a position for closing said access opening, said door having a first section, when said door is closed said first section extending through said access opening into said heating cavity to form a first portion of a biplanar transmission line, said first portion of said biplanar transmission line having an origin adjacent the inner end of said first section;

said door having a second section extending from the outer end of said first section in overlapping relationship with said member to form a second portion of said biplanar transmission line;

a first electromagnetic wave filter provided in one of said walls and said first section, said first filter having a first opening extending through said one of said wall and said first section, said first opening being located at a given distance from said origin; and

a second electromagnetic wave filter provided in one of said member and said second section, said second filter having a second opening formed through said one of said member and said second section and spaced from said origin by about an odd number of one-fourth wavelengths.

2. A microwave oven according to claim 1, in which:

said given distance is less than one-fourth wavelength; and

said first and second openings are located in difierent planes and are spaced from each other by less than one-fourth wavelength.

3. A microwave oven according to claim 1, in which:

said first filter includes a terminating surface and a side surface opposite to said one of said wall and said first section for defining one side of a transmission path from said first opening to said terminating surface; and

a partition is mounted in said first filter for defining the other side of said path so that the distance from said origin through said first opening and along said path to said terminating surface is about an even multiple of onefourth wavelength.

4. A microwave oven according to claim 3, in which:

said distance from said origin to said terminating surface is about 1 wavelength.

5. A microwave oven according to claim 3, in which:

said first filter is provided in said wall and said first opening.

extends through said wall to said first filter; and

said second filter is provided in said member and said second opening extends through said member to said second filter.

6. A microwave oven according to claim 3, in which:

said first filter is provided in said first section and said first opening extends through said first section to said first filter; and

said second filter is provided in said second section and said second opening extends through said second section to said second filter.

7. A microwave oven according to claim 3, in which:

said first filter is provided in said wall and said first opening extends through said wall to said first filter; and

said second filter is provided in said second section and said second opening extends through said second section to said second filter.

8. in a microwave oven comprising:

a heating cavity having an access opening, said heating cavity being defined by at least one wall;

a member extending from said access opening in a given plane other than that of said wall;

a door having a first section extending from an origin within said cavity along said wall and through said access opening out of said heating cavity, said first section and said wall defining a first part of a biplanar transmission line;

a first electromagnetic wave filter provided in said first section and having a first opening thereto formed through said first section at a first distance from said origin;

said door having a second section extending from said first section in overlapping relationship with said member, said second section and said member defining a second part of said biplanar transmission line; and

a second electromagnetic wave filter having a second opening formed through said second section at a second distance of about an odd number of one-fourth wavelengths from said origin.

9. A microwave oven according toclaim 8, in which:

said first filter includes a cavity having a partition mounted therein to establish in said cavity a selected electrical length such that the sum of said first distance plus said selected length equals about an even multiple of onefourth wavelength.

10. A microwave oven according to claim 9, in which:

said second distance equals about one-fourth wavelength;

and

said sum of said first distance and said selected electrical length equals about one wavelength.

1 1. A microwave oven according toclaim 8, in which:

said first section extends from said origin to said second section for a distance less than one-fourth wavelength.

12. A microwave oven according to claim 1 l, in which:

said first filter includes a cavity having a terminating surface; and

a partition is mounted within said cavity, said partition being efiective to define a biplanar path from said first opening'to said terminating surface, the distance from said origin through said first opening and along said path to said terminating surface being equal to about an even number of one-fourth wavelengths.

13. A microwave oven according toclaim 8, in which:

said one wall and said first section establish a low electrical impedance at said origin during the operation of said oven;

said first filter includes a cavity having a terminating surface spaced an even number of one-quarter wavelengths from said origin such that a high electric impedance appears at a selected location within said cavity during the operation of said oven; and

heat-resistant means are mounted in said first cavity at said selected location, said means being electrically lossy for absorbing electromagnetic wave energy during no-load operation of said oven.

14. A microwave oven according to claim 13, in which:

said heat-resistant means are fabricated from a material taken from the group consisting of high-temperature silicon carbide, water extended polyethylene, and silicone rubber graphite.

15. in a microwave oven, comprising a heating cavity defined by a plurality of intersecting sidewalls and a rear wall;

a cavity frame secured to said sidewalls and extending in a plane other than that of said sidewalls, said frame being v rovided with an aperture to permit access to said heating cavity;

a door having an inner wall, said inner wall being received within said heating cavity for closing said heating cavity when said door is in a closed position, the perimeter of said inner wall being spaced from said sidewalls by a selected gap when said door is in said closed position, said door having a first transmission line element extending from said perimeter of said inner wall along a portion of said sidewalls and out of said heating cavity for a selected distance, said door having a second transmission line element extending from the outer end of said first element across said cavity frame, said second element being spaced from said cavity frame by said selected gap when said door is in said closed position;

said first element and said portion of said sidewalls forming a first section of a biplanar transmission line, said line having an origin adjacent the perimeter of said inner wall;

said second element and said cavity frame forming a second section of said biplanar transmission line;

a first filter cavity having a first opening extending through said first element, said first opening being spaced from said origin by a first distance; and

a second filter cavity having a second opening extending through said second element, said second opening being spaced from said first opening by a second distance;

the sum of said first and second distances being about an odd multiple of one-fourth wavelength.

16. A microwave oven according to claim 15, in which:

said sum of said first and second distances is equal to about one-fourth wavelength.

17. In a microwave oven including a heating cavity defined by a rear wall and a plurality of sidewalls, said sidewalls defining an access opening, a frame extending from a first of said sidewalls on one side of said access opening to a second of said walls on a second opposite side of said access opening, the improvement in said oven which comprises:

said door being provided with a first side and a second side, said first side of said door having a first section extending along said first wall into said heating cavity when said door is in said closed position to form a first transmission line element, said first section being spaced from said first wall by a gap having a predetermined width when said door is in said closed position to form a first portion of a biplanar transmission line, said first side of said door having a second section overlapping said frame when said door is in said closed position to form a second portion of said biplanar transmission line;

a pair of electromagnetic wave filters, a first of said filters having a first opening extending through one of said first wall and said first section and the other of said filters having a second opening extending through one of said frame and said second section; and

means mounting said second side of said door on said second side of said access opening for pivotal movement relative to said closed position, said mounting means being effective to mount said door in said closed position so that said door is tilted at a first angle relative to said first wall, said first angle being obtuse.

18. A microwave oven according to claim 17, in which:

said mounting means is also effective to mount said door in said closed position so that said door is tilted at a second angle relative to said second wall, said second angle being acute.

19. A microwave oven according to claim 17, in which:

said frame includes a top portion adjacent said first sidewall and spaced from said rear wall by a first distance, a bottom portion adjacent said second sidewall and spaced from said rear wall by a second distance, and side portions connecting said top and bottom portions, id first distance being shorter than said second distance so that said heating cavity is truncated; and

said mounting means is provided adjacent said bottom portion of said frame so that said door slopes toward said rear wall when said door is in said closed position.

20. A microwave open according to claim 17, in which:

said first section and said first wall define a first transmission line having an origin at the end of said first section, said origin being received in said heating cavity when said door is in said closed position; and

said first opening is spaced from said origin by less than onequarter wavelength.

2]. A microwave oven according toclaim 20, in which:

a partition is mounted in said first of said filters to define therein a given electrical length such that the total distance from said origin to said first opening plus said given electrical length equal about an even number of onequarter wavelengths.

22. A microwave oven according to claim 21, in which:

said total distance equals about one wavelength; and

the distance from said origin to said second opening equals about one-quarter wavelength.

23. In a microwave oven, comprising:

a heating cavity defined by at least a first wall and a second wall opposite to said first wall, said heating cavity having an access opening between said first wall and said second wall;

a frame secured to said first and second walls and extending in a plane other than the planes of said first and second w l a door mounted adjacent said second wall for movement into a position for closing said access opening, said door having an inner wall extending toward said first wall at a selected obtuse angle when said door is in said closed position, said angle being measured relative to the plane defined by said first wall;

said door having a first transmission line element extending from the end of said inner wall for a given length in opposed relationship to said first wall, said first element extending out of said heating cavity;

said door having a second transmission line element extending along said frame;

said first wall and said first element forming a first transmission line having a given width and said given length when said door is in said closed position;

said second element and said frame forming a second transmission line; and

a pair of filter cavities mounted in said door, a first of said cavities having a first opening thereto formed in said first element, a second of said cavities having a second opening thereto formed in said second element;

said first element being movable toward said first wall to decrease both said given width and said given length of said first transmission line as said door moves from said closed position so as to decrease said obtuse angle.

24. A microwave oven according to claim 23, in which:

said door is pivotally mounted to said second wall such that said first element moves in an arcuate path during said movement of said door.

25. A microwave oven according to claim 23, win which:

a rear wall extends between said first wall and said second wall; and

said first and second walls extend different distances from said rear wall to said door when said door is in said closed position:

26. In a microwave oven, comprising:

a heating cavity defined by top, bottom side and rear walls, said bottom wall terminating at a given vertical plane and said top wall terminating at a location spaced toward said rear wall from said given plane so that said cavity is truncated;

a cavity frame secured to and extending outwardly from said top, bottom and sidewalls at the truncated end of said cavity, said frame having an aperture to permit access to said cavity;

a closure member having top, bottom and side portions, said bottom portion being mounted adjacent said bottom wall for pivotal movement, said closure member being movable into a closed position so that a first section thereof overlaps said cavity frame to form a first transmission line element and a second section of said member extends into said heating cavity along said top, bottom and sidewalls to form a second transmission line element;

a top part of said second section being spaced from said top wall by a gap having a selected width and length when said closure member is in said closed position;

said closure member having a closure wall bounded by said top, bottom and side portions, said closure wall sloping toward said rear wall when said closure member is in said closed position;

a first filter provided in said first section, said first filter having a first opening through said first section; and

during the initial movement of said closure member from said closed position into an open position so that as the length of said gap decreases the width of said gap decreases.

l l i i l

Claims (26)

1. In a microwave oven comprising: a heating cavity having an access opening, said heating cavity being defined by at least one wall; a member secured to said wall in a plane other than that of said wall; a door movable into a position for closing said access opening, said door having a first section, when said door is closed said first section extending through said access opening into said heating cavity to form a first portion of a biplanar transmission line, said first portion of said biplanar transmission line having an origin adjacent the inner end of said first section; said door having a second section extending from the outer end of said first section in overlapping relationship with said member to form a second portion of said biplanar transmission line; a first electromagnetic wave filter provided in one of said walls and said first section, said first filter having a first opening extending through said one of said wall and said first section, said first opening being located at a given distance from said origin; and a second electromagnetic wave filter provided in one of said member and said second section, said second filter having a second opening formed through said one of said member and said second section and spaced from said origin by about an odd number of one-fourth wavelengths.

2. A microwave oven according to claim 1, in which: said given distance is less than one-fourth wavelength; and said first and second openings are located in different planes and are spaced from each other by less than one-fourth wavelength.

3. A microwave oven according to claim 1, in which: said first filter includes a terminating surface and a side surface opposite to said one of said wall and said first section for defining one side of a transmission path from said first opening to said terminating surface; and a partition is mounted in said first filter for defining the other side of said path so that the distance from said origin through said first opening and along said path to said terminating surface is about an even multiple of one-fourth wavelength.

4. A microwave oven according to claim 3, in which: said distance from said origin to said terminating surface is about 1 wavelength.

5. A microwave oven according to claim 3, in which: said first filter is provided in said wall and said first opening extends through said wall to said first filter; and said second filter is provided in said member and said second opening extends through said member to said second filter.

6. A microwave oven according to claim 3, in which: said first filter is provided in said first section and said first opening extends through said first section to said first filter; and said second filter is provided in said second section and said second opening extends through said second section to said second filter.

7. A microwave oven according to claim 3, in which: said first filter is provided in said wall and said first opening extends through said wall to said first filter; and said second filter is provided in said second section and said second opening extends through said second section to said second filter.

8. In a microwave oven comprising: a heating cavity having an access opening, said heating cavity being defined by at least one wall; a member extending from said access opening in a given plane other than that of said wall; a door having a first section extending from an origin within said cavity along said wall and through said access opening out of said heating cavity, said first section and said wall defining a first part of a biplanar transmission line; a first electromagnetic wave filter provided in said first section and having a first opening thereto formed through said first section at a first distance from said origin; said door having a second section Extending from said first section in overlapping relationship with said member, said second section and said member defining a second part of said biplanar transmission line; and a second electromagnetic wave filter having a second opening formed through said second section at a second distance of about an odd number of one-fourth wavelengths from said origin.

9. A microwave oven according to claim 8, in which: said first filter includes a cavity having a partition mounted therein to establish in said cavity a selected electrical length such that the sum of said first distance plus said selected length equals about an even multiple of one-fourth wavelength.

10. A microwave oven according to claim 9, in which: said second distance equals about one-fourth wavelength; and said sum of said first distance and said selected electrical length equals about one wavelength.

11. A microwave oven according to claim 8, in which: said first section extends from said origin to said second section for a distance less than one-fourth wavelength.

12. A microwave oven according to claim 11, in which: said first filter includes a cavity having a terminating surface; and a partition is mounted within said cavity, said partition being effective to define a biplanar path from said first opening to said terminating surface, the distance from said origin through said first opening and along said path to said terminating surface being equal to about an even number of one-fourth wavelengths.

13. A microwave oven according to claim 8, in which: said one wall and said first section establish a low electrical impedance at said origin during the operation of said oven; said first filter includes a cavity having a terminating surface spaced an even number of one-quarter wavelengths from said origin such that a high electric impedance appears at a selected location within said cavity during the operation of said oven; and heat-resistant means are mounted in said first cavity at said selected location, said means being electrically lossy for absorbing electromagnetic wave energy during no-load operation of said oven.

14. A microwave oven according to claim 13, in which: said heat-resistant means are fabricated from a material taken from the group consisting of high-temperature silicon carbide, water extended polyethylene, and silicone rubber graphite.

15. In a microwave oven, comprising a heating cavity defined by a plurality of intersecting sidewalls and a rear wall; a cavity frame secured to said sidewalls and extending in a plane other than that of said sidewalls, said frame being provided with an aperture to permit access to said heating cavity; a door having an inner wall, said inner wall being received within said heating cavity for closing said heating cavity when said door is in a closed position, the perimeter of said inner wall being spaced from said sidewalls by a selected gap when said door is in said closed position, said door having a first transmission line element extending from said perimeter of said inner wall along a portion of said sidewalls and out of said heating cavity for a selected distance, said door having a second transmission line element extending from the outer end of said first element across said cavity frame, said second element being spaced from said cavity frame by said selected gap when said door is in said closed position; said first element and said portion of said sidewalls forming a first section of a biplanar transmission line, said line having an origin adjacent the perimeter of said inner wall; said second element and said cavity frame forming a second section of said biplanar transmission line; a first filter cavity having a first opening extending through said first element, said first opening being spaced from said origin by a first distance; and a second filter cavity having a second opening extending through said second element, said second opening beIng spaced from said first opening by a second distance; the sum of said first and second distances being about an odd multiple of one-fourth wavelength.

16. A microwave oven according to claim 15, in which: said sum of said first and second distances is equal to about one-fourth wavelength.

17. In a microwave oven including a heating cavity defined by a rear wall and a plurality of sidewalls, said sidewalls defining an access opening, a frame extending from a first of said sidewalls on one side of said access opening to a second of said walls on a second opposite side of said access opening, the improvement in said oven which comprises: said door being provided with a first side and a second side, said first side of said door having a first section extending along said first wall into said heating cavity when said door is in said closed position to form a first transmission line element, said first section being spaced from said first wall by a gap having a predetermined width when said door is in said closed position to form a first portion of a biplanar transmission line, said first side of said door having a second section overlapping said frame when said door is in said closed position to form a second portion of said biplanar transmission line; a pair of electromagnetic wave filters, a first of said filters having a first opening extending through one of said first wall and said first section and the other of said filters having a second opening extending through one of said frame and said second section; and means mounting said second side of said door on said second side of said access opening for pivotal movement relative to said closed position, said mounting means being effective to mount said door in said closed position so that said door is tilted at a first angle relative to said first wall, said first angle being obtuse.

18. A microwave oven according to claim 17, in which: said mounting means is also effective to mount said door in said closed position so that said door is tilted at a second angle relative to said second wall, said second angle being acute.

19. A microwave oven according to claim 17, in which: said frame includes a top portion adjacent said first sidewall and spaced from said rear wall by a first distance, a bottom portion adjacent said second sidewall and spaced from said rear wall by a second distance, and side portions connecting said top and bottom portions, id first distance being shorter than said second distance so that said heating cavity is truncated; and said mounting means is provided adjacent said bottom portion of said frame so that said door slopes toward said rear wall when said door is in said closed position.

20. A microwave open according to claim 17, in which: said first section and said first wall define a first transmission line having an origin at the end of said first section, said origin being received in said heating cavity when said door is in said closed position; and said first opening is spaced from said origin by less than one-quarter wavelength.

21. A microwave oven according to claim 20, in which: a partition is mounted in said first of said filters to define therein a given electrical length such that the total distance from said origin to said first opening plus said given electrical length equal about an even number of one-quarter wavelengths.

22. A microwave oven according to claim 21, in which: said total distance equals about one wavelength; and the distance from said origin to said second opening equals about one-quarter wavelength.

23. In a microwave oven, comprising: a heating cavity defined by at least a first wall and a second wall opposite to said first wall, said heating cavity having an access opening between said first wall and said second wall; a frame secured to said first and second walls and extending in a plane other than the planes of said first and second walls; a door mounTed adjacent said second wall for movement into a position for closing said access opening, said door having an inner wall extending toward said first wall at a selected obtuse angle when said door is in said closed position, said angle being measured relative to the plane defined by said first wall; said door having a first transmission line element extending from the end of said inner wall for a given length in opposed relationship to said first wall, said first element extending out of said heating cavity; said door having a second transmission line element extending along said frame; said first wall and said first element forming a first transmission line having a given width and said given length when said door is in said closed position; said second element and said frame forming a second transmission line; and a pair of filter cavities mounted in said door, a first of said cavities having a first opening thereto formed in said first element, a second of said cavities having a second opening thereto formed in said second element; said first element being movable toward said first wall to decrease both said given width and said given length of said first transmission line as said door moves from said closed position so as to decrease said obtuse angle.

24. A microwave oven according to claim 23, in which: said door is pivotally mounted to said second wall such that said first element moves in an arcuate path during said movement of said door.

25. A microwave oven according to claim 23, win which: a rear wall extends between said first wall and said second wall; and said first and second walls extend different distances from said rear wall to said door when said door is in said closed position.

26. In a microwave oven, comprising: a heating cavity defined by top, bottom , side and rear walls, said bottom wall terminating at a given vertical plane and said top wall terminating at a location spaced toward said rear wall from said given plane so that said cavity is truncated; a cavity frame secured to and extending outwardly from said top, bottom and sidewalls at the truncated end of said cavity, said frame having an aperture to permit access to said cavity; a closure member having top, bottom and side portions, said bottom portion being mounted adjacent said bottom wall for pivotal movement, said closure member being movable into a closed position so that a first section thereof overlaps said cavity frame to form a first transmission line element and a second section of said member extends into said heating cavity along said top, bottom and sidewalls to form a second transmission line element; a top part of said second section being spaced from said top wall by a gap having a selected width and length when said closure member is in said closed position; said closure member having a closure wall bounded by said top, bottom and side portions, said closure wall sloping toward said rear wall when said closure member is in said closed position; a first filter provided in said first section, said first filter having a first opening through said first section; and a second filter provided in said second section. said second filter having a second opening through said second section so that said first and second openings are in different planes; said top part being movable upwardly toward said top wall during the initial movement of said closure member from said closed position into an open position so that as the length of said gap decreases the width of said gap decreases.

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