US3629537A

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

Info

Publication number: US3629537A
Application number: US70641A
Authority: US
Inventors: Duane Buford Haagensen
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1970-09-09
Filing date: 1970-09-09
Publication date: 1971-12-21
Anticipated expiration: 1988-12-21
Also published as: NL7112263A; NL161961B; DE2144548A1; DE2144548C3; DE2144548B2; JPS5145341B1; NL161961C; FR2107439A5; GB1368105A

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, a microwave oven 10 embodying the principles of the present invention is shown including an enclosure or outer housing 12 containing a heating chamber or cavity 14 and a power source 16 connected to a waveguide 18 for supplying electromagnetic wave energy to the heating cavity 14 for heating a product (not shown) received in the heating cavity 14. A door 20 is shown provided with a handle 22 for opening the heating cavity to receive the product. When the door 20 is closed, an electromagnetic wave energy door seal 24 is effective to limit the amount of electromagnetic wave energy which leaks from the heating cavity 14 as the product is heated. The door is also shown having a frontal face 26 which borders a window or screen 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, the width 30 of the frontal face 26 is minimized so that the observation screen 28 may have a maximum area for a given overall height 32 and width 34 of the door 20. Further, according to another object, the door seal 24 is provided while retaining a maximum amount of the heating 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 the outer housing 12 and the heating cavity 14 received therein. The door 20 is shown in an open position to expose an opening 38 at one side of the outer housing 12. The opening 38 permits access to the heating cavity 14 for placing products (not shown) in the heating cavity 14.

As shown in FIGS. 2 and 3, the door 20 is provided with an inner wall 40 which is positioned in the heating cavity when the door is closed (FIG. 3). The perimeter 42 of the inner wall 40 is spaced from the sidewalls 44 of the enclosure 12 by a selected gap 46. The electromagnetic wave energy seal 24 is provided for limiting the amount of energy which leaks through the gap 46 and out of the oven 10 when the door 20 is closed and the electromagnetic wave energy is supplied to the heating cavity 14. The seal 24 includes a first portion 48 of a biplanar transmission line 50 which has an origin 52 adjacent the perimeter 42 of the inner wall 46. The first portion 48 extends out of the heating cavity and feeds a second portion 54 of the transmission line 50. The second portion 54 extends along a cavity frame or border member 56 of the enclosure in a plane other than that of the first portion 48 so that the transmission line 50 is biplanar.

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

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

Each of the sidewalls 44 extends forwardly from the rear wall 76 to a terminus 82 which fonns a perimeter 84 of the access opening 38. A dashed line 86 is shown in FIG. 2 to define an outer portion 88 of each of the sidewalls 44. The outer portion 88 extends rearwardly from each terminus 82 for a selected distance 90.

Although the enclosure 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 an enclosure 92 having a single wall structure as shown in FIG. 5. There, a rear wall 94 and sidewalls 96 form both a housing and define a heating cavity 98. Also, forward ends 100 of the sidewalls 96 have a border section or cavity frame 102 provided with a return lip 104 which extends back into the heating cavity 98. An inner surface 106 of the lip 104 defines the perimeter of an access opening 108 and also corresponds to the outer portion 88 of the sidewalls 44 shown in FIG. 2.

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

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

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

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

The biplanar transmission line 50 includes a first transmission line element 116 extending from each side of the perimeter 42 of the inner wall 40 of the door 20. Each of the first elements 1 16 extends generally parallel to the outer portion 88 of the sidewall 44 opposite thereto and is spaced therefrom by the width of the gap 46. The first elements 116 extend out of the heating cavity 14 and past the plane 118 of the cavity frame 56 by an amount equal to the width of the gap 46 whereupon each intersects a second transmission line element 120. Each of the second elements 120 extends in parallel, overlapping relationship with the cavity frame 56.

The first transmission line elements 116 and the portions 88 of the sidewall 44 opposite thereto form the first portion 48 of the transmission line 50 having a width equal to the width of the gap 46. The first portion 48 extends from each side of the inner wall 40 of the door 20 and has a length which is less than an odd multiple of one-quarter wavelength as measured from the origin 52 of the transmission line 50.

Each second transmission line element 120 and the section of the cavity frame 56 which is overlapped thereby form the second portion 54 of the transmission line 50. As shown in FIG. 4, the first and

second portions

48 and 54, respectively, of the transmission line 50 extend in different planes. As will become clear, the biplanar arrangement permits the width 122 and the height 124 of the access opening 38 to be maximized for a given height 32 and width 34 of the enclosure 12 without significantly reducing the usable depth 36 of the heating cavity 14. Moreover, the biplanar arrangement uses a minimum amount of the width 30 of the frontal face 26 of the door 20 so that the area of the observation screen 28 is maximized for a given width and height of the door 20.

Still referring to FIG. 4, the first opening 62 is in the form of a slot which extends through each first transmission line element 116 at the first distance 64 from the origin 52 so that electromagnetic wave energy is fed from the transmission line 50 into the first filter 58. The first filter 58 is provided with a cavity 126 defined by

respective surfaces

128 and 130 of the inner wall 40 and the first element 116. Further, a cavity wall 132 mounted opposite to the first opening 62 is provided with a surface 134 which extends from the inner wall surface 128 to a surface 136 of the frontal face 26 of the door 20. In view of the low electrical impedance which appears at the origin 52, the electrical length from the origin 52 to the terminating surface 74 of the first cavity 126 must be equal to about an even multiple of onequarter wavelength so that a low electrical impedance appears at the terminating surface 74 of the first filter cavity 126. For this purpose, the partition 70 is mounted to the closed end 74 of the first cavity 126. The partition 70 extends generally parallel to the cavity wall 132 and defines a serpentine or tortuous electrical path 138 within the first cavity 126. The path 138 defined by the partition 70 has the selected electrical length 72 from the first opening 62 to the terminating surface 74 so that the selected electrical length 72 plus the first distance 64 are equal to about an even multiple of onequarter wavelength. As a result, at least one high impedance appears along the electrical path 138 inside the first cavity 126 between the first opening 62 and the terminating surface 74.

The second opening 66 is in the form of a slot which extends through each second transmission line element 120 at the distance 68 from the origin 52 so that electromagnetic wave energy is fed from the transmission line 50 into the second filter 60. The second filter 60 is provided with a second cavity 140 defined by a surface 142 of the second element 120 and by a surface 144 of a cavity wall 146 which is opposite to the surface 142. The frontal face 26 extends around a corner toward the cavity frame 56 to provide a terminating surface 148 for closing the second filter cavity 140. With a low electrical impedance appearing at the origin 52, the second distance 68 is about an odd multiple of one-quarter wavelength. Because the second cavity 60 is closed ended, the distance from the second opening 66 to the terminating surface 148 is selected such that a low electrical impedance also appears at the terminating surface 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 of distances 64 and 150) is an odd multiple of one-fourth wavelength and that the sum of the distance 64 and the length 72 is about an even multiple of one-fourth wavelength. To minimize the amount of cavity depth 36 used, the distance 64 may be 0.5 units, for example, and the distance 68 may be 1 unit. Further, if the total distance (distance 64 length 72) is selected as 4 units, for example, the partition 70 will be effective to reduce the door depth and the width 30 of the frontal face 26 required for the seal 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 64 length 72= 4 units, thus:

0.5 length 72= 4 units length 72= 3.5 units Further, because the path 138 from the opening 62 to the terminating surface 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 the path 138 from the opening 62 to the terminating surface 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, the

dual filters

58 and 60 of the first embodiment are shown in a different arrangement which permits an even smaller door face width 30 and a resulting greater area for the observation screen 28. The heating cavity 14 is shown defined by the inner door wall 40 and the side wall 44. The origin 52 of the biplanar transmission line 50 is adjacent the intersection of the inner door wall 40 and the first section 1 16. The transmission line 50 extends from the origin 52 along the first portion 48 and the second portion 54. However, a first opening 62' extends through the wall 44 to permit electromagnetic wave energy to enter a cavity 126' of the first filter 58 which is provided in a space 151 between the inner walls 44 and outer walls 154. The first cavity 126 is defined by a surface 156 of a wall 158 and a surface 160 of a wall 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 the electrical path 138. It may be understood that because the first cavity 126' is provided in the space 151, the width 30 of the frontal face 26 is decreased so that the area of the observation screen 28 may be increased.

The second filter 60 is similar to the second filter described above in reference to FIGS. 3 and 4. Also, the spacing of the first and

second openings

62 and 66, respectively, as well as the dimensions of the cavities 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 the door seal 24 provided with facilities 166 to protect the oven 10 in the event the power source 16 supplies electromagnetic wave energy to the heating 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 the biplanar transmission line 50 and the

dual filters

58 and 60 positioned to limit the amount of radiation emitted from the heating cavity 14. Normally, a product or other lossy material is placed in the heating 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 the transmission line 50 past the low impedance which appears at the origin 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 the heating cavity 14, the microwave 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 the origin 52 and into the transmission line 50. To render the seal effective in reducing the amount of electromagnetic wave energy which leaks completely out of the oven 10, the high impedance which exists in the cavity 126 of the first filter $8 is used to advantage. In particular, it may be recalled that because of the location of the first opening 62 relative to the origin 52, one or more high impedances appear within the first cavity 126. A block 168 of material having selected properties is mounted within the first 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.

The block 168 mounted at the location of one of the high impedances is effective to absorb the electromagnetic wave energy which leaks into the biplanar transmission line 50. As a result, a substantial portion of the electromagnetic wave energy supplied to the heating cavity 14 is absorbed so that only a limited amount of such energy is emitted from the oven 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, the door 20 may be held tightly closed by the lock and the lock must be secured before the power 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 the power source 16 in the event the door 20 is open more than a very slight amount. After a period of usage, the part of such devices which senses the position of the door 20 may wear or otherwise require adjustment. As a result, the device may permit the power source 16 to continue to operate even though the door 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 the microwave 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 a microwave oven 180. The oven 180 is similar in design to the oven 10 in that it is provided with an outer housing 182 which defines a heating cavity 184. Also, electromagnetic wave energy is supplied from the source 16 to the heating cavity 184 through the waveguide 18. In combination with the

dual filters

58 and 60 and the transmission line 50 of the first embodiment, the oven 180 is provided with a heating cavity frame 186 and a door 188 which are mounted in a new relationship with respect to the walls 44 of the oven 180. This new relationship enhances the effectiveness of the radiation seal provided by a top section 190 of the first portion 48 of the biplanar transmission line 50 and the first filter 58 as the door 188 is opened. More particularly, as shown in FIG. 9, in its closed position the door 188 slants or slopes toward the rear wall 76 of the oven 180 and is positioned at an acute angle relative to a vertical line 192. The door 188 also extends toward the top wall 78 at an obtuse angle therewith and extends away from the bottom wall at an acute angle.

As the door 188 is moved from the closed position (FIG. 9) to the open position (FIG. 8), the length 194 (FIG. 10) of the top section 190 of the first portion 48 of the transmission line 50 decreases. However, because of the initial positional relationship between the door 188 and the cavity frame 186, there is at the same time a decrease in the width of the gap 46 between the outer portion 88 of the top wall 78 and the first transmission line element 116. The first portion 48 of the transmission line 50 consists of inductive and capacitive elements, thus, the capacitance thereof increases as the width of the gap 46 decreases. The increase in the capacitance decreases the length 194 of the top section 190 which is required to produce a given resonant frequency for the top section of the transmission line. Therefore, with the door 188 mounted to the bottom of the enclosure 182 at the obtuse angle relative to the outer portion 88, it may be appreciated that as the door 188 is initially opened, the decrease of the width of the gap 46 across the top section 190 is counteracted by the decrease in the length 194 of the top section 190 so that the resonant frequency of the top section of the transmission line 50 remains relatively constant during the initial open ing movement of the door 188. As a result, even though the door 188 is rotated clockwise from the closed position through a small initial opening angle, the top section 190 of the first portion 48 of the transmission line 50 and the first filter 58 will continue to limit the amount of electromagnetic wave energy which leaks out of the heating cavity 184. The provision of such improved radiation seal along the top section 190 of the transmission 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 the power source 16.

Considering FIGS. 810 in greater detail, the outer housing 182 is shown containing the rear wall 76 and the sidewalls 44 which extend vertically between the top and

bottom walls

78 and 80, respectively. In the second embodiment, the housing 182 is truncated so that the inner wall 40 of the door 188 will be positioned at an acute angle with respect to the top wall 78. In addition, when the rear wall 76 is perpendicular to the sidewalls 44, it may be observed that the top wall 78 extends forwardly from the rear wall 76 a shorter distance than the bottom wall 80 extends forwardly from the same wall. As a result, a terminus 196 of the bottom wall 80 is spaced from the rear wall 76 by a greater distance than the spacing of a terminus 198 of the top wall 78 from the rear wall 76. The

termini

196 and 198 cooperate with termini 200 of the sidewalls 44 to form the perimeter of a tilted access opening 202. The outer 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 dashed line 86 as shown in FIG. 8.

Referring to FIGS. 8 and 9, it may be appreciated that the cavity frame 186 has an inner perimeter 204 which is coextensive with the perimeter of the tilted access opening 202. The cavity frame 186 is positioned at an acute angle relative to the vertical line 192 and extends from the inner perimeter 204 to the outer housing 182. Further, the cavity frame 186 extends at an acute angle relative to the outer portion 88 and at an obtuse angle relative to the bottom wall 80.

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

Alternatively, other well-known devices, such as spacers or the like, may be provided to limit the inward movement of either of the

doors

20 or 188 so that the gap 46 exists between the respective cavity frames 56 and 186 and the second transmission line elements 120.

When the door 188 is closed, the inner wall 40 closes the access opening 202 except for the gap 46 which exists between the perimeter 42 of the inner wall 40 and the top, bottom and sidewalls 78, 80 and 44, respectively.

As in the first embodiment, the oven 188 is provided with the biplanar transmission line 50 which includes the first portion 48 and the second portion 54 extending around the four sides of the door 188. The first portion 48 includes one of the first transmission line elements 116 extending from each side of the perimeter 42 of the inner wall 40. Because the inner wall 40 is not perpendicular to the top wall 78, a top transmis sion line element 206 intersects the inner wall 40 at an acute angle. The top element 206 extends generally parallel to the outer portion 88 of the top wall 78, which is opposed thereto and is spaced therefrom by the width of the gap 46 when the door 188 is in the closed position (FIG. 9'). The first transmis sion line element 206 extends out of the heating cavity 184 and intersects the respective second transmission line element 120. As shown in FIG. 9, in the closed position of the door 188, the second position of the door 188, the second transmission line element 120 is spaced from the cavity frame 186 by the width of the gap 46 and extends at an acute angle relative to the vertical line 192 in parallel relationship with the cavity frame 186, which is overlapped thereby form the second portion 54 of the biplanar transmission line 50.

As in the first embodiment, the first and

second filters

58 and 60, respectively, are mounted in the door 188 and cooperate with the transmission line 50 so that a low impedance appears at the origin 52 of the gap 46 when the door 188 is closed.

The significance of the positioning of the closed door 188 relative to the sloped cavity frame 186 and the top wall 78 may be more fully appreciated by referring to FIG. 10 where the initial opening movement of the door 188 is shown in successive steps. There, the sloped cavity frame 186 and the outer portion 88 are shown in cross section, whereas (for the closed l 0 position of the door) the first and second

transmission line elements

206 and 120, respectively, are shown in solid lines. The gap 62 and the length 194 of the first portion 48 of the transmission line 50 are also shown for the door 188 in the closed position.

As the door 188 is initially opened, the top transmission line element 206 and the other structure of the door assume the dashed line position in which the width of the gap 46 has decreased to a smaller gap 220 and the length 194 of the first portion 48 of the transmission line 50 has shortened to a length 222.

Because the width of thev gap 46 and the length 194 of the first portion 48 of the transmission line 50 decrease simultaneously, the resonant frequency of the top section 190 of the transmission line 50 remains relatively constant during the de picted initial opening movement of the door 188. As a result, the top section 190 of the first portion 48 of the transmission line 50 and the first filter 58 fed thereby will continue to limit the amount of electromagnetic wave energy which leaks through the top section 190 out of the heating cavity 184.

In the practice of the present invention, a microwave oven door 20 having a biplanar transmission line 50 extending across the upper section of the cavity 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 the door 20 for the electromagnetic wave frequency (2,450 megacycles per second) used with the door 20 in its normal closed position.

The door constructed had a cavity frame 56 having a width of 1.156 inches and a length of 17.0 inches. When the door 20 was in the closed position, the gap was 0.125 inches between the cavity frame 56 and the second element and between the outer portion 88 of the top wall 78 and the first element 116. The inner wall 40 extended 1.875 inches into the heating cavity 14 past the access opening 38.

The

cavities

126 and 140 were constructed from metal having a thickness of 0. inch. The first distance 64 was 0.875 inch and the size of the first opening 62 was 0.25 inch by the width of the cavity frame 56.

The door 20 was constructed to illustrate the radiation sealing effect without minimizing the size of the seal, thus the second 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, the second cavity 140 was filled with polypropylene which decreased the distance 68 to 3,563 inches. However, the data taken are still representative of the improved sealing characteristics of the oven 10 shown in FIG. 4.

The size of the second opening 66 was 0.1875 inch and the distance 212 was 0.938 inch, and the internal depth of the second 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 terminating surface 74 was located 0.625 inch from the surface 136 and the partition 70 had a length of 1.0 inch. The width 214 of the electrical path 138 was 0.406 inch and the width 216 of such path 138 was 0.250 inch. The length of the partition 70 was 1.0 inch.

With this construction, the power source 16 provided electromagnetic wave energy at the frequency of 2,450 megacycles per second, the door 20 was in its closed position and the amount of radiation was measured with a sensitive radiation detector. The constructed door 20 was found to effectively prevent the electromagnetic wave energy from being emitted from the heating cavity 14 through the gap 46 and out of the oven 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

8. in a microwave oven comprising:

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 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 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 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 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 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 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 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

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.