US3736398A

US3736398A - Method and means for controlling electromagnetic wave energy leakage in microwave ovens

Info

Publication number: US3736398A
Application number: US00198802A
Authority: US
Inventors: G Jarvis
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1971-11-15
Filing date: 1971-11-15
Publication date: 1973-05-29
Anticipated expiration: 1990-05-29
Also published as: JPS4859441A

Abstract

A method and means for controlling electromagnetic wave energy leakage in microwave ovens. The hinges, latches, and other pairs of metallic protruding members that are located in proximity of the perimeter of the door of the microwave oven are separated by distance equal to a multiple of 1/2 wave lengths of the fundamental operating frequency of the magnetron. A method for preventing the escape of an excessive amount of electromagnetic wave energy from around the perimeter of the microwave oven door including the steps of monitoring the field of electromagnetic wave energy in the vicinity of the perimeter of the microwave oven door and mounting a pair of metallic reflective projections across the path of any excessive leakage of electromagnetic wave energy.

Description

United States Patent 1 Jarvis 1 May 29, 1973 [54] METHOD AND MEANS FOR 3 Primary ExaminerJ. V. Truhe CONTROLLING ELECTROMAGNETIC Assistant ExaminerHugh D. Jaeger w v ENERGY LEAKAGE N Attorney-Franklin D. Jankosky, Walter R. Thiel, MICROWAVE OVENS Alfred B. Levine [75] Inventor: George A. Jarvis, Minneapolis, [57] ABSTRACT Mmn. Assigneez Litton y Inc, Beverly Hills, A method and means foncontrollmg electromagnetic Califwave energy leakage n microwave ovens. The hinges,

- latches, and other pairs of metallic protruding mem- [22] Filed: Nov. 15, 1971 bers that are located in proximity of the perimeter of the door of the microwave oven are separated by [21] Appl' 198802 distance equal to a multiple of k wave lengths of the fundamental operating frequency of the magnetron. A [52] US. Cl ..219/l0.55 method for preventing the escape of an excessive lnt. amount of electromagnetic wave energy from around of Search the perimeter of the microwave oven door including t the steps of monitoring the field of electromagnetic References cued wave energy in the vicinity of the perimeter of the UNITED STATES PATENTS microwave oven door and mounting a pair of metallic reflective projections across the path of any excessive 3,448,232 6/1969 Kluck t ..2l9/l0.55 leakage of electromagnetic wave energy. 3,633,564 1/1972 Togashi ..2l9/10.55 X 3,629,537 12/1971 Haagensen ..219/10.55 2 Claims, 3 Drawing Figures Patented Ma 29, 1973 3,736,398

B E W GEORGE A. JARV/S 27 INVENTOR wmjg lwusghu ATTORNEY FIELD OF THE INVENTION This invention relates to electromagnetic wave energy seals for microwave ovens and more particularly to auxiliary means and a method for preventing the escape of electromagnetic wave energy from such ovens.

DESCRIPTION OF THE PRIOR ART The typical microwave oven includes a cavity wherein foodstuffs may be placed to be cooked by exposing the foodstuffs to electromagnetic wave energy radiated by a magnetron. As a practical matter, access must be provided to facilitate the insertion and the removal of the foodstuffs to and from the cavity of the microwave oven. Pivotally supported doors have been generally utilized to provide such access. As is wellknown, electromagnetic wave energy in the vicinity of the perimeter of the door will escape from the cavity of the microwave oven unless proper measures are taken to block such escape of electromagnetic wave energy.

In the past, microwave oven manufactures have utilized various types of electromagnetic wave energy seals to prevent the escape of electromagnetic wave energy from the cavity of the microwave oven. Originally, microwave ovens featured direct metallic contact between the inside surface of a metallic door and the front surface of a metallic housing, with the addition of spring biasing of the door to maintain contact between the two surfaces. However, the spring biasing method did not prove satisfactory over an extended period of use because of warping of the metallic surfaces or because of deterioration of the springs. In either case, arcing between the respective surfaces of the door and the housing and subsequent excessive leakage of electromagnetic wave energy occurred unless proper maintenance produres were strictly observed.

Subsequent approaches included the insertion of a number of non-metallic spacers or a conductive rubber gasket between the door and the microwave oven housing to establish a gap so as to prevent arcing of the electromagnetic wave energy. Usually these approaches included a recessed area having a terminating conducting surface located an integral number of k wave lengths of the fundamental frequency of the magnetron from the origin of the gap, or included a series of recessed areas having a depth equal to approximately wave length of the fundamental frequency of the magnetron. In some instances, such recessed areas have been made more compact and have been filled with a dielectric material to maintain the same electrical wave length.

While these prior electromagnetic wave energy seals were originally acceptable, the recently adopted standards of the Department of Health, Education and Welfare requiring all microwave ovens to have a radiation leakage level of less than I mw/cm upon sale and being capable of maintaining a radiation leakage level of less than 5 mw/cm under all operating conditions preclude many such electromagnetic wave energy seals from being utilized in microwave ovens sold in the United States. It is well-known that household appliances are subjected to abuse, which in the case of microwave ovens, could impair the effectiveness of the door seal. The cost of replacing the door entirely, or even of the door seal, can be quite expensive.

Accordingly, it is the object of the present invention to provide auxiliary means to cooperate with the original door seal to prevent the escape of electromagnetic wave energy from the cooking cavity of the microwave oven.

It is a further object of the present invention to provide a door for a microwave oven wherein any pair of projections located either on the inner surface of the door, or the surface defining the access aperture of the cavity of the microwave oven, are spaced in accordance with the operating frequency of the magnetron of the microwave oven to prevent the escape of electromagnetic wave energy from the cooking cavity of the microwave oven.

It is a still further object of the present invention to provide the microwave oven Serviceman with a method for correcting any excessive leakage of electromagnetic wave energy from the cooking cavity of the microwave oven.

It is another object of the present invention to provide stick-on metallic strips which may be readily placed on the interior surface of the microwave oven door, or on the surface defining the access aperture of the cooking cavity, to prevent the escape of electromagnetic wave energy from the cooking cavity of the microwave oven.

SUMMARY OF THE INVENTION In accordance with the objects set forth above, the present invention provides auxiliary means for preventing the escape of electromagnetic wave energy from the cooking cavity of a microwave oven. In the construction of the microwave oven, any pair of metallic protruding members, located in proximity of the perimeter of the door of the microwave oven, for example, the hinges or the latching members, are separated by a multiple of A wave lengths of the fundamental operating frequency. If excessive leakage of electromagnetic wave energy from the cooking cavity occurs after use of the microwave oven, a method of attenuating such leakage includes the placing of a pair of metallic reflective projections, spaced at a multiple of A wave lengths of the fundamental operating frequency, either on the interior surfaces of the microwave oven door or on the surface defining the access aperture of the cooking cavity.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects, advantages and characteristic features of the present invention will become readily apparent from the following detailed description of the preferred embodiments of the invention when taken in conjunction with the drawings, in which:

FIG. 1 is a perspective view of a microwave oven in accordance with the present invention;

FIG. 2 is an elevational view of the interior side of the microwave oven door in accordance with the present invention; and

FIG. 3 is a side elevational view of the microwave oven door taken along line 3-3 of FIG. 2 in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a perspective view of a microwave oven 10, in accordance with the principles of the present invention. The exterior of the microwave oven door is generally comprised of a housing 11, a door 12 and a control panel 13. The control panel 13 includes a conventional timer and an indicator light. The door 12 includes a window 14, or viewing area, which consists of an RF screen sandwiched between two panes of glass which enables one to view the cooking of the foodstuffs which are placed within the cooking cavity 15 of the microwave oven 10.

The door 12 is shown in its open position so that features of the present invention may be more clearly illustrated. The door 12 is movably supported by a pair of

hinges

16a and 16b which are partially shown in FIG. 1, and are more clearly shown in FIGS. 2 and 3. The door 12 also includes a handle 17 and a pair of

latching members

18a and 18b which, upon closure of the door 12, engage a pair of concealed cooperating latching and interlock members located behind the

openings

19a and 19b of the front panel 20. Further illustrated is movable member 21, one end of which is mounted within the door 12 via opening 21a, the other end of the movable member 21 cooperates with a concealed door stop within the microwave oven 10 to limit the swing of the door 12.

Referring now to FIGS. 2 and 3, there is respectively shown an elevational view of the interior side of the door 12 and side elevational view of the door 12 in accordance with the principles of the present invention. The interior side of the door 12 is principally comprised of an interior surface panel 22, a choke 23, a flexible seal plate 24 and the window 14. The choke 23 and the flexible seal plate 24 cooperate to form an effective seal to prevent the escape of electromagnetic wave energy from the cooking cavity 15. The choke 23 is comprised of a continuous piece of suitable dielectric material, such as polypropyleyne, which is located within a recessed metallic area of the interior surface panel 22. The primary purpose of the choke 23 is to prevent the escape of the fundamental operating frequency of 2,450 Ml-Iz from the cooking cavity 15. The flexible seal plate 24 abuts the front panel when the door 12 is closed to form a capacitive seal to prevent the escape of electromagnetic wave energy of harmonics of the fundamental operating frequency from the cooking cavity 15.

During the development of the microwave oven 10, it was discovered that if a pair of metallic reflective projections separated by some multiple of wave length of the fundamentalfrequency of 2,450 MHz, were placed on either the front panel 20 or-on the interior surface panel 22 in the path of escaping electromagnetic wave energy, the level of such leakage was decreased. The door 12 was then held in its closed posi tion without the aid of any hinges or latches, and an acceptable level of leakage of electromagnetic wave energy existed. When the necessary respective pairs of hinges and latches were placed at respective random locations on the door 12, the leakage of electromagnetic wave energy increased. However, if the respective pair of hinges and latches were separated by some multiple of k wave length of the fundamental operating frequency of 2,450 MHz, the leakage of electromagnetic wave energy was decreased. Thus, it was determined that strategic placement of any pair of metallic reflective projections would serve as additional means to prevent the escape of electromagnetic wave energy from the cooking cavity 15. The pair of hinges 16a and I6 b are separated by the distance A, as shown, and

the pair of

latching members

18a and 18b are separated by the distance B, as shown. The respective lengths of A" and 8" are multiples of A wave lengths of the fundamental operating frequency of 2,450 MHz, more specifically, the distances A and B are 2 lambda and l-k lambda of 2,450 MHz.

Referring now to FIGS. 1, 2 and 3, a plurality of pairs of metallic reflective projections are illustrated which may be placed either on the interior surface panel 22 or on the front panel 20 to attenuate the escape of electromagnetic wave energy from the cooking cavity 15. As is well-known, if a microwave oven is subjected to abuse, the door seal will not effectively attenuate electromagnetic wave energy attempting to escape from the cooking cavity via any gap existing around the perimeter of the door of the microwave oven. During an exhaustive test conducted on the microwave oven illustrated in FIG. 1, wherein the door 12 was opened and closed a great many times, it was also found that the escape of electromagnetic wave energy would increase on random points around the perimeter of the door 12. Pairs of metallic reflective projections were strategically placed, across such points of escape, either on in terior surface panel 22 or the front panel 20 at a distance apart equal to, or approximately equal to, k wave lengths of the operating frequency, and escape of electromagnetic wave energy from the cooking cavity 15 was attenuated.

Depending upon the gap set up between the interior surface panel 22 and the front panel 20, the metallic reflective projections varied in thickness between 0.01 inches and 0.05 inches. In FIG. 1, metallic

reflective projections

25a and 25b are located on the front panel 20 at a distance C which is multiple 9% wave lengths of operating frequency. In FIGS. 2 and 3, the respective pairs of metallic

reflective projections

26a and 26b, and 27a and 27b are respectively located on the interior surface panel 22 at distances D" and E, which are approximate multiples of 1% wave lengths of the fundamental operating frequencies.

In the past, microwave oven servicemen would periodically check for any excessive leakage of electromagnetic wave energy from the perimeter of thedoors from microwave ovens that were in the field. In order to obtain an accurate reading of the escape of such electromagnetic wave energy, a power density meter is placed within 5 cm. of the perimeter of the door 12 and the power density meter is moved around the entire perim eter of the door 12. In the case of the method ofproviding auxiliary means to prevent the escape of electromagnetic wave energy from around the perimeter of the door, as practiced by the present invention, a serviceman may monitor the field of electromagnetic wave energy existing around the perimeter of the door 12. If the field of electromagnetic wave energy is approximately 5 mw/cm or higher at any point around the perimeter of the door, the serviceman may then mount a pair of metallic reflective projections on either the interior surface panel 22 or on the front panel 20, as illustrated in FIGS. 1, 2 and 3 in the path of such escape of electromagnetic wave energy. The respective electromagnetic wave energy in such paths then encounters the pair of metallic reflective projections and a current is established between the two metallic reflective projections. As the electromagnetic wave energy is reflected back and forth between the two metallic reflective projections, the electromagnetic wave energy is attenuated to a level safely below 5 mw/cm In some cases, it was found when the electromagnetic wave energy in one point was attenuated by the use of a pair of metallic reflective projections, there was an increase of electromagnetic wave energy at some other point along the perimeter of the door, though such increased level of leakage of electromagnetic wave energy at the other was not necessarily above the minimum safe level. Thus, in effect, the serviceman is sometimes balancing the leakage of electromagnetic wave energy around the perimeter of the door 12 so that the leakage level is not above 5 mw/cm at any single point.

The serviceman may be provided with metallic reflective projections which may be mounted on the interior surface panel 22 or on the front panel by means of a conventional adhesive, or the serviceman may be provided with metallic reflective means which are comprised of metallic strips coated on one side with an adhesive that is protected by a removable cover. The serviceman may monitor the microwave oven to discover any point of excessive leakage, and upon such discovery, he may readily tear off the protective covers of a pair of metallic reflective means and properly mount them on the microwave oven as described earlier.

It should be understood that the present invention is not limited to pairs of hinges, latches or mounted metallic reflective projections separated by a distance equal to some multiple of approximately 1% wave length of the fundamental operating frequency. For example, in some cases, because of the physical dimensions of the cooking cavity, or because of the gap set up between the interior surface panel of the door and the front panel of the microwave housing, it may be necessary to build a microwave oven wherein pairs of embossed projections are fabricated as part of the metallic interior surface panel or the metallic front housing panel to provide an effective seal against the leakage of electromagnetic wave energy. In addition, if the microwave oven utilizes only one latch, a metallic reflective projection may be paired with it to provide an effective auxiliary seal in accordance with the principles of the present invention.

Thus, although the present invention has been shown and described with reference to particular embodiments, for example, pairs of metallic reflective projections which may be mounted apart at a distance equal to multiples of h wave lengths of the fundamental operating frequency, nevertheless, various changes and modifications obvious to persons skilled in the art to which this invention pertains, for example, the use of three metallic reflective projections spaced to provide two separate multiples of h wave lengths of the fundamental operating frequency, are deemed to lie within the spirit, scope and contemplation of the invention as set forth in the appended claims.

What is claimed is:

l. A method for controlling electromagnetic wave energy leakage between a door and a front face ofa microwave oven comprised of the steps of:

measuring the magnitude of electromagnetic wave energy leakage existing in proximity of said door of said microwave oven; and placing a pair of metallic reflective members on said front surface to form a hollow rectangular slot within the path of said electromagnetic wave energy leakage at a distance apart that is approximately equal to a multiple of /2 wavelengths of the fundamental frequency of the electromagnetic wave energy generated within said microwave oven.

2. A method for controlling electromagnetic wave energy leakage between a door and a front face of a microwave oven as recited in claim 1 which further includes the step of:

removing a pair of adhesive coated metallic reflective members from a protective cover prior to placing said pair of metallic reflective members on said front surface of the microwave oven.

Claims

1. A method for controlling electromagnetic wave energy leakage between a door and a front face of a microwave oven comprised of the steps of: measuring the magnitude of electromagnetic wave energy leakage existing in proximity of said door of said microwave oven; and placing a pair of metallic reflective members on said front surface to form a hollow rectangular slot within the path of said electromagnetic wave energy leakage at a distance apart that is approximately equal to a multiple of 1/2 wavelengths of the fundamental frequency of the electromagnetic wave energy generated within said microwave oven.

2. A method for controlling electromagnetic wave energy leakage between a door and a front face of a microwave oven as recited in claim 1 which further includes the step of: removing a pair of adhesive coated metallic reflective members from a protective cover prior to placing said pair of metallic reflective members on said front surface of the microwave oven.