US3448232A - Microwave unit seal - Google Patents

Description

. J. H. KLUCK Mano WAVE uui'r SEAL June 3, 1969 Sheet or-2 Filed Jan. 11. 1967 6) .lllqlllll lllllllllllll lllll'llllllllllll lillllllllllllllllllIIIIIHIHIIIIIHH /7 \\\\\\\\\\\\\\\\\\\\\\&\\\\\\\\\\\\\\\ June 3,1969 .1. HJKLUCK 3,448,232 MICROWAVE UNIT SEAL I Sheet 2 of2 Filed Jan. 11,' 19a? I N VENTOR. Mama K4042 United States Patent 3,448,232 MICROWAVE UNIT SEAL James H. Kluck, Altadena, Califi, assignor to Hammtronics Systems, Inc., Pasadena, Calif., a corporation of Delaware Filed Jan. 11, 1967, Ser. No. 608,521 Int. Cl. H05b 9/06 US. Cl. 219-1055 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to microwave units and more particularly to microwave units or electronic ovens having seals for preventing the leakage of radiation from the microwave unit proper.

In a microwave oven operating at frequencies above nine hundred megacycles (900,000,000 cycles) it is necessary to provide an effective seal around the access door openings to prevent the leakage of the propagating energy from the microwave chamber or oven. When the frequency of the propagating energy has a wavelength that is smaller than the principal dimensions of the door a significant amount of energy can escape through the gaps between the door and the chamber proper when these gaps are only a few thousand-tbs of an inch. Although the energy radiated from such a gap may be only a small fraction of the total energy directed into the microwave unit, it can readily be at a high enough level to be dangerous to personnel working in the vicinity of the unit. The leakage field will be particularly severe in cases of a lightly loaded microwave unit where very high field strengths are present in the unit interior. The maximum tolerable field strength for humans is generally considered to be .01 watts per square centimeter. It can be readily appreciated that only a small amount of leakage radiation could produce field strengths of this magnitude or greater when a microwave unit or oven is operated at power levels in the neighborhood of one thousand watts or more, as is the usual case. It is therefore necessary to provide a door closure which provides an effective seal to microwave energy for use in such units directly by making good metal-to-metal contact with the door frame around the entire perimeter of the access opening or door.

It has been obsreved that under the prolonged service normally expected of a microwave unit or oven the contacting plate type of seal eventually becomes deformed and its sealing effectiveness is degraded. A more effective seal is provided by a contacting type seal used in conjunction with a quarter wave transmission line technique which reduces the voltage appearing at the contact point of the door seal. This latter type of seal, however, adds a complication to the apparatus and is frequency sensitive so its effectiveness is reduced for operation at frequencies slightly removed from the operating frequency for which the quarter wave section is designed. The quarter wave transmission line technique used alone is unsatisfactory since it requires precise tolerances on dimensions in order to be effective and is satisfactory only within a frequency band of about one percent (1%). This is considerably less than the frequency excursions encountered in practice which can be four percent (4%) or more within the Federal Communications Commission (FCC) requirements for this type of equipment. Furthermore the quarter wave choke design is only effective for the particular mode of propagation-for which it is designed and in the case of a large volume electronic oven a number of, modes can be propagated in the vicinity of the gap which are not appreciably affected by the presence of the choke and hence are not appreciably reduced by it. Other contacting types of seals make use of braided wire gaskets which are easily damaged, are readily contaminated by food particles, and are difiicult to clean. Furthermore, both the contacting plate type and the metalized gasket type of door seal require considerable closing pressure in order to effect a good seal against the leakage of radiation.

The type of microwave unit under consideration in the present application and towhich the present invention is applicable is disclosed in US. Patent 3,235,971 granted on Feb. 22, 1966 to the same assignee as the present application. The microwave unit disclosed in the aforementioned patent is directed to a microwave unit or electronic oven for drying food pieces.

The present invention provides an improved microwave unit having a durable, easily cleaned seal against the leakage of harmful radiation which does not impede the opening and closing of the closure or door, for the microwave unit, without resorting to the use of a closing bias pressure on the closing member and yet maintains an effective seal against the escape or leakage of harmful radiation. The sealing means of the present invention is constructed and adapted to be substantially independent of frequency over the allowable operating frequency range of this type of microwave unit as specified for the ISM bands by the Federal Communications Commission. For example, the frequency range may be between 915 and 2450 megacycles. The sealing means of the present invention requires a minimum closing pressure and yet does not require uniform contact between the cooperating parts in order to be effective. When the invention is employed with a pressure vessel, the seal will not interfere with the conventional pressure seals such as conventional O-rings or gaskets for sealing against external or internal pressures.

From a structural standpoint, the present invention cOmIprehends an electrical conductive shell defining a microwave chamber adapted for the propagation of microwave energy therein. The shell is provided with an access opening and a closure member or door for the shell to seal off the access opening of the chamber to restrict the propagation of radiation in the thus defined chamber. To provide the sealing means for the chamber at the access opening the chamber and the closure member are defined to include a plurality of coacting electrical conductive elements defined to effectively short circuit any microwave energy tending to leak from the chamber through the gap between the closure member and the adjacent chamber wall. The electrical conductive elements are constructed to be dimensionally defined with respect to the operating microwave range of the unit and spaced apart based on this operating range whereby when the closure member and the chamber are arranged in a closed fashion both the vertical and parallel components representative of the propagating electrical field are cut off thereby preventing the leakage of the propagating energy. The microwave unit of the present invention may further include a compressible gasket mounted to completely surround the series of conductive elements and of a dimension to be engaged by a closure member in its normal closed position.

These and other features of the present invention may be more fully appreciated when considered in the light of the following specification and drawings, in which:

'FIG. 1 is a perspective view of a microwave unit showing the closure member in an open position and embodying the invention;

FIG. 2 is an enlarged, partial view of the elements comprising the sealing means arranged on one of the coacting surfaces illustrated in FIG. 1;

FIG. 3 is an enlarged, partial view showing the cooperating wall of the microwave unit and the closure member in a closed and sealing relationship;

FIG. 4 is a partial, top plan view of a modified microwave unit embodying the invention; and

FIG. 5 is an enlarged, partial view of the sealing means of the present invention for the microwave unit illustrated in FIG. 4 and shown in a sealing relationship.

Now referring to the drawings, the microwave unit embodying the present invention will be examined in more detail. It should be understood at the outset that the microwave unit 10 may be of any conventional construction such as the microwave unit disclosed in the aforementioned U.S. Patent 3,235,971. Accordingly, it will be appreciated by those skilled in the art that any conventional source for providing the microwave energy to be propagated within the microwave unit 10 proper may be provided for the purposes of this invention, the source of microwave energy and means for propagating same within the unit 10 is not illustrated to simplify the description of the present invention. The construction for these means illustrated in U.S. Patent 3,235,971 may be employed, for example.

The microwave unit 10 is defined by a plurality of walls 11 arranged in a substantially rectangular configuration with each wall having its interior surfaces 11a of an electrical conducting material. The inner walls 11a then define the microwave chamber 12 in which an article to be dried or heated is placed for subjection to the microwave energy propagating within the thus defined chamber. One of the walls, the front wall 11, as illustrated in FIG. 1 is provided with an access opening 13 to allow an operator access to the interior of the microwave chamber for placing the article into the chamber or for removing the article from the chamber. As is conventional, the microwave chamber 12 is defined as a completely enclosed chamber by means of a closure or door 14 illustrated as being hinged (not shown) to the front wall 11 at a point identified by the reference numeral 15 and dimensionally defined to completely cover the access opening 13. The inner surface 14 for the closure member 14 is constructed of an electrically conductive material. It will be recognized by those skilled in the microwave art that the above-described construction of the Inicrowave unit 10 is of conventional construction.

An important feature of the present invention is the provision of the sealing means between the front wall 11 of the microwave unit 10 and the closure member 14. For this purpose a sealing means is illustrated mounted on the front wall 11 as a plurality of electrical conductive elements 16 spaced around the access opening 13. The conductive elements 16 are mounted so that they protrude from the front face of the wall. Coacting with the elements 16 mounted on the front wall 11 are similarly defined and constructed elements 17 mounted to protrude from the electrically conductive surface or inner surface 14 of the closure member 14. The conductive elements 16 and 17 are defined to interfit with one another when the closure member 14 is placed in its normal closed position and are electrically defined to effectively cut off both the vertical and parallel components of the electrical field for the microwave energy propagating in the chamber 12 as will be made more evident immediately hereafter.

As is best illustrated in FIG. 2, the electrically conductive elements 16 mounted on the front wall 11 of the unit are dimensionally defined with a long and short dimension and with the elements 16 arranged with the long dimension substantially perpendicular to the line defining the outer periphery of the access opening 13. The elements 16 are arranged in a spaced-apart relationship and which distance is defined by the electrical characteristics of the energy propagating in the chamber 12 as will be noted hereinafter. The arrangement of the electrical conductive elements 16 in the corner of the wall 11 corresponding to the corner of the access opening -13, as illustrated in FIG. 2, should be noted along with the spacing of the elements 16 a preselected distance from the access opening 13.

With the above-described structure in mind, the operating principles of the sealing means of the present invention will be examined. The electric field pattern of the energy tending to leak out through the gap between the front wall 11 and the closure surface 14a can be resolved into a vector component, E which is perpendicular to the gap and a vector component, E which is parallel to the gap, as shown in FIG. 3. Both of these electric field vectors must be suppressed to eliminate energy leakage through the gap. As is well known to those skilled in the microwave art, as the distance between the conducting walls parallel to the electric field vector is reduced, a point is reached at which this transmission system will no longer support propagation and the system is considered to be at its cutoff point. In the parallel plate transmission which is approximated by the microwave unit door gap, the cutoff point occurs when the wall spacing is equal to one-half the wavelength of the exciting energy, at which point the transmission of real energy ceases. The electric and magnetic fields still extend into the transmission region but diminish exponentially with distance so that at the exit end of this transmission system the radiation field excited by them is greatl reduced. The magnitude of the fields extending into the cutoff region is reduced by reducing the spacing between the conducting walls parallel to the electric field vector. Likewise the magnitude of the fields appearing at the exit end of the transmission system is further reduced by increasing the length of the transmission path. In such a cutoff waveguide system the attenuation is given reasonably accurately by the expression wherein A is the attenuation in decibels, db, 1 is the length of the cutoff region and cl is the distance between the walls which are parallel to the electric field. This attenuation is almost wholly reactive so that the real power absorbed in the cutoff region is negligible. Furthermore, it can be determined from the above equation, that since the frequency is not a factor thereof, the attenuation is independent of frequency. These properties are very advantageous when applied to the microwave unit 10 where maximum efficiency is required and Where the frequency of operation may vary considerably.

An examination of FIG. 3 will reveal that the electric field vector, E directed parallel to the gap is effectively suppressed by reducing the gap formed by conducting walls 11 and 14 so that they form a region which is well below cutoff. For the highest frequency commonly used for microwave units employed as electronic ovens, 2450 megacycles, the cutoff point, occurs for a spacing of 2.41 inches. Therefore, it should be apparent that under normal conditions that when the door 14 is closed the gap formed between the door 14 and the front wall 11 will be small and substantially below cutoff for the electric field polarized parallel to the gap. The gap spacing and the length of the cutoff region may then be adjusted to provide the discrete attenuation and suppression of this field in accordance with the cutoff attenuation specified above. For example, applying the above principles, a gap spacing of 0.20 inch for for a length of 0.50 inch will provide '68 decibels of leakage suppression.

The electric field polarized perpendicularly to the gap in the absence of the elements 16 and 17 is not appreciably suppressed since the width of the gap for the usual microwave unit is so large that a below cutoff situation is not attained. Hence, it has been established that the interlocking elements 16 and 17 approximate the necessary walls parallel to the electric field to produce a cutoff condition. A true cutoff condition could only be achieved if the elements 16 and 17 made a solid connection to the opposite wall. It has been determined, however, that a considerable gap may exist, while still pro viding a high degree of suppression. It is believed that the effect of the gap is to increase the width of the cutoff dimension to the width between adjacent conductive elements 16 on the same mounting surface rather than the width between adjacent elements when the closure member 14 is closed (the distance between an element 17 and an element 16 when the door 14 is in the closed position).

The cutoff equation specified above may be applied to the configuration of the conducting elements 16 and 17 to obtain dimensional data using the spacing between adjacent elements on the same mounting surface as the distance d in the equation. For example, using elements having a thickness of 0.040 inch, a separation of 0.25 inch between centers, and a transmission length of 0.5 inch would provide a suppression of 65 decibels.

In accordance with the teaching of this invention, a microwave unit has been constructed employing the conductive elements 16 and 17. For this purpose, elements 16 were 0.50 inch long, .040 inch thick, and extended 0.19 inch above their mounting surface, and were spaced 0.25 inch between centers. The elements 17 mounted on the door 14 were identically dimensionally defined. One set of the elements 16 or 17 is displaced as shown in FIG. 3 so that the elements 17 on the door 14 bisected the space between the elements 16 on the front wall 11. With such a microwave unit 10, microwave energy was supplied to the chamber 12 at a power level of 1400 watts and which chamber was loaded with 1400 grams of water. Under these operating conditions, a maximum total field strength of 1 milliwatt per square centimeter was measured close to the sealed ggap. This is well below the tolerance level of milliwatts per square centimeter and is approximately the same level as the radiation intensity measured from a commercial oven employing a thin plate contacting closure in conjunction with a quarter wave choke recess. It has also been determined as a result of such operation that the leakage was not affected by small changes in the gap between the door 14 and the front wall 11. To this end, a gap of between 0.03 and 0.13 inch between the elements 16 and 17 and the opposite surface could exist without noticeably affecting the level of leakage energy.

Another important aspect of the invention that has been determined is that improved sealing action results when the elements 16 are set back from the access opening 13 a small distance s. It has been found that this arrangement of the elements 16 in conjunction with a similar arrangement of the elements 17 provides added suppression to leakage energy polarized parallel to the gap. In microwave units 10, then, operating at power levels in excess of 1000 watts, it is preferred to space the elements 16, one-fourth inch away from the access opening 13 (s=% inch) to achieve the benefit of this additional leakage suppression.

In FIGS. 4 and 5 another embodiment of the microwave unit 10 is illustrated embodying the invention. This embodiment is essentially the same as discussed hereinabove except that the access opening 13 is of a circular construction and the door 14' is also defined as circular in geometry. The sealing elements 16 and 17, however, are similarly defined and arranged around the access opening 13 and on the inner conductive face of the door 14'; see FIG. 5 in particular.

When it is desired to provide an air or pressure seal in addition to the microwave energy seal, a conventional compressible gasket may be provided around the external perimeter of the door seal. One such compressible gasket is illustrate-d in FIG. 4 and is identified by the reference character 20. The non-critical nature of the allowable gap between the elements 16 and 17 and their opposing walls facilitates the use of such a gasket. The gasket 20 may be advantageously constructed to serve not only as an air seal but also can be constructed of a conductive or microwave absorbing material to provide further suppression of the leakage of energy.

What is claimed is:

1. In a microwave unit including a microwave chamber adapted for the propagation of microwave energy therein and having an opening in at least one wall therein to provide access to the interior of the chamber, the wall providing the access opening being constructed and defined a preselected distance from the outer periphery of the wall to provide a sealing surface, a plurality of solid electrically conductive elements mounted on the thus defined sealing surface at preselected spaced apart locations and completely encircling the access openings, and a closure member for the access opening adapted for completing the microwave chamber, the closure member having a plurality of solid electrically conductive elements mounted adjacent and around its outer periphery at preselected spaced apart locations to interfit with the similarly defined conductive elements on the sealing surface when the closure member is applied to close the access opening whereby the interfitted elements coact to effectively short circuit the components of the field pattern of any microwave energy tending to leak from the chamber and thereby effectively seal the microwave energy within the chamber.

2. In a microwave unit as defined in claim 1 wherein the electrically conductive elements on the sealing surface and the closure member are dimensionally defined with a long and a short dimension and the elements are arranged with the long dimension being substantially perpendicu-lar to the line defining the outer periphery of the access opening.

3. In a microwave unit as defined in claim 2 wherein the electrically conductive elements on the closure member are spaced to interfit with the elements on the sealing surface at a point substantially centrally of the distance between adjacent elements on the sealing surface.

4. In a microwave unit as defined in claim 1 wherein the conductive elements interfit without contacting one another or the adjacent surface.

5. In a microwave unit as defined in claim 1 including a compressible gasket mounted on the chamber to completely surround the conductive elements and of a dimension to be firmly engaged by the closure member in its normal closed position.

6. In a microwave unit as defined in claim 5 wherein the compressible gasket is constructed of an electrical conductive material.

7. In a microwave unit as defined in claim 6 wherein the compressible gasket is constructed of a material capable of absorbing microwave energy.

8. In a microwave unit as defined in claim 1 wherein the electrically conductive elements are mounted on the sealing surface a preselected distance from the outer periphery of the access opening.

9. In a microwave unit including an electrically conductive shell defining a microwave chamber adapted for the propagation of microwave energy therein, the shell having an access opening therein, and a closure member for the shell to seal off the access opening of the chamber to restrict the propagation of the microwave energy within the thus defined chamber, said chamber and said clo sure member including a plurality of solid electrically conductive elements spaced apart and interfitted to coact with each other to effectively short circuit any microwave energy tending to leak from the chamber through the gap between the closure member and the adjacent chamber wall thereby sealing off the chamber, the electrically conductive elements being dimensionally defined and spaced apart to provide an electrical cutoff of the components of 7 8 the electrical field pattern that are both perpendicular and 3,196,242 7/1965 De Vries et al. 219-10.55 parallel to the gap between the closure member and the 3,219,747 11/1965 McAdams 21910.55 chamber and thereby provide a seal independent of the 9 3,260,832 7/1966 Johnson 219-10.55

frequency of the propagating energy within the chamber.

RICHARD M. WOOD, Primary Examiner.

5 References Cited L. H. BENDER, Assistant Examiner. UNITED STATES PATENTS 2,956,143 10/1960 Schall 219 10.s5 1 10 US 2,958,754 11/1960 Hahn 219 10.55 2

3,182,164 5/1965 Ironfield 21910.55 10

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