Waveguide Filter For Microwave Heating Apparatus

Abstract

A low pass filter is provided for rectangular waveguide of the type employed for launching electromagnetic energy in microwave ovens. The filter substantially rejects the transmission of harmonic frequencies of the fundamental generator operating frequency as well as higher order waveguide modes such as the TE.sub.20, TE.sub.30 etc. The structure described has a corrugated configuration with ribs defining alternate cavities and constrictions having cutoff frequency characteristics. Each of the ribs are slotted to substantially suppress such higher order modes of the harmonic frequenices particularly the second harmonic. The structure is spaced from the broad rectangular waveguide walls to define two propagation paths for the electromagnetic energy. Lightweight conductive materials may be utilized. Rib structure is symmetrically disposed on opposing sides of a reference plane extending along the longitudinal axis of the waveguide.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 262,396, filed June 13, 1972 now U.S. Pat. No. 3,758,737 issued Sept. 11, 1973 and assigned to the assignee of the present invention. Another related application is copending application Ser. No. 359,031 issued May 10, 1973 which is a division of application Ser. No. 262,396 filed June 13, 1972.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a waveguide filter for microwave electromagnetic energy transmission in heating apparatus.

2. Description of the Prior Art

Microwave heating includes the generation of electromagnetic energy by such popular sources as the magnetron oscillator used in World War II radar applications. Such generators provide a source of microwave energy which is fed to the interior of an enclosure through hollow-pipe waveguide. The oven supports a plurality of energy modes. Magnetrons are operated by domestic low frequency AC line voltages energizing power supplies capable of generating very high DC voltages. An interaction region is defined between a central emissive cathode and circumferentially disposed anode cavity resonators. The electrons are accelerated toward the cavity resonators and a spoke-like electrical space charge rotates in a substantially helical path in the interaction region to initiate the high frequency oscillations. In such devices the preferred operating TE.sub.10 mode is referred to as the .pi.-mode and, to assure the proper phasing of the energy, the mode of operation most frequently utilized is the N/2 mode where N indicates the number of cavity resonators.

In the microwave heating apparatus art the escape of high frequency energy from the oven cavity is necessarily controlled in order to comply with standards established by Federal and State regulatory agencies, such as the Department of Health, Education and Welfare, Federal Communications Commission and the United States of America Standards Institute. The assigned frequencies for mcirowave heating apparatus is either 915 or 2,450 MHz. For domestic use the latter frequency is most frequently utilized. The term "microwave" as used in this specification is intended to refer to that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to one millimeter and frequencies of 300 MHz to 300 GHz.

One of the problems encountered in the efficient protection of microwave apparatus by appropriate elongated energy seals around the access opening is created by the fact that magnetron energy generators generate harmonic frequencies, as well as, the operating frequency of 2,450 MHz. Additionally, such generators provide energy oscillations in adjacent operating modes such as, for example, the N/ 2-1 mode. It is necessary, therefore, to provide means in the energy seal structures for the absorption of the harmonic frequencies of the fundamental magnetron operating frequency. Such safety measures include additional elongated energy absorbing bodies of such materials as rubber or plastic loaded with carbon derivatives or ferrite materials and the like disposed adjacent to the oven access opening to provide a second energy seal. These bodies become rapidly heated during operation which tends to shorten life and create other problems.

A conventional microwave oven apparatus launching section for coupling the energy from the magnetron generator has a rectangular configuration. Such waveguides are intrinsically high pass filters having a threshold or cutoff frequency below which the excitation fields die exponentially. The cutoff frequency depends on the geometry of the waveguide as well as the particular energy mode excited. For normal TE.sub.10 mode propagation the cutoff frequency is equivalent to .lambda..sub.c = 2a where a represents the side dimension of the waveguide. The provision of filters in rectangular waveguides for the attenuation of higher order waveguide modes of the magnetron harmonic frequencies, such as the second harmonic 4,900 MHz of the microwave oven frequency (2,450 MHz) has been difficult to achieve. This has led to the control by absorption in the energy seal materials disposed adjacent to the oven access opening.

Some primary fundamental energy seals which have evolved in the art forming electrical chokes are described and discussed in U.S. Pat. Nos. 3,182,169 issued to Richard Ironfield, dated May 4, 1965, and 3,584,177 issued to Arnold M. Bucksbaum, dated June 8, 1971, all assigned to the assignee of the present invention. These electrical choke arrangements commonly are provided adjacent to the access opening or in the door closure means. Such choketype energy seals, however, are dimensioned for handling primarily only escaping energy at the fundamental frequency. The present invention, therefore, is directed to the control of substantially all waveguide modes of any harmonic frequencies of the fundamental operating frequency.

SUMMARY OF THE INVENTION

In accordance with the present invention a low pass waveguide filter structure is provided for an energy launching section. A series of open and short circuit admittances having predetermined electrical cutoff parameters are formed by half sections arranged symmetrically having identical image parameters. Rib members define a series of constrictions and spaces to provide the required inductances and capacitances and are slotted. The spacings between the rib members may be uniform or nonuniform. In microwave heating applications requiring an operating frequency of, for example, 2,450 MHz the low pass filter has a pass band or transmission region below a cutoff frequency of, illustratively, 2,800 to 3,200 MHz. The stop band or rejection region extends well above the transmission frequencies or, illustratively, 3,000 to 6,000 MHz with virtually no energy propagation in this frequency range. Adjacent magnetron modes such as the N/2 -1 mode generally have frequency ranges of 3,900 to 4,200 MHz. Second and higher harmonics of the fundamental operating frequency arise at approximately 4,900 MHz. All these energy transmission components, as well as higher order waveguide modes of these frequencies, will be effectively rejected. Transforming end sections are utilized to provide improved matching of the filter structure to the magnetron energy generator and oven cavity.

The filtering of harmonic frequencies has substantially reduced or eliminated the need for providing elongated energy absorbing bodies surrounding the oven access opening in addition to the primary energy escape seals. Hence, in place of the bodies or gaskets of a rubber or plastic composition loaded with carbon derivatives or ferrite materials and the like, unloaded plastic or rubber gasket materials can be utilized with a saving in material cost.

The waveguide filter structure may be fabricated of any lightweight metallic material, such as aluminum, to evolve the overall corrugated configuration with the slotted rib members and intervening spaces. The waveguide filter structure is secured in the waveguide launching section by any means secured to the opposite waveguide walls.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the illustrative embodiments of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view, partially broken away, of an illustrative microwave oven apparatus;

FIG. 2 is a vertical cross-sectional view of the embodiment illustrated in FIG. 1;

FIG. 3 is an enlarged perspective view of the waveguide filter structure embodying the present invention;

FIG. 4 is a diagrammatic illustration of an alternative embodiment of the invention;

FIGS. 4a and 4b are partial diagrammatic views of the embodiment in FIG. 4;

FIG. 5 is a diagrammatic view of another alternative embodiment;

FIG. 6 is an enlargement of the portion of the embodiment illustrated in FIG. 1 showing a portion of a choke-type door energy seal arrangement exemplifying the prior art, taken along the line 6--6 in FIG. 1;

FIG. 7 is a perspective view of a slotted waveguide filter structure with a portion of the launching energy section broken away;

FIGS. 8a and 8b are diagrammatic presentations of the electric and magnetic fields of the TE.sub.20 mode;

FIGS. 9a and 9b are similar presentations for the TE.sub.30 mode;

FIG. 10 is a plan view of an embodiment of the invention for effectively suppressing TE.sub.20, TE.sub.30 modes of the harmonic frequencies;

FIG. 11 is a cross-sectional view taken along the line 11--11 in FIG. 10; and

FIG. 12 is an end view of the embodiment shown in FIGS. 10 and 11.

Description of the Preferred Embodiments

Referring to the drawings, particularly FIGS. 1 and 2, the microwave oven apparatus 10 will be described. The oven comprises top and bottom conductive walls 12 as well as side walls 14 defining enclosure 16 having an access opening (not visible closed by means of door assembly 18 which may be side or bottom-hinged, as well as of the sliding type actuated by handle 20. An escutcheon control panel member 22 is disposed adjacent to the door assembly and is utilized for mounting timers 24 and 26, as well as start, stop and light buttons 28, 30 and 32.

The electromagnetic energy is generated by a magnetron energy generator assembly indicated generally by the box 34. Such devices are considered to be well known in the art and further details may be obtained by referring to the text "Microwave Magnetrons," Radiation Laboratory Series, Vol. 6, by G. B. Collins, McGraw-Hill Book Co., Inc., 1948 and U.S. Pat. No. 3,531,613 issued Sept. 29, 1970, to C. P. Domenichini et al., and assigned to the assignee of the present invention. Such devices are energized by high voltage supply circuits also considered to be within the scope of the knowledge of the prior art and have been designated by box 36. The generated energy is coupled to the oven enclosure by a probe antenna 38 housed within the dielectric dome 40. The energy is launched into a rectangular waveguide launching section 42 with the dome extending through an aperture 44 in the waveguide section. Top wall 12 of the oven enclosure supports the waveguide section 42. The section is closed at one end by a terminating wall 46 and the antenna 38 is spaced from this wall a predetermined distance to provide for optimum launching of the microwave energy. Such energy is coupled into the oven enclosure 16 through the inner open end 48 of the waveguide section. Distribution of the energy to evolve the desired heating pattern is accomplished by any of the well-known means including, for example, mode stirrer 50 having a plurality of paddle members 52 actuated by motor 54 also supported by top wall 12. The articles to be heated within the enclosure 16 are supported on a dielectric plate member 56 spanning an indentation in the bottom wall 12 to facilitate the distribution of the electromagnetic energy on all sides of the articles.

The waveguide filter structure 58 having a substantially corrugated configuration is disposed within the launching waveguide section 42 between the energy antenna 38 and the open end 48. The spacings are preferably evenly matched between the end of the waveguide filter structure 58 and the inner edge 60 of the inner end and antenna 38 to obtain efficient matching between the respective structures. The waveguide filter structure 58 is supported by the narrow walls 62 and spaced from the broad waveguide walls 64 to define essentially two equal and identical paths for the propagation of energy indicated by arrows 66 and 68.

Attention is now directed to FIGS. 3 and 4 to proceed with an explanation and analysis of the corrugated waveguide filter 58. In an article entitled "Analysis of a Wide-Band Waveguide Filter" by Seymour B. Cohn, Proceedings of the IRE, June 1949, pps. 651-656, the author presents a discussion and analysis of a hollow highpass rectangular waveguide filter. Normally such propagation means have an inherent characteristic cutoff frequency which for the TE.sub.10 mode is twice the dimension a of the broad waveguide walls. Any energy below the cut-off frequency dies exponentially and there has been little demand in the art, therefore, for any further filter structures for rectangular waveguides in view of the intrinsic propagation characteristics. The aforementioned article seeks to increase the bandwidth of such filter structures by providing a series alternating rib members and spaces defining constrictions and cavities which provide a nondissipative filter for energy propagating within its boundaries. Each section of the overall filter structure is fabricated to have a predetermined cutoff frequency and the overall admittance (reciprocal of impedance) of the filter is related to the short and open circuit admittances of each section. All the admittances are normalized with respect to the characteristic admittances of the rectangular waveguide having a width a and height b. The characteristic admittance of the guide is inversely proportional to b if width a is held constant. The filter structure may be provided split in identical mirror image sections and the parameters of classical filter theory are utilized in the article to determine the height of the rib sections as well as constriction in between to evolve a stop band or rejection region having a guide wavelength .lambda.g.infin..

In FIG. 4 a half section is designated by the numeral 70. A series of formulae and design considerations have been presented in the article "Design Relations for the Wide-Band Waveguide Filter" by the same author in the Proceedings of the IRE, July 1950, pps. 799-803. In these articles the symbol l refers to the width of the cavities through which the energy is propagating and the symbol l' indicates the constrictions between the cavities. The symbol b.sub.T indicates the height of the terminating waveguide section through which the energy passes. One-half of each of the waveguide filter sections 70 (l/2) may be reduced to a short-circuited section indicated in drawings FIG. 4a. Similarly, the open circuit admittances have been analyzed by the aforereferenced author by the model indicated in FIG. 4b.

Now in the instance of microwave heating predetermined assigned frequencies of 2,450 MHz or 915 MHz are involved and the energy to be distributed within the oven enclosure is desirably as close to these frequencies as possible. Wide band propagation, therefore, is of little consequence and our attention is directed solely to the suppression of extraneous magnetron operating modes and harmonic frequencies of the operating frequency. In FIG. 4 the half section indicated by the solid line 72 represents the model utilized by the aforereferenced author for computing the image admittances of the overall filter structure as well as the short and open circuit admittances of each section. Dashed line 74 indicates a reference plane forming a boundary in the author's analysis of the cutoff wavelength computations for the pass and stop bands for rectangular waveguide. In the present invention line 74 coincides with the longitudinal axis of a waveguide section. From the aforereferenced analysis an equation has evolved to derive a value b.sub.0 equal to b.sub.T .sqroot. 1-(.lambda..sub.g1 /.lambda..sub.g 2).sup.2. Wherein the symbol .lambda..sub.g1 indicates the cutoff guide wavelength of the filter structure, .lambda..sub.g2 indicates the matching point wavelength where filter impedance will match the terminating inpedance and b.sub.T indicates the height of the terminating guide section. Utilizing the image parameters it is possible to calculate the height of the rib member and end portions, as well as, thickness of the intervening constricted portions. The symmetrical structure is evolved by the identical model defined by boundary line 72'.

In accordance with the invention instead of transmitting energy through the waveguide filter arrangement a solid or dissipative configuration has evolved. Energy transmitted along the path indicated by arrow 66 sees a half filter section 72 and an identical path indicated by arrow 68 is formed by section 72' for the remaining electromagnetic energy. The equivalent capacitances and inductances are theoretically determined and normalized to determine respective cutoff guide wavelengths for the overall filter structure and define a stop band or rejection region having a frequency higher than the waveguide cutoff frequency. Such devices are commonly referred to in the art as low pass filters and have an upper limit of the stop bands of a finite value, illustratively, six times the cutoff frequency of the filter. By appropriate transformation of theory the ribs define the restrictions and the spaces therebetween are the cavities. The electromagnetic energy instead of traversing and being propagated through the filter arrangement is transmitted around the filter with the ends thereof terminating in transforming sections 76 and 78 for more efficient matching of the impedances of the waveguide filter to the remaining structure.

As shown in FIG. 3, the waveguide filter 58 comprises a plurality of rib members 80 and 82 defining a corrugated configuration which are substantially of the same height on opposing sides of the body member 84. Intervening alternating spaces 86 are formed between the rib members and for the narrow transmitting band and wide stop band are generally narrower than the cavity width (l) in the previously referenced articles.

The normalized characteristic cutoff guide wavelength of the filter structure .lambda..sub.g1 as well as the desired stop band minimum limit for a predetermined height b.sub.T of the rectangular waveguide launching section can be determined with a stop band or rejection region preferably extending from 3,000 to 6,000 MHz. In an exemplary embodiment for operation in the microwave oven apparatus at 2,450 MHz the filter cutoff frequency fell between 2,800 to 3,200 MHz. The rejection band of from 3,000- - 6,000 MHz encompasses the adjacent magnetron energy modes as well as second harmonics of the operating frequency. For optimum matching the antenna 38 was spaced from the terminating end wall 46 of the waveguide launching section a distance of approximately 1.25 inches or one-quarter wavelength. The end of the waveguide filter structure 58 or the end of the matching transformer section 76 is positioned approximately 1.4375 inches from the antenna 38. The end of matching transformer section 78 of the waveguide filter is disposed approximately 1.4375 inches from the inner edge 60 of the open end 48 of the waveguide launching section. Suitable tapped holes 88 are provided in the body member 84 to support the waveguide structure 58 by metal or nylon screws to the sidewalls 62 and provide for clearance and centering of the ends of the rib members 80 and 82 with relation to the top walls 64 of the waveguide structure. In FIG. 3 the rib members have been illustrated as being nonuniformly spaced which may be desirable in some embodiments while the rib members 90 have been shown as being uniformly spaced in FIGS. 1 and 2 for other applications. The corrugated waveguide structure may be fabricated of any lightweight metal, such as aluminum, and large strips of such structures may be fabricated quite inexpensively to be cut to any desired length. The spacing of the rib members is identical on opposing sides of the body member. Any number of sections may be utilized in the waveguide filter structure and the number of such sections is not limited by the illustrations hereinbefore described.

A large number of dissimilar sections may also be utilized with each section providing a sharper characteristic cutoff frequency for different rejection regions. Such a structure may have a tapered appearance which also assists in the matching of the filter structure impedances to those of the rectangular waveguide launching section. In FIG. 5 the first rib members 92 could, illustratively, have a stop band frequency around 3,500 MHz. Subsequent rib members 94 have a cutoff frequency characteristic value around 4,500 MHz. Rib members 96 then provide a cutoff frequency around 5,500 MHz. Matching transformer sections 98 are also provided. This dissimilar arrangement of a series of sections having different cutoff frequency values may enhance the removal of all the spurious energy signals outside the desired operating mode and frequency for efficient microwave heating.

A unique low pass filter arrangement has now been evolved for rectangular waveguide sections. Due to the elimination or substantial reduction in the undesirable energy oscillations certain distinct advantages are noted over prior art microwave oven apparatus particularly, those utilizing peripheral elongated high energy-absorbing gasket materials surrounding the access opening in conjunction with a primary choke-type energy seal as shown in FIG. 6. Door assembly 18 comprises a panel member 100 and ring member 102 secured together by any conventional metallurgical means to form a unitary assembly. Perforations 104 in metal panel member 100 allow for visual observation of the oven interior during cooking while preventing the escape of any energy radiated within the oven enclosure 16. An outer window member 106 is supported within ring member 102. An inner window assembly 108 with a transparent region also renders the perforations inaccessible to damage and simplifies cleaning of the oven interior. Both windows may be fabricated of a thermoplastic material. A stud 110 secured to frame member 112 provides for press fitting into apertures in panel member 100. Window member 106 may be secured in position by a suitable adhesive as well as door trim members 114 shown in FIG. 1.

A door-type electrical choke arrangement 116 extends peripherally around the door to form an elongated primary energy seal mating with slightly tapered walls 118 of the oven conductive walls 12 and 14 to assure a snug fit after closing. The choke arrangement comprises a peripheral upstanding wall section 120 which defines with the opposing tapered conductive wall 118 an elongated electromagnetic energy escape path 122 extending peripherally around the access opening. The point of origin of the energy is indicated at the gap 124. Ring member 102 has a substantial step configuration to provide a conductive wall surface 126 forming a part of the choke arrangement as well as a front lateral member 128 overlapping the peripheral walls of the oven. Ring member 102 defines with wall section 120 a second electromagnetic energy path 130 of predetermined dimensions for energy entering through the peripheral gap 124. The parallel paths 122 and 130 are filled with bodies 132 of a dielectric material, such as polystyrene or polypropylene. The entrance and exit to the frequency sensitive cavity defined by the choke walls circumscribing the path 130 is provided by gap 134. The foregoing arrangement may be considered to be the primary high frequency energy seal offering a path of least resistance for any energy at the operating frequency escaping around the peripheral gap 124.

In accordance with the principles of energy transmission, such a choke arrangement is selected primarily to provide a high series reactance at the choke opening and to reflect a short circuit from a terminating wall surface 136 to the energy. The choke dimensions are typically selected to provide a short circuit at the point of origin or gap 124 of the escaping energy or approximately one-half a wavelength of the operating frequency. The operating frequency is effectively attenuated by such energy seals, however, harmonics of the operating frequency will not be effectively controlled by the described choke arrangement. In addition to the primary energy seal, prior art microwave oven apparatus have employed elongated high energy absorbing bodies in the form of gaskets 138 and 140 between the wall surface 128 and the peripheral wall surfaces 142. Such lossy high energy absorbing material bodies include rubber or plastic materials loaded with carbon derivatives or ferrite material and the like and may be secured to the metallic walls by suitable adhesive materials. Such secondary energy seals become heated over extended periods of operation by absorption and may become warped, charred or cease to function properly.

In the practice of the present invention, the elimination or substantial reduction of the harmonic frequencies will substantially reduce or eliminate the need for the additional high energy absorbing bodies of the prior art adjacent to the door opening. In place of such peripheral gaskets, commonly of a black composition, more esthetic colors may be employed such as white or vivid colors in plastic materials which will render the oven apparatus more appealing in addition to being easy to clean. Prior art carbon loaded gasket materials are costly and may now be readily replaced by materials costing one-tenth of the prior art amount. The additional cost of the low pass waveguide structure to effectively suppress the undesired modes and harmonic frequencies involves relatively minimal expense so that overall savings of 50 percent or better of the original energy absorbing material gasket cost may be realized.

FIG. 7 illustrates means for substantially suppressing higher order waveguide modes such as the TE.sub.20 shown in FIGS. 8a and 8b wherein the electric field distribution is indicated by arrows 150 along the waveguide length. The magnetic flux lines are designated 152 in the surface view of FIG. 8b. The TE.sub.30 mode is shown by the electric field lines 154 in FIG. 9a and magnetic flux lines 156 in FIG. 9b.

In accordance with the invention the provision of a plurality of slots 158 in spaced rib members 80 and 82 will effectively suppress the higher order waveguide modes of the harmonic frequencies of the magnetron operating frequency. In this structure three slots are provided in each rib member and this total number represents the first number in the highest order mode to be suppressed i.e., TE.sub.30. Hence, for TE.sub.40 etc., four or more slots are provided.

FIGS. 10-12 inclusive represent an exemplary embodiment for microwave heating apparatus operating at 2,450 MHz. The body member 160 is diecast of a material such as zinc and comprises transverse rib members 162, 164 and 166 symmetrically disposed on opposing sides. Each of the rib members are slightly tapered for tooling purposes and mounting holes 168 and 170 are provided in body member 160. Each rib member is slotted to provide three aligned slots 172, 174 and 176, which can also be tapered. The total number of slots will effectively suppress all higher order waveguide modes of the harmonic frequencies up to and including the TE.sub.30 mode.

Body member 160 is also provided with rectangular recesses 172 in opposing surfaces 174 and 176 in the region between the rib members. This structure will materially reduce cost of fabrication and lighten the overall assembly. Additionally, stepped impedance matching structures 178 and 180 are provided at the ends of the body member. The outer upper edges of all the rib members may be provided with curved surfaces 162a, 164a and 166a in those embodiments where waveguide section 42 of the microwave heating apparatus 10 is formed by a drawing process to result in somewhat of a "bathtub" configuration. The curved edges will prevent any sharp projections and will assure a proper fitting of the waveguide filter within the launching section.

There is thus disclosed an effective low pass filter for rectangular waveguide, particularly for use in microwave oven apparatus to suppress harmonic frequencies of the operating frequency. The filter structure described has a corrugated configuration. The alternate rib members defining the intervening spaces and constricted body portion may be uniformly or nonumiformly disposed in each section. The ribs are slotted to suppress higher order waveguide modes of the higher harmonic frequencies. Variations, modifications and alterations of the illustrative embodiments will be evident to those skilled in the art. It is intended, therefore, that the foregoing description of the invention be considered in the broadest aspects and not in a limiting sense.

Claims

1. In an electromagnetic energy transmission system for microwave heating apparatus including waveguide energy coupling means and low pass filter means mounted within said waveguide means for providing predetermined propagating characteristics to substantially suppress transmission of energy at harmonic frequencies of a desired operating frequency, said filter means comprising a body member having rib members and alternate spaces defining an inductance and capacitance in series along said waveguide means extending transversely to the direction of energy propagating along said waveguide means, the improvement comprising:

a plurality of aligned slots in each of said rib members disposed to substantially suppress higher order transmission modes of said harmonic frequencies (i.e., TE.sub.20, TE.sub.30, etc.).

2. The filter means according to claim 1 wherein the total number of slots in each rib member is substantially equal to the first numeral in the designation characterizing the highest of the transmission modes to be suppressed.

3. The filter means according to claim 1 wherein said slotted rib members are symmetrically disposed from opposite sides of said body member.