DE2424778A1 - WAVE GUIDE FILTERS, IN PARTICULAR FOR MICROWAVE DEVICES - Google Patents

Description

PATENT LAWYERS .... _{,, οο}

DR. PHIL. G. NICKEL-DR.-1NC J. DORNER ^MunChen » ^{άθ11 2 2} '

8M0NCHEN15 Lawyer files: 27 - Pat.

LANDWEHRSTR. 35 POST BOX 104

TEL. IO8 1I) S5S7I9

Amana Refrigeration Inc., Amana, Iowa, United States of America.

Waveguide filters, in particular for microwave devices

The invention relates to a waveguide filter, in particular for an electromagnetic energy transmission system for microwave devices with a low-pass filter to achieve a predetermined propagation characteristic, A transfer of energy in the case of harmonics of the desired operating frequency to suppress essentially, as well as with a body that is spaced apart, has ribs extending transversely to the direction of energy propagation in this system.

Such energy transmission systems for microwave devices are used to heat objects. as Any microwave source can be used, e.g. a magnetron, as used in the Radar equipment was used in World War II. The microwave energy from the generator is through guided a waveguide into the interior of a chamber. Several energy oscillation states are possible in the chamber. The generators are operated with low

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frequent mains alternating current operated, which power supply device en feeds that generate high DC voltages. The interaction area is defined by a central emission cathode and an in Circumferentially arranged cavity resonator anode determined. The electrons are heading towards the Cavity resonators are accelerated and a space charge distributed like a spoke rotates along an im essential spiral-like path in the interaction area and releases the high-frequency Vibrations off. In such devices, the preferred TE-jQ vibrational state is called alsIT mode, and mostly used to ensure the correct phase state of the Energy, the w mode, where N is the number of cavity resonators.

In various countries authorities and offices have issued regulations which stipulate that at Microwave oven the leakage of high-frequency energy is controlled or suppressed. For microwave oven are frequencies of either 915 MHz or 2450 MHz required. The latter frequency is predominantly used in the private sector. The term "Microwave" means in the following description Share of the electromagnetic energy spectrum, its Wavelengths on the order of. 1 m to 1 mm and its frequencies 300 MHz to 300 GHz be.

In the protection of microwave devices, the elongated energy seals form around the access opening a particular problem. This problem results from the fact that the generators in addition to the operational

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frequency of 245o MHz, the corresponding harmonics produce. In addition, the generators deliver vibrations in adjacent vibration states, e.g. in S - -1 mode. It is therefore necessary to provide energy seals that the Absorb harmonics of the fundamental oscillation of the magnetron used as a generator. Such Safety devices include additional, elongated, energy-absorbing bodies made of rubber or Plastic, which carbon derivatives, ferrites or the like; these bodies are provided next to the oven door and form a second energy seal. However, these bodies are quickly heated during operation, which shortens their lifespan and other problems are caused.

In conventional microwave ovens, rectangular waveguides are used to transmit the energy from the generator. Such waveguides naturally represent high-pass filters with a threshold or cut-off frequency below which the excitation fields decrease exponentially. The cut-off frequency depends both on the geometry of the waveguide and on the particular, excited oscillation state. For the normal TE ₁₀ vibrational state, the wavelength of the cutoff frequency is approximately A _c = 2a, where "a" is the width of the waveguide. It is therefore difficult to provide special filters in the rectangular waveguides for attenuating waveguide oscillation states of higher order of the harmonics, such as the first harmonic of 4900 MHz at a microwave oven frequency of 2450 MHz. This has led to the fact that one has tolerated related energy, which was influenced by absorption in the sealing material, which was arranged in the area of the oven door.

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Some primary, basic energy seals, which Forming electrical chokes are known from "US Pat. Nos. 3,182,169 and 3,584,177. These electrical Chokes are generally provided in the area of the opening of the furnace or in the furnace door itself. However, such energy chokes are generally designed so that they primarily only the escaping Influence the energy of the fundamental frequency.

The object of the invention is to provide a waveguide filter for microwave devices of the type mentioned so that it is essentially in all waveguide oscillation states responds to any harmonics of the operating frequency.

This object is achieved according to the invention in that each of these ribs has a number of mutually aligned slots in order to _{substantially suppress transmission forms of higher order of these harmonics (these are types of vibration TE 20} , TE, etc.).

Advantageously, the total number of slots in each rib is substantially equal to the first digit in FIG Designation that identifies the highest of the forms of transmission to be suppressed.

The slotted ribs are conveniently symmetrical on opposite sides of the body arranged.

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The low-pass waveguide filter according to the invention is thus provided for an energy delivery section of the microwave device. A series of open circuit conductance and short circuit conductance, which have predetermined electrical limit parameters are arranged symmetrically Half areas formed with identical image parameters. The ribs that are slit define a row of limitations and distances to provide the necessary inductances and capacitances. The distances between the ribs can be uniform or uneven. For microwave devices that require an operating frequency of, for example, 2450 MHz the low-pass filter has a transmission or transmission range that is below the cut-off frequency, for example from 2800-3200 MHz. The interrupt band or rejection area extends over the Transmission frequencies addition, that amounts to, for example 3000 - 6000 MHz, with practically no energy propagation in this frequency range.

Have neighboring vibration states, such as the «- 1 mode generally frequency ranges from 3900 - 42oo MHz, first and higher harmonics of the fundamental frequency are around 4-900 MHz. All of these energy transfer components as well as waveguide oscillation states of higher order of these frequencies become effective rejected. Transforming end sections are used to improve the adaptation of the filter to the dated Generator generated energy and provided to the furnace capacity.

By filtering out harmonics, the ' Need or is this at least reduced to _

in addition to the primary energy poetry, it is still extended

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to place absorbent bodies around the oven door. Therefore, instead of the previously used seals made of rubber or plastic, which contained carbon derivatives, ferrites or the like, simple Plastic or rubber materials are used, which are much cheaper.

The waveguide filter can be made of any light metal material, such as aluminum, can be made to create the grooved arrangement with the slotted ribs and the spaces in between. The waveguide filter is at the edema waveguide output section by any fastening means present on opposite waveguide walls attached.

The invention is described in more detail below using an exemplary embodiment and in conjunction with the drawing. Show in detail:

Figure 1 is a perspective view of a microwave oven with a waveguide filter, partially in a broken section,

Pigur 2 shows a vertical section through the embodiment shown in FIG.

Figure 3 is an enlarged, perspective view of a waveguide filter,

FIG. 4 shows another embodiment of a waveguide filter,

Figures are partial views of the embodiment shown in Figure 4, 4a and 4b,

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Figure 5

a view of another embodiment,

Figure 6

part of the embodiment shown in Figure 1 in an enlarged view position, which shows part of a conventional door seal, which is designed as a throttle,

Figure 7

a perspective view of a waveguide filter according to the invention with slotted ribs and a broken off part of the energy release section,

characters
8a and 8b

Illustrations of the electric and magnetic fields, the type of vibration,

characters 9a and 9b

similar illustrations for the type of vibration,

Figure 10

a plan view of an embodiment of the invention for suppressing the TE ₂₀ , TE, _Q modes of the harmonics,

Figure 11

a cross section along the section line 11-11 in Figure 10 and

Figure 12

a side view of the embodiments shown in FIGS.

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In Figures 1 and 2, a microwave oven 10 is shown. The furnace includes conductive floor and ceiling walls 12 and side walls 14 »which delimit a chamber 16 which has an opening (not visible), which is closed by a door 18, which is hinged to the side or on the floor or is also displaceable and can be operated with the aid of a handle 20. There are time switches on a control plate 22 next to the door and 26 and switch-on, switch-off and light buttons 28, 30 and 32, respectively.

The electromagnetic energy is generated by a magnetron generated. Such generators are known and further details can be found in the article "Microwave Magnetrons ", Radiation Laboratory Series, Vol. 6 by G.B. Collins, McGraw-Hill Book Company, Inc., 1948 and from U.S. Patent 3,531,613. Such generators are powered by high voltage supply circuits 36, which are also known. The generated Energy is transmitted via a probe antenna 38 »which is located in a dielectric dome 40, passed on to the furnace chamber. The energy is in one Delivery section 42 of a rectangular waveguide, into which the dome protrudes through an opening 44, submitted. The ceiling 12 of the furnace chamber supports the waveguide section 42. This section is on one End closed by an end wall 46 and the antenna 38 is at a predetermined distance from this wall 46 so that the microwave energy can be optimally emitted. This energy is delivered to the furnace chamber 16 via the inner, open end 48 of the waveguide section. The distribution the energy to develop the desired heating pattern is obtained with the help of known devices, e.g. with the help of an impeller 50

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which has a plurality of paddles 52 and is driven by a motor 54 which is supported by the ceiling 12 is held. The objects to be heated are carried in the chamber 16 on a dielectric plate 56, which spans a recess in the bottom wall 12, so that the distribution of the electromagnetic '. Energy is relieved on all sides of the objects.

A waveguide filter 58, which has a substantially corrugated shape, is in the dispensing area 42 of the waveguide between the antenna 38 and the open end 48. The distances between the end of the Waveguide filter 58 and the inner edge 60 of the inner end and between the waveguide filter 58 and the antenna 38 are preferably the same size so an effective match between the corresponding parts is obtained. The waveguide filter 58 is attached to the narrow walls 62 arranged at such a distance from the wide walls 64 of the waveguide that two substantially equal and identical paths 66 and 68 for energy propagation are defined.

With reference to Figures 3 and 4, the corrugated waveguide filter 58 will now be described and analyzed. In an article "Analysis of a Wide-Band Waveguide Filter" by Seymour B. Cohn, published in Proceedings of the IRE, June 1949, 651-656, a hollow high-pass filter for a rectangular waveguide is described and discussed. Such propagation devices normally have an inherent, characteristic cut-off frequency which, for the TE _1Q mode of vibration, is twice the dimension "a" of the wide waveguide walls. An energy below this cut-off frequency decays exponentially and therefore existed so far, with regard to the

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the body's own spreading properties, hardly any need for further filters. In the above An attempt is made to increase the bandwidth of such filters by adding a series of alternating filters Ribs and spaces are provided which limit the constrictions and chambers that are used for the Energy propagation within its limits form a lossless filter..Each section of the whole The filter has a predetermined cutoff frequency and the total conductance of the filter (the reciprocal the impedance) is related to the short-circuit and no-load conductance of each section. All conductance values are with respect to the characteristic conductance values of a rectangular waveguide with a Width "a" and a height "b" normalized. Of the characteristic conductance of the conductor is inversely proportional to "b" if "a" is kept constant will. The filter can be divided into identical, mirror-image sections, and the parameters the classical filter theory are in the essay for determining the height of the ribs and for Determine the constrictions between these used to create a disruptive band or a rejection area with a principal wavelength Aejco to develop.

A half area 70 is shown in FIG. 4. A number of formulas and design considerations are known from the article "Design Relations for Wide-Band Waveguide Filters" by the same author in Proceedings of the IRE, July 1950, 799-803. In this paper, the symbol Z denotes the width of the chambers through which the energy propagates and the symbol JL denotes the boundaries between the chambers. The symbol by denotes the height of the

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section of the waveguide through which the energy passes. Half of each waveguide section 70 ( £ / n) can be reduced to a short-circuited section shown in Figure 4a. In a similar way, the named author has analyzed the idle conductance values with the aid of a model shown in FIG. 4b.

Allocated frequencies of 2450 MHz or 915 MHz are used for microwave heating, and the energy that is to be distributed within the furnace chamber should be as close as possible to these frequencies. Broadband propagation therefore has little consequences, and attention is directed exclusively to the suppression of oscillation states that are foreign to the magnetron and of harmonics of the operating frequency. In the figure, half the section indicated by the solid line 72 represents the model which the named author uses to calculate the image conductance of the entire filter and the short-circuit and open-circuit conductance of each section. The dashed line 74 denotes a reference plane which, in the analysis mentioned, forms a limit in the calculation of the cut-off wavelength for the pass band and for the interruption area of a rectangular waveguide. In the exemplary embodiment, the line 74 coincides with the longitudinal axis of a waveguide section. From the analysis mentioned, an equation was developed to derive a value b _Q which is equal to ^ m "ta- (Xa ₁ / X ^) * '. The symbol A ^ denotes the cut-off wavelength of the filter, Agj. Denotes the tuning wavelength, in which the filter impedance is matched to the terminating impedance, and bm denotes the height of the terminating area of the conductor. Using the image parameters it is possible to calculate the height of the ribs and the end sections as well as the thickness of the boundary sections in between. The symmetrical structure is developed from the identical model determined by the boundary line 72 '.

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According to an earlier proposal, which is also used by the present invention, instead of transmitting the energy via a waveguide filter arrangement, a fixed or dielectric arrangement was developed. The energy is along a path 66 which is formed by a half filter section 72, forwarded _r - 'fet, and the rest of the electromagnetic energy is / directed along an identical V ege 68 of the portion 72' is formed. The equivalent capacitances and inductances are theoretically determined and normalized in order to determine the corresponding cut-off conductor wavelengths of the entire filter arrangement and to limit an interrupt band or a rejection region whose frequency is higher than the cut-off frequency of the waveguide. Such devices, which are generally called low-pass filters, have an upper limit of the interruption bands at a fixed value, for example at six times the cut-off frequency of the filter. By appropriately transforming the theory, the ribs determine the boundaries, and the spaces between them form the chambers. The electromagnetic energy does not pass through the filter arrangement, but instead is passed around the filter, the ends of which end in the transforming sections 76 and 78 for more effective matching of the impedances of the waveguide filter to the rest of the structure.

As can be seen in Figure 3, the waveguide filter includes a number of ribs 80 and 82 of equal height on either side of a body 84, creating a grooved arrangement. The intervening spaces 86 are formed by the ribs and are narrower than the chamber width (£) in the above-mentioned articles for both the narrow transfer tape and the wide interrupt or blocking tape.

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Both the normalized, characteristic cut-off wavelength Xg ^ of the filter as well as the one you want Minimum limit of the stop band for a predetermined height b ™ of the delivery section of a rectangular waveguide can be determined with an interrupt band or restricted area, which is preferably of 3000 MHz to 6000 MHz is enough. Pur the operation of the microwave oven with a frequency of 2450 MHz lay the cutoff frequency in one case between 2800 and 3200 MHz. The interrupt or blocking tape was sufficient from 3000 to 6000 MHz and included both the neighboring Energy oscillation states as well as the first harmonic of the operating frequency. The antenna 38 for optimal adjustment at a distance of approximately 3.175 cm or a quarter of a / ½ length of the End wall 46 of the output portion of the waveguide arranged. The end of the waveguide filter 58 or the end of the adjustment and conversion area 76 was approximately 3.6513 cm away from antenna 38. The end of the matching and converting section 78 of the waveguide filter was located at a distance of approximately 3.6513 cm from the inner edge 60 of the open end 48 of the delivery region of the waveguide, in which body 84 are. Holes 88 are provided, which are used to attach the waveguide 58 by means of metal or nylon screws to attach the side walls 62 so that the ends of the ribs 80 and 82 at a distance from the ceiling 64 of the Waveguide and be arranged centered relative to this. In Figure 3, the ribs are uneven Distance from each other shown, which is favorable for some applications, while the ribs 90 in Figures 1 and 2 are shown equally spaced from each other, what for other types of applications is cheap. The grooved waveguide can be made of any light metal, e.g. aluminum.

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and large strips of such structures can be produced very cheaply and then converted into any Lengths are cut. The ribs point the same on opposite sides of the body Distances on. The waveguide filter can have any number of sections.

A greater number of different sections can also be used, with each section being one has sharper, characteristic cutoff frequency for different stop ranges. Such a structure can be tapered, which allows the pilter impedance to be matched to the impedance of the output area of the rectangular Waveguide contributes. The first ribs 92 in Figure 5 have a stopband frequency around 3500 MHz. The other ribs 94 have a characteristic cutoff frequency at 4500 MHz. The ribs 96 have one Cutoff frequency at 5500 MHz. Adaptation and transmission sections 98 are also provided. This diverse Arranging a series of sections has different cutoff frequency values, all of which suppresses unwanted or incorrect energy components that are outside the desired operating oscillation state and the desired operating frequencies, thereby increasing the efficiency of the microwave heating is increased.

A special low-pass filter for rectangular waveguide sections was therefore developed. Through the Suppression or substantial reduction of undesired energy oscillations result in certain advantages compared to the known microwave ovens, which use peripheral, elongated, strongly energy-absorbing sealing materials that surround the access opening,

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in conjunction with a primary, choke-like energy seal shown in FIG. the Door 18 comprises a panel 100 and annular members 102 which, in a conventional manner, are integrally formed into one Arrangement are interconnected. Allow perforations 104- in the metal plate 100 It is advisable to look into the oven during the cooking process, while at the same time avoiding the loss of the energy in the chamber 16 Prevent radiated energy. An outer window 106 is held in the annular portion 102. An inner one Window 108 with a clear area protects the perforations from damage and facilitates them Cleaning the interior of the furnace. Both windows can be made from a thermoplastic. A Bolt 110 attached to a frame 112, sits with a press fit in an opening in the plate 100. Both the window 106 and the door frames 114 in Figure 1 can be secured in place with suitable adhesives.

An electrical choke 116 extends circumferentially around the door and forms an elongated, primary energy seal which tightly fits into the slightly beveled walls 118 of the conductive sidewalls 12 and 14 fits. The choke includes an upright peripheral wall portion 120 which is connected to the opposite, inclined conductive wall 118 for the electromagnetic energy an outlet path 122 which extends circumferentially around the access opening. The starting point of energy is indicated at the gap 124. The annular part 102 is formed in a step shape, and it forms both a conductive outer wall 126, which is part of the choke, and a front side part 128, which lies over the peripheral walls of the furnace. The ring part 102 defines together with the Y / and 120 one second path 130 of predetermined dimensions for the

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electromagnetic energy, which through the .peripheral Gap 124 enters. The parallel paths 122 and 130 are provided with bodies 132 of dielectric material, e.g. Polystyrene or polypropylene filled. The gap 134 forms both the input and the output of the frequency-sensitive Chamber which is delimited by the throttle walls which surround the path 130. The described Arrangement can be referred to as a primary seal for high frequency energy, which for any other Energy escaping around peripheral gap 124 at the operating frequency, a path of least resistance forms.

In accordance with the principles of energy transfer, such a throttle arrangement is primarily selected so that it has a high series reactance at the choke inlet and a short circuit from the surface 136 of the end wall reflected forth for the energy. The dimensions of the throttle are usually selected so that they are at the Origin point or the gap 124 a short circuit for the cause escaping energy or be approximately half the wavelength of the operating frequency. By such Energy seals, the operating frequency is effectively attenuated, while the harmonics of the operating frequency are not effectively attenuated by the described choke will. In addition to the primary energy seal, known microwave ovens have elongated, strong energy-absorbing body, which acts as seals and 140 between the V / and 128 and the peripheral walls are arranged. Such bodies made of highly absorbent material can be made of rubber or plastic, which contains carbon derivatives, ferrites or similar substances; they can be attached with suitable adhesives be attached to the metallic walls. These secondary energy seals are due to prolonged operation

ι;

The absorption of energy warms them up, and they can warp, char, and no longer function properly.

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The present arrangement, which eliminates the harmonics or at least significantly ΓβάμζΙβΓΐ, makes the use of additional energy-absorbing bodies next to the access door is superfluous. Instead of such Peripheral seals, which are normally black, can now be white or plastic materials can be used with vivid colors to match the stove give a more appealing appearance and also - · are easy to clean. The previously used carbon-containing sealing materials are expensive and can now be easily replaced with other materials that are only a tenth as expensive. The added cost of the low pass waveguide that serves to suppress the undesired oscillation states and the harmonics minimal cost, making a total of 50 # or more of the cost of the previously used sealing material can be saved.

FIG. 7 shows an arrangement according to the invention for largely suppressing waveguide oscillation stood en higher order., such as those in Figures 8a and 8b shown TEg vibration modes. The electric Field distribution along the waveguide is shown in these figures by means of arrows 150. the Magnetic lines of force are denoted by 152 in the plan view according to FIG. 8b. The TE, Q mode of vibration is by means of the electric field lines 154 in FIG. 9a and shown by means of the magnetic lines of force 156 in Figure 9b »

The inventive arrangement of a number of slots 158 in the spatially separated ribs 80 and 82 one obtains an effective suppression of waveguide oscillation states of higher order of the upper

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oscillations of the magnetron operating frequency _v In the embodiment shown, three slots are provided in each rib and the total number represents the first digit of the highest order oscillation state to be suppressed, which is the lE - ^ - oscillation type here. Four or more slots are therefore provided for the TE ^ Q mode, etc.

Figures 10-12 illustrate an exemplary embodiment of a microwave oven operating at 2450 MHz. The body 160 is die cast from a material such as zinc and has transverse ribs 162, 164 and 166 symmetrically disposed on opposite sides of the body 160. Each of these ribs is slightly beveled for machining reasons. Mounting holes 168 and 170 are provided in the body 160. Each rib is slotted to create three aligned slots 172, 174 and 176 which can also be beveled. The total number of slots effectively suppresses all higher order waveguide oscillation states, the harmonics up to and including the TE, ₀ oscillation aait.

The body 160 is also provided with rectangular recesses 182 in opposing surfaces in the area between the Ribs. This training noticeably reduces the manufacturing costs and makes the entire arrangement easier. Additional, stepped impedance matching elements 178 and 180 are at the ends of the body 160 intended. The outer top edges of all of the ribs can have curved surfaces 162a, 164a and 166a be provided in those versions in which the

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The waveguide portion 42 of the microwave oven 10 is drawn to form an ¹¹ bathtub "configuration. The curved edges prevent sharp protrusions and ensure a proper fit of the waveguide filter within the dispensing portion.

The above is an effective low pass filter for rectangular waveguides, especially for use disclosed in microwave oven for suppressing harmonics of the operating frequency been. The low-pass filter described has a corrugated shape. The alternates consecutive Ribs that delimit the spaces in between and the constricted areas of the body can be in everyone Section may be arranged uniformly or non-uniformly. The ribs are slotted to provide waveguide vibration conditions to suppress higher order of higher harmonics. It should be emphasized that the Microwave ovens represent only a preferred field of application for the filter described and claimed

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Claims (3)

PATENT CLAIMS 1.J Waveguide filter, especially for an electromagnetic transmission system for microwave devices with a low-pass filter to achieve a predetermined propagation characteristic to a transfer of energy at harmonics of the desired operating frequency substantially suppress, as well as with a body that is spaced apart, transverse to the direction of the Energy propagation in this system has extending ribs, characterized in that each of these ribs (80, 82) has a number of mutually aligned slots (158) in order to transfer forms of higher order of these harmonics (the are modes of vibration TEp, TE, etc.) to suppress. 2. Filter according to claim 1, characterized in that that the total number of slots (158) in each rib (80, 82) is substantially equal to that first digit in the name, which identifies the highest of the forms of transmission to be suppressed. 3. Filter according to claim 1, characterized in that that the slotted ribs (80, 82) symmetrically on opposite sides of the Body (84) are arranged. -2o- 4 0 9 881/1096 Blank page