US3511958A

US3511958A - Electronic oven including microwave coupling structure and folded cavity filters therefor

Info

Publication number: US3511958A
Application number: US788204A
Authority: US
Inventors: James E Staats
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-12-31
Filing date: 1968-12-31
Publication date: 1970-05-12
Anticipated expiration: 1987-05-12

Description

3,511,958 UCTURE AND ELECTRONIC OVEN INCLUD UPLING STR FOLDED CAVITY FILTERS THEREFOR 5 Sheets-Sheet 1 Filed Dec. 51, 1968 Fla.

7 0 8 6 MN 6 0 w 7 V 2 w 2 g 5 6 H u M 2 2 w 2 2 2 f K V. 2 L 12 4p 24 W Y, 6 7 6 m2 w wq a /3 W a w .2 4 a 2 q// 3 0m HA 1 O0 w m 22 w my l O m a Q n& 2 3 k m w 2 3 j 3 F|l1lir|||L b \6 7 IH/Z 3m m E "Q Q 3 m 6 A 11 T n T C 2 INV'ENTOR JA MES E. STAA Ts Q BY May 12, 1970 J. E. STAATS 3, ,95 ELECTRONIC OVEN INCLUDING MICROWAVE COUPLING STRUCTURE AND FOLDED CAVITY FILTERS THEREFOR Filed Dec. 31, 1968 5 Shets-Sheet 2 May 12, 1970 J. E. STAATS ING MICROWAVE COUPLING STR CAVITY FILTERS THEREFOR 3,51 1,958 UCTURE AND ELECTRONIC OVEN INCLUD FOLDED Filed Dec. 31, 1968 3% m 1 vmx E@G 3 12 j fi 2 5 Sheets-Sheet 5 III III il I l I I n Em own m mkm Man

Vkm, mm mkm mum Nmm

omn 1mm \hmmm mmm Mmm SNS n -X QM? .7 I mm mwm NNM mm m 3,511,958 CTURE AND 1 J. E. STAATS May 12, 1970 MICROWAVE COUPLING sTRu FOLDED CAVITY FILTERS THEREFOR 5 Sheets-Sheet L m vwm mmm mwm \gfg 7 n4 i r mmm E; E j u 1i m? fin kw Rm mm Em in MMV kNv 3m I412! ild: i

May 12, 1970 J. E. STAATS 3,511,953 ELECTRONIC OVEN INCLUDING-MICROWAVE COUPLING STRUCTURE AND FOLDED CAVITY FILTERS THEREFOR I Filed Dec. 31, 1968 Sheets-Sheet 5 j W W 64 67 62 65 L FIG. 9

5 a: k E STOP BAND Q 5 Ii k 4- E E 2 72 k 7/ O I I I I I I l l FREQUENCY IN. G/GAHERTZ United States Patent 3,511,958 ELECTRONIC OVEN INCLUDING MICROWAVE COUPLING STRUCTURE AND FOLDED CAVITY FILTERS THEREFOR James E. Staats, Louisville, Ky., assignor to General Electric Company, a corporation of New York Filed Dec. 31, 1968, Ser. No. 788,204 Int. Cl. H05b 9/06; H03h 7/10 US. Cl. 219-1055 ABSTRACT OF THE DISCLOSURE There is disclosed an electronic oven comprising a generator of microwave energy of a predetermined frequency coupled to a cooking cavity and to a source of DC and 60-cycle AC operating potentials by a coaxial transmission line, the inner conductor of the transmission line including structure defining a folded resonant cavity rejection filter for the second harmonic of the predetermined frequency formed by two cup-like conductors facing in opposite directions and disposed one within the other, and also including a capacitive bypass filter between the inner and outer conductors for the second and higher harmonics of the predetermined frequency.

15 Claims This invention is concerned with an improved transmission line forming a part of an electronic oven and, more particularly, with improved microwave filtering elements for the transmission line.

It is a general object of this invention to provide an electronic heating apparatus including structure defining a cooking cavity, a generator for generating electromagnetic wave energy and a transmission line for coupling the generator to the cooking cavity, wherein the transmission line includes improved resonant cavity filters for attenuatingthe second harmonic of the operating frequency of the generator, which filters comprise a compact structure occupying a relatively small portion of the transmission line.

It is a further object of this invention to provide in an electronic heating apparatus, structure defining a cooking cavity, a generator for generating electromagnetic wave energy of a predetermined ultra-high frequency and having a pair of output terminals, a transmission line coupling the generator to the cooking cavity and including a hollow outer conductor coupled to one terminal of the generator and an inner conductor coupled to the other terminal of the generator and a filter section in the inner conductor and comprising structure defining an inner space and an outer space disposed about the inner space and a passage interconnecting the inner space and the outer space to provide a folded cavity, the filter section structure having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, the folded cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency, the filter section presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along the inner conductor while having a length equal to about one-eighth of the wave- 3,511,958 Patented May 12, 1970 "ice length of the second harmonic of the predetermined frequency, whereby during the transmission of the electromagnetic wave energy along the transmission line, the second harmonic of the predetermined frequency is highly attenuated by the filter section while the predetermined frequency is propagated to the cooking cavity substantially without attenuation.

In connection with the foregoing object it is another object of this invention to provide an electronic heating appartus of the type set forth, wherein the transmission line includes a first cup-like filter member including a cylindrical side wall surrounding the adjacent portion of the inner conductor concentric therewith and spaced therefrom and an end wall closing the side Wall at one end thereof and connected to the inner conductor, the cylindrical side Wall having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, and a second cup-like filter member including a cylindrical side wall surrounding the cylindrical side wall of the first filter member concentric therewith and spaced therefrom and an end wall closing the side wall of the second filter member at one end thereof and connected to the inner conductor, the cylindrical side wall of the second filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, the first and second cup-like filter members cooperating with each other and with the inner conductor to form a folded cavity having an effective electrical length equal to one-quarter of the Wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency.

Yet another object of this invention is to provide an electronic heating apparatus of the type set forth, wherein the transmission line includes a filter section in the inner conductor and comprising structure defining first and second inner spaces and first and second outer spaces respectively disposed about the first and second inner spaces and first and second passages respectively interconnecting the first inner and outer spaces and the second inner and outer spaces to provide first and second folded cavities, the filter section having a length equal to about one-quarter of the Wavelength of the second harmonic of the predetermined frequency, the first and second folded cavities each having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and each being resonant at the second harmonic of the predetermined frequency, the filter section thereby providing two resonant cavities each presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along the inner conductor.

In connection with the foregoing object, a further object of this invention is to provide an electronic heating apparatus of the type set forth wherein each of the first and second folded cavities is defined by first and second cup-like filter members concentrically disposed one within the other along the inner conductor and so arranged that the end walls of the inner ones of the cup-like members abut one another.

It is another object of this invention to provide a transmission line for interconnecting a generator for generating electromagnetic Wave energy of a predetermined ultra-high frequency and a load wherein the generator has a pair of output terminals, the transmission line comprising a hollow outer conductor coupled to one terminal of the generator, an inner conductor disposed within the outer conductor concentric therewith and coupled to the other terminal of the generator, and a filter section in the inner conductor and comprising structure defining an inner space and an outer space disposed about the inner space and a passage interconnecting the inner space and the outer space to provide a folded cavity, the filter section structure having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, the folded cavity having an effective electrical length equal to onequarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency, the filter section presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along the inner conductor while having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, whereby during the transmission of electromagnetic wave energy along the transmission line the second harmonic of the predetermined frequency is highly attenuated by the filter section while the predetermined frequency is propagated to the load substantially without attenuation.

In connection with the foregoing object, it is another object of this invention to provide a transmission line of the type set forth comprising a first cup-like filter member including a cylindrical side wall surrounding the adjacent portion of the inner conductor concentric therewith and spaced therefrom and an end wall closing the side Wall at one end thereof and connected to the inner conductor, the cylindrical side wall having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, and a second cup-like filter member including a cylindrical side wall surrounding the cylindrical side Wall of the first filter member concentric therewith and spaced therefrom and an end wall closing the side wall of the second filter member at one end thereof and connected to the inner conductor, the cylindrical side wall of the second filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, the first and second cup-like filter members cooperating with each other and with the inner conductor to form a folded cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency.

In connection with the foregoing objects, yet another object of this invention is to provide a transmission line of the type set forth comprising structure defining first and second inner spaces and first and second outer spaces respectively disposed about the first and second inner spaces and first and second passages respectively interconnecting the first inner and outer spaces and the second inner and outer spaces to provide first and second folded cavities, the filter section structure having a length equal to about one-quarter of the wavelength of the second harmonic of the predetermined frequency, the first and second folded cavities each having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and each being resonant at the second harmonic of the predetermined frequency, the filter section thereby providing two resonant cavities each presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along the inner conductor.

In connection with the foregoing object, another object of the invention is to provide a transmission line of the type set forth wherein each of the first and second folded cavities is defined by first and second cup-like filter members concentrically disposed one within the other along the inner conductor and so arranged that the end walls of the inner ones of the cup-like filter members abut one another.

Further features of the electronic heating apparatus pertain to the particular arrangement of the parts of the transmission line and coupling assembly forming a part thereof, whereby the above-outlined and additional operating features thereof are attained.

The invention both as to its organization and its method of operation, together with objects and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view with certain portions broken away of an electronic heating apparatus made in accordance with and embodying the principles of the present invention;

FIG. 2 is a fragmentary view on an enlarged scale in vertical section of a crossed-field discharge device forming a part of the generator for the apparatus of FIG. 1;

FIG. 3 is a fragmentary view in vertical section through the transmission line forming a part of the apparatus of FIG. 1 and interconnecting the device of FIG. 2 to the cooking cavity of the apparatus and including a harmonic filter according to a first embodiment of the present invention;

FIG. 4 is an enlarged fragmentary view of that portion of the transmission line of FIG. 3 including the harmonic filter according to a first embodiment of the invention;

FIG. 5 is a view in vertical section along the line 5-5 of FIG. 4;

FIG. 6 is a fragmentary view in vertical section through the transmission line forming a part of the apparatus of FIG. 1 and including a harmonic filter according to a second embodiment of the invention;

FIG. 7 is an enlarged fragmentary view of that portion of the transmission line of FIG. 6 including the harmonic filter according to a second embodiment of the present invention;

FIG. 8 is a schematic representation of the equivalent electrical circuit for the harmonic filter of FIG. 7; and

FIG. 9 is a chart showing the frequency response of the filter of FIG. 4.

Referring now to FIG. 1 of the drawings, the electronicheating apparatus 10 there illustrated, and embodying the features of the present invention, is in the form of a combination electric and electronic range that is especially designed for home use. More particularly, the range 10 comprises an upstanding substantially boxlike casing 11 formed of steel and including a pair of side walls 12, a rear wall 13 having a removable closure member or panel 15 disposed therein, and a top wall -14 and a bottom wall 16, the removable panel 15 being removably held in place by a plurality of screws 27. The casing 11 houses in the upper central portion thereof a metal liner 20 defining a heating or cooking cavity 21 therein, the metal liner 20 preferably being formed of steel, and essentially comprising a box-like structure provided with a top wall 22, a bottom wall 23, a rear wall 24, and a pair of opposed side walls 25; whereby the liner 20 is provided with an upstanding front opening into the heating cavity 21 defined therein. Further, the casing 11 is provided with a front door 28 arranged in a front opening formed therein and cooperating with the front opening provided in the liner 20, the front door 28 being mounted adjacent to the lower end thereof upon associated hinge structure 29; whereby the front door 28 is movable between a substantially vertical closed position and a substantially horizontal open position with respect to the front opening provided in the liner 20.

An electric heating unit 36 is arranged in the upper portion of the heating cavity 21 adjacent to and below the top wall 22, and an electric heating unit 37 is arranged in the lower portion of the heating cavity 21 adjacent to and above the bottom wall 23; which

electric heating units

36 and 37 are utilized in the carrying out of conventional baking and broiling cooking operations in the heating cavity 21, as explained more fully hereinafter. Also, a temperature sensing bulb 38 is arranged in a pocket provided in one of the side walls 25; which temperature sensing bulb 38 forms a part of an oven switch and temperature controller and is utilized in carrying out the previously mentioned conventional baking and broilingoperations in the heating cavity 21. The side walls 25 of the liner 20 further carry thereon a plurality of shelf supports 26 for the supporting of shelves (not shown) that in turn support items to be cooked within the heating cavity 21, There also is provided below the front door 28 a lower front panel 19 that closes a front opening in the lower portion of the casing 11, the bottom wall 16 further being provided with a reticulated member or screen 17 and the lower portion of the rear wall 13 being provided with a reticulated member or screen 18, the

screens

17 and 18 permitting the passage of air through the lower portion of the casing 11 to cool the electronic apparatus therein as will be described more fully hereinafter.

Disposed below the liner 20 and spaced therefrom is a generally horizontally arranged lower baffle 30 extending laterally across the casing 11 between the side walls '12 thereof and extending rearwardly from the front of the casing 11 to a point spaced forwardly of the rear Wall 13, the baffle 30 in cooperation with the casing 11 defining a bottom machinery compartment in the lower portion of the casing 11, the lower front panel 19 being removable to provide access to the bottom machinery compartment 35.

Mounted on the underside of the baflle 30 is an electric motor 31 having an output drive shaft 32 including a reduced portion 33 extending upwardly into the liner 20 and supporting thereon a turntable 34 formed of expanded sheet metal and arranged in the lower portion of the heating cavity 21. The turntable 34 is mounted for rotation and upon operation of the motor 31 is adapted to support food to be heated or cooked in the electronic operation that is carried out in the heating cavity 21, as explained more fully hereinafter. The motor 31 has an associated gear train (not shown) that reduces the speed of the shaft 33 to approximately 6 rpm.

A rear baffle is provided to the rear of the liner 20 and extends across the casing -11 between the side walls 12 thereof, the baffie 40 including a main wall 41 carrying centrally thereof a rearwardly offset wall 42 that is in general horizontal alignment with the rear of the liner 20 and spaced rearwardly therefrom. Disposed in the lower portion of the main wall 41 is an opening around which is disposed a flange 43 connecting with and air duct 44 that communicates with the screen 18 in the rear wall 13 of the casing 11. It further will be noted that the bottom bafiie 30 carries on the rear thereof an upwardly and rearwardly extending baflle section 39 that extends toward the offset wall 42 but is spaced therefrom, the

baffles

30 and 40 being formed of metal, such as steel, whereby the spacing between the baflie section 39 and the rear bafile 40 minimizes the conduction of heat therebetween during the operation of the range 10. The rear baffie 40 cooperates with the casing 11 to provide a rear machinery compartment 45, the rear machinery compartment 45 being disposed behind the liner 20 and access thereto being provided through the removable panel 15 that covers the opening in the rear wall 13 described above.

There is arranged to the right (as viewed in FIG. 1) of the bottom machinery compartment 35 a generator for supplying ultra-high frequency electromagnetic wave energy for electronic cooking within the cooking cavity 21, the generator 50 including a crossed-field electronic discharge device 100 of the construction and arrangement disclosed in the copending application of James E. Staats, Ser. No. 559,267, filed June 21, 1966 now Pat. No. 3,458,755. Referring to FIGS. 1 and 2, it will be seen that the device is disposed within a box-like structure or casing 101 that extends completely about the device 100 but is open on two opposed sides thereof, the sides disposed to the left and right in FIG. 2, the device 100 and the associated parts therefor being mounted within and electrically connected to the casing 101. As will be explained more fully hereinafter, high operating DC potentials are present on the casing 101, whereby it is desirable electrically to isolate and shield the casing 101, and to this end a second box-like structure 102 has been provided that surrounds the casing 101 and is also provided on an opposed pair of sides thereof with openings in alignment with the opposed open sides of the casing 101, all to accommodate the passage of air through the casing 101 and the box-like structure 103 to cool the device 100 and the associated parts housed therein. However, in order to prevent contact of the user with the casing 101 and the device 100 disposed therein, reticulated metal covers 103 are provided for covering the openings in the opposed sides of the boxlike structure 102, the covers 103 permitting passage of air therethrough and thus through the casing 101 and the structure 102 while preventing contact of a user with the casing 101 and the parts disposed therein that carry the high operating DC potentials. Further, the casing 101 is mounted upon insulators 104 carried by the structure 102, thereby electrically to insulate the casing 101 from the structure 102.

As viewed in FIG. 1, the structure 102 and the casing 101 disposed therein are disposed to the rear of the bottom apparatus 35, and are disposed to the right within the bottom apparatus compartment 35 when the range 10 is viewed from the front. In order to provide cooling air for passage through the reticulated covers 103 and across the device 100, there has been provided an open ended housing 105 disposed to the left in FIG. 1 or in front of the structure 102 and housing therein at least a part of a voltage doubler and rectifier circuit that supplies DC operating potentials to the device 100 and also houses therein a fan 106 powered by a motor 55 within the housing 105, an air duct 107 being provided about the fan 106 to direct air therefrom into the structure 102 and into the casing 101 and about the device 100. More specifically, the fan 106 operates to draw air through the screen 17 at the bottom of the apparatus compartment 35, the air being formed into a stream by the housing 105 and passed over the rectifier 130, the motor 55 and through the air duct 107 into the structure 102; the air stream within the structure 102 passes into the casing 101 and about the device 100 and cooling fins 118 disposed thereon and passes therefrom and into the air duct 44 to be discharged through the screen 18 in the rear wall 13 of the casing 11.

In accordance with the present invention, the air stream generated by the fan 106 is used to cool all of the various parts of the generator 50, and specifically the crossed-field discharge device 100 and the voltage doubler and rectifier circuit 130 associated therewith. To this end the housing 105 and the air duct 107 and the air duct 44 have been provided so as to concentrate the air stream upon the parts noted, all while attempting to deflect the air stream away from the

baffies

30 and 40. The

baffles

30 and 40 further protect the liner 20 and the cooking cavity 21 disposed therein from the air stream thus generated so as to maintain more uniform cooking conditions within the cooking cavity 21 and thus to improve the cooking therein.

Referring now particularly to FIG. 2 the generator 50 there illustrated will be described in greater detail. An electron discharge device 100 is contained within a substantially cylindrical metal envelope 111 and includes anode and cathode structure (not shown). Surrounding the envelope 111 and connected thereto is a plurality of cooling fins 118 for dissipating heat from the device 100 as explained above. In order to establish a unidirectional magnetic field within the device 100 there is provided a composite magnetic field winding 120a and 1201; disposed at the upper and lower ends of the device 100 and connected in series relation by a conductor 123. A DC operating potential from a voltage doubler and rectifier 130 is applied to the winding 12% by a conductor 122, and from the winding 120a to the device 100 by a conductor 124 which is connected to one of the cooling fins 118 as at 125. Further details of the construction and operation of the device 100 are disclosed in the previously mentioned Staats application Ser. No. 559,267 now Pat. No. 3,458,755, the disclosure of which is incorporated herein by reference.

The generator 50 is arranged to be advantageously operated in connection with suitable control and power supply apparatus, details of the construction and operation of which are disclosed in the copending US. applications of James E. Staats, Ser. No. 656,977, filed June 12, 1967 now Pat. No. 3,421,115, Ser. No. 569,006, filed June 27, 1966 now abandoned and Ser. No. 181,144, filed Mar. 20, 1962, the disclosures of all of which applications are incorporated herein by reference.

The device 100 is operative to supply microwave energy at an ultra-high frequency of about 915 mHZ., with a power output at the output terminals in the general range of 50 to 800 watts. The device is arranged to supply the RF power for cooking and to this end a lower transmission line 300 extends from the device 100 and is coupled to an upper transmission line 200 which extends to the cooking cavity 21, the

transmission lines

200 and 300 being of the coaxial type including an inner conductor and an enclosing outer conductor electrically insulated therefrom. Both of the output terminals of the device 100 are at a substantial voltage DC with respect to ground potential, so that the output terminals are electrically insulated from ground potential, as well as from each other. One of the output terminals is coupled by a capacitor 317 to the adjacent end of the outer output conductor, and the other output terminal is coupled by a capacitor 318 to the adjacent end of the inner output conductor, the remote end of the inner conductor projecting as an antenna 201 into the oven cavity 21; the remote end of the outer conductor is electrically connected to the metal liner 20. Thus the RF power produced by operation of the device 100 is radiated from the antenna 201 into the cooking cavity 21, so as to produce cooking effects upon food arranged therein, all in a conventional manner.

An important feature of the present invention resides in the construction and connection of the

transmission lines

200 and 300 interconnecting the crossed-field discharge device 100 and the liner 20, a detailed description of the

transmission lines

200 and 300 being now presented with particular reference to FIGS. 3 through 5 of the drawings. The crossed-field discharge device 100 is provided with a magnet yoke 126 at the upper end thereof which connects to the anode of the device 100 (not shown) and forms an outer conductor and an output terminal for the device 100, the lower end of the magnetic yoke 126 being more particularly connected to the anode of the device 100 and the upper end extending upwardly through the field winding 12012 and being connected to a magnet flange 127. The lower end of the device 100 is likewise provided with a magnet yoke 128 having the upper end thereof connected to the anode of the device 100 and the other end extending downwardly through the field winding 120a and being connected at the lower end to a magnet flange 129. The cathode of the device 100 (not shown) has connected thereto a stud 381 forming a part of the upper coupling structure 380, the stud 381 and the magnet yoke 128 forming a coaxial output connection for the device 100.

Referring specifically to FIG, 3, it will be seen that 8 the output from the other end of the upper transmission line 200 is coupled to the antenna 201 disposed in the heating cavity 21, the antenna 201 being essentially a rod having a threaded stud 202 at one end thereof, the antenna 201 extending into the cavity 21 a short distance below the top wall 22 and essentially midway between the side walls 25, see FIG. 1. The stud 202 extends into a complementarily threaded opening at the end of a rod 217 forming an inner conductor that extends into the cavity 21, the conductor 217 being held in position by a bracket 204 and a cooperating electrical insulator 208. More specifically, the bracket 204 includes an annular mounting flange 205 which is suitably secured as by welding to the rear wall 24 of the liner 20 and disposed about an opening therein, the flange 205 in turn carrying a forwardly directed annular flange 206 terminating in an inturned holding flange 207 disposed agaist the forward surface of the insulator 208.

A top section 210 of the upper transmission line 200 is formed by a first tube 211 forming the outer conductor therefor and the inner conductor 217. The forward edge of the tube 211 has an outwardly directed flange 212 thereon which is suitably secured as by welding on the rear side of the rear wall 24 and surrounding the opening therein and assisting in holding the insulator 208 in the operative position in cooperation with the bracket 204.

The rear end of the tube 211 is received in a T 220, the T 220 including a body 221 and a pair of

arms

222 and 124 and a leg or arm 226 all provided with a seat or

shoulder

223, 225 and 227, respectively internally thereof. The rear end of the tube 211 is disposed within the arm 222 and in telescopic relationship therewith and has the rearmost end thereof spaced slightly forwardly with respect to the shoulder 223, an insulator 218 being disposed between the tube 211 and the shoulder and held in position thereby, the insulator having a central opening therein receiving the inner conductor 217 therethrough. Preferably the outer end of the arm 222 has longitudinally extending slots (not shown) formed therein and receives therearound a clamp 215 having associated therewith a screw 216 (see FIG. 1) for tightening the clamp 215 about the outer end of the arm 222, thus releasably to clamp the arm 222 about the rear end of the tube 211 to hold it in the operative position. It further is pointed out that the arm 222 extends through an opening 46 in the batfle wall 42, whereby the major portion of the T 220 is disposed to the rear of the baflle wall 42 and thus in the rear machinery compartment 45, whereby the T 220 is accessible for service and maintenance through the removable panel 15 (see FIG. 1 also The other arm 224 carries on the outer end thereof a dust cover 228 that serves to close the same to prevent the entry of dust, dirt, water and the like into the interior of the T 220.

A rear section 230 for the upper transmission line 200 is provided by an outer tube 231 and an inner conductor 232 disposed within the outer tube 231. The upper end of the tube 231 extends into the lower arm or leg 226 of the T 220 and is spaced from the shoulder 227 thereof, an insulator .234 being disposed between the upper end of the tube 231 and the shoulder 227 and held in position therebetween. The insulator 234 has an opening therethrough to receive the upper end of the inner conductor 232, the inner conductor 232 having an upper flattened end 233-having an opening therein receiving therethrough a screw 219 engaging in a complementarily threaded opening in the rear end of the rod 217, whereby to interconnect the rear end of the inner conductor 217 and the upper end of the inner conductor 232. There further is provided around the lower portion of the leg 226 one of the clamps 215, the lower end of the leg 226 preferably being slotted .(not shown) whereby tightening of the screw 216 associated with the clamp 216 serves to tighten the leg 226 about the upper end of the tube 231 releasably to hold the parts in the assembled telescoping positions.

The lower end of the rear transmission line section 230 is received in a T 240, the T 240 including a body 241 having a pair of

arms

242 and 244 and a leg on arm 246 each provided with a cooperating internal seat or

shoulder

243, 245 and 247, respectively. More particularly, the leg 246 is positioned upwardly and receives telescopically therein the lower end of the tube 231, the lower end of the tube 231 being spaced upwardly with respect to the seat 247 to receive therebetween an insulator 236 to mount the same Within the T 240, the insulator 236 having an opening therethrough receiving the lower end of the inner conductor 232 therethrough. The outer end of the leg 246 is preferably slotted (not shown) and receives therearound one of the clamps 215, whereby by means of the associated screw 216, the clamp 215 can be tightened about the outer end of the leg 246 to clamp it against the lower end of the tube 231 releasably to hold the parts in the assembled position. The arm 244 preferably carries thereon a dust cover 248 which cover serves to prevent the entry of dust, dirt, water and other debris into the interior of the T 240 and the

upper transmission lines

200 and 300.

There is operatively associated with the other arm 242 of the T 240 a rear section 310 of the lower transmission line 300, the rear section 310 including a tube 311 as the outer conductor and a tube 314 as the inner conductor spaced therefrom and electrically insulated therefrom. The tube 311 extends into and is telescopically associated with the arm 242 of the T 240, the inner end of the tube 311 being spaced from the shoulder 243 and receiving therebetween an insulator 238 that is held in position thereby. One of the clamps 215 is disposed about the outer end of the arm 242 which is preferably slotted (not shown), whereby by means of the associated screw 216 the clamp 215 can be tightened to press the arm 242 about the adjacent end of the tube 311 releasably to hold the parts in the assembled position. The insulator 238 has an opening therein receiving therethrough the inner conductor 314, the rear end of the inner conductor 314 carrying a plug 315 secured thereto and having a threaded opening therein, the lower end of the rear inner conductor 232 having a flattened portion as at 235 having an opening therein receiving a screw 237 that threadedly engages th opening in the plug 315, thus to interconnect the lower end of the inner conductor 232 and the rear end of the inner conductor 314. The forward end (the end disposed to the left of FIG. 3) of the inner conductor 314 is outwardly flared as at 316 to permit ready assembly with other portions of the lower transmission line 300 as will be described more fully hereinafter. Telescopically overlapping the tube 311, in surrounding relationship therewith, is an annular coupling member 313, having an enlarged diameter portion 319 extending to the left of the tube 311 as viewed in FIG. 3. Telescopically received within the enlarged portion 319 of the coupling member 313 is a tube 312 having a diameter slightly greater than the diameter of the tube 311.

The rear lower section 310 cooperates with and is telescopically associated with a front lower transmission line section 320, the front lower section 320 including a tube 321 serving as the outer conductor and an annular inner conductor 322 disposed within the tube 321 and spaced therefrom and electrically insulated therefrom. The tube 321 has an external diameter slightly less than the internal diameter of the tube 312 whereby the rear end of the tube 321 is telescopically received therein, a layer of insulation 317 being disposed therebetween to provide a capacitive coupling at RF frequencies. The annular inner conductor 322 has received in an opening 323 therethrough and threadedly engaged therewith a probe or inner conductor 324, the probe 324 having an external diameter slightly less than the internal diameter of the inner conductor 314, whereby the rear end of the probe 324 can be disposed therein, a layer of insulation 31 8 being provided therebetween and forming therewith a capacitive coupling at RF frequencies.

The rearward end of the inner conductor 322 has a hollow portion generally designated 325 and including a relatively thin cylindrical side wall 327 and an annular end wall 328. The cylindrical side wall 327 is disposed in surrounding relationship with the adjacent portion of the inner conductor 324 concentric therewith and spaced therefrom. Disposed within the hollow end portion 325 of the inner conductor 322 is a cup-like electrically conductive filter member 330 having a cylindrical side wall 331 closed at the rearward end thereof by an annular end wall 332. The cylindrical side wall 331 is disposed in surrounding relationship with the adjacent portion of the inner conductor 324 concentric therewith and spaced therefrom. The side wall 331 is also spaced slightly from the side wall 327 of the inner conductor 322, and the open end of the side wall 331 is spaced longitudinally rearwardly from the end wall 328 of the inner conductor 322. The end wall 332 of the cup-like member 330 has an internally threaded opening 333 therein, the inner conductor 324 being received therethrough and threadedly engaged thcreat with the cup-like member 330. The inner conductor 324, the hollow end 325 of the inner conductor 322 and the cup-like member 330 all cooperate to define a folded cavity 340 comprising an annular inner space 337 between the inner conductor 324 and the side wall 331, an outer annular space 338 defined between the

side walls

327 and 331 and a passage 339 interconnecting the inner and

outer spaces

337 and 338 at the forward ends thereof. An annular insulator 335 is disposed about the inner conductor 324 and abuts the outer surface of the end wall 332 and the open end of the cylindrical side wall 327, thereby closing the folded cavity 340 and insulating the

side walls

327 and 331 from each other. A lock nut 336 is threadedly engaged with the inner conductor 324 and bears against the insulating spacer 335 fixedly to retain the parts in their assembled position. The folded cavity 340 is of such dimensions as to be resonant at the second harmonic of the operating frequency of the device to provide a rejection filter for the second harmonic as will be more fully described hereinafter.

The forward end of the front lower transmission line section 320 is coupled to the crossed-field discharge device 100 by means of a T 370, the T 370 including a body 371 having a pair of

arms

372 and 374 and a leg or arm 376 each provided with a cooperating internal seat or

shoulder

373, 375 and 377, respectively. More particularly, the leg 376 is positioned downwardly and is telescopically received in the upper end of the magnet yoke 126 for coupling thereto, the T 370 forming a part of the upper coupling structure 380. Also forming a part of the upper coupling structure 380 is a bullet 382 receiving into the lower end thereof the stud 381 that is coupled to the cathode of the device 100, the bullet 382 having a shoulder 383 thereon that cooperates with the shoulder 377 to hold in operative position an insulator 378 that serves to center the bullet 382 with respect to the leg 376. The upper end of the bullet 382 abuts the forward end of the inner conductor 322 and carries an internally threaded eye 384 through which is threadedly inserted the probe 324, the rear end of the probe 324 being threadedly received in the complementarily threaded opening 323 in the inner conductor 322, thereby to support the forward end of the inner conductor 322, there also being provided therearound a supporting insulator 367. The forward end of the tube 321 is telescopically received within the leg 372 and is spaced a short distance away from the shoulder 373, thereby to clamp the insulator 367 in the operative position therebetween. Preferably the forward end of the tube 321 is fixedly secured as by soldering to the arm 372.

A decoupling structure is carried by the other arm 374 of the T 370, and more particularly comprises an inner conductor 385 having a rear end connected to the forward end of the probe 324 by means of a threaded connection therebetween, the inner conductor 385 having a reduced forward portion 386. Fixedly mounted on the arm 374 and telescopically received therein is an outer conductor 387 within which is disposed an insulating sleeve 379. Disposed within the insulating sleeve 379 is a cup-like inner conductor 388 having the outer end closed by a wall 389, a screw passing through an opening in the outer wall 389 and engaging in a threaded opening in the forward end of the reduced portion 386 of the inner conductor 385 fixedly to interconnect the parts and to provide the input terminal 368.

It is noted that the

outer conductors

312 and 321 overlap a distance corresponding to about one-quarter wavelength at the freqeuncy of operation of the device 100 and the inner conductor 314 overlaps the probe 324 a distance corresponding to about one-quarter of the wavelength of the frequency of operation of the device 100, thereby to provide a filter for the second and higher even harmonics of the frequency of operation of the device 100. However, in the operation of microwave generators of the type set forth herein for use with electronic cooking apparatus some harmonics of the frequency of operation of generator, particularly the second, may be of sufficient power level to require a more effective filter than that provided by the usual choke and capacitive filters. For example, the above-described capacitive filter has a fairly narrow band width on the order of about mHz. which is insufficient to cover the wide frequency pulling range caused by various food load impedances in an electronic oven. Accordingly, there is provided by this invention a fairly broad band rejection filter for the second harmonic of the frequency of operation of the device 100, the method of operation of which willnow be more fully described. The cylindrical side wall 327 of the hollow end portion 325 of the inner conductor 322 has a length equal to about one-eighth of the wavelength of the second harmonic of the frequency of operation of the device 100. However, the side wall 327, in cooperation with the side wall 331 and the inner conductor 324 and the end wall 328 defines a cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic, the cavity being formed by the outer space 338, the connecting passage 339 and the inner space 337. There is thereby defined a folded cavity 340 of such dimensions as to form a quarter-wave resonant cavity at the second harmonic of the frequency of operation of the device 100, which resonant cavity presents a high series impedance of the tarnsmission of this second harmonic along the transmission line 300 thereby producing high attenuation of this second harmonic frequency. The above-described filter structure is particularly advantageous in that it provides a quarter-wave resonant cavity in a section of transmission line having a length equal to only about oneeighth of the wavelength of the frequency of resonance, thereby affording a valuable economy of space. Furthermore, the folded-cavity harmonic filter of this invention has been found to provide low cost construction, low insertion loss, high power capability, a high degree of harmonic attenuation and fairly broad band operation.

In FIG. 9, there is shown a curve 70 representing the frequency response of the folded-cavity harmonic filter of this invention, the power transmitted through the filter in microwatts being plotted along the ordinate and the frequency in gigahertz being plotted along the abscissa. The response curve 70 has a relatively flat portion 75 of high attenuation. From FIG. 9 it can be seen that the stop band of the filter measured from the point 71 to the point 72 has a width of about 40 mHz., the filter providing at least 40 db attenuation over this stop band. It is noted that the filter has fairly sharp cutoff at the low frequency end of the stop band, it being understood, of course, that only the bottom of the response curve is shown, the curve 70 extending nearly vertically upward to approximately 800 watts. Thus, while the filter provides very high attenuation of the second harmonic, the fundamental (915 mHz.) frequency of operation of 12 the device is passed by the filter substantially without attenuation.

In the transmission line 300, the bullet 382, the T 370, the inner conductor 322 and the tube 321 are shaped and arranged to provide a quarter-wave transformer section at the frequency of operation of the device 100- More particularly, the shouldered portion of the bullet 382 and the conductor 322 have dimensions such that the impedance of the device 100 is matched to the impedance of the

transmission lines

200 and 300 that are in turn matched to the impedance of the heating cavity 21. Likewise, the bullet 382, the inner conductor 385, the outer conductor 387 and the inner conductor 388 are all shaped and arranged to provide a quarter wave transformer section that assists in decoupling RF energy from the input terminal 368 to prevent the propagation of RF energy into the power supply. Furthermore, the inner conductor 385, the outer conductor 387, the insulator 379, the inner conductor 388 and the end wall 389 cooperate to provide the parallel capacitive impedance and reactive impedance. To this end the distance between the inner surface of the end wall 389 and the center of the bullet 382 preferably corresponds to about one-quarter wavelength of the frequency of operation of the device 100, and the

conductors

387 and 388 overlap a distance corresponding to about one-eighth wavelength of the frequency of operation of the device 100. It is noted that the stepped configuration of the inner conductor 385 permits a shorter mechanical connection while maintaining an electrical characteristic equavilent to one-quarter wavelength of the operating frequency of the device 100.

Referring now to FIGS. 6 and 7, there is shown a second embodiment of the folded-cavity harmonic filter according to this inventoin whereby even greater attenuation of the second harmonic of the frequency of operation of the device 100 is attained by providing two folded cavity filters connected in series along the inner conductor of the transmission line 300. The structure of FIG. 6 is essentially the same as the structure of FIG. 3 with the exception of the inner conductor portion of the forward section 320 of the lower transmission line 300. Therefore, the description of that portion of the structure of FIG. 6 which is identical to the structure of FIG. 3 will not be here repeated and the identical portions of FIGS. 3 and 6 are shown with the same reference numerals in both figures.

The rear lower section 310 of the transmission line 300 cooperates with and is telescopically associated with the front lower section 320, as described above, the front lower section 320 including the tube 321 serving as the outer conductor and the probe 324 serving as the inner conductor. There is provided along the inner conductor 324 a filter section 420 comprising a pair of rejection filters for the second harmonic of the frequency of operation of the device 100. Disposed adjacent to the forward end of the inner conductor 324 is a cup-like electrically conductive filter member 425 comprising a cylindrical side wall 426 surrounding the adjacent portion of the inner conductor 324 concentric therewith and spaced therefrom and being closed at the forward end thereof by an end Wall 427, the outer surface of which abuts the eye portion of the stud 382. An inner filter member 430 is disposed rearwardly from the end wall 427 about the inner conductor 324. The inner filter member 430 is cylindrical in shape comprising a solid central portion 431 and a pair of

hollow end portions

432 and 438, the

end portion

432 and 438 respectively comprising

cylindrical side walls

433 and 439 closed at the inner ends thereof by the central portion 431. The cylindrical side wall 433 is disposed within the cylindrical side wall 426 of the cup-like member 425 and is concentric with and disposed approximately midway between the side wall 426 and the inner conductor 324. Surrounding the other cylindrical end 438 of the inner filter member 430 is a second cup-like filter member 435 comprising a cylindrical side wall 436 surrounding the side wall 439 of the inner filter member 430 concentric therewith, the side wall 436 being closed at the rearward end thereof by an end wall 437. The side wall 439 of the inner filter member 431 is surrounded by the cup-like member 435 and is disposed approximately midway between the side wall 436 and the inner conductor 324 concentric therewith. Disposed around the central portion 431 of the inner filter member 430 is an annular insulating spacer 421 insulating the open ends of the

cylindrical side walls

426 and 436 from each other and from the inner filter member 430. An annular insulating spacer 422 is disposed about the inner conductor 324 and abuts the outer surface of the end wall 437 of the cup-like member 435. The parts of the filter section 420 are all held in their assembled position by a lock nut 423 threadedly engaged with the inner conductor 324 and bearing against the insulating spacer 422.

It can be seen from FIGS. 6 and 7 that the filter section 420 is essentially a combination of two folded-cavity filters of the type shown in FIG. 3 arranged back-to-back. The cup-

like filter members

425 and 435 each have a length corresponding to about one-eighth of the wavelength of the second harmonic of the frequency of operation of the device 100. The inner conductor 324, the inner filter member 430 and the cup-

like filter members

425 and 435 cooperate with one another to define two cavities 440, each having an effective electrical length equal to one-quarter of the wavelength of the second harmonic frequency, and each comprising an inner annular space 428, an outer annular space 429 and a connecting passage 434 interconnecting the

spaces

428 and 429 at one end thereof. There are thereby defined two folded cavities 440, each having such dimensions as to be resonant at the second harmonic of the frequency of o eration of the device 100, and each thereby presenting a high series impedance to the transmission along the transmission line 300 of the second harmonic to effect high attenuation thereof. Each of the folded cavities in the filter section 420 provides all of the advantages afforded by the filter of FIG. 3, but the filter section 420 achieves a compact and economical series arrangement of two such filter sections to afford increased attenuation of the second harmonic.

There is shown in FIG. 8 the schematic representation of a lumped-impedance equivalent circuit, generally designated 60, for the filter section 420 of FIG. 6. The circuit includes a pair of

conductors

61 and 68, respectively corresponding to the inner and outer conductors of the transmission line 300, the conductor 68 being grounded as at N. Connected in series relationship in the conductor 61 are two parallel

resonant circuits

62 and 65, each comprising an inductive impedance and a capacitive impedance. More particularly, the parallel resonant circuit 62 comprises a capacitance 63 and an inductance 64 while the parallel resonant circuit 65 comprises a capacitance 66 and an inductance 67. Each of the

parallel circuits

62 and 65 is tuned to resonance at the second harmonic of the frequency of operation of the device 100 and they respectively correspond to the two folded resonant cavities of filter section 420. The

resonant circuits

62 and 65 thus present a high series impedance to transmission of the second harmonic along the conductor 61 while transmitting the fundamental frequency of operation of the device 100 substantially without attenuation. Connected in parallel with the

resonant circuits

62 and 65 is a capacitance 69 corresponding to the insulating spacer 421 in FIG. 6 and having a value such as to present a low impedance to the frequency of operation of the device 100 while presenting a high impedance to the second harmonic thereof.

Because the potentials for operating the device 100 are derived from the voltage doubler and rectifier circuit 130, neither the stud 381 forming the inner conductor or any of the parts such as the yoke 126 and the T 370 forming the outer conductor of the coupling structure 380 can be grounded. However, it is highly desirable to ground the portion of the transmission line 200 disposed to the rear of the rear baffle 40, and to this end the capacitive coupling at 317 and 318 has been provided between the rear lower transmission line portion 310 and the front lower transmission line portion 320. Furthermore, the

outer conductors

312 and 321 overlap a dis tance corresponding to about one-quarter wavelength at the frequency of operation of the device and the inner conductor 314 overlaps the probe 324 a distance corresponding to about one-quarter wavelength of the frequency of operation of the device 100, whereby to provide a filter for the second and higher even harmonics as well as providing a DC insulation between the parts named. Accordingly, the outer conductor 312 can be grounded as on the casing 11 and the bafile member 40, thereby to present only grounded parts to workmen gaining access to the rear machinery compartment 45 through the removable panel 15.

It further is pointed out that the T 220, the entire rear transmission line section 230, the T 240 and the rear lower transmission line section 310 form a removable transmission line assembly that can be bodily moved rearwardly through the opening provided by the removable panel 15 for maintenance and repair of the parts. Such movement of the transmission line assembly rearwardly is accomplished by simply loosening the clamp 215 about the leg 222 which frees the T 220 from the tube 211 and removing the screw 219 to free the inner conductor 232 from the inner conductor 217. Due to the telescoping arrangement of the lower transmission line portions there is no need to remove or disconnect any parts other than by the relative sliding movement of the

conductors

314 and 324 with respect to each other and of the coupling member 313 and tube 312 with respect to each other. Reassembly of the parts is facilitated by the flared end 316 on the inner conductor 314 and by the enlarged diameter portion 319 on the coupling member 313. It further is necessary to hold the removable transmission line assembly in the assembled position, and to this end a spring 249 under tension has been provided interconnecting the T 240 and the baffie wall 42, thus continually to urge the removable transmission line assembly into the assembled operative position. It is also noted that the front panel 19 of the range 10 is preferably removable to afford easy access to the device 100 which, together with the front portion 320 of the transmission line 300, may be disengaged from the rear portion 310 of the line and removed through the front of the machinery compartment 35 for servicing.

Another important feature of the

transmission lines

200 and 300 resides in the fact that the

tubes

211, 231, 312, 321 and 387 can all be formed essentially of standard tubing shaped as required and cut to length, the tubing preferably being formed of copper, brass or other good electrically conductive metal. The

inner conductors

232 and 314 also can be formed essentially of standard tubing shaped as required and cut to length, this tubing also preferably being formed of copper, brass or the like. The

Ts

220, 240 and 370 are also of standard configuration and are all identical one to the other, the Ts preferably being formed of copper, brass or other material having good electrical conductivity. Finally, the

insulators

218, 234, 236, 238, 335, 367, 378, 421 and 422 are preferably all formed of a polytetrafiuoroethylene resin such as that sold under the trademarkTeflon.

A lower coupling structure 390 cooperates with the magnet yoke 128 serving as an outer conductor of a transmission line and a tube 391 connected at the upper end to one end of the heater of the device 100 (not shown) and serving as an inner conductor of a coaxial transmission line, the lower end of the tube 391 carrying an insert 392 therein and receiving the screw 396 serving as an input terminal. Disposed within and essentially lining the magnet yoke 128 is a sleve 393 of electrically insulating material, an inner conductor 394 being disposed against the sleeve 393 and telescopically overlapping a portion of the yoke 1'28 and having the outer end thereof closed by an end wall 395, the end wall 395 having an opening therethrough receiving the shank of the screw 396 that engages in a complementarily threaded opening in the insert 392. The yoke 128, the tube 391, the insulating sleeve 393, the inner conductor 394 and the end wall 395 cooperate to provide a parallel resonant circuit including a reactive impedance and a capacitive impedance, the structure comprising a high impedance to RF energy to prevent propagation thereof onto the conductor 155. More specifically, the distance between the lower adjacent end of the anode of the device 100 and the inner surface of the end wall 395 is equivalent to a quarter wavelength at the operating frequency of the device 100, and the yoke 128 and the inner conductor 394 telescopically overlap a distance equivalent to one-eighth wavelength at the operating frequency of the device 100.

From the above it will be seen that the liner is effectively isolated from the bottom machinery compartment 35 and the rear machinery compartment 45 by the

baffle members

30 and 40, respectively, thereby to provide a more uniform distribution of heat within the liner 20 and thus to permit good cooking therein. The entire generator 50 including the crossed-field discharge device 100 and the voltage doubler and rectifier circuit 130 therefor are housed within the bottom machinery compartment 35 which provides a protecting housing therefor. The fan 106 serves to cool all of the electrical components of the generator 50 by drawing air inwardly through the screen 17 into the bottom machinery compartment 35 and across the crossed-field discharge device 100 and outwardly through the screen 18. The stream of air thus created is effectively prevented from coming into contact with the liner 20 due to the presence of the

baffle members

30 and 40. Also, the improved coupler structure and transmission line 200 has been provided, the major portion of which can be readily removed from the assembled relation with the liner 20 and the device 100 for repair and services purposes through the removable panel 15, and can thereafter be readily reassembled therewith.

Finally, there has been provided a, folded-cavity rejection filter for the second harmonic of the frequency of operation for the device 100, which filter affords economical and compact construction allowing the construction of a quarter-wave resonant cavity in a section of transmission line having a length equal to only about one-eighth of the wavelength of the frequency of resonance. There has also been provided a harmonic filter which affords broad-band operation, low insertion loss, high power capability and a high degree of harmonic attenuation. It will be understood, of course, that while the filters described above are designed to filter the second harmonic of the frequency of operation of the device 100, they may be designed to filter any desired harmonic with appropriate changes in the dimensions of the various parts.

From the above it will be seen that there has been provided an improved electronic heating apparatus 10 and microwave coupling structure 380390 and

improved transmission lines

200 and 300 therefore and improved harmonic filters in the transmission line 300 which fulfill all of the objects and advantages set forth above.

While there has been described what is at present considered to be the preferred embodiments of the invention, it will be understood that various modifications can be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an electronic heating apparatus, structure defining a cooking cavity, a generator for generating electromagnetic wave energy of a predetermined ultra-high frequency and having a pair of output terminals, a transmission line coupling said generator to said cooking cavity and including a hollow outer conductor coupled to one terminal of said generator and an inner conductor coupled to the other terminal of said generator, and a filter section in said inner conductor and comprising structure defining an inner space and an outer space disposed about said inner space and a passage interconnecting said inner space and said outer space to provide a folded cavity, said filter section structure having a length equal to about one-eighth of the wavelength of the second harmonic of the predeten mined frequency, said folded. cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency, said filter section presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along said inner conductor while having a length equal to about one-eighth of the wave length of the second harmonic of the predetermined frequency, whereby during the transmission of electromagnetic wave energy along said transmission line the second harmonic of the predetermined frequency is highly attenuated by said filter section while the predetermined frequency is propagated to said cooking cavity substantially without attenuation.

'2. The electronic heating apparatus set forth in claim 1, and further including a second filter section in said transmission line including a high impedance path in said inner and outer conductors for the second and higher even harmonics of the predetermined frequency to effect high attenuation thereof.

3. In an electronic heating apparatus, structure defining a cooking cavity, a generator for generating electromagnetic wave energy of a predetermined ultra-high frequency and having a pair of output terminals, a transmission line coupling said generator to said cooking cavity and including a hollow outer conductor coupled to one terminal of said generator and an inner conductor coupled to the other terminal of said generator, a first cup-like filter member including a cylindrical side wall surrounding the adjacent portion of said inner conductor concentric therewith and spaced therefrom and an end wall closing said side wall at one end thereof and connected to said inner conductor, said cylindrical side wall having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, and a second cup-like filter member including a cylindrical side wall surrounding the cylindrical side wall of said first filter member concentric therewith and spaced therefrom and an end wall closing said side wall of said second filter member having a length equal to nected to said inner conductor, said cylindrical side wall of said second filter members having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, said first and second cup-like filter members cooperating with each other and with said inner conductor to form a folded cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency, said first filter member being disposed within said second filter member and said second filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency while presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along said inner conductor, whereby during the transmission of electromagnetic Wave energy along said transmission line the second harmonic of the predetermined frequency is highly attenuated by said filter members while the predetermined frequency 4. The electronic heating apparatus set forth in claim 3, and further including a filter section in said transmission line including a high impedance path in said inner and outer conductors for the second and higher even harmonic of the predetermined frequency to effect high attenuation thereof.

5. The electronic heating apparatus set forth in claim 3, wherein each of said inner and outer conductors comprises two portions insulated from each other and telescopically overlapping a distance equal to about onequarter of the wavelength of the predetermined frequency to provide a low impedance capacitive bypass path between said inner and outer conductors for the second and higher harmonics of the predetermined frequency.

6. The electronic heating apparatus set forth in claim 3, wherein the end walls of said first and second cup-like filter members are respectively disposed in opposite directions along said inner conductor and wherein said second cup-like filter member surrounds and is spaced from said first cup-like filter member.

7. The electronic heating apparatus set forth in claim 3, wherein the end walls of each of said first and second cup-like filter members has an internally threaded opening therein, said inner conductor being disposed through said internally threaded openings and being threadedly engaged thereat with said first and second cup-like filter members.

8. In an electronic heating apparatus, structure defining a cooking cavity, a generator for generating electromagnetic wave energy of a predetermined ultra-high frequency and having a pair of output terminals, a transmission line coupling said generator to said cooking cavity and including a hollow outer conductor coupled to one terminal of said generator and an inner conductor coupled to the other terminal of said generator, and a filter section in said inner conductor and comprising structure defining first and second inner spaces and first and second outer spaces respectively disposed about said first and second inner spaces and first and second passage respectively interconnecting said first inner and other spaces and said second inner and outer spaces to provide first and second folded cavities, said filter section structure having a length equal to about one-quarter of the wavelength of the second harmonic of the predetermined frequency, said first and second folded cavities each having an effective electrical length equal to one-quarter of-the Wavelength of the second harmonic of the predetermined frequency and each being resonant at the second harmonic of the predetermined frequency, said filter section having a length equal to about one-quarter of the Wavelength of the second harmonic of the predetermined frequency while providing two resonant cavities, each presenting a high impedance to the transmission of the second harmonic of the predetermined frequency along said inner conductor, whereby during the transmission of electromagnetic wave energy along said transmission line the second harmonic of the predetermined frequency is highly attenuated by said filter section while the predetermined frequency is propagated to said cooking cavity substantially without attenuation.

9. The electronic heating apparatus set forth in claim 8, and further including a second filter section in said transmission line including a high impedance path in said inner and outer conductors for the second and higher even harmonics of the predetermined frequency to effect high attenuation thereof.

10. In an electronic heating apparatus, structure defining a cooking cavity, a generator for generating electromagnetic wave energy of a predetermined ultra-high frequency and having a pair of output terminals, 2. transmission line coupling said generator to said cooking cavity and including a hollow outer conductor coupled to one terminal of said generator and an inner conductor coupled to the other terminal of said generator, and a first cup-like filter member including a cylindrical side wall surrounding the adjacent portion of said inner conductor concentric therewith and spaced therefrom and an end wall closing said side wall at one end thereof and connected to said inner conductor, said cylindrical side wall having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, and a second cup-like filter member including a cylindrical side wall surrounding the cylindrical side wall of said first filter member concentric therewith and spaced therefrom and an end wall closing said side wall of said second filter member at one end thereof and connected to said inner conductor, said cylindrical side wall of said second filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, said first and second cup-like filter members cooperating with each other and with said inner conductor to form a first folded cavity having an effective electrical length equal to onequarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency to effect high attenuation thereof, a third cup-like filter member including a cylindrical side wall surrounding the adjacent portion of said inner conductor concentric therewith and spaced therefrom and an end wall closing said side wall of said third filter member at one end thereof and abutting the end wall of said first filter member and connected to said inner conductor, said cylindrical side of said third filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, and a fourth cup-like'filter member including a cylindrical side wall surrounding the cylindrical side wall of said third filter member concentric therewith and spaced therefrom and an end wall closing said side wall of said fourth filter member at one end thereof and connected to said inner conductor, said cylindrical side wall of said fourth filter member having a length equal to about one-eighth of the wavelength of the second harmonic of the predetermined frequency, said third and fourth cup-like filter members cooperating with each other and with said inner conductor to form a second folded cavity having an effective electrical length equal to one-quarter of the wavelength of the second harmonic of the predetermined frequency and being resonant at the second harmonic of the predetermined frequency to effect high attenuation thereof, whereby during the transmission of electromagnetic wave energy along said transmission line the second harmonic of the predetermined frequency is highly attenuated by said filter members while the predetermined frequency is propagated to said cooking cavity substantially without attenuation.

11. The electronic heating apparatus set forth in claim 10, and further including a filter section in said transmission line including a high impedance path in said inner and outer conductors for the second and higher even harmonics of the predetermined frequency to effect high attenuation thereof.

12. The electronic heating apparatus set forth in claim 10, wherein each of said inner and outer conductors comprises two portions insulated from each other and telescopically overlapping a distance equal to about onequarter of the wavelength of the predetermined frequency to provide a high impedance path in said inner and outer conductors for the second and higher even harmonics of the predetermined frequency.

13. The electronic heating apparatus set forth in claim 10, wherein the end walls of said first and second cuplike filter members are respectively disposed in opposite directions along said inner conductor and the end walls of said third and fourth cup-like filter members are respectively disposed in opposite directions along said inner conductor, and wherein said first and third cup-like filter members are respectively spaced from said second and fourth cup-like filter members.

14. The electronic heating apparatus set forth in claim 19 2o 13, wherein the end walls of said first and third cup-like 2,877,433 3/1959 Devot. filter members are integral 'With each other. 3,187,277 6/1965 Wantuch 15. The electronic heating apparatus set forth in claim l 14, and further including an annular insulating spacer 3439144 4/1969 et a1 219-1055 disposed about the end Walls of said first and third cup- 5 3,411,114 11/1968 Schmld 3 like filter members and separating the cylindrical side Walls of said second and fourth cup-like filter members JOSEPH Primary EXaminer from each other' References Cited L. H. BENDER, Assistant Examiner UNITED STATES PATENTS 10 Us. Cl. 2,700,136 1/1955 Devot. 33373 2,851,666 9/1958 K'aich.