US3439144A - Electronic oven - Google Patents

Description

April 15, 1969 ELECTRONIC OVEN Sheet Filed June 29, 1967 7 mm \m 3 Y om wn A 0 0 "0... m w .0 0 k x m 0 W mm 6 w o m Q l v.0 Q NE E S m W6 i w Km. a m g Q a 3 mm Qwi w 5 m9 o: www v I I 11|IrlI|i J, m: n9 wwm \vn \wn HI? ///4- w: I n: E m 9 A won mmm \w 5 NR mom wmw 8L \ww v9 Q9 i 4 Now N9 N9 \9 mww QQMQH m Em cow NM. Qmm m U Dvm 4. m w Y ow Q2 w 2m 8m April 15, 1969 ELECTRON IC OVEN Sheet 3 of 5 Filed June 29, 1967 mon Qmm Nmm Gm mm an R? United States Patent 3,439,144 ELECTRONIC OVEN James M. Boswell, Jr., Louis H. Fitzmayer, and Richard V. Prucha, Louisville, Ky., assignors to General Electric Company, a corporation of New York Filed June 29, 1967, Ser. No. 649,993 Int. Cl. H05b 9/06 US. 'Cl. 219-10.55 18 Claims ABSTRACT OF THE DISCLOSURE The present invention is directed to an electronic oven, and particularly to an improved transmission line and filter therefor.

The present invention is particularly useful when the generator for the electronic oven includes a magnetron deriving its operating potentials from a voltage doubler and rectifier power supply wherein neither the anode nor the cathode of the magnetron may be grounded for safety purposes. In such cases it is necessary to provide a DC. break or choke joint in at least the outer conductor, and preferably also the inner conductor, of the transmission line coupling the output of the magnetron to the cooking cavity so that the outer conductor and the liner defining the cooking cavity can be grounded without interfering with the operation of the magnetron and its power supply. In such systems ditficulty has been encountered in preventing escape of the third harmonic and multiples of the third harmonic from the choke joint used in providing the DC. insulation between the generator and the cooking cavity, and there also has been encountered excessive propagation of the third harmonic and multiples thereof of the operating frequency of the magnetron to the cooking cavity.

It is an important object of the present invention to provide an improved electronic oven of the type set forth including a coaxial transmission line having therein a choke tube surrounding the outer conductor at the choke joint providing a DC. insulation between the magnetron and the cooking cavity, the choke tube being spaced from the associated outer conductor and having one end thereof disposed adjacent to the open end of the associated outer conductor and having the other end thereof connected to the associated outer conductor, the distance between the electrical connection of the choke tube to the associated outer conductor and the end of the choke tube adjacent to the free end of the outer conductor being equivalent to one quarter wavelength of the frequency of operation of the magnetron to provide a high impedance at the open end of the associated outer conductor to the third and higher harmonics of the frequency of operation of the magnetron.

Another object of the invention is to provide an electronic oven having a transmission line including therein a pair of telescopically overlapping outer conductors electrically insulated from each other to provide a choke joint, the inner one of the overlapping outer conductors ice having a plurality of spaced-apart lands on the surface thereof disposed toward the other outer conductor, the width of the lands and the spaces therebetween being such that in cooperation with the other outer conductor there is provided a choke filter for the third and higher harmonics of the frequency of operation of the magnetron further to attenuate radiation of the third and higher harmonics of the frequency of operation of the magnetron from the joint between the overlapping outer conductors.

A further object of the invention is to provide an improved electronic oven of the type set forth having a transmission line including therein a first outer conductor and a first inner conductor coaxially arranged and coupled to the output terminals of the magnetron, a second outer conductor electrically connected to the first outer conductor and having the longitudinal axis thereof disposed at an angle with respect thereto and a second inner conductor disposed within the second outer conductor and insulated therefrom and having one end thereof connected to the first inner conductor and having the longitudinal axis thereof disposed at an angle with respect to the first inner conductor, and a tuning slug extending longitudinally outwardly from the first inner conductor at the juncture thereof with a second inner conductor, the tuning slug providing a capacitance that cooperates with the distributed capacitance and inductance in the transmission line to provide a low pass filter for frequencies below the third harmonic and multiples of the third harmonic of the frequency of operation of the magnetron, whereby in the transmission of electromagnetic wavelength between the first and second inner conductors there is effected high attenuation of the third harmonic and multiples of the third harmonic of the frequency of operation of the magnetron.

Further features of the invention pertain to the particular arrangement of the parts and elements of the electronic oven and the transmission line forming a part thereof, whereby the above-outlined and additional operating features thereof are attained.

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

FIGURE 1 is a side view with certain parts broken away of an electronic oven embodying therein the present invention;

FIG. 2 is a fragmentary view of an enlarged scale in vertical section with certain portions broken away of the lower portion of the transmission line of the present invention; and

FIG. 3 is a fragmentary view on an enlarged scale in vertical section with certain portions broken away of the upper portion of the transmission line of the present invention.

Referring to FIG. 1 of the drawings, the electronic oven 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 electronic oven 10' comprises an upstanding box-like casing 11 formed of steel including a pair of side walls 12, a rear wall 13 which is preferably removable, a top wall 14 and a bottom wall 16. 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 oppose-d side walls 25; whereby the liner 20 is provided with an upstanding front opening into the cooking cavity 21 defined therein. Further, the casing 111 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 cooking cavity 21 adjacent to and below the top wall 22 and an electric heating unit 37 is arranged in the lower portion of the cooking cavity 21 adjacent to and above the bottom wall 23; which electric heating units 36 and 37 are of the sheathed resistance type and are utilized in carrying out the conventional baking and broiling cooking operations in the cooking cavity 21. The side walls 25 of the liner 20 further carry thereon a plurality of shelf supports 26 for supporting shelfs (not shown) that in turn support items to be cooked within the cooking cavity 21.

Disposed below the bottom wall 23 is a bottom machinery compartment in the lower portion of the casing 111, a lower front panel 19 being provided that is removable to give access to the bottom machinery compartment 35. Mounted in the bottom machinery compartment 35 is an electric motor 31 having an output drive shaft 32 that drives a gear reducer 33, the output shaft of which extends upwardly through the bottom wall 23 and carries on the upper end thereof a turntable 34 arranged in the lower portion of the cooking cavity 21. The turntable 34 is mounted for rotation upon operation of the motor 31 and is adapted to support food to be heated or cooked in an electronic cooking operation that is carried out in the cooking cavity 21. The motor 31 is operative to turn the turntable 34 at a rate of approximately six revolutions per minute.

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 bafiie 40 being formed of metal, such as steel, and cooperating with the casing 11 to provide a rear machinery compartment 45, the rear machinery compartment being disposed behind the liner 20 and access thereto being provided through the removable rear wall 13.

There is arranged in the bottom machinery compartment 35 a magnetron-type generator for supplying ultra-high frequency electromagnetic wave energy for electronic cooking within the cooking cavity 21, the magnetron 50 being of the construction and arrangement disclosed in United States Letters Patent No. 3,315,121 granted Apr. 18, 1967 to James E. Staats for Crossed- Field Electric Discharge Device. Referring to FIGS. 1 and 2, it will be seen that the magnetron 50 is disposed within a box-like structure or casing 51 that extends completely about the magnetron 50 but is open on two opposed sides thereof, i.e., the sides disposed to the left and right in FIGS. 1 and 2. The magnetron 50 and the associated parts therefor are electrically insulated from the casing 51 since high operating D.C. potentials are prescut on those parts electrically connected directly to the magnetron 50. Preferably the openings of the casing 51 are covered with reticulated metal members or screens 52 that permit passage of cooling air therethrough and about the magnetron 50 but prevent contact of the user with the magnetron 50 and the parts disposed thereabout that carry the high operating D.C. potentials.

The magnetron 50 includes an outer wall 55 that is directly connected to the anode thereof and which is provided at the upper end thereof with an end bell 56 carrying thereon an inner output terminal 57, the exterior surface of the end bell 56 providing an annular outer output terminal for the magnetron 50. Surrounding the end bell 56 is an annular magnetic pole piece 58 that is connected to a yoke 59 providing magnetic coupling to a second pole piece disposed at the other end of the magnetron 50 to establish a magnetic field through the magnetron 50, a magnetic field coil 60 surrounding the magnetic pole piece 58 to create a strong magnetic field therethrough. As illustrated, an O-ring 61 of silicone rubber is disposed between the field coil 60 and the end bell 56. An electrical connection is made to the annular outer output terminal provided by the end bell 56 by means of a wire mesh ring 62 resting on the upper surfaces thereof and in turn resting against a flange 63 formed on the lower end of an annular outer conductor 64, a second annular member 65 having an outwardly flared flange 66 being provided around the annular conductor 64 and resting upon the flange 63 thereof and having a wire mesh ring 67 disposed between the outer conductor 6-4 and the flange 66. The inner output terminal 57 has threadedly connected thereto an upstanding conductor 68 that is disposed within the annular outer conductor 64 and generally concentric therewith and extending upwardly through an opening in a ceramic insulator 69 that rests upon the upper end of the outer conductor 64. It will be understood that the RF output from the magnetron 50 appears between the outer conductor 64 and the inner conductor 68 serving as the components of a coaxial transmission line for ultra-high frequency electromagnetic wave energy.

A transmission line is provided to interconnect the output of the magnetron 50 to the cooking cavity 21, the transmission line being particularly illustrated in FIGS. 2 and 3 of the drawings and including a short vertical section 100, a first horizontal section 200, a second longer vertical section 300 and an upper horizontal section 400 connecting to an antenna extending into the rear of the cooking cavity 21 for the introduction of microwave energy thereinto. More specifically, it will be seen that the horizontal transmission line section 200 extends essentially rearwardly from the magnetron 50 and through the wall 40 and into the rear machinery compaltment 45 and there connects with the upstanding transmission line section 300 which in turn connects with the horizontal transmission line 400 that extends forwardly through the wall 40 and to the rear wall 24 of the cooking cavity 21 for connection to the antenna 80 disposed therein.

Referring specifically to FIG. 2 of the drawings, it will be seen that the transmission line section comprises a T 101 including a body 102 having a pair of arms 104 and 10 6 and a leg or arm 108, each provided with a cooperating internal seat or shoulder 105, 107 and 109, respectively. More particularly, the leg 108 is positioned downwardly and receives telescopically therein the upper end of the annular outer conductor 64, the ceramic spacer 69 also being disposed therein and abutting against the shoulder 109, the lower end of the leg 108 pressing against the wire mesh gasket 67 whereby the outer conductor 64 is coupled to the leg 10-8. Disposed within the T 101 is a bullet 110 having a lower end 111 formed as a plurality of prongs that fit into the annular opening in the upper end of the inner conductor 68, a screw 112 extending into an opening in the upper end of the bullet 110 and downwardly therethrough and forcing the prongs on the lower end 111 outwardly firmly to connect to the conductor 68, thereby to provide an electrical connection therewith; preferably a lock washer 113 is provided to hold the screw 112 in the adjusted position thereof. it will be understood that a suitable opening is formed in the T 101 above the screw 112 to provide access thereto during the assembly operation and a spring 114 is also provided in the conductor 68, the spring 114 being useful during assembly of the parts to assure the proper positioning of the bullet 110 with respect to the conductor 68. Finally, a dust cap 115 is provided closing the outer end of the leg 104.

The horizontal transmission line section 200 includes a first tube or outer conductor 201, a first inner conductor 220, a second tube or outer conductor 240 and a second inner conductor 250 which is also preferably in the form of a tube. The outer conductor 201 is annular in form and includes a mounting portion 202 having an external diameter slightly less than the internal diameter of the leg 106 of the T 101 and telescopically received therein and abutting against a washer 203 of polytetrafiuoroethylene resin, the preferred resin being that sold under the trademark Teflon positioned against the shoulder 107 in the leg 106. A suitable clamp 204 is provided fixedly to interconnect the leg 106 and the mounting portion 202, thus firmly to mount the outer conductor 201 on the T 101 and to provide a good electrical connection therebetween. The portion of the outer conductor 201 extending beyond the leg 106 and to the right in FIG. 2 has a first recess therein as at 205 and a second recess therein as at 207, the recesses 205 and 207 cooperating with the outer free end of the conductor 201 to provide a pair of spaced- apart lands 206 and 208.

The inner conductor 220 includes a body portion 221 disposed within the T 101 and having integral therewith an ear 222 having an opening therethrough receiving the shank ofthe screw 112, the washer 113 clamping the car 222 against the bullet 110 to provide a good mechanical connection and a good electrical connection between the bullet 110 and the inner conductor 220. Extending outwardly to the right from the body 221 and integral therewith is a cylindrical probe 225 that extends through an opening in the washer 203 and generally concentric with and spaced from the inner surface 209 of the outer conductor 201. A first ring 226 of low-loss dielectric material is carried by the probe 225 adjacent to the insulator 203 and in surrounding relationship therewith, and a second ring 227 of low-loss dielectric material is carried by the probe 225 intermediate the ends thereof but disposed toward the outer end thereof and in surrounding relationship therewith. The rings 226 and 227 are firmly affixed to the probe 225, the preferred material of construction of the rings 226 and 227 being a synthetic organic plastic resin, the preferred resin being polytetrafluoroethylene resin.

The outer conductor 201 and the inner conductor 220 form a section of a coaxial transmission line to which are applied both the RF output from the magnetron 50 and the D.C. operating potentials applied thereto. In order to isolate electrically with respect to D.C. potentials the magnetron 50 from the cooking cavity 21, and more particularly to make it possible to ground the outer conductor of the transmission line and the liner 20 defining the cooking cavity 21, it is necessary electrically to insulate with respect to D.C. potentials the outer conductor 201 from the cooperating outer conductor 240 and also preferably electrically to insulate with respect to DC. potentials the inner conductor 220 from the cooperating inner conductor 250. To this end a layer or sleeve 210 of electrically insulating and dielectric material is provided covering the major portion of the length of the conductor 201, and specifically that portion thereof which is telescopically received within the cooperating outer conductor 240. More specifically, the inner diameter of the outer conductor 240 is slightly greater than the outer diameter of the insulating sleeve 210 so as to receive the associated end of the outer conductor 201 and the surrounding sleeve 210 therein in telescoping relationship. Also, the outer or lefthand end of the outer conductor 240 is outwardly flared as at 241 so as to provide for easy assembly of the outer conductor 240 over the inner conductor 201 and its enclosing insulating sleeve 210.

The inner conductor 250 is tubular in form and has an inner diameter only slightly greater than the outer diameter of the rings 226 and 227 carried by the inner conductor 228, the lefthand end of the inner conductor 250 telescopically receiving therein the adjacent end of the inner conductor 220 and more specifically the probe 225 and the rings 226 and 227 carried thereon, the lefthand end of the conductor 250 abutting against the insulator 203.

Finally, there is provided about the outer conductor 240 a choke sleeve or tube 270 having an inner diameter substantially greater than the outer diameter of the outer conductor 240 and spaced uniformly therefrom therearound a predetermined distance and having the lefthand end thereof as viewed in FIG. 2 terminated a short distance away from the outwardly flared portion 241, all for reasons that will be described in detail hereinafter.

The vertically extending transmission line section 300 includes at the lower end thereof a T 301 including a body 302 having a pair of arms 304 and 306 and a leg or arm 308 each provided with a cooperating internal seat or shoulder 305, 307 and 309, respectively. More specifically, the leg 304 is disposed to the left as viewed in FIG. 2 and carries therein a washer or insulator 311 of Teflon disposed against the shoulder 305 and extends from the rear machinery compartment 45 through an opening 41 in the wall 40 and into the bottom machinery compartment 35. The inner diameter of the leg 304 is slightly greater than the outer diameter of the adjacent end of the outer conductor 240 so that the adjacent or righthand end of the outer conductor 240 is telescopically received in the leg 304 and abuts against the ceramic washer 311. The outer diameter of the leg 304 is slightly smaller than the inner diameter of the choke tube 270 so that the choke tube 270 is telescopically received thereover. An annular clamp 312 is provided about the adjacent end of the choke tube 270 to clamp the choke tube 270 on the leg 304 and to clamp the leg 304 against the adjacent end of the outer conductor 240 to provide a good mechanical connection between the outer conductor 240, the choke tube 270 and the T 301, and also to provide for good electrical connection between the outer conductor 240 and the choke tube 270 at the outer end of the leg 304-. The righthand end of the inner conductor 250 as viewed in FIG. 2 extends outwardly and to the right through the opening disposed centrally in the washer 311, the free end of the inner conductor 250 fitting over and afiixed to a shouldered member 321. A tuning slug 320 carries a screw 322 therein for the purpose of aflixing the tuning slug 320 to the member 321 and the conductor 250.

The vertical transmission line section 300 further includes an outer annular conductor 330 and an inner annular conductor 340 extending upwardly from the T 301 to a second T 351 (see FIG. 3). More specifically, the lower end of the outer conductor has an outer diameter that is slightly less than the inner diameter of the leg 308 and is telescopically received therein and bears downwardly against a washer or insulator 331 of Teflon supported on the shoulder 309 of the T 301. An annular clamp 332 is provided about the leg 308 to clamp it about the lower end of the outer conductor 330 to provide both a good mechanical connection and a good electrical connection therebetween. The lower end of the inner conductor 340 extends downwardly through an opening disposed centrally in the ceramic washer 331 and carries a connecting portion 341 that is connected to the tuning slug 320 so that the inner conductor 340 and the tuning slug 320 are connected both mechanically and electrically. There also is provided a dust cap 342 to close the leg 306.

The upper end of the vertical transmission line section 300 is provided by the T 351 that includes a body 352 having a pair of arms 354 and 356 and a leg or arm 358 each provided with a cooperating internal seat or shoulder 355, 357 and 359, respectively. More particularly, the leg 358 is positioned downwardly and has an internal diameter slightly greater than the external diameter of the upper end of the outer conductor 330, whereby the upper end of the outer conductor 330 is telescopically received in the leg 358, a ceramic washer or insulator 361 being provided abutting against the shoulder 359 and against the upper end of the outer conductor 330. In order to provide both a good mechanical connection and a good electrical connection between the leg 358 and the outer conductor 330, an annular clamp 362 is provided about the lower end of the leg 358 to clamp it tightly about the upper end of the outer conductor 330. The upper end of the inner conductor 340 extends upwardly through an opening disposed centrally in the insulating washer 361 and into the body 352 of the T 351, a connecting portion 342 being provided on the upper end thereof. The outer end of the leg 356 is closed by a dust cover 363.

The upper horizontal transmission line section 400 comprises a tube or outer conductor 401 and an inner conductor 410. The outer diameter of the outer conductor 401 is slightly less than the inner diameter of the leg 354 on the T 351 and is telescopically received therein and abuts against a ceramic washer or insulator 402 that bears against the shoulder 355 in the leg 354. It will be noted that the leg 354 extends forwardly through an opening 42 in the wall 40 and thus from the rear machinery compartment 45 to the rear wall 24 of the liner 20 and through an opening therein. The forward or lefthand end of the outer conductor 401 is viewed in FIG. 3 has an outwardly directed flange 403 therearound that is suitably secured to the rear wall 24 of the liner 20. The righthand end of the outer conductor 401 is held in the operative position with respect to the leg 354 by an annular clamp 409 disposed about the forward end of the leg 354 and clamping it against the associated end of the outer conductor 401. Covering the outer end of the outer conductor 401 is a ceramic washer or insulator 404 that is held in place by a mounting member 405 that includes a flange 406 secured to the flange 403, an annular body 407 extending to the left and an inwardly directed retaining flange 408 that serves to hold the insulator 404 against the flange 403 and centered With respect to the outer conductor 401.

The inner conductor 410 is in the form of. a rod that extends through aligned openings in the insulators 402 and 404, the rear end of the inner conductor 410 being secured as by the screw 411 to a connecting portion 342 at the upper end of the inner conductor 340. The forward end of the rod 410 has a threaded opening 412 therein that receives a threaded stud 81 on the rear end of the antenna 80, the antenna 80 being in the form of a probe extending into the cooking cavity 21 defined by the liner 20.

A coaxial transformer is provided by the parts in the transmission line section 200 to transform the effective impedance of the cooking cavity 21, the transmission line v and the magnetron 50 to match the higher impedance of the average load in the cooking cavity 21. More specifically, the transformer determines the coupling between the magnetron 50 and the cooking cavity 21 and serves to decouple the load in the cooking cavity 21 from the magnetron 50 to improve the stability of operation of the electronic oven. The critical parts in the coaxial transformer are the outer conductor 201 and the inner conductor 250, the ratio between the inner diameter of the outer conductor 201 and the outer diameter of the inner conductor 250 being approximately 1.5 :1, thereby providing the necessary transformer action and decoupling as described.

As has been explained heretofore, the preferred power supply for the magnetron 50 is a voltage doubler and rectifier system which can be connected directly to a household three-wire Edison supply to provide the necessary potentials for operating the magnetron 50 without the use of any power transformer therein. A voltage doubler system, however, cannot have either of the output terminals thereof grounded, whereby both the output terminal in the form of the conductor 64 and the output conductor 68 from the magnetron 50 carry thereon high potentials with respect to ground potential during the operation of the magnetron 50. For safety reasons it is essential that the oven liner 20 and the various parts connected thereto be grounded, and therefore it is necessary to provide a DC. decoupling insulation in the transmission line between the liner 20 and the magnetron 50 so as to prevent the application of the high potentials on the magnetron 50 to the oven liner 20. Accordingly, a DC. insulation break has been provided in the outer conductor, and specifically, the insulating sleeve 210 has been provided between the outer conductor 201 and the outer conductor 240, whereby there is a transmission of microwave energy therebetween but no transmission of D0. potentials therebetween. Likewise, the inner conductor 220 is insulated with respect to the inner conductor 250 so as to prevent transmission of DC. potential therebetween while providing for an efficient transmission of microwave energy therebetween. It will be understood therefore that the outer conductor 240 and the inner conductor 250 are insulated wtih respect to D.C. potentials from the associated outer conductor 201 and inner conductor 220, respectively, while propagating therebetween the RF energy from the magnetron 50 to the oven cavity 21.

These overlapping conductors 201-240 and 220-250 also provide a filter for the second and fourth harmonic of the fundamental operating frequency of the magnetron 50, substantially to attenuate the transmission of the second and fourth harmonics from the magnetron 50 to the cooking cavity 21. To this end the outer conductors 201 and 240 overlap a length that is substantially equivalent to a quarter wavelength of the operating frequency of the magnetron 50, and the inner conductors 220 and 250 likewise overlap a length equivalent substantially to one quarter wavelength of the operating frequency of the magnetron 50. The provision of the ring 227 on the probe 225 also causes preferential attenuation of the fourth harmonic of the fundamental operating frequency of the magnetron 50, the ring 227 being positioned a predetermined distance from the outer end of the probe 225 equal substantially to a quarter wavelength of the fourth harmonic of the fundamental operating frequency of the magnetron 50. As a result, the overlapping relationship of the outer conductors 201 and 240, the overlapping relationship of the inner conductors 220 and 250 and the position of the rings 226 and 227 on the probe 225 serve to provide an efficient filter for the second and fourth harmonics of the fundamental operating frequency of the magnetron 50, thus highly to attenuate the transmission of the second and fourth harmonics from the magnetron 50 to the cooking cavity 21.

It will be appreciated that the construction of the horiozntal transmission line section 200 is such as to permit ready assembly and disassembly between the parts thereof. More specifically, the outer conductor 240 is slidingly received over the insulating sleeve 210 and can be readily removed therefrom by movement to the right as viewed in FIG. 2. Likewise, the inner conductor 250 is simply slidingly fitted over the probe 225 and the rings 226 and 227 thereon, whereby the inner conductor 250 can be removed therefrom by simply moving it to the right as viewed in FIG. 2. In short, those parts mounted on and connected to the T 301 can be bodily moved to the right for dissassem'bly with respect to those parts mounted on the T 101, thus providing for a ready disassembly of the transmission line section 200 into two telescoping parts. To facilitate assembly of the two telescoping transmission line sections, the forward or righthand end of the ring 227 is beveled as at 228 to facilitate the insertion of the conductor 250 thereover, and the outer conductor 240 has the lefthand end thereof flared outwardly as at 241 to facilitate assembly thereof over the insulating sleeve 210. In order to hold the parts in the assembled position with the lefthand end of the inner conductor 250 firmly pressed against the insulating washer 203, a spring 370 has been provided interconnecting the T 301 and the wall 40, the spring 370 being under tension and urging the T 301 and the parts mounted thereon to the left as viewed in-FIG. 2 and into the assembled position thereof.

While the above described choke joint structure admirably fulfills the desired purpose of providing a DC. insulation between the outer conductors 201-240 and between the inner conductors 220-250, while providing for ready assembly and disassembly thereof, a problem is encountered in that there is a tendency for the third and higher harmonics of the fundamental operating frequency of the magnetron 50' to escape from the resultant choke joint to an undesirable degree. Accordingly, there has been provided in accordance with the present invention the coke tube 270 and a choke filter formed by the outer conductors 201 and 240 substantially to attenuate and essentially to eliminate the propagation of the third harmonic and higher harmonics of the operating frequency of the magnetron 50 from the choke joint. Refer ring first to the choke tube 270, the righthand end thereof as viewed in FIG. 2 is electrically connected to or shorted to the outer conductor 240 at the lefthand end of the leg 304 on the T 301, and the distance between this point of shorting and the forward or lefthand end of the choke tube 270 is equal to one quarter wavelength of the fundamental operating frequency of the magnetron 50. The dimension recited offers a high impedance to the third and higher harmonics at the lefthand end of the choke tube 270 in the space between the choke tube 270 and the outer conductor 240 and particularly the flared end 241 thereof. The choke tube 270 also isolates the outer conductor 240 from the field coil housing or magnetic yoke 59, thus preventing any transportation of third harmonic wave energy in the space between the choke tube 270 and the yoke 59. As a result there is substantially no leakage of the third and higher harmonics from the juncture between the outer conductors 201 and 240, although there is a low impedance for the fundamental frequency of the magnetron 50 at the outer end of the inner conductor 201, thereby to provide for efficient transmission of the fundamental frequency from the outer conductor 201 to the outer conductor 240 for further transmission to the cooking cavity 21.

The choke filter provided .by the outer conductor 201 in cooperation with the outer conductor 240 provides a short for the third and higher harmonics of the fundamental operating frequency of the magnetron 50- so as further to attenuate the radiation of these frequencies from the choke joint between the outer conductors 201 and 240, and specifically at the flared end 241 of the outer conductor 240. This choke filter is provided by the lands 206 and 208 and the spaces 205 and 207, the dimensions of these parts being adjusted and arranged so that the distances between any two points of impedance discontinuity on the lands 206 and 208 are integral multiples of one-half the wavelength of the third, fourth, fifth, sixth, seventh and eight harmonics of the fundamental operating frequency of the magnetron 50. More specially, the dimensions of the outer conductor 201 are chosen and arranged so that the width of the lands 206 and 208 is equal to approximately 0.039 times the fundamental wavelength of the magnetron 50, the space 207 has a width equal to 0.049 times the fundamental wavelength of the magnetron 50, the overall length of the outer conductor 201 is equal to one quarter wavelength of the fundamental frequency of the magnetron 50 and the internal diameter of the outer conductor 201 is equal to 0.058 times the fundamental wavelength of the magnetron 50. When the parts are so dimensioned, there can be no points of resonance for the wavelength or one-half the wavelength of the third, fourth, fifth, sixth, seventh and eighth harmonics of the fundamental frequency of the magnetron 50 in the choke filter. Accordingly, there is a high attenuation of the third and higher harmonics of the fundamental frequency of the magnetron 50 in passing from the outer conductor 201 to the outer conductor 240, thus highly to attenuate radiation of the third and higher harmonics of the fundamental frequency of the magnetron 50 from the joint therebetween and specifically at the flared end 241 of the outer conductor 240.

Turning now to the third harmonic filter provided by the tuning slug 320 in FIG. 2, the tuning slug 320 adds capacitance to the transmission line at the right angle turn or juncture of the inner conductors 250 and 340 in the T 301. This added capacitance acts with existing distributed capacitance and inductance in the transmission line to give a rejection filter or notch filter effect at the third harmonic and multiples of the third harmonic of the fundamental frequency of the magnetron 50. More specifically, there is provided at the righthand end of the tuning slug 320 a high impedance to the third harmonic and multiples thereof and at the connector 341 a low impedance to the third harmonic and multiple thereof which tends to reflect the third harmonic and multiples thereof back toward the magnetron 50, thus substantially to attenuate and prevent transmission of the third harmonic and multiples thereof from the inner conductor 250 onto the inner conductor 340. As a consequence, the tuning slug 320 has the effect of effectively filtering the third harmonic and multiples thereof from the energy transmitted into the transmission line section provided by the conductor 330-340, thus highly to attenuate the transmission of the third harmonic and multiples thereof of the operating frequency of the magnetron '50 to the cooking cavity 21.

In a typical construction of the electronic oven 10, the TS 101, 301 and 351 are formed of copper, the outer conductor 201, the inner conductor 220, the inner conductor 250 and the inner conductor 340 are formed of aluminum. The outer conductor 240, the choke sleeve 270 and the outer conductor 330 are formed of copper. The outer conductor 401, the inner conductor 410, and the antenna are formed of nickel plated steel. In a preferred embodiment of the invention, the magnetron 50 has a frequency of operation of 915 megacycles in which case the outer conductor 201 has an overall length of 3.25 inches, an outer diameter of the mounting portion 202 of 1.125 inches, an inner diameter of 0 .75 inch, an external diameter of the lands 206 and 208 of 1.028 inches, an external diameter of the spaces 205 and 207 of 0.970 inch, a length of the lands 206 and 208 of 0 .50 inch and a length of the space 207 of 0.63 inch.

From the above it will be seen that there has been provided an electronic oven which fulfills all of the objects and advantages set forth above.

While there has been described what is at present considered to be the preferred embodiment 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. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultrahigh frequency electromagnetic energy of a predetermined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said generator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third inner con-ductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor dispose-d within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, a choke tube surrounding said third tube and having one end thereof disposed adjacent to the flared end of said third tube and spaced away therefrom and having the other end thereof electrically connected to said third tube, the distance between the electrical connection of said choke tube to said third tube and the end of said choke tube adjacent to the flared end of said third tube being equivalent to one quarter wavelength of the frequency of operation of said generator to provide a high impedance at the flared end of said third tube to the third and higher harmonics of the frequency of operation of said generator thereby highly to attenuate radiation of the third and higher harmonies of the frequency of operation of said generator from the joint between said second and third tubes, said second tube having a plurality of spaced-apart lands on the surface thereof disposed toward said third tube, the width of said lands and the spaces therebetween being such that in cooperation with said third tube there is provided a choke filter for the third and higher harmonics of the frequency of operation of said generator further to attenuate radiation of the third and higher harmonics of the frequency of operation of said generator from the joint between said second and third tubes, and a tuning slug extending longitudinally outwardly from said third inner conductor at the juncture thereof with said fourth inner conductor, said tuning slug providing a capacitance that cooperates with the distributed capacitance and inductance in said transmission line to provide a rejection filter for the third harmonic of the frequency of operation of said generator and multiples of said third harmonic, whereby in the transmission of the electromagnetic wave energy between said third and fourth inner conductors there is effected high attenuation of the third harmonic of the frequency of operation of said generator and multiples of said third harmonic.

2. The electronic oven set forth in claim 1, wherein said second tube and said third inner conductor cooperate to provide a coaxial transformer to transform the impedance of said generator and said transmission line to match the higher impedance of the average load in said cooking cavity, the ratio between the inner diameter of said second tube and the outer diameter of said third inner conductor being approximately 1.5 :l.

3. The electronic oven set forth in claim 2, wherein the inner diameter of said second tube is equivalent in length to 0.058 times the predetermined wavelength of operation of said generator.

4. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultra-high frequency electromagnetic energy of a predeto said generator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third ou'ter conductor, a third inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor disposed within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connect-ed to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, and a choke tube surrounding said third tube and having one end thereof disposed adjacent to the flared end of said third tube and spaced away therefrom and having the other end thereof electrically connected to said third tube, the distance between the electrical connection of said choke tube to said third tube and the end of said choke tube adjacent to the flared end of said third tube being equivalent to one quarter wavelength of the frequency of operation of said generator to provide a high impedance at the flared end of said third tube to the third and higher harmonics of the frequency of operation of said generator thereby highly to attenuate radiation of the third and higher harmonics of the frequency of operation of said generator from the joint between said second and third tubes, said second tube having a plurality of spaced apart lands on the surface thereof disposed toward said third tube, the width of said lands and the spaces therebetween being such that in cooperation with said third tube there is provided a choke filter for the third and higher harmonics of the frequency of operation of said generator further to attenuate radiation of the third and higher harmonics of the frequency of operation of c said generator from the joint between said second and third tubes.

5. The electronic oven set forth in claim 4, wherein said lands and the spaces therebetween are arranged so that the distances between any tWo points of impedance discontinuity on adjacent lands are multiples of the wavelength or one-half the wavelength of the third and fourth and fifth and sixth and seventh and eighth harmonics of the predetermined frequency of operation of said generator.

6. The electronic oven set forth in claim 4, wherein two of said lands are provided having a space therebetween and on each side thereof, the Width of each of said lands being equivalent in length to about 0.039 times the wavelength of the predetermined frequency of operation of said generator, and the width of the space between said lands being equivalent in length to about 0.049 times the wavelength of the predetermined frequency of operation of said generator.

7. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultrahigh frequency electromagnetic energy of a predetermined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said generator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first I inner conductor disposed within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor disposed within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, a choke tube surrounding said third tube and having one end thereof disposed adjacent to the flared end of said third tube and spaced away therefrom and having the other end thereof electrically connected to said third tube, the distance between the electrical connection of said choke tube to said third tube and the end of said choke tube adjacent to the flared end of said third tube being equivalent to one quarter wavelength of the frequency of operation of said generator to provide a high impedance at the flared end of said third tube to the third and higher harmonics of the frequency of operation of said generator thereby highly to attenuate radiation of the third and higher harmonies of the frequency of operation of said generator from the joint between said second and third tubes, and a tuning slug extending longitudinally outwardly from said third inner conductor at the juncture thereof with said fourth inner conductor, said tuning slug providing a capacitance that cooperates with the distributed capacitance and inductance in said transmission line to provide a low pass filter for frequencies below the third harmonic of the frequency of operation of said generator and multiples of said third harmonic, whereby in the transmission of the electromagnetic wave energy between said third and fourth inner conductors there is effected high attenuation of the third harmonic of the frequency of operation of said generator and multiples of said third harmonic.

8. The electronic oven set forth in claim 7, wherein said second tube and said third tube telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, and said second inner conductor and said third inner conductor telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, thereby to provide high attenuation of the second harmonic and multiples thereof of the predetermined frequency of operation of said generator in transmission of electromagnetic wave energy along said transmission line.

9. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultrahigh frequency electromagnetic energy of a predetermined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said gene erator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor disposed Within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, said second tube having a plurality of spaced apart lands on the surface thereof disposed toward said third tube, the width of said lands and the spaces therebetween being such that in cooperation with said third tube there is provided a choke filter for the third and higher harmonics of the frequency of operation of said generator further to attenuate radiation of the third and higher harmonics of the frequency of operation of said generator from the joint between said second and third tubes, and a tuning slug extending longitudinally outwardly from said third inner conductor at the juncture thereof with said fourth inner conductor, said tuning slug providing a capacitance that cooperates with the distributed capacitance and inductance in said transmission line to provide a low pass filter for frequencies below the third harmonic of the frequency of operation of said generator and multiples of said third harmonic, whereby in the transmission of the electromagnetic wave energy between said third and fourth inner conductors there is effected high attenuation of the third harmonic of the frequency of operation of said generator and multiples of said third harmonic.

10. The electronic oven set forth in claim 9, wherein the lands and the spaces therebetween are arranged so that the distances between any two points of impedance discontinuity on adjacent lands are multiples of the wavelength or one-half the wavelength of the third and fourth and fifth and sixth and seventh and eighth harmonics of the predetermined frequency of operation of said generator.

11. The electronic oven set forth in claim 9, wherein tWo of said lands are provided having a space therebetween and on each side thereof, the width of each of said lands being equivalent in length to about 0.039 times the wavelength of the predetermined frequency of operation of said generator, and the width of the space between said lands being equivalent in length to about 0.049 times the wavelength of the predetermined frequency of operation of said generator.

12. The electronic oven set forth in claim 9, wherein said second tube and said third tube telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, and said second inner conductor and said third inner conductor telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, thereby to provide high attenuation of the second harmonic and multiples thereof of the predetermined frequency of operation of said generator in the transmission of electromagnetic wave energy along said transmission line.

13. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultra-high frequency electromagnetic energy of a predetermined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said generator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor disposed within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, and a choke tube surrounding said third tube and having one end thereof disposed adjacent to the flared end of said third tube and spaced away therefrom and having the other end thereof electrically connected to said third tube, the distance between the electrical connection of said choke tube to said third tube and the end of said choke tube adjacent to the flared end of said third conductor being equivalent to one quarter wavelength of the frequency of operation of said generator to provide a high impedance at the flared end of said third conductor to the third and higher harmonics of the frequency of operation of said generator thereby highly to attenuate radiation of the third and higher harmonics of the frequency of operation of said generator from the joint between second and third tubes.

14. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultra-high frequency electromagnetic energy of a pre determined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and

a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said generator and an intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed Within said first tube and insulated therefrom and having the outer end thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a second inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, and a fourth inner conductor disposed within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, the end of said third tube disposed toward said generator being outwardly flared to facilitate insertion thereinto of the adjacent end of said second tube, said second tube having a plurality of spaced apart lands on the surface thereof disposed toward said third tube, the Width of said lands and the spaces therebetween being such that in cooperation with said third tube there is provided a choke filter for the third and higher harmonics of the frequency of operation of said generator further to attenuate radiation of the third and higher harmonics of the frequency of operation of said generator from the joint between said second and third tubes.

15. The electronic oven set forth in claim 14, wherein said lands and the spaces therebetween are arranged so that the distances between any two points of impedance discontinuity on adjacent lands are multiples of the wavelength or one-half the wavelength of the third and fourth and fifth and sixth and seventh and eighth harmonics of the predetermined frequency of operation of said generator.

16. The electronic oven set forth in claim 14, wherein two of said lands are provided having a space therebetween and on each side thereof, the width of each of said lands being equivalent in length to about 0.039 times the wavelength of the predetermined frequency of operation of said generator, and the width of the space between said lands being equivalent in length to about 0.049 times the wavelength of the predetermined frequency of operation of said generator.

17. An electronic oven comprising a box-like metal liner defining a cooking cavity, a generator for supplying ultra-high frequency electromagnetic energy of a predetermined frequency, said generator having an annular outer output terminal and an inner output terminal both at high potentials with respect to ground potential, and a coaxial transmission line interconnecting said generator and said cooking cavity and including a top section coupled to said cooking cavity and a bottom section coupled to said generator and in intermediate section interconnecting said top and bottom sections, said top section including a first tube connected to said liner and extending outwardly therefrom to provide a first outer conductor and a first inner conductor disposed Within said first tube and insulated therefrom and having the outer end 17 thereof coupled to said cooking cavity, said bottom section including a second tube connected to said annular outer output terminal and providing a second outer conductor and a secOnd inner conductor connected to said inner output terminal and disposed within said second tube and insulated therefrom, a third tube telescopically overlapping said second tube and insulated therefrom and extending outwardly therefrom to provide a third outer conductor, a third, inner conductor disposed within said third tube and insulated therefrom and telescopically overlapping said second inner conductor and insulated therefrom, said intermediate section including a fourth tube providing a fourth outer conductor interconnecting and electrically connected to both said first tube and said third tube and having the longitudinal axis thereof disposed at an angle with respect to said third tube, a fourth inner conductor disposed within said fourth tube and insulated therefrom and having one end thereof connected to said first inner conductor and having the other end thereof connected to said third inner conductor and having the longitudinal axis thereof disposed at an angle with respect to said third inner conductor, and a tuning slug extending longitudinally outwardly from said third inner conductor at the juncture thereof with said fourth inner conductor, said tuning slug providing a capacitance that cooperates with the distributed capacitance and inductance in said transmission line to provide a low pass filter for frequencies below the third harmonic of the frequency of operation of said generator and multiples of said third harmonic, whereby in the transmission of the electromagnetic Wave energy between said third and fourth inner conductors there is effected high attenuation of the third harmonic of the frequency of operation of said generator and multiples of said third harmonic.

18. The electronic oven set forth in claim 17, wherein said second tube and said third tube telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, and said second inner conductor and said third inner conductor telescopically overlap a distance equivalent to a quarter wavelength of the predetermined frequency of operation of said generator, thereby to provide high attenuation of the second harmonic and multiples thereof of the predetermined frequency of operation of said generator in the transmission of electromagnetic wave energy along said transmission line.

References Cited UNITED STATES PATENTS 3,172,987 3/1965 Fitzmayer 219 10.55 3,321,605 5/1969 Reker 219-1055 3,388,231 6/1968 Staats 219 -1o.55

RICHARD M. WOOD, Primary Examiner,

L. H. BENDER, Assistant Examiner.

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