US3727098A

US3727098A - Magnetron filter box

Info

Publication number: US3727098A
Application number: US00165113A
Authority: US
Inventors: P Crapuchettes
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1971-07-22
Filing date: 1971-07-22
Publication date: 1973-04-10
Anticipated expiration: 1990-04-10
Also published as: DE2229743B2; JPS5412034B1; DE2229743A1

Abstract

A magnetron filter box of novel construction serves to dissipate and filter out spurious signals appearing at the filament terminals of a magnetron. The filter box includes: two (or more) elongated electrical conductors spaced and insulated from one another which are provided with adjacent end portions of each adapted for connection to a respective one of the filament terminals of the magnetron which provide a current carrying path to the filament; microwave dissipative material of a predetermined volume is placed about and surrounds the electrical conductors minimally all along a portion of the lengths; and a metal container of predetermined dimension and volume encloses the microwave dissipative material and underlying conductors and is adapted to enclose additionally the stem or terminal end of the magnetron either with or without the metal pole piece as part of the container combination. The relative geometry between the dissipative material and the metal enclosure is such as to form a microwave cavity of a low-Q factor, suitably less than 100, and adjacent portions of the metal enclosure and the dissipative material is such as to define by analogy a transmission line with a ''''lossy'''' characteristic impedance, suitably less than 50 ohms.

Description

atent Crapuchettes 21 Appl. No. 165,113

[52] US. Cl. ..31S/39.51, 333/69, 315/3953,

v 315/75, 331/86 [51] Int. Cl ..l-l0lj 25/50 [58] Field of Search ..3l5/39.5l, 39.53,

[56] 1 References Cited 1 I V UNITED STATES PATENTS 3,531,613 9/1970 Domenichini ..333/79 X 3,551,858 12/1970 Cielof... ..333/79 3,020,447 2/1962 .Schall et al. 315/3951 X 3,543,082 11/1970 Boehm ..3l5/39.53 3,551,735 12/1970 Staats ...3l5/39.53- 3,458,753 7/1969 Staats ..315/39.53

MAGNETRON FILTER BOX Inventor: Paul Wythe Crapuchettes, Woodside, Calif.

Assignee: Litton Systems, Inc., San Carlos,

Calif.

Filed: July22 1971 Primary Examiner--l-lerman Karl Saalbach Assistant .Examiner--Saxfield Chatmon, Jr. Attorney-Ronald M. Goldman et a1.

1 Apr. 10, 1973 [5 7 ABSTRACT A magnetron filter box of novel construction serves to dissipate and filter out spurious signals appearing at the filament terminals of a magnetron. The filter box includes: two (or more) elongated electrical conduc tors spaced and insulated from one another which are provided with adjacent end portions of each adapted for connection to a respective one of the filament terminals of the magnetron which provide a current carrying path to the filament; microwave dissipative material of a predetermined volume is placed about and surrounds the electrical conductors minimally all along a portion of the lengths; and a metal container of predetermined dimension and volume encloses the microwave dissipative material and underlying conductors and is adapted to enclose additionally the stem or terminal end of the magnetron either with or without the metal pole piece as part of the container combination. The relative geometry between the dissipative material and the metal enclosure is such as to form a microwave cavity of a lowQ factor, suitably less than 100, and adjacent portions of the metal enclosure and the dissipative material is such as to define by analogy a transmission line with a lossy characteristic impedance, suitably less than 50 ohms.

' l Clairns, 11 Drawing Figures PATH-HEB APR 1 01975 3; 727, 098

suwsur INVENTOR. PAUL w. CRAPUCHETTES EW "1 M ATTORNEY MAGNETRON FILTER BOX BACKGROUND OF THE INVENTION This invention relates to a magnetron filter box and, more particularly, to a dissipative filter which is adapted to be used in conjunction with the filament leads of a magnetron.

Microwave vacuum tubes have heretofore been used as generators of microwave frequency energy, microwave sources or, as variously termed, microwave oscillators, and as such are found in common use in many electronic systems. One such microwave tube is the magnetron. The magnetron is, essentially, a selfcontained device in that the elements which cooperate to generate the microwave frequency energy are internal of the envelope. Terminals for connection of various electrical circuitry, including filament terminals, which permit current to be applied to the magnetron filaments, a cathode terminal, an anode terminal, and an output terminal or window are provided outside the tube envelope. Very simplyexplained, when the appropriate sources of filament current are applied to the filament terminals and an appropriate source of high electrical voltage is applied between the anode and cathode terminals, the microwave frequency energy is generated internally within the tube and this energy is delivered at the magnetron output window.

In magnetrons, as with all utilitarian devices, there is always some degree of imperfection, however slight, which requires correction, compensation, minimization, or adjustment. In particular, the magnetron by design provides the predetermined microwave frequency at. the designed power level. However, there are in fact other frequencies generated in varying degrees in varying levels, usually substantially less than the predominating output signal, such as harmonics of the fundamental frequency, and these signals may be termed spurious." Included in this definition of spurious signals are any portions. of the fundamental frequency which leak out through other than the output window. These spurious signals in practice have either been so small as to be disregarded or were reduced to acceptable levels by conventional electrical filters, electrical devices which filter out or eliminate, ideally, the undesired spurious signals. In .recent years, however, the magnetron has found a prime commercial application as the source of microwave energy used in microwave cooking appliances or ovens. And it is in connection with this appliance-that the magnetron and spurious signals is next considered as background of my invention.

The microwave oven provides the means by which one can cook or heat food by exposure to microwave radiation. In this manner food is heated or cooked very, very rapidly in comparison to cookingwith conventional ovens and such a decrease in cooking time is an advantage and represents a significant convenience for many people.

As with all technicallysophisticated appliances, however, special care is given to the design and maintenance of the appliance in order to maintain its utilitarian values while avoiding abuse. While microwave radiation is of utility in cooking food, it is not to be exposed to humans or interfere with other electronic equipment in any significant degree.

walls prevent radiation leakage from these areas. A

door closes the cooking chamber during operation of the magnetron.

Any leakage of radiation, either from around the edges of the door closing the cooking cavity or from the back panel of the oven housing in which the magnetron and associated power supplies are physically enclosed,

is to be avoided.

A first consideration must be made regarding leakage of microwave energy. The Federal'Communications Commission, another Government agency, prescribes and allocates the frequencies which may be used for various purposes and has assigned the frequency of 2,450 MHz for microwave cooking. By regulation, the FCC requires the radiating unit to be of such a construction that intensity of radiation leakage at frequencies outside of a percent bandwidth of the assigned frequency range of 2,400 to 2,500 MHz must be less than 25 microvolts per meter measured at a distance 'of1000 feet per 500 watts of output. The purpose of this limit is to prevent signals from the microwave oven from interfering with other forms of radio communications and television.

A second consideration regarding the degree of radiation is made. Initially, such radiation leakage as was permissible and harmless to humans was originally determined to be on the order of 10 milliwatts per square centimeter at a distance of 2 inches. This represents a level of radiation comparable to that normally emanating from the sun on a clear spring day. When one considers that the magnetron delivers into the oven cavity some 500 to 1,000 wattsof microwave energy, such leakage represents a rather small portion. Oven designs and equipment then available met this standard and were thus considered safe. However, recently the heretofore permissive radiation limits were re-examined, particularly having in mind the increasing number of electronic appliances being introduced into the home by the consumer and to the possibly cumulative effects of harmless doses of radiation from each. Thus, to provide additional protection the levels of permissible radiation were reduced even further as an attempt to account in some way for the unknown amountsof radiation that the consumer may be exposed from an unknown quantity of additional appliances, each of which by themselves are harmless. Accordingly, by Government regulation, the permissible radiation leakage whether from the door seal area attention to manufacturing tolerances and more sturdy and expensive sheet metal work.

By comparing regulations of the two agencies one determines essentially that of the two, the restrictions on spurious out-of-band radiation promulgated by the FCC to prevent interference with radio and television reception stations is more restrictive by some 60 db than the permissive radiation levels announced by the other Government agency. However, the permissible limits of radiation of the latter are more strict in a different sense, they include leakage at the assigned frequency of 2,450 MHZ or in-band radiation.

One type of magnetron widely used in microwave ovens is the L-500l or variations thereof,v manufactured by Litton Industries, Electron Tube Division, San Carlos, Cal., which are perhaps better illustrated in US. Pat. Nos. D-215,298, D-208,86l,' D- 2l8,l69, and D-2l2,758. I

The filament terminals on the magnetron, as previously noted, physically appear as bands of conductive material on a stemlike projection at one end of the magnetron. The stem is actually divided into'electrically insulated parts separated by aluminum oxide, an insulating material, with the tip of the stem comprising a pinched-off copper tube. This stem end of the tube was inserted within or surrounded by a large metal or large shielding enclosure referredto as the choke box or magnetron filter box, and this latter unit'was added during assembly of the magnetron into the equipment. Such an element appears in two of the cited patents. Typically, one end of this enclosure was formed of a metal screen material or a perforated metal sheet so that air could be introduced into that region. The openings in the enclosure are, however, of such dimension as to prevent the passage of any spurious signals above a predetermined intensity from the vicinityof the cathode stem. The power supply leads including the high voltage cathode supply and the two leads through which filament current was coupled to thefilament ter-' such sources considered, the normal level of spurious radiation in present ovens is below the FCC limits.

Conservative design principles require the manufacturer to provide an oven with components which have leakage, if at all, substantially below the 25 microvolt per meter level prescribed by the FCC so that considering the vagaries of normal mass production manufacturing techniques, all ovens built should comply with the radiation standards, regardless of the degree of per} fection of assembly by personnel on any given day and in this way waste or scrap in the manufacturing process is'eliminated.

and filament, which also serves as the cathode, inuring due to asymmetries in the anode. From the filament the microwave energypasses through the terminal 'on the stem and appears on the power supply leads, unlessthere filtered out or dissipated.

A second source of spurious signals are subharmoriics. These are microwave-signals of some submultiple 'of the fundamental frequency of 2,450 MHz which are inherently generated in the tube but in very, very low' levels. A prime objection to such subharrnonic signals is that they may fall within the television frequencies and thus could potentially interfere with normal television reception during operation of the magnetron.

minals of the magnetron extendedfrom the outside of the enclosure through an opening to the magnetron stem. r

The filter box typically included wire inductors and capacitors. The inductors were placed in series with the respective filament leads and the capacitors were placed between a filament lead and electrical ground to form a low pass filter. Such a filter prevented beyond permissible limits any high frequency signals which might leak from the cathode to the filament terminals from traveling along the electrical leads to the outside of the oven enclosure, and the capacitors served to shortcircuit to ground as much high frequency energy as was possible. In addition, or as an alternative construction, beads of lossy" or dissipative microwave material such as ferrites were included in such a filter box to positively absorb and dissipate any microwave energy in that vicinity. While such constructions proved acceptable in the past, costs must be reduced and the recent reduction in level of permissible radiation leakage requires reevaluation of design and manufacturing procedures.

A third form of spurious signal derives from the multiplicity of resonant modes of the multi-segment anode found withinthe magnetron tube. If the anode has N resonators there will be N resonant-modes of the first order, second order, etc. Since the normal mode of operation, called 1' mode, has been optimized for impedance all other signals are comparatively lower. Since these other resonant modes will always be outside of the allotted frequency range band allotted fro microwave cooking and heating, care must be exer- I cised to assure that energy produced and radiated is less than the permissible level.

A fourth form of spurious signal derives from an electronic interaction between the impedances associated with the normal 41" mode and unintended systematic impedances. Generally such signals, if generated, are within 2,415 and 2,485 MHz where their magnitude can exceed 25 uv/meter, as described above.

OBJECTS OF Tl-lElNVENTION microwave frequency signals that may appearat the filament terminals of a magnetron.

And it is a still further object of the invention to provide magnetrons with integral filters for use in microwave ovens that reduce further any potential leakage from the microwave oven.

BRIEF SUMMARY OF THE INVENTION In accordance with my invention, at least two elongated electrical conductors are spaced from one another and have adjacent end portions adapted for connection to filament terminals of a magnetron and form a portion of the electrical path by which filament current is supplied to such magnetron. Microwave dissipative material is located along the length and substantially surrounds the two conductors. In addition, a microwave cavity comprising metal walls surrounds and contains within the loss material and underlying conductor portion, the conductor ends and the magnetron filament terminals. In accordance with the invention an opening is provided in this metal cavity into which a magnetron stern portion containing the filament and high voltage terminals can bev inserted for connection to the respective end portions of said electrical conductors and a passage is provided in a remote end portion of the cavityto permit suitable connections to be made with said conductors.

Further, in accordance with the invention, the microwave cavity is of a low Q factor, suitably less than 100. In accordance with said low Q factor, the ratio of the volume defined by the microwave cavity to the volume of the loss material is a number of fraction greater than I and lesser, approximately, than 20.

In accordance with an additional aspect of the invention the outer cavity walls and the elongated conductive wires and attached loss material are considered analogous to a coaxial transmission line and the geometry of such line is defined to have a characteristic impedance Z, preferably less than 50 ohms.

In accordance with a still further aspect of the invention one wall of the microwave cavity may comprise the flat pole piece section of the magnetron.

The foregoing and other objects and advantages of the invention together with the elements comprising the invention, including equivalents and substitutions for the elements thereof, become apparent to one of ordinary skill in the art from a consideration of the following detailed description taken together with the figures of the drawings in which:

DESCRIPTION OF DRAWINGS of the elements in the preferred embodiment and their method of assembly; and

FIG. 6 illustrates a top view of another embodiment of the invention with the upper container wall removed; FIG. 7 and 8 illustrate side and end views of the embodiment of FIG. 6;

FIG. 9 illustrates still another embodiment of the invention;

FIG. 10a represents in cross section the conductors and loss material of FIG. 9; and

FIG. 10b represents a modified form of the conductors and loss material suitable for the embodiment of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates, symbolically, the elements of my invention in a magnetron filter box and its relationship to a magnetron, symbolically illustrated, used in microwave oven. The magnetron is of the type L- 5260A sold by Litton Industries, Electron Tube Division, illustrated in greater detail in U.S. Pat. No. D- 212,758 and U.S. Pat. No. 3,493,810. The magnetron includes a body portion I, typically a hollow copper cylinder, containing therewithin in a vacuum the elements of the magnetron that are necessary for the generation of microwave energy including cathode, anode, filament, and magnetic pole piece extensions, which details are not illustrated or otherwise explained since they form no part of the instant invention and are available elsewhere in the prior art. At one end of the magnetron body is an output terminal of window 3 which is the exit for the microwave energy generated within body 1. At the other end is a stem portion 5. Stem 5 extends from the body 1 and includes spaced metal portions or terminals 7 and 9 which internally complete a circuit to the magnetron filament winding. The terminals are insulated from one another by a vacuum tight aluminum oxide insulating material or spacer. Both stem 5 and window 3 protrude through openings provided in

pole pieces

13 and 15. The pole pieces are of a flat rectangular shape and consist of ferromagnetic material, typically annealed iron, and function as part of the magnetic circuit necessary to the operation of the magnetron. A first permanent magnet 17 is located between and at one end of

pole pieces

13 and 15, and a like second permanent magnet 19 is located at the left end and between the same pole pieces. Both magnet 17 and magnet 19 have their North Poles facing pole piece 13. The magnets provide the magnetic flux which is carried through the magnetic path formed by the pole pieces and by means of magnetic material connected at the front and back ends of magnetron body 1', and internal pole pieces (not illustrated) present the magnetic flux internally of the tube where such forms a necessary ingredient to the generation of microwave energy. The horizontal lines 21 are symbolic of the metal, suitably aluminum, cooling fins which are stacked in between the pole pieces in a heat conducting relationship with the body 1. The cooling fins contain suitable openings to permit them'to be fitted over the magnetron body 1 and

magnets

17 and 19. Obviously in the variation of the magnetron illustrated in U.S. Pat. No. D-2l8,l69 the cooling fins would be arranged vertically and radially outward from body portion 1.

A pair of elongated electrical conductors or

wires

31 and 33 are illustrated spaced from one another above pole piece 13.

Electrical conductors

31 and 33 include

respective end portions

35 and 37 located proximate one another and connected, respectively, to filament terminals 7 and 9. These conductors extend through passages in and support three bodies of a dissipative or loss material 39. The passages, not Clearly illustrated in 1 this figure, permit the loss material to be mounted so that it surrounds all, portions of each conductor and at the same time serves to maintain the predetermined spacing between

conductors

31 and 33.

Dashed line- 41 represents the walls of a compact metal container which encloses the ferrite material and a portion of the

wires

31 and 33 including the

end portions

35 and 37 thereof. Thecontainer covers, also, and

encloses the stem end of the magnetron. In this illustration one wall of the metal container 41 is effectively formed by the flat surface of pole piece 13. The relationship between the metal, can or cover and the dissipative material will be discussed hereinafter in greater detail. Loss material in addition to 39 is included in the metal cover. An additional slab of microwave loss material 40 is attached to an end wall of the container.

An insulator 43 is inserted in a passage in an end wall of the container. The insulator contains two passages spaced from one another through which

wires

31 and 33 extend, and the insulator serves to prevent the wires from contacting the metal walls of container 41. It is apparent that the

particular conductors

31 and 33 need not be made so long as to extend beyond the confines of container 41 but instead may he -terminated at a socket fixed at the end wall. However in such an instance the extension of electrical circuitry would require theinclusion of a plug fitted to be received with such a socket so as to permit anextension of the electricalpath through the containers beyond the walls of container 41 to the appropriate source of filament current. The foregoing description provides background to the more exact description of the preferred embodiment'. As can be seen the magnetron filter box is formed in a small and convenient package. suitably mounted integral with the magnetron. In connection with the remaining figures, it is noted that when an element is assigned a numeral in a preceding figure that same numeral is thereafter used, for convenience of the reader, in subsequent figures where the same element appears.

The top view of the preferred embodiment of the magnetron filter box is illustrated in FIG. 2 with a top wall or sideremoved inorder to provide better illustration of the contentsrA container or box which forms a microwave cavity is of an elongated narrow construction and is formed of metal, suitably aluminum or stainless steel. This includes a pair of

end walls

50 and 51, a front wall 53,'a back wall 54, a bottom wall 57 and a top wall (removed 55 in FIG. 3). End wall 51 includes a v passage in which is mounted an insulator, 43 in FIG. 1.

dashed lines, is mounted spaced from and generally parallel with the first conductor 31. Conductor 33 includes a front end portion 37 proximate end portion 35 of conductor 31. Suitably,

conductors

31 and 33 used are Teflon insulated, as made by Gore Industries, to eliminate corona. End portions'35 and 37 are bent so as to engage or contact satisfactorily the filament ter- I minals of the magnetron stem, represented by the dashed lines 9 and 7. In this

embodiment conductors

31 and 33 are of such a length as to extend through wall 51 and the insulator 43, mounted in wall 51, so that they are maintained spaced at thatend and insulated from electrical contact with the metal container walls.

Beads

39a, 39b and 390, of microwave dissipative or loss material, suitably ferrite, are mounted on

conductors

31 and 33 along the straight elongated portion thereof within the metal container. Each of the ferrite beads is a standard material,- usually

grade

9 or 11, and has an elliptical cylindrical geometry and the consistency of a solid sintered pressed powder body. Each of the beads has two spaced passages therethrough through which

conductors

31 and 33 extend. In this way the

conductors

31 and 33 support the ferrite beads and, in turn the ferrite beads assist to maintain the conductors spaced apart. In addition, it is seen that the ferrite A sheath 4 2 of heat shrinkable tubing, suitably irradiated polyvinyl tubing, such as that sold by Raychem Company, which may include additional loss material, covers the

beads

39a, 39b and 39c. To permit a clear illustration of the beads the cover tubing is represented solely by dashed lines. i

In addition, a standard size slab of ferrite microwave loss material 38 is attached to end wall 50, suitably with epoxy.

Bottom wall 57, represented by a dashed line in FIG. 3, of the filter box. in the preferred embodiment is in fact a portion-of the flat iron pole piece of the magnetron, such as illustrated in FIG. 1. An opening 59 represents the opening in the pole piece through which the magnetron stern containing'the filament terminals and cathode terminals extends. The

side walls

50, 51, 53 and 54, are simply joined to the metal pole piece in a manner which eliminates any gaps or vcracks therebetween. This is suitably accomplished by welding or by electrically conductive epoxy material. The magnetron pole piece thus serves to provide an electrically conductive body or wall to the metal container with an opening adapted to receive the end of the magnetron. However, it is apparent that in an obvious variation of this embodiment of the invention, a'metal wall can be provided to close the bottom of the container before the box is placed atop the magnetron pole piece. Of course, a suitable opening or passage such as 59 would be formed at the location in the figure to permit the insertion therethrough of the magnetronstem. In such a construction the bottom wall is fastened to the magnetron pole piece and the pole piece serves only to support the filter and, eliminates its function as an ele.ctrically conductive boundary. I

FIG. 3 illustrates the filter boxfrom one side facing wall 53 but with side wall 53 and attached ferrite removed, the embodiment of FIG. 2. End wall 50, insulator 43, installed in the passage through side wall 51,

electrical conductors

31 and 33, in part illustrated'by dashed lines, and the respective

front end portions

35 and 37 thereof, are presented. In this figure the conductors are also shown extending through the

ferrite beads

39a, 39b and 390, and the ferrite beads are enclosed in tubing 42. In addition, ferrite slab 38 is shown attached to the end wall 50, and air openings 56 are visible in back wall 54. Bottom wall 57 in this view is represented by dashed lines since in the preferred embodiment the electrically conductive wall is formed by a portion of the surface of the pole piece of the magnetron and in the alternative construction, discussed in connection with FIG. 2, may be a separate metal wall. The stem portion of the magnetron is represented by the dashed lines and includes a pinch off tube and filament terminals 7 and 9 shown extendingthrough the opening59 in this bottom wall. The top wall 55 of the container includes a raised portion, bulge, or dome 60 at the end of the container adjacent end wall 50 and over the opening 59. This raised portion forms a dome over the end of the stem of the magnetron and serves to maintain a proper insulating space between the metal parts and the end of the magnetron stem while otherwise permitting a smaller dimension or height to the container elsewhere than at this location, thus reducing cavity Q.

The dome portion 60 of top wall 55 includes several holes or passages therethrough, such as 61a, 61b and 610, visible in this figure. These openings are generally less than 1/ 10th of the wavelength of the fundamental microwave frequency of 2,450 MHz so as to preclude any leakage of microwave frequency therethrough and at the same time permitting a passage for air to cool the magnetron stem. I

In the structure of the preferred embodiment the top wall 55 and bottom wall 57 are enlarged at the right hand ends in FIGS. 2 and 3, primarily to make the container large enough in volume at that end to receive the stem of the magnetron and allow sufficient clearance therebetween so as to provide appropriate high voltage insulation and prevent corona or high voltage breakdown therebetween. At the left hand end of these walls the width of the wall is significantly more narrow so as to provide a slender appearance, preferably with a slight spacing between the end walls and the ferrite beads. Overall the top view of the top and bottom walls is seen to resemble in shape paddles with handles. As a result, the front and back walls, 53 and 54, are bent in shape so as to conform to edge of the top and bottom walls. End front wall 50 is rectangular in shape but of a larger area than the other end wall 51.

The end view of 'FIG. 4 illustrates the filter box with end wall 51 and insulator 43 removed. Viewing the filter box from that end,

electrical conductors

31 and 33 are visible and extend through passages in ferrite bead 39a. For clarity, sheath tubing 42 and ferrite slab 38 illustrated in FIGS. 2 and 3 is omitted. Top wall 55 is seen to include the dome portion 60 and in this view an additional hole, 61d, is visible, in addition to the holes 61a and 61b visible from theview of FIG. 3.

The metal walls of the container form essentially a closed microwave cavity in which any microwave energy is confined. By design, the microwave cavity in the filter box of the invention is of a low Q, 0 representing the quality factor of the cavity, and thus is considerably smaller in size than the shielding cavities of the prior art constructions. The ferrite material installed within the cavity occupies a predetermined volume, represented by V,. In turn, the volume of the cavity is of a predetermined volume, represented as V,,. In order to obtain a low Q cavity on the order of one-hundred or less, the relationship or ratio between V and V, or V /V, is such as to be a number, including any fraction thereof, approximately less than 20 andat least slightly greater than the number 1 (one).

A second factor used to determine the relative geometry and actual dimension of the metal container and ferrite of the preferred embodiment, FIGS. 2 through 4, is best illustrated in connection with FIG. 4. Considering the narrow length of the metal container (apart from that over the stem of the magnetron) as analogous to a transmission line in which the

electrical conductors

31 and 33 and the surrounding ferrite are considered as the center conductor thereof, an approximate characteristic impedance 2,, can be assigned. In the preferred embodiment of the invention this characteristic impedance is as low as feasible, preferably less than 50 ohms. An approximation in obtaining such impedance is that there is a mean perimeter such as represented by the dashed lines P in FIG. 4. This means perimeter is the average between the perimeter of the ferrite material and the perimeter of the inner surface of the metal container. In addition, a mean or average spacing s between the ferrite material and inner surface of the walls of the metal container is defined, disregarding the end ferrite slab 38 which is not a part of the cross-section. Accordingly, the impedance characteristic Z is approximated as 377 (S P). It is noted that in the instance where the metal container is formed such as to directly contact theenclosed ferrite material, then S reduces to approximately 0 and the perimeters of the ferrite, the container, and the mean perimeter P approach a common value for which there is no limiting impedance 2,. The addition of ferrite such as 38 at the end wall 50 reduces further 0,, or quality factor of the entire enclosed cavity.

In an actual construction of this embodiment of the invention the following dimensions for the container were chosen and are presented here by way of an example. At its widest end top and

bottom walls

55 and 57 of the filter box are 1% inches wide; at their narrow end these walls are /8 inch wide. In addition, the transition between wide to narrow width occurs at a location 1% inches from either end, and the length of the top and side walls is 3 inch overall. The height of the front and back walls is of an inch.

By contrast to the invention, one prior art construction had a filter box 1% inches deep by 4 inches in width and 5 inches in length. Such prior art filter box contained four 1 inch balun coils and two filter capacitors. In addition to requiring many difficult assembly operations, the costs were high and the results were barely equivalent to that obtained in the filter box of the invention. In another prior art construction the filter box was 1% inches deep by 4 inches in length and 3 inches in width, and incorporated therewithin inductive components. Not only is this of a larger size and of a relatively more expensive cost of manufacture, but the results were poorer by some 20 dbs, which increased the possibility of television interference and required the power supply enclosure to be very tight.

I construction is given. Head 39 cross-section is /16" X 9/16" so that its perimeter, P,, is [5/16 1r 5/16) I X 2] which reduces to 1,605 inches The height of the box is inches and its width is "/a of. an inch so its perimeter, P,,, is "/s "/a] or'3.25 inches. The mean perimeter, P, is approximately equal to the sum of P, and P b (1.605 3.250) =32 (4.855)

2.428 inches. To determine, S, the mean spacing as heretofore defined, the outer diameter of the ferrite bead 39 is the effective working surface, since, the dielectric constant, for the bead is 13.5. Thus S inch-the height of the boxless the height of the bead, 5/16 multiplied by k 0.219 inches. Thus, Z, 377 X 0.219/2.428 34.0 ohms. Note that if in' determining, S,-the width of the box and the widthof the ferrite bead were used to determine the average spacing a value of Z as defined, is obtained of 24ohms.

To determine the quality factor Q in this practical construction as herein defined and taught, the volume, V of the enclosed ferrite material is first determined and equals the sum of volumes of the three ferrites' 39a,

In operation a suitable source of filament current is supplied to electrical leads 3] and 33 and the latter conductors carry this current to the filament terminals of the magnetron. Should any microwave frequency energy be coupled from the cathode terminal into the stem area it is ideally absorbed by the ferrite material 39a through 39c and 38. The ferrite material thus serves to eliminate any high frequency energy which couples from the cathode to the filament leads 31 and ,33. In'addition, the entire combination of metal cavity and ferrite material constructed according to the preceding teachings present a load to both the fundamentaL'to any. harmonics of the fundamental, and to any subharmonics of the fundamental energy. This essentially loads down the tube and inasmuch as the characteristic impedance Z, of the unit considered as a transmission line is low,'.suitably below 50 ohms, any

' possibility of propagating beyond the filter box any so- 0.470 cubic inches. The volume of the container, V is according to the exemplary dimensions previously I V,,V, z 2.657/O.470 5.6

The measured attenuation for this unit was 60 dbm and the measured attenuation for the FCC limit on the same scale was 54 dbm.

The exploded view of the filter box illustrated in FIG. Sillustrates one method of assembly of the filter box of FIGS. 2-4. The insulation coated

electrical conductors

31 and 33 are relatively rigid and their

end portions

35 and 37 may be first formed to the desired shape. The

ferrite beads

39a, 39b, and 39s, are thereupon slipped over the insulated conductors. Additionally, the ferrite slab 38 is cemented in place to the end wall of the can or metal container. The heat shrinkable tubing is placed over the ferrite beads and shrunk in place to I form a good fit. Thereupon the ends'of the

wires

35 and 37 are attached to the ends of the magnetron, suitably by soldering, to form a firm electrical path and mechanical joint therewith. Next the can is moved down and the

wires

31 and 33 slipped into the passage in the can and the can is then placed over the entire assemblyand in abutment with the pole piece, not illustrated, of the magnetron. Subsequently, insulator 43 is slipped over the ends of electrical leads 3] and 33 and pushed into position within the opening in the can. Subsequently the can is welded to the pole piece of the magnetron to form a complete unit.

dissipated inthe air atmosphere surrounding the metal called fast modes is eliminated.

Another embodiment of the invention is illustrated in FIGS. 6, 7 and 8 showing the magnetron filter box from various views. For convenience, elements in FIGS. 6, 7, and'8, which find a corresponding element in the preceding'embodiment of the invention of FIGS. 2 through 5, are similarly identified with a corresponding numeral which is primed. Moreover, since each of those'elements has been discussed in connection with the preceding preferred embodiment of the invention and have the same'function and purpose, it would appear-redundant to repeat in detail that description which is already in the readers understanding. For description of those elements reference should be made to the preceding'description. For conciseness, therefore, the description of structure following is, in

I essence, a description of features different from that found in the preferred embodiment with some comparison between the two. 3 I

In this embodiment it is first noted that the container walls are essentially rectangular in shape. Accordingly, that portion of the metal container surrounding the ferrite beads, 39a through 39c, is spaced fromthose beads by a slightly larger distance and in this forms a basic transmission line, analogously, of a lossy characteristic impedance Z, slightly larger than that of the embodiment of FIGS. 2-4. In addition, slabs of ferrite material, 40a, 40b, 40c and 40d, are installed within the metal container and provide additional loss material. Ferrite slabs 40a and 40b are cemented or epoxied to front walls 53', and

ferrite slabs

40c and 40d are cemented or epoxied to back wall 54'. While it is possible to obtain the same volume of ferrite within the container byv simply increasing the

dimensions'of ferrite beads

39a, 39b and 39c, to do so would result in a design that requires ferrite beads of anonstandard size and hence of a substantially more expensive per unit volume cost. It is noted that the size and geometry of ferrite slabs 40a through 40d are such as is commercially available as standard items. An additional advantage of locating these standard ferrite elements 39' along the walls of the metal container accrues: any heat built up in the loss material through dissipation of microwave-energy is conducted through the metal wall on which they are attached and this heat is thereafter can. In other respects the design mode of operation and The width of the box is 1 inches wide. The length overall is 3 inches and the height is it of an inch. The results obtained with this embodiment are comparable with that of the preferred embodiment although numerically a larger number of elements are required and the overall volume of the box is larger with this construction. The ferrite pieces 40a, 40b, 40cand 40d located on the walls 53 and 54' are approximately 5/16 inches in thickness while the dimensions of ferrite beads 39 are the same as in the preceding example. Thus, althoughthe wall separation is actually greater than in the preceding example, and is 1 inches in width, the distance or spacing is the same. ThusLS" 2 (5/ I6) of an inch. Hence the calculation of effective impedance, Z, 377 (SIP) 33.51 ohms.

The 0,, in this example is determined as in the preceding case: V, volume of 7 ferrite beads 0.740 cubic inches. V 3" X 1%" X 54" 3.37 cubic inches. Thus Q V lV, 3.37/0.740 4.6. The measured value of attenuation was 59 dbm.

The embodiment of FIG. 9 represents a special case I stantially the same as the inner perimeter of the metal cavity. This embodiment includes the two spaced

electrical conductors

61 and 63 which are insulated from one another and are adapted to be connected to a suitable supply of filament current and high voltage. The conductors are first surrounded or packed with a lossy dielectric material such as Eccosorb CR8 or FDS, illustrated as element 65. In turn, the lossy dielectric is covered by a metal wrapping 67 which may be in the form of a metal tape. In addition, a metal container 69 is fitted over the ends of conductors 61 and 63and in electrical contact with the metal tape 67. Metal container 69 forms a dome which covers the stem section of the magnetron and serves a shielding function. As illustrated the magnetron filter box is mounted on a section of the pole .piece 71 of the magnetron, the pole piece serving as one of the electrically conductive walls of the filter box in the cavity otherwise formed by the domelike member 69. In turn, this wall effectively has a passage through which the magnetron stem portion is inserted and introduced within the filter box. FIG. 10a shows in cross-section a portion of the filter box which includes the

conductors

61 and 63, the packing 65 of electrically lossy dielectric material, and the underlying layer of metal tape 67. A modification of the foregoing is illustrated in FIG. 10b in which conductors 61' and 63' are first ensleeved by a dielectric material 73, and

this material is in turn covered by a heat shrinkable tube consisting of lossy material such as ferrite powder or both, within a large shielding metal enclosure, but no attempt was made to integrate the shielding enclosure with the dissipative networks to form a compact and more efficient magnetron filter box that comprised a fewer number of lower cost parts;

By the teachings of my invention the shielding is integrated into the simple compact arrangement of the I drawings with the dissipative material so as to form structure analogous both to a low Q microwave cavity and a lossy transmission line, and in this more efficiently eliminates the leakage of the fundamental frequency, leakage of subharmonics, and any fast modes which in the prior art structures were capable of circulating between the filament lines and the large cavity.

It is understood that the foregoing embodiments of the invention are presented as illustrative of the invention and not by way of limitation, since one skilled in the art, upon reviewing this specification, finds many equivalents and substitutions for the details of the elements illustrated and which substitutions or equivalents do not depart from the spirit and scope of the disclosed invention.

Accordingly, it is specifically understood that my invention is to be broadly construed within the spirit and scope of the appended claims.

What I claim is:

1. A combination of a magnetron and magnetron filter box comprising:

a magnetron which includes a body portion, said body portion having an elongated stemlike end, and said stemlike end containing cathode and filament terminals, a pole piece of ferrous material, said pole piece containing an opening therethrough, said body portion positioned with said stemlike end extending through said opening and projecting outward from the surface of said pole piece, and means preventing microwave energy leakage through said pole piece opening between said body portion and said pole piece;

first and second elongated electrical conductor means for carrying electrical filament current and a high cathode electrical voltage, said electrical conductor means being insulated electrically from one another and having first respective end portions adjacent one another and each connected to a respective one of the cathode and filament terminals of said microwave tube;

microwave dissipative material for dissipating microwave energy and comprising a predetermined volume, V,, said microwave dissipative material surrounding said first and second conductor means and extending therealong a major portion of the length thereof each to a point proximate said end portions of said conductor means;

microwave cavity means defining a predetermined region of volume V, for confining microwave energy therewithin, said cavity comprising walls of electrically conductive material and including as one wall thereof a portion of said pole piece surface, said microwave cavity including therein said electrical conductor means, said stemlike ends of said magnetron body and said microwave dissipative material;

, 15 means for providing a filament current and high voltage path to said conductor means from outside said cavity; and wherein the volume of said cavity, V and the volume of said dissipative material, V,, define a ratio of V to V,, which ratio comprises a quantity equal to or less than 20 whereby said microwave cavityis of a low Q factor electrical characteristic;

and i 1 wherein a predetermined impedance Z is defined as approximately equal to 377 (S P) ohms, wherein said relationship of S and P being such that said Z is equal to or less than 50 ohms, where S is defined as an average spacing distance between the outer surface of said dissipative material and the inner wall surfaces of said cavity means and where P is defined as amean perimeter length about the space between said dissipative material and said cavity walls,

I 2. The invention as defined in claim 1 further com-' prising a slab of microwave dissipative material located on a wall of said microwave cavity at a location adjacent said st'emlike end of said magnetron body portion.

3. The invention as defined in claim 2 further comprising a plurality of small holes less than 1/ th A in diameter located, in at least some walls of said microwave cavity to provide a path out of and into said cavity for air.

4. An; electromagnetic energy filter means comprismg: g

first and second elongated electrical conductor means forcarrying electrical filament current and a high cathode electrical voltage, said electrical conductor means being spaced and having end portions adjacent one another for connection to the respective terminals of a microwave tube; microwave dissipative 7 material for dissipating microwave energy, said microwave dissipative material surrounding said first and second conduc-- tor means and extending along a predetermined length thereof upto 'a point proximate said end portions of said conductor means, said dissipative 7 material further comprising a predetermined volume V,;

microwave cavity means for confining microwave energy within a predetermined region of volume V',, said cavity comprising walls of electrically conductive material, said cavity means including therewithin said microwave dissipative material, and said first and second conductor means, including said end portion of each said conductor means;

and wherein the ratio of cavity. volume, V,,, to dissipative material volume,,V,, comprises a quantity equal to or less than 100 to define a low Q cavity;

a first opening in a first wall of said cavity means, said first opening being-of sufficient size to permit positioning within saidcavity means of a terminal socket end of a microwave tube for permitting connection within said cavity means between terminals on said socket and the corresponding end portions of said conductor means and to permit I said microwave tube to close said first opening;

means for permitting application of filament voltage and high'voltage to said conductor means from outside'said cavity;

I and wherein an average spacing'distance is defined between the outer surface of said dissipative material and an inner surface of said cavity means which comprises a predetermined lengthS, and wherein a mean perimeter length, P, is defined about the space between said. dissipative material and said cavity walls, and wherein a predetermined impedance, Z,,, is defined as approximately equal to the quantity 377 8/? ohms, said relationship of S and P being such that said Z, is equal to or less than ohms.

5. The invention as defined in claim 4 wherein said electrical conductor means are'sufficiently spaced from said cavity walls to provide high voltage insulation therebetween.

6.- The invention as defined in claim 5 whereinsaid ratio of V /V, is equal to or less than 20.

7. The invention as defined inclaim 4 wherein said cavity means comprises the metal box open along one side, and a metal pole piece of a microwave tube closes said open side to complete said metal cavity, and wherein said tube socket extends through said pole piece into said cavity,

- 8. The invention as defined in claim 7 further comprising a plurality of additional openings in said metal box for permitting passage of air, each of said openings having as its longest dimension-a length equal to or less.

than the quantity 1/10.), where A is the wavelength of the operating frequency of the magnetron.

9. The invention as defined in claim 8 further comprising an additional quantity of microwave dissipative material located along an end wall of said cavity adjacent said first opening.

10. The invention as defined in claim 8 wherein said dissipative material further comprises a plurality of discrete portions arranged in side by side relationship,

each of' said portions having a passage therethrough and wherein said conductor extends through said passages for supporting said portions, and a single sheath of heat shrinkable polyvinyl tubing material extending over and covering all said portions to retain said discreet-portions in said side by side relationship.

' l1.-T he combination of a magnetron and magnetron filter box comprising:

a magnetron microwave tube for generating microwave frequency energy of a wavelength A; said magnetron microwave tube including a tube body, microwave passage means for permitting passage of generated microwave energy from said tube bodyto a load, and filament and high voltage terminal means'for coupling filament current and high voltage to within said tube body; low Q microwave cavity of a predetermined volume, V,, covering saidterminal means, said low Q microwave cavity including therewithin elongated electrical conductor means adapted for connection to an external source of filament current and high voltage for providing an electrically conductive path to said'terminals, and microwave dissipative material of a predetermined volume, V surrounding said conductor means along a substantial portion of the length of said conductor means for dissipating microwave energy exiting from said magnetron tube via said terminals, and wherein the outer surface of said-dissipative material'and the inner surface of said microwave cavity is separated by an average spacing distance of a predetermined length, S, and wherein a mean perimeter length, P, is defined about said space between said dissipative material and the surrounding walls of said cavity; and wherein a predetermined impedance, Z,,, is defined as approximately equal to the quantity 377 (SP) ohms,

, said relationship of S and P being such that Z. is

equal to or less than 50 ohms, and wherein the ratio V /V, is a quantity equal to or less than 100.

12. The invention as defined in claim 11 wherein said dissipative material further comprises a'plurality of discreet portions arranged in side by side relationship,

each of said portions having a passage therethrough and wherein said conductor extends through said passages for supporting said portions, and a single sheath of heat shrinkable polyvinyl tubing material extending over and covering all said portions to retain said discreet portions in said side by side relationship.

13. The invention as defined in claim 11 wherein said microwave cavity means includes a plurality of openings through the walls thereof to permit passage of cooling air into said cavity, each of said passages having as its longest dimension a length less than 1/1 0th A.

14. The invention as defined in claim 1 1 wherein said ratio of V to V, is approximately 20.

Claims

1. A combination of a magnetron and magnetron filter box comprising: a magnetron which includes a body portion, said body portion having an elongated stemlike end, and said stemlike end containing cathode and filament terminals, a pole piece of ferrous material, said pole piece containing an opening therethrough, said body portion positioned with said stemlike end extending through said opening and projecting outward from the surface of said pole piece, and means preventing microwave energy leakage through said pole piece opening between said body portion and said pole piece; first and second elongated electrical conductor means for carrying electrical filament current and a high cathode electrical voltage, said electrical conductor means being insulated electrically from one another and having first respective end portions adjacent one another and each connected to a respective one of the cathode and filament terminals of said microwave tube; microwave dissipative material for dissipating microwave energy and comprising a predetermined volume, Vl, said microwave dissipative material surrounding said first and second conductor means and extending therealong a major portion of the length thereof each to a point proximate said end portions of said conductor means; microwave cavity means defining a predetermined region of volume Vc for confining microwave energy therewithin, said cavity comprising walls of electrically conductive material and including as one wall thereof a portion of said pole piece surface, said microwave cavity including therein said electrical conductor means, said stemlike ends of said magnetron body and said microwave dissipative material; means for providing a filament current and high voltage path to said conductor means from outside said cavity; and wherein the volume of said cavity, Vc, and the volume of said dissipative material, Vl, define a ratio of Vc to Vl, which ratio comprises a quantity equal to or less than 20 whereby said microwave cavity is of a low Q factor electrical characteristic; and wherein a predetermined impedance Zo is defined as approximately equal to 377 (S divided by P) ohms, wherein said relationship of S and P being such that said Zo is equal to or less than 50 ohms, where S is defined as an average spacing distance between the outer surface of said dissipative material and the inner wall surfaces of said cavity means and where P is defined as a mean perimeter length about the space between said dissipative material and said cavity walls.

2. The invention as defined in claim 1 further comprising a slab of microwave dissipative material located on a wall of said microwave cavity at a location adjacent said stemlike end of said magnetron body portion.

3. The invention as defined in claim 2 further comprising a plurality of small holes less than 1/10th lambda in diameter located in at least some walls of said microwave cavity to provide a path out of and into said cavity for air.

4. An electromagnetic energy filter means comprising: first and second elongated electrical conductor means for carrying electrical filament current and a high cathode electrical voltage, said electrical conductor means being spaced and having end portions adjacent one another for connection to the respective terminals of a microwave tube; microwave dissipative material for dissipating microwave energy, said microwave dissipative material surrounding said first and second conductor means and extending along a predetermined length thereof up to a point proximate said end portions of said conductor means, said dissipative material further comprising a predetermined volume Vl; microwave cavity means for confining microwave energy within a predetermined region of volume Vc, said cavity comprising walls of electrically conductive material, said cavity means including therewithin said microwave dissipative material, and said first and second conductor means, including said end portion of each said conductor means; and wherein the ratio of cavity volume, Vc, to dissipative material volume, Vl, comprises a quantity equal to or less than 100 to define a low Q cavity; a first opening in a first wall of said cavity means, said first opening being of sufficient size to permit positioning within said cavity means of a terminal socket end of a microwave tube for permitting connection within said cavity means between terminals on said socket and the corresponding end portions of said conductor means and to permit said microwave tube to close said first opening; means for permitting application of filament voltage and high voltage to said conductor means from outside said cavity; and wherein an average spacing distance is defined between the outer surface of said dissipative material and an inner surface of said cavity means which comprises a predetermined length S, and wherein a mean perimeter length, P, is defined about the space between said dissipative material and said cavity walls, and wherein a predetermined impedance, Zo, is defined as approximately equal to the quantity 377 S/P ohms, said relationship of S and P being such that said Zo is equal to or less than 50 ohms.

5. The invention as defined in claim 4 wherein said electrical conductor means are sufficiently spaced from said cavity walls to provide high voltage insulation therebetween.

6. The invention as defined in claim 5 wherein said ratio of Vc/Vl is equal to or less than 20.

7. The invention as defined in claim 4 wherein said cavity means comprises the metal box open along one side, and a metal pole piece of a microwave tube closes said open side to complete said metal cavity, and wherein said tube socket extends through said pole piece into said cavity.

8. The invention as defined in claim 7 further comprising a plurality of additional openings in said metal box for permitting passage of air, each of said openings having as its longest dimension a length equal to or less than the quantity 1/10 lambda where lambda is the wavelength of the operating frequency of the magnetron.

10. The invention as defined in claim 8 wherein said dissipative material further comprises a plurality of discrete portions arranged in side by side relationship, each of said portions having a passage therethrough and wherein said conductor extends through said passages for supporting said portions, and a single sheath of heat shrinkable polyvinyl tubing material extending over and covering all said portions to retain said discreet portions in said side by side relationship.

11. The combination of a magnetron and magnetron filter box comprising: a magnetron microwave tube for generating microwave frequency energy of a wavelength lambda ; said magnetron microwave tube including a tube body, microwave passage means for permitting passage of generated microwave energy from said tube body to a load, and filament and high voltage terminal means for coupling filament current and high voltage to within said tube body; a low Q microwave cavity of a predetermined Volume, Vc, covering said terminal means, said low Q microwave cavity including therewithin elongated electrical conductor means adapted for connection to an external source of filament current and high voltage for providing an electrically conductive path to said terminals, and microwave dissipative material of a predetermined volume, Vl, surrounding said conductor means along a substantial portion of the length of said conductor means for dissipating microwave energy exiting from said magnetron tube via said terminals, and wherein the outer surface of said dissipative material and the inner surface of said microwave cavity is separated by an average spacing distance of a predetermined length, S, and wherein a mean perimeter length, P, is defined about said space between said dissipative material and the surrounding walls of said cavity; and wherein a predetermined impedance, Zo, is defined as approximately equal to the quantity 377 (S Divided by P) ohms, said relationship of S and P being such that Zo is equal to or less than 50 ohms, and wherein the ratio Vc/Vl is a quantity equal to or less than 100.

12. The invention as defined in claim 11 wherein said dissipative material further comprises a plurality of discreet portions arranged in side by side relationship, each of said portions having a passage therethrough and wherein said conductor extends through said passages for supporting said portions, and a single sheath of heat shrinkable polyvinyl tubing material extending over and covering all said portions to retain said discreet portions in said side by side relationship.

13. The invention as defined in claim 11 wherein said microwave cavity means includes a plurality of openings through the walls thereof to permit passage of cooling air into said cavity, each of said passages having as its longest dimension a length less than 1/10th lambda .

14. The invention as defined in claim 11 wherein said ratio of Vc to Vl is approximately 20.