CA1114456A - Dual feed, horizontally polarized microwave oven - Google Patents

Abstract

DUAL FEED, HORIZONTALLY POLARIZED
MICROWAVE OVEN

A B S T R A C T
A microwave oven providing for the introduction of microwave energy from a magnetron into the oven cavity from each of two opposite walls is disclosed. The oven includes a waveguide and feed box arrangement which allows for optimum power coupling and simplified magnetron impedance matching to provide an oven having both good efficiency and cooking pattern.

Description

B}~CICGROUMD OF THE INVENTION

This applqcation i9 a division of Can~dian Application 296,458, filed February 8, 1978.
This invention relates to electrvnic cooklrlg oven3 ln general, and more speciflcally to domestic cooklng appllances for cooking foods by the applicatio~l of ~nergy itl tlle mlcro-wave frequency range. The use of such ovens l)as ~ecome increas-ingly widesprea~ largely due to the speed Or cooking offered over conventional techniques, Microwave ovens have heretofore had some character-istics of cooking performance that were less tllan satisfactory,expecially in the area o~ the eveness oE tlle energy pa~tcrn throughout the oven cavity and it i8 well-known that ~lcrowave ovens frequently exhibit a pattern of "hot spots" or "cold spots" within the oven.
There are many causes of uneven cooking patterns and performance, and in subs-tantial part such pa-tterns are determined by the method in which microwave energy is intro-duced into the cavity. Early ovens provided coaxial antenna proJecting through the wall of the oven into the coo~ing cavity.
Other arrangements provided for slotted waveguides which trans-mitted the energy from the magnetron to the cavity. Still other arrangements coupled the energy into a feed ~ox, or intermediate zone between the waveguide and the cavity ~Id added some type of rotating, energy re~lecting device to aid in breaking up standing wave pattern~.
It is common practice to use such a stirrer device in the oven cavity itself or in a feed box to change the number of modes present during an interval when food ~s ~oin~
~eated. A single stationary mode in a microwavc oven cnv1ly will exhibit itsel~ as alternate hot spots and cold SpO ~9 in the heated food.

,, .,,. . , . -- , . ,. , . : . , , . ~, . . : .

4~

e }lO'~ spo~s are about ~.5 inch~?s apclrt in arl ovon oi~e~ tin(~

2 at 2ll50 MIIZ. 'l`~le p~lrpose of the stirrer :is to attempt to shif't3 tile positloll of tlle }10 t spo-ts by changing the phase rela-l;ion-4 ship of the ~/aves that comblrle to ~orm the slngle stationnry mode.
6 While these -tec]-ln;~lues have providecl solne irnprove-7 ment, ideal performance has not 'been achi-!ved. Moreover, 8 because of -these limi-tations "nicrowave ovcns have largely 9 been limited to cooking one type of Loodstuff a-t a t:iMe.
The inven-tion disclosed herein establishes new li -techniques for overcoming many of -the performance lilnitatiorls 12 of prior art microwave ovens, especially as -those lin~itations 13 involve eveness of coolcing pa-ttern, magnetron to wavegulde 14 to cavi-ty impedance matching, power coup'ling efficiency, and the like. Beneficial use has been made of certain microwave 16 characteristics and techniques whic}l, although Icnown in other 17 fields of microwave technology, are considered undesirable 18 in those fields. One such characteristic which is known in 19 the fields of radar and long distance microwave communica-tions is ealled the 'llong lines effect", and equipment used in -those 21 fields is generally designed and eonstructed to eliminate 22 the effect insofar as possible. X have discovered that those 23 same effeets ean be intentionally designed i~nto microwave 24 ovens to produce suprisin~ly 5uperior r~sults.
The oven disclosed herein also departs from the '' .
26 conventions of -the indllstry and supplies energy from the 27 ma~ne-tron into the coolcing eavity throu~h eaeh of two opposite 28 walls, ra-ther than from either the top or bottom of the cavity 29 as is almost univers.ll]y the case with ovens sol(l today. As a r~sult, the cavity is horizontally rather than ver-tically ~, .

.
' :

polarized and the lnterference attributed to the food load in the oven is greatly reduced.
SUMMARY OF THE INV~NTION
The present invention provides an oven cavity wavequide-feed box combination which allows for the introduction of microwave energy to the cavity through each of two opposite side walls. The waveguide is configured to allow for optimal coupling of energy from the magnetron and optimal matching of the waveguide to the cavity with a minimum of tunihg adJustments.
The invention relates to a microwave oven for cooking foodstuffs including a cooking chamber, generating means providing microwave energy to the chamber, waveguide means for conducting the energy and having a plurality of energy exit ports wherein the output of the generating means is introduced into the waveguide at a point at least two guide wavelengths from at least one of the exit ports.

b ' ' ' `
.~:

' '-:. : .',,, ',: . '' . :' .: :: :, ', ':' . .,.' .. ' ' ," ' , '' ' .: ' .' ' . ' : :', ' '~ ' . . . ' ' .. ' "''' ' ': ''. ' . ' . . : ' ' In one embodiment a box-like oven cavity has a cut-away window portion on each of two opposite side walls. A feed box is mounted to each side wall ad~acent the windows, each feed box communicating directly with a waveguide. The waveguide in turn lies along the top surface of the cavity and extends outwar~ly of one of the feed boxes. A backwall is provided at the end of the outward extension and the magnetron is mounted to allow its antenna to extend into the waveguide in proximity to said back wall. A single tuning stub is also provided in the waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a front view in elevatlon of a microwave oven incorporating the design of the present invention wherein the view has been partly cut-away to expose interior components of the oven;

.

. :.

-mbl~ - 3a -1 ~igure 2 :is ~ si~c view of thc~ ovcn of Fi.eulc :l.
2 ln cross secti.on takcn a'l,ong l,ine 2-~ in Figure l;

3 Figure 3 is a top plan vie~/ of the oven of F'igure 1

4 par-tially cu-t a~Jay to expose in-tcrior e].ements;
Figure 4 is an illustra-tion of the display of a 6 spectrum analyzer sl~owi.ng operating charac:t~ri,stics of a 7 prior art oven;
~ Figure 5 is an illustrat;ion of the di~;pl.3y of a 9 spectrum analyzer showing operatine cllaractelis-tics of the oven incorporating thc present inven-tion.

DESCRIPTION OF PR~FERR~D EMBODI~N'rS

11 The essential configuration of -the oven incorporating 12 the features of the presen-t inven-tion can be readily secn in 13 Figures 1 - ~ in which the numeral 10 denotes the ov~n generally 14 which includes an outer cabine-t or wrap 11, a cooking cavity 12, a waveguide 13 for directing microwave energy into a pair of 16 feed boxes 14 mounted on each side of the cavity 12. A magne-17 tron 17 is mounted to the waveguide 13 having its antenna 18 portion lB extending into the waveguide in the near vicini-ty 19 of the back wall 19 of the waveguide.
As the drawings illustrate, the energy emitted by 21 the magnetron 17 travels throuFh waveguide 13 entering the 22 feed boxes 14 through ports 20 and 21. Al-though port 20 23 is located relatively noar magnetron 17 as in prior art designs, 24 port 21 is spaced a much greater distance away. A major por-tion of the energy travels through waveguide 1~, port 21, 26 feedbox 14 and into cavity 12 wherein some of the energy is 27 transmitted or refle~ted back to the magnetron along the same . ' ' .

' ~ .
- . ... ........

1 lerl~tl-ly l~at]~ or tllrou~h ti~o feo(l box 1l~ i.m~l ~)ort 20 I)"~k ;ll~o 2 thc wavoc~ ide. In either case, a subslclnt:ial port:ion Or the 3 enerey trnvels a relative].y long I)ath bcforc arri.ving back 4 to the m~gnetron 1'7. 'l'hi~; is be:licved to gi.ve rise to a "lon~ lines e:Cfect" as clescribecl more fully hereinarter.
6 A stirrer 16 is mountecl in each of tile feed boxcs 7 14, each stirrer bcing driven ln ro-tary mo-tion by a ~lotor ]5.
8 The s-tirrers serve to ran~omly ref]ect and mi~ the incoming 9 energy to change the phase relationship of the ener~y w~ves to assist in avoiding the formation of a single stati.onary 11 mode within cavity 12 12 Additional ways in which more uniform dis~ribution 13 of microwave energy can be achievecl in the cavity include 14 changing the frequency of the incoming microwave energy, changi~g the phase coherence of the wave, and changing the 16 amplitude of the wave in a random mamler. ~nfortunately, 17 it has not been heretofore known how to accomplish ~Iny or 18 all of these changes in an effective, commercially practical 19 manner. On the other hand, the use of microwave energy in the fields of radar and com~unications attempts to hold these 21 changes to a minimum. It is believed that the changes may 22 be caused by the so ealled "long lines effect". In the presen-t 23 inven-tion the long lines effect is purposely enhanced to im-24 prove the heating and cooking pattern, and to increase the efficiency of the magnetron. ' 26 While it is believed that the improved results 27 observed in the present design are in major part attributable ~
28 to the beneficial use of the long lines effect, it will be ..
29 understood ~hat the ~recise causes of energy distribution patterns are difficul-t to identify. The invention doscribed ~ , ~ . ~5~
~ .
: ' .

~ .
:

~ ~ ~ : - ;',, ' - . ,: , : :

: ' .

1 ai~d c].aimecl~lerein shou].d not ~e viewcd a; li.mite~ to e ~ro-2 cise theory O:r opera-t:i.on although cvery erfort has been Inade 3 to identiry and c~:plai.rl its tlloory of operation for the benc-4 Iit of worl~ers in the art.
General di-;cuss;.on of the causes an(l effcct~ of the 6 lon~ lines effect can bc found in tile pal~er "Lone-:l.incs i.r.lcct 7 and Pulsed Magnetrons" by W. L. Pritchard ia the IRE Trans-8 acti.fns on M.icrowave Theory and Techni.tll~es MTT-/-I, No 2, _ . , .
9 1956 pp. 97-110. A slightly revisecl version of the a~ovc paper is found in the boolc Crossed - Field M.icrowave ~c~i.ces, 11 E. Okness ed3.tor, ACaCICllliC Press, 1961, s-tarting at p. ~2-~.
12 The long line~ effect can occur in a hollow wave-13 guide because it is a di.spersive circuit eleme~t, which is 14 defined as a circuit element in which the phase velocity changes with frequency. E'or example, in a s-tandard ~ized ~fR~ 340 16 waveguide the phase velocity is such that a wavelength at 17 2425 MHZ is 17.5113 centimeters while at 2475 ~Z a wavelength 18 is 16.9917 centimeters. Accordingly, for a 360 phase change 19 at 2425 MHZ there is an additional 11.0 change in phase at 2475 M~IZ. In a ten wa~elength section of guide a reflected 21 wave at 2475 MHZ will be 220.0 out of phase with a wave at 22 2L~25 ~IZ.
23 The phase change effects the opera-tion of the mag-24 netron. For e~ample, the efficiency can be calculated as Z5 follows:
26 (1) Efficiency =
1 + 0.~17 fo/PQo 27 Where Qo is the unloaded quality factor of the magne-tron, P
28 is the pulling figure, and fo is the center frequency of :
29 oscillation. .
' ' `

:

.

1 'l'lle tcrln f' or ~)ulJ.in~ fiGure is tl~e lo~ xcur.;ion o.f t)~e 2 flctl~cnc.y, :f, w~en the vo:Ltalr,e s-tancling wave ratio (VS~Itl) 3 i3 e~lual to 1.5 a1ld i5 varied throllc~ a].l possi~)le phasc:,.
For a stand:inlr wave rati.o Or 1 . 5 about /Il)elcorlt of translllitte(l I)owel is rcflected ~c-lck to tlle n~aGnetron. With thj.s anlount 6 of reLlec-te(l power the pulling figure i.s:
7 (2) P -- Lz - r.~ - 0-/~]-7 f/Qe 8 Wherein :Lc is the maxillm11n fre(luerlcy~ r~ ~ i3 the 9 n~i.nil~uDl, fo is the center frequoncy as ~bove, alld QQ i.s -the qu~lity factor o:E the external circuit.
11 Note in equation (].) that as -the product PQo bec~ )es 12 larger the elficiency increases. The ef:Eiciency is also 13 calculated to be:
14 (~) Efficiency =
'' 1 ~ Qe/Uo From equation (3) it is seen that when the quality.
16 factor of the ex-ternal circuit is low and the unloaded quali-ty 17 fac-tor is high the efficiency will be high.
18 The quality factor of a circuit is equal to:
19 ~(4) Q = 2~ x ener~y stored energy dissipated per cycle . 20 The ener~y reflected back to the ~agnetron can cause : 21 the magnetron to start oscillating at another frequency. If 22 the amount of reflected power is high enough, that is the VSWR
23 is high, the magnetron will oscillate at two frequencies.at 24 the same time. The spread between -the two frequencies is calculated from -the equation:
2~ (5) ~ f = _ ~ . c :- 2 1~ g 27 Where 1 is the length of the waveguide,~ is the 28 wave length in free space, ~g is the wavelength in the wave-.
29 qulde, and c i~ the speed of light.

~7~

:. ~ . ~ .

: .-: : , :. . : : : , : :

lt (,all bc` :0en ~ t tllC I.()nlrOr Ih6 ~.IVeL~U:j~O tl1e sm~1]1er~ ti1C
2 diLl`crc1lcc in tht~ two frequ~l1cie- will be 3 Accor~inlr]y, i~ can ~c scen froln the ~recc~ir1~
4 discussion ~31at t3~e p~llling ligurc P is increased by ]o~orinG

5 -the qua]ity f~ctor Q o:f t11e ex-~ernal circuit or cal/ity. A

6 larger pulling figure in turn increases t11e ofriciency an~l

7 provides for the gencration of ~di-tional frec;uer1cies by tl1e

8 magnetron. The long lines ef~ect is in turn ~1isper~ive cal1sirlg

9 a change in phase rela-tio1lshirJ betwcen waves of dirleren~
lO wavelengths as they arrive baclc at the magnetron. Moreover, ll two or more frequencies are generated when -the VSWI~ excee~1s lZ about l.5 beeause O:r the long lines effect.
13 In dornestic microwave ovens in use today it is lL~ common practice to either introdl1ce the microwave energy di-15 rectly into the cavity without a waveguide or to use a relatively 16 shor-t section of waveguide ex-tending from one side Or the oven 17 to the center of the -top or bottom wall. The waveguides 18 in common use are approxin1ately lO - 20 cm in length. For l9 a standard WR-340 waveguide at 2l~50 ~-IZ one wavelength is about 20 17.3 cm. Thus it can be seen that such waveguides are about 21 one wavelength in leng-th or often somewhat less than one wave-22 lengt}1.
23 It i5 also known -that the load placed in the ovcn 24 will effect the output frequency of the magnetron) variations 25 f ~ 5 MHZ being common A~plying the equations shown above, .
26 it can be calculated that the change in phase between 2~l45 MHZ
27 and 2455 M~1Z, assuming 2450 MHZ as the center frequency, will ;~ ~ Z8 be about three degrees in one wavelength of waveguide. Ac-29 cordingly, a~rerlected wave at 2455 MHZ would arrive back at ;~ ~ 30 the magnetron about six degrees ou-t of phase with a 2445 ~lZ
` 31 ~ wave. - ~ ~
:, .: ~ , . ..
; : , " .,:
: :

: .- . - " ~ . ., : . - . :,, : , : .

- l$~

1 Thls pl~ase di:rler(~ ifll. i.s no-t ~surf.i.cierlt to hclve a S.i.g~
2 ficant:Ly measurabLe i.mpact on the ener6y d:ispersion pattern 3 or cookirlg patte]r) in the overl.
4 The oven describe(l herein s-tancls in rather s-tri.k:irlg conl:rast. The len~th.of -the waveguide l.~ frorn thc ~ntclln;.l 1 6 of -the magnetron to -the por~ ;'1 i5 a~out Go cm or aboul ~,l 7 gui.die wavelcngths in a WR-~40 waveguiclc.
8 Applying -the samc ecluat.ions as in the casc of lhe 9 15 cm waveguide tl:iscus.ed a~ove, it can be seen th.ll tl-lcrc wou].d exist about a twcnty-follr degree phase cliffererlce at 11 the mag~lletron between a wave at 2445 M~IZ and one at Z~55 ~
12 In addition, since micro~ave energy can enter and leave the 13 oven cavity 12 through cither of the two feed bo~ces 14 thcreby14 creating a number of potential pa-ths of travel, and because of the presence of -the stirrcrs 16 in each feed box, it is 16 estimated that at least five to six additional degrees of 17 phase shift may take place.
18 The large phase shift thus created in the present 19 oven ~rovides favorable conditions for a more dispersed energy pattern in the cavity. While the litera-ture dealing with 21 radar applications suggest that long lines effects appear in 22 waveguides of ten guide wavelengthis or more, the present in-23 vention indica-tes that at least in the microwave oven closed 24 circuit envirollment the long lines effec-t begins to have impact in a waveguide of at least three guide wavelengths in lcngth.
26 The result of these effects upon the energy di.~-tri-27 bution in the cavity is illustrated in Figures 4 and 5.
28 Microwave oven energy pat-terns can be accurately and graph-29 ically measured using a spectrum analyzer which measures fre- -.
quency and power and converts it to an oscilloscope display 31 in which frequency is plotted along the abscissa and voltage : ~ .
~ . 9_ ~.
~. . .

': ' ' , . ,:,, .. ., . . . , . , . .. :. . :-.. , :. , . . , . :
- , . ~ ~ , . . , .. , : . . :
:- , : . , : i , . -:' . . i ' ' ' : ~ . . :
. ~ . : : , ; . .:
" .:' , . ' ' . :'. ' :' l i.n doci~els alol~ the ordinate. The vo].ta~re i.n dec~ eli :i.,;
2 a lo~arithm:;.c rat:io of the meclsllred voltclt~e at e~ch l're(luency 3 to a presel.ecte~ base vol-ta~,e. Polaroi.cl ~ otograph~ of the l~ scope prescnt;ation were madc from which t~lC i].lustrati.or1s in Fi~ures 4 and 5 werc ~r(?parc(1.
6 The arc.1 oL ~rcatest inl.eres~ frolll tl1e st~ po~ t 7 of ener~y distri.b~ltion patt;crn and hencc coolci.nG patt,crn is ~ tl1at lyin~ betwecn t}-1e ~10 d~ an(l 50 db :L:t.ncs. The ~at,tcrn g in Figure 4 wa~ obta:ined us:in~ a Mo~lcl ~]5 mi.crowave ovcn manufactured by I,it-ton Sys-tems, Inc. cmd having a WR~ O
ll waveguide about 15 cm in len~th. The pa t tcrn ShOWII in ~ u~e 5 12 was obtained USill~ an oven as described hereln. As can be 13 seen, the pattern ln Figure 4 is largely concentra-ted in the l4 frequency range frorn about 2452 MHZ to about 2466 M~IZ, wi-th the tr~lly significallt concen-tration in the range of about 16 2462 to about 2466, a very narrow band. This indicates the 17 exis-tance of a single dominant mode and wave pattern.
18 On the other hand, the pattern shown in Figure 5 l9 is spread much more broadly with significant power levels found in the range from about 2430 M~IZ to about 2464 MHZ.
21 This indicates multiple significant wave patterns in the oven 22 cavity and hence a much improved cooking pattern as compared 23 with tha-t shown in F`igure 4. The long lines effec-t has caused 24 a "srnearing" of the power across a frequency bandj an unde-sirable result in radar and comrnunications applications, but 26 a result sought after ~.n microwave coolsing, 27 In the structure of the present invention -the side-28 walls 23 of the cavity 12 each contain a large cut-away portion 29 or window 24... The feed boxes 14 are attached to the e~terior.
surface of the side walls to colncide with the windows 24.
31 Each window is preferabl~- covered wi-th a microwave transparent 32 cover 29, ::
:

.. . ...

1`1-e bot,'.om wal~. 27 oî the cavi ty 12 is lorme~l to 2 provide a ledee 3() or. which she~f c"B :i.s sul)E~ortc~, 'I'hc s1lolf 2 3 is prerera~ly ml~]e from a glass or celc~ le l~at~licll WiliCIl lf;
tr~nsl)arellt to micr(>wlve energy. Sincc cnerGy cn ter g tl~c c~vil.y ~l~rougl~ the si~e walls 23 l~at~ler tllall IhlouLrll the t~
6 i,lle tol wal1 2(, i s substanlially flut al-l(l so]l./l. Tll(3 OVell .i';
7 ol cour se provi~led wi l;l-~ a cloor 25 wllicll ca~ e cl oscd OVel' l,l~e - ~ front o~ -tlle cavity during cooking.
9 ln order to effectively colli)Je tlle OU~ llt ~OWel' of the magnetron 17 to the cavity 12 thlougll llle wavegui(le 13 11 it is neeessary to matcll the wclveguide iolp~darlce to tl~e inl~)edallce 12 of the cavity. While this ean be done to some extent throulrh 13 design dim~nsions oth~r measures are necessary to aceomp:Lish 1/l "fine tuning". Stub 22 is provided for this purpose, exl.onding into the wa~eguide at a preselected point. Again using the 16 spectrum analyzer, the optimum position for -the stu~ 22 Elnd 17 its length can be determined. The present design allows the 18 tuning operation to be carried out with a single stub even 19 thouGh energy is fed into the eavity from two feed boxes.
In prior art designs eaeh feed entrance has generally requlred 21 a separate tuning stu~.
22 The oven illustrated herein has been eonfigured 2} for operation at 2450 ~-IZ although the same prineiples app~y 24 to ove~ls sized for operation at a different au-thorized ire~luencyO
For optimu~ operation, ~he waveguide 13 is approximately 25.5 26 inches in length with the cen-ter line of the magnetron spaced approximately 0~7 inches from the back wall 19 of the waveguide.
28 The stùb 2~ is preferably located approximately 20.875 inches 29 ~rom the wave guide baek wall 19. The waveguide 13 in cross-seetion is approximately 1.7 inches high and 3.4 inehes wide.

. .
... , ~ .
' - . .: , - - , ~ .. . .

,: : : . , . '. . ' . . ' ~
.. .. . . : . , ;, - , :.. , .: : . :
. .

14~S6 It i3 plcferrccl to f~r~ the cav.lty 12, the ~eed bo~e~ 14 and the wavegu~de 1~ int~ a un:Ltary weldcd sl;bar;scml~ly to w~ich the rem~i.nirlg operating comporlerlls rnay he udde(l, arld the cntire struc-ture housed wl-thirl cab:inc~ 11.. It i.s furt.ller preferred to h~ve ledge 30 froln about 0.5 to 0.75 illChC5 above bottom wfll.l. 27 in orcler to locatc flat or ].ow prufile food items, sucl- as bacon strips, withill ~1l al'C.I Or' }ljtrh energy field strength.
While in the foregoing speci.ficati~rl tlle inve1)lior lias beerl descli~ed .in great detail, it will ~e un~rst~od that 5uch detail is intended to be illustl.allve and tha-t modifications can ~e made by -those skilled in the art wi~houl departing from the scope of the invention as defined in the appended claims.
Certain aspects of the foregoina description are disclosed and clairned in Canadian application 296,458, filed February 8, 1978 to the assignee of the present application.

.

, . ,,~ , ' .'.... : ,~ ; ' : :: . ~: '. ' ' . ,, :
-: . : ~ : . : . ' ; : - . . . . : ' :

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A microwave oven for cooking foodstuffs including a cooking chamber, generating means providing microwave energy to said chamber, waveguide means for conducting said energy and having a plurality of energy exit ports wherein the output of said generating means is introduced into said waveguide at a point at least two guide wavelengths from at least one of said exit ports.

2. An oven according to claim 1 having energy receiving means communicating with each of two opposite side walls of said cooking chamber, said exit ports communicating with said energy receiving means.

3. The oven according to claim 1 including energy stirrer means positioned in the energy path between said generating means and said cooking chamber, said stirrer means being rotated within said energy path.

4. The oven according to claim 2 wherein said energy receiving means comprise an energy reflective compartment mounted adjacent each of said opposite side walls adapted to receive energy from said waveguide means through said exit ports.

5. The oven according to claim 4 including energy stirrer means rotatably mounted in said compartments.