Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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.
,, .,,. . , . -
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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
~,
.
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.
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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.
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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;
.
. :.
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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
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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. .
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.
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.
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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. - ~ ~
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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
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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.
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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.
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