Microwave oven feed system

A microwave oven feed system having the output probe of the magnetron inserted directly into the microwave enclosure. A rotating feed structure positioned within the enclosure couples the microwave energy from the probe into a directive radiation pattern towards the food. The feed structure may be located in a well extending from the enclosure and separated from the processing cavity by a layer of microwave transparent material, the functions of the layer being to provide thermal isolation and to provide a protective covering for the feed structure. A microwave choke around the periphery of the well prevents leakage of microwave energy from the enclosure. The choke may be elevated from the floor of the cavity to prevent food drippings and spilled soups from running into the feed structure well.

BACKGROUND OF THE INVENTION 
Two design objectives of a microwave oven are (1) that the energy 
distribution within the cavity be such as to provide uniform heating in 
food and (2) that there be an acceptable load impedance on the magnetron 
with any of a variety of food loads in the cavity. With regard to the 
second objective, an acceptable load impedance is one which will provide 
sufficient loading for the magnetron to prevent excessive anode heating 
without loading the magnetron so heavily that it will fail to oscillate at 
the correct frequency and shift to another mode. In other words, the 
magnetron should be coupled tightly enough so as to get good efficiency or 
maximum power output but loosely enough to give good frequency stability. 
The magnetron performance effects of impedance matches are well known and 
generally specified by magnetron manufactureres on Reike Diagrams. 
When microwave ovens were first introduced for food cooking and industrial 
processing, some models had the output probe of the magnetron inserted 
directly into the microwave enclosure. It was found that some improvement 
could be gained in heating uniformity by positioning a moving device 
commonly referred to as a mode stirrer in the enclosure. However, with the 
direct insertion configuration, little was done to provide the magnetron 
with an acceptable impedance load with a variety of food loads. 
Accordingly, it was common to have the magnetron operating inefficiently 
and/or with poor frequency stability. 
One way of providing an acceptable impedance match for the magnetron is to 
couple it into a waveguide; this has become the conventional microwave 
feed system. Typically, the output probe of the magnetron is inserted into 
a waveguide approximately one-quarter wavelength from a shorted end so 
that substantially all the microwave energy couples in the opposite 
direction. Generally, the end opposite the shorted end opens into the 
microwave enclosure. A mode stirrer means is commonly positioned in the 
waveguide or adjacent to it within the microwave enclosure. Coupling the 
magnetron output probe into a waveguide and the waveguide into the cavity 
provided for smaller impedance variations on the magnetron as a result of 
different food loads. 
The use of a waveguide external to the microwave cavity has several 
significant disadvantages. First, there is the cost of the waveguide that 
obviously must be included in the price of the oven. Second, there are 
microwave energy losses in the waveguide which reduce the efficiency of 
the system. Third, the coupling of microwave energy into the cavity from a 
waveguide to set up standing waves which are varied by a mode stirrer has 
not produced the most desirable uniformity in cooking. 
The elimination of the external waveguide creates many significant 
problems. For example, an acceptable impedance match must be provided for 
the magnetron for a variety of food loads. Also, uniformity of heating 
within the foods must be provided. Furthermore, if the microwave feed 
system is used in a combination oven which has an additional heat source 
for self-cleaning by pyrolysis, a means for isolating the magnetron from 
the self-cleaning temperatures must be provided. Also, there must be a 
method for sealing the feed system to prevent leakage of microwave energy. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a microwave feed system which 
eliminates the waveguide external to the cavity. Furthermore, it is an 
object to provide a feed system that provides heating uniformity within 
the food and at the same time provides an optimum matched load for the 
magnetron with a variety of food loads. More specifically, it is 
preferably that the optimum combination of power, efficiency and frequency 
stability be provided by properly matching the impedance of the magnetron 
for a variety of food loads. 
It is also an object of the invention to provide a microwave feed system 
that may be used in a combination microwave oven where cavity temperatures 
may be about 900.degree. F. in the self clean mode. Specifically, the feed 
system must isolate the magnetron from the self-cleaning temperatures. 
Also, it may be an objective of the invention to provide a choking 
structure to prevent microwave energy from leaking between the feed well 
and the floor of the microwave cavity. It is also an objective that the 
feed structure prevent food drippings or spilled soup from getting into 
the feed well. 
These and other objects and advantages will become apparent from the 
reading of the attached detailed description with reference to the 
drawings. The invention discloses an enclosure for exposing bodies to 
microwave energy comprising a plurality of metallic surfaces one of which 
has an aperture, a magnetron mounted external to the enclosure and having 
its output probe inserted through the aperture into the enclosure, means 
positioned adjacent to the output probe within the enclosure for coupling 
microwave energy from the output probe into a directive radiation pattern 
and means for rotating the coupling means about an axis passing through 
the probe. It may be preferable that the coupling means comprises a flat 
plate having a plurality of slots therein for transferring microwave 
energy from the output probe through the slots. Substantially all of the 
microwave energy introduced into the enclosure may be radiated from the 
slots or a similar plurality of antennas. The enclosure may be defined as 
a rectangular cross-section box having a cylinder extending from a 
circular orifice in one of the surfaces of the box. Furthermore, the means 
for rotating the coupling means may comprise air driven means. 
The invention may also be practiced by the combination of a microwave open 
cavity having an orifice in one of the surfaces of the cavity, a metallic 
cylinder attached to one of said surfaces extending outward from the 
orifice to a bottom having an aperture therein, a magnetron mounted 
external to the interior defined by the cylinder and the bottom with the 
magnetron having its output probe inserted through the aperture into the 
cylinder, and means for coupling microwave energy from the probe into a 
directive radiation pattern directed through the cylinder into the cavity. 
It may be preferable to provide means for rotating the coupling means. 
Also, a heating element may be positioned within the cavity. 
Furthermore, the invention may be disclosed by the combination of a cavity 
for exposing bodies to microwave energy comprising a plurality of metallic 
surfaces with the bottom surface having an orifice, a tunnel extending 
through the orifice into the cavity, the bottom surface around the tunnel 
having a raised portion comprising first and second surfaces parallel to 
the tunnel with at least a portion of the second surface being above the 
top of the first surface, the distance from the first surface to the 
tunnel being approximately one quarter wavelength of the energy, the 
distance from the second surface being less than one quarter inch from the 
tunnel. It may be preferable that the second surface have a plurality of 
vertical slots to prevent the transmission of energy in the peripheral 
direction around the second surface. Also, means for introducing microwave 
energy into the tunnel directed towards the cavity is provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a free standing combination 
microwave/electric range 6 which embodies the invention to advantage. The 
invention may also be embodied in a combination microwave/gas range or 
just a plain microwave oven. The range has conventional surface heating 
elements 82 and a control panel 84 for operating both the surface heating 
elements and the oven. Additional knobs would generally be provided for 
selecting the individual operation of microwave and electric heating with 
various power settings, cooking modes, and time controls. The oven has a 
heating element 80 positioned at the bottom of the cavity to provide heat 
for normal bake and self-cleaning. As is well known, self-cleaning by 
pyrolysis typically requires temperatures in the range from 880.degree. F. 
to 1100.degree. F. A second heating element 86 is spaced a short distance 
from the top of the oven and provides broiling. 
The source of microwave energy is microwave feed system 8 which embodies 
the invention. It is described in detail with reference to FIGS. 2 and 3. 
Range 6 includes many features such as, for example, door 88 which 
prevents the leakage of microwave energy, thermal insulation (not shown) 
in the walls, and a top vent for exhausting self-cleaning gases. These as 
well as other features are conventional and are, therefore, not described 
in detail herein. 
Referring to FIG. 2, there is shown a partially cut away elevation view of 
microwave feed system 8 including a microwave feed well 10 which is 
attached to the floor 12 of the microwave cavity 14. Along a circle with a 
radius of approximately 6.5 inches from a center in the middle of floor 
12, the floor is shaped upwards at a right angle 16 for approximately 0.5 
inches along surface 17 to another right angle bend 18 towards the center 
for 0.5 inches to an upward 45.degree. bend 20 for approximately one inch 
to a rounded 135.degree. bend 22 back down towards the floor for almost 
one inch along surface 23. The floor is fabricated of porcelain enameled 
steel and the bends so described forming a mound around circular hole 24 
are manufactured by stamping. Circular hole 24 is approximately 10 inches 
in diameter. 
In circular hole 24 is attached microwave feed well 10 which comprises a 
cylinder 26 with bottom disk 28 having a circular hole 30 in the center 
for insertion of output probe 32 of magnetron 34 which is connected to 
disk 28 by bolts 33. The inner edge of disk 28 is formed downward as shown 
in FIG. 2 to make contact with wire mesh gasket 36 of the magnetron 
thereby preventing leakage of microwave energy from well 10 toward the 
magnetron. Well 10 is secured to the floor of the oven by bracket 38 which 
is a circular plate that preferably is welded along its outer 
circumference to the bottom of floor 12. An approximately 10 inch 
concentric circular aperture is cut from bracket 38 and the inner edge is 
bent downward at a right angle as shown to provide a surface 39 through 
which rivets 40 connect well 10 to bracket 38. For a reason to be 
described later herein, only three rivets 40 are used around the 
circumference of cylinder 26 of well 10 to connect the same to bracket 38. 
Feed structure 50 couples the microwave energy from the magnetron output 
probe 32 into a directive radiation pattern that is not coaxial with the 
axis of rotation that will be described later herein. The feed structure 
first comprises flat plate 52 that has a circular planar surface 
approximately nine inches in diameter. A first slot 54a which is the 
closest to the geometric center has dimensions 3.times.1 inches with the 
length being perpendicular and centered on a first radii of the plate. The 
near side of the slot is approximately 0.69 inches from the center. A 
second slot 54b which is next closest to the geometric center has 
dimensions of 3.times.1.31 inches with the length being perpendicular and 
centered on a second radii of the plate. A third slot 54c which is 
farthest from the geometric center has dimensions of 4.times.0.95 inches 
with the length being perpendicular and centered on a third radii of the 
plate. The first, second and third radii are spaced 120.degree. apart. 
Feed structure 50 further comprises dish 56 which is connected to flat 
plate 52 by means such as a plurality of rivets or spot welds. Dish 56 is 
shaped so as to substantially form three separate waveguides from the axis 
of rotation at the output probe of the magnetron to the individual slots 
which function as antennas. The width of each waveguide is approximately 
four inches and each side runs inward until it intersects a side from the 
adjacent waveguide. The general form of the dish is shown by the dotted 
line in FIG. 3. Microwave energy is introduced into the feed structure 
cavity 58 formed by flat plate 52 and dish 56 by the magnetron output 
probe at the center junction or common excitation point. The energy 
travels outward through the three waveguides to the respective slots. At 
the slots, the energy couples into the well with the E field substantially 
altered by approximately 90.degree. during the transition from waveguide 
to free space. The microwave energy passes through cover 60 which is 
substantially transparent to microwave energy. It may be preferable that 
cover 60 be fabricated from Pyroceram material because it provides good 
thermal insulation. Feed structure 50 substantially provides a directional 
antenna and the pattern may be described according to conventional near 
field power pattern theory. In a heavily loaded oven, the energy 
distribution from the feed structure directly into the food can be likened 
to a system having no oven walls; this is substantially different than 
conventional coupling of microwave energy into the cavity through a 
waveguide with a mode stirrer positioned somewhere in the cavity to alter 
the modes as set up between the walls of the cavity. 
It has been found that very desirable heating characteristics are created 
by a feed structure which is rotated and which in a stationary position 
provides a directive radiation pattern which is not coaxial with the axis 
of rotation. The specific feed structure 50 described in detail earlier 
herein provides these desirable heating characteristics. It will be 
understood by those skilled in the art, however, that the particular 
details of the feed structure may be modified without departing from the 
inventive concept. For example, although three slots 54a-54c are shown, it 
may be preferable to provide a different number. It may also be preferable 
that the slots be positioned at different distances from the geometric 
center of the plate than shown and have different dimensions than 
described. It has been discovered that the radiation pattern becomes more 
directive when the number of slots is increased. Also, positioning the 
slots further from the geometric center of the flat plate generally 
contributes to making the pattern more directive. Although directivity in 
general is a desirable characteristic, there obviously is a limit to the 
amount of directivity that is desirable. Further, there are mechanical 
limitations as to the number of slots that can be provided. Also, the size 
of the flat plate limits the distance from the center at which the slots 
can be located. Generally speaking, the slots should be on the order of 
one quarter wavelength or less wide and greater than one half wavelength 
long. It is apparent that the amount of energy radiated from a particular 
slot is in part a function of the size of the slot and its position on the 
plate relative to the output probe. Furthermore, a plurality of antennas 
other than slot antennas could be used. 
As shown in FIG. 2, microwave energy is coupled to the three waveguides 
from a common excitation point by inserting the magnetron output probe 32 
directly into the feed structure. As mentioned earlier herein, it is 
advantageous that the feed system provide a distribution of power within 
the cavity which affects uniform cooking. With the feed structure shown in 
FIGS. 2 and 3, a very desirable pattern is radiated which, when rotated, 
provides uniform cooking. However, it is also advantageous that the feed 
system for coupling the output of the magnetron into the oven cavity 
provides an acceptable load impedance to the magnetron with any of a 
variety of food loads in the oven. As is known to those skilled in the 
art, this acceptable load impedance is one which provides sufficient 
loading for the magnetron to prevent excessive anode heating and yet does 
not load the magnetron so heavily that it will fail to oscillate at the 
specified frequency and shift to another mode. In other words, the 
magnetron must be coupled tightly enough to get good efficiency or maximum 
power output but loosely enough to give good frequency stability. The 
magnetron performance effects of impedance matches are well known and are 
generally specified by magnetron manufacturers on Reike Diagrams. 
In addition to providing support for feed structure 50 resting on output 
probe 32, bearing 62 also serves as a dielectric material for providing a 
desirable impedance match for the magnetron to the waveguide transitions. 
More specifically, bearing 62 provides capacitive loading between the 
output probe and flat plate 52 which is induced towards the instantaneous 
voltage potential of the output probe. The most preferable dimensions of 
bearing 62 depend on the material used, the particular magnetron model, 
and the feed structure. For example, in the preferred embodiment, bearing 
62 comprises Teflon or a similar synthetic resin polymer which has the 
additional advantages of being transparent to microwave energy and having 
a favorable coefficient of friction with the cap 63 of the output probe. 
The magnetron used in a demonstration model was a Hitachi Model 2M170 and 
the feed structure dimensions were as described earlier herein. For this 
example configuration, it was found that optimum coupling results were 
obtained using a bearing 62 having a one sixteenth to one eighth inch 
layer between flat plate 52 and the cap 63 of output probe 32. The outside 
diameter of the bearing cylinder encasing output probe 32 is 0.665 inches. 
Furthermore, it is preferable that the cylinder extend down over the 
output probe for at least one half inch to minimize feed structure 50 
wobbling while rotating in a horizontal plane. Bearing 62 is attached to 
flat plate 52 by pressing a circular projection of the soft Teflon through 
a slightly smaller circular hole in the center of the plate. The inside of 
the cylinder of the bearing fits snugly enough over the cap 63 of the 
output probe to provide support to prevent feed structure 50 from tilting 
from the horizontal plane; however, it is loose enough so as to minimize 
friction which would inhibit the rotation of the bearing over the cap. 
Blower 64 directs a stream of air across the fins (not shown) of the 
magnetron for cooling. The air is then channeled by duct 66 up to the 
bottom disk 28 of the well where it passes through angled perforation 68 
as shown into the well. Perforations 68 are small enough in diameter so as 
to prevent microwave energy from propagating from the interior of the well 
out. The air pressure created in the well interior by the introduction of 
air through perforations 68 causes air to be vented from the well in 
either or both of two preferable locations. First, because it is 
advantageous to circulate air through a microwave cavity while cooking to 
remove water vapor among other effluents, it may be preferable to direct 
air into microwave cavity 14. Gaps 70 are provided between the upper 
support surface of 135.degree. bend 22 and cover 60. Also, air passage 
space between the two surfaces may be provided by such means as bumps 
along the ridge of bend 22 or horizontal grooves in cover 60. It is 
advantageous not to have any vertical air passages from cavity 14 into 
well 10; drippings from cooking foods or spilled soup could pass through 
vertical passages and become deposited within well 10 causing undesirable 
effects. Second, air may be vented from well 10 through perforations 72 in 
cylinder 26. The function of perforations 72 may be to create an air flow 
path from perforations 68 which passes across blades 74 to accomplish air 
driven rotation of feed structure 50. Even if perforation 68 had not been 
angled and perforations 72 were not provided in cylinder 26, air driven 
rotation could still be created by the slight build up of air pressure 
underneath flat plate 52 and the outward movement across blades 74 which 
are angularly positioned from radial lines. Perforations 72 may also serve 
to decrease the pressure inside well 10 and thereby controllably reduce 
the amount of air flowing into cavity 14 through gaps 70. Vent 76 may be 
cut into duct 66 to reduce the amount of air passing across blades 74 
without reducing the required amount of air passing across the magnetron 
fins for cooling. 
As described earlier herein, flat plate 52 has a diameter of 9 inches. 
Although this dimension is not critical in the design, the flat surface 
was formed from an 11 inch diameter disk. A plurality of one inch slits 
were cut inward from the circumference of the disk along radial lines. 
Also, small notches were angularly cut from the inward ends of the slits 
so that the surface areas between the slits could be folded down and 
twisted at an angle to form blades 74. Teflon rivet 78 may be engaged to 
dish 56 so as to eliminate the possibility of feed structure 50 making 
contact with bottom disk 28 of well 10 caused by wobbling during rotation 
of the feed structure. Arcing is not considered to be a serious problem 
anyway because the voltage potential difference between dish 56 near disk 
28 is very small. Furthermore, other steps were taken to insure that feed 
structure 50 remains in a horizontal plane during rotation. As described 
earlier herein, the cylinder of bearing 62 preferably extends downward 
over cap 63 for at least one half inch to provide stability. Also, weights 
(not shown) may be attached to feed structure 50 to compensate for the 
unbalance caused by the nonsymmetric dish. 
The combination of the cavity floor 12 portion formed by bends 16, 18, 20 
and 22, the upper portion of cylinder 26 of well 10 and bracket 38 form a 
microwave choke which prevents microwave leakage from the region between 
the cavity floor and the well. Specifically, the distance between cylinder 
26 and surface 17 is one quarter wavelength of the microwave energy. 
According to well known guide stub theory, energy attempting to propagate 
between cylinder 26 and surface 23 of the cavity floor 12 sees the 
reflection from surface 17 and the resulting high impedance. Thin 
vertical, rectangular sections 25 are cut around the periphery of surface 
23 to form gaps which substantially prevent the propagation of energy in a 
peripheral mode around surface 23. The general technique and theory of 
this type choke is taught in U.S. Pat. No. 3,767,884 to Osepchuk, assigned 
to the same assignee herein, which patent is hereby incorporated by 
reference. It is preferable that the spacing between surface 23 and 
cylinder 26 is one eighth of an inch .+-.one sixteenth of an inch. 
Further, it is preferable that surface 23 be parallel in a vertical 
direction to cylinder 26 for a distance of at least one half inch. 
The raised choke structure formed by bends 16, 18, 20, and 22 also prevents 
food drippings and spilled soups from running along the cavity floor down 
into the well to create cleaning problems. In an alternate embodiment of 
the choke structure, however, the floor 12 of cavity 14 is not raised. 
Rather, the upper edge of cylinder 26 is bent outward, positioned down 
against the floor 12 of the cavity and then riveted or spot welded around 
the periphery at a spacing of not greater than 1.5 inches to create the 
seal. 
As described earlier herein, the microwave feed well 10 may be used to 
advantage in a combination microwave oven wherein a second heat source 
such as conventional electric or gas is used. For example, electric 
heating element 80 provides normal bake and self-cleaning heat for the 
oven. For self-cleaning pyrolysis, cavity temperatures conventionally must 
rise to the range from 880.degree. to 1100.degree. F. As typical 
magnetrons in use today may be damaged if heated above 500.degree. F., a 
means of thermally isolating the magnetron from the high temperatures in 
the cavity is required. First, the porcelain enamel walls exhibit some 
thermal insulation. Second, as mentioned earlier herein, only three rivets 
40 were used around the circumference of cylinder 26 of well 10 to attach 
it to bracket 38. These poor thermal joints substantially reduce the 
conduction of heat from floor 12 to well 10 through bracket 38. Third, 
Pyroceram cover 60 which provides protective covering for well 10 also 
serves as a good heat insulator. The cover is held in place by clips 81. 
It was also found that the air space between cover 60 and flat plate 52 
provided some thermal insulation and that making the well deeper provided 
more thermal insulation. However, it was observed that making the distance 
between cover 60 and flat plate 52 more than three inches adversely 
affected the power distribution within the cavity. It may be preferable to 
position a layer of insulation between cover 60 and flat plate 52. 
Furthermore, in the self-cleaning mode, it is desirable to have a flow of 
air through the cavity by chimney effect to remove the gaseous by-products 
of pyrolysis. During this operation, even without the blower being turned 
on, there is a natural flow of air up through well 10 into cavity 14 to 
provide further thermal isolation of the well bottom and magnetron. 
This completes the description of the preferred embodiment. However, it is 
understood, that those of ordinary skill in the art will see many 
modifications and alterations without departing from the spirit and scope 
of the invention. Therefore, it is intended that the scope of the 
invention be limited only by the appended claims.