Note: Descriptions are shown in the official language in which they were submitted.
1 330066
SUSCEPTOR IN COMBINATION WITH
GRID FOR MICROWAVE OVEN PACKAGE
s
Microwave cooking often offers advantages of speed
and convenience in heating foods. However, in the past,
microwave cooking has been unsatisfactory for a number of
food products. Because microwave cooking relies upon the
dielectric heating of foods responsive to microwave radi-
ation, the heating characteristics in a microwave oven for
some food products is dramatically different from that
experienced in a conventional oven. Problems with micro-
wave cooking for a number of food products include the
problem of undesirable temperature differentials. Often-
times, food products cooked in a microwave oven will heat
more in ~he interior due to dielectric heating caused by
microwave radiation, than at the surface. This is
directly contrary to the temperature differential achieved
in a conventional oven, which is oftentimes desirable for
foods which require a crisp surface or brown crust in
order to have desirable taste characteristics. An addi-
tional problem with microwave cooking is that necessarytemperatures for browning and crisping of the surface of
food products have not been achieved. This is an old
1 330066
--2--
problem in the art, and many attempts have been made to
solve it.
A related problem is the problem of moisture
differentials which result in moisture migrating in an
undesirable manner in a food product. Oftenti~es, instead
of evaporating or migrating from the surface of the food
to the center of the food product, moisture will migrate
the wrong way, i.e., from the center of the food product
t~ the surface during microwave cooking. This has the
effect of leaving the food surface soggy, which is
oftentimes undesirable and detrimental to the texture and
taste of the food.
Microwave cooking may also have a problem regarding
the time characteristics of microwave cooking. For
example, microwave cooking of cookies or bread may occur
so rapidly that the cookie batter does not have time to
spread properly, and the bread does not have time to rise
Z0 properly.
In the past, attempts to solve some problems with
microwave cooking have involved the use of susceptors
which heat in response to microwave radiation. Typically,
susceptors have been used which contain a thin film of
metal, usually aluminum, deposited upon a substrate. Such
susceptors typically have been characterized by surface
resistivities in the range of 10 to 500 ohms per square.
Such thin film susceptors have exhibited problems in the
past related to the deterioration of such susceptors, when
exposed to microwave radiation. Typically, susceptors
deteriorate or break up, and become less reflective and
more transmissive to microwave radiation as they are
heated in a microwave oven. For many food products, this
is undesirable. At present, there is no way known to
applicants in which a practical and disposable susceptor
1 330066
can be manufactured economically which will not break up
and significantly change its performance characteristics
when exposed to microwave radiation. In the past, it has
not been possible, to applicants' knowledge, to produce a
susceptor with a particular combination of reflected,
transmitted and absorbed power which would substantially
remain at the same percentages of reflected, transmitted
and absorbed power during microwave radiation. In the
past, there has been no practical way of control]ing
deterioration of a susceptor during microwave irradiation.
Susceptors used in the past have also suffered from
nonuniform heating. Thus, prior attempts to use suscep-
tors to minimize some of the problems of microwave cooking
have resulted in additional problems of food products
which are overheated in some places and unberheated in
other places due to the nonuniform heating of the suscep-
tor. For example, attempts to heat large pizzas have
generally resulted in overheating of the outside of the
pizza, and underheating of the center of the pizza.
In the past, there has been no practical way of
controlling the rate of temperature rise of the susceptor.
The only variables available to affect the temperature
rise have been choosing the resistivity of the susceptor
material, and the strength with which the metallized film
is glued to its paper support. ~owever, the surface
resistance of the susceptor material changes during
microwave irradiation, and the metal film breaks up.
Thus, both variables available to affect the rate of
temperature rise change during microwave heating.
From the above discussion it will be clear that prior
art microwave cooking systems have been unsatisfactory in
many respects.
1 330066
--4--
The present invention permits a package to be
designed for microwave cooking in which the percentage of
reflected, transmitted and absorbed power can be prede-
termined. More importantly, during microwave cooking, the
percentage of reflected, transmitted and absorbed power
will remain relatively stable. The present invention
provides a method for substantially maintaining perform-
ance characteristics in spite of the deterioration of
susceptors used in connection with the invention. The
deterioration of reflection, absorption and, most import-
antly, transmission characteristics can be controlled.
In addition, the present invention provides the
advantage of uniform heating of a food product. The
present invention also allows the rate of temperature rise
to be controlled.
With the techniques and design flexibility provided
by the present invention, a package can be designed to
provide cooking characteristics desired for a particular
food. A particular food may have a prèdetermined desired
set of cooking characteristics, e.g., overall rate of
heating, temperature, surface versus internal rate of
heating, etc., for optimum preparation. The present
invention provides techniques to design a package to
conform to such desired characteristics. Significantly,
the rate of internal dielectric microwave heating may be
separately controlled from the rate of surface heating.
The present invention utilizes an electrically
conductive grid in combination with a susceptor to produce
desirable microwave cooking characteristics. Preferably,
the grid and susceptor will be placed in close proximity
to each other. The grid and susceptor may be used in
combination with an otherwise totally or partially
shielded food package. A grid/susceptor combination can
1 330066
-- 5
also be used in eombination with an unshielded food
package.
This invention provides the food package for a
microwave o~en, having a first ~heet of material defining
susceptor means for heating in response to microwave
radiation, a ~econd ~heet of material definin~ a grid in
close pro~imity to the æusceptor means, a food item
located in heating relat~on~hip with the susceptor means,
and, the grid and the susceptor means being located on the
same side of the food item.
FIG. 1 is a graph ~epicting curves for reflected
~ransmitted and absorbed power for a susceptor in free
space.
FIG. 2 is a graph depict~ng a tricoordinate plot
showing absorbed power, reflected power and transmitted
power for susceptors of various resistivities, both before
and after heating.
FIG. 3 is an eYploded perspective view of a preferred
embodiment of a susceptor/grid combination useful for
microwave cooking of pizza.
FIG. 3A is a partially cut-away top view of the grid
shown in the FIG. 3.
FIG. 4 is a cross-sectional ~iew of the microwave
package depicted in FIG. 3.
FIG. 4A is a partially cut-away cross-sectional view
of the joint between the top and the bottom of the packaqe
shown in FIG. 4.
6681b
1 330066
- SA -
FIG. 4B is a partially cut-away cros~-sectional view
of the grid, ~usceptor and food item shown in FIG. 4.
FIG. 5 is a graph depicting a tricoordinate plot of
the transmitted, reflected and absorbed power
characteristics of the grid~susceptor combination
according to the present invention, both before and after
heating.
FIG. 6 is a graph depiciting an espanded view of a
portion of the graph shown in FIG. 5.
6681b
1 330066
--6--
FIG. 7A is a schematic cross-sectional representation
of a preferred grid and susceptor combination for pizza
and the like.
FIG. 7B is a cross-sectional schematic representation
of an alternative grid and susceptor configuration.
FIG. 7C is a cross-sectional schematic representation
of a grid and susceptor combination for use with an
unshielded food container.
FIG. 7D is a schematic cross-sectional representation
of a grid and susceptor combination for use with an
unshielded food container.
FIG. 8 is a graph depicting absorbance versus hole
size for various susceptor resistivities.
~ IG. 9 is a graph plotting temperature versus hole
size for various resistivity susceptors.
PIG. lO is a graph depicting percentage of power
reflected and transmitted for various size holes based
upon a mathematical model.
PIG. 11 is a graph plotting reflectance as a function
of hole size for various grid geometries based upon a
mathematical model.
FIG. 12 is a graph depicting a contour plot of
resistivity versus hole diameter showing observed heating
performance for grid and susceptor combinations.
FIG. 13 is a graph depicting percentage of microwave
power reflected as a function of the thickness of the foil
grid.
1 330066
FIG. 14A is a copy of an image taken with an infrared
camera showing the heating effects upon a particular grid
geometry.
FIG. 14B is a copy of an image taken with an infrared
camera showing the heating effects upon a particular grid
geometry.
FIG. 14C is a copy of an image taken with an infrared
camera showing the healing pattern for a susceptor alone,
illustrating hot spots encountered with known ~usceptors.
FIG. 14D is a copy of an image taken with an infrared
ca~era showing the heating pattern for a susceptor in
combination with a grid, illustrating uniformity of heat-
ing which may be compared with FIG. 14C.
FIG. 15 is a graph depicting temperature due to
heating as a function of spacing between holes.
FIG. 16 is a graph depicting uniformity of heating by
plotting the standard deviation of experimental tempera-
ture measurements taken with an infrared camera as a
function of spacing between holes.
~ IG. 17 is a graph depicting absorbed power as a
function of hole size for various resistivity susceptors.
FIG. 18 is a graph depicting percentage of trans-
mitted power as a function of hole size for variousresistivity susceptors.
FIG. 19 is a graph depicting percentage of reflected
power as a function of hole size for various resistivity
and various grid and susceptor combinations.
1 330066
--8--
FIG. 20 illustrates an alternative embodiment of the
invention.
FIG. 21 illustrates an alternative embodiment of the
invention.
~ IG. 22 illustrates an alternative embodiment of the
invention.
PIG. 23 illustrates an alternative embodiment of the
nventlon.
FIG. 24 illustrates an alternative embodiment of the
invention.
FIG. 25 illustrates an alternative embodiment of the
invention.
FIG. 26 illustrates an alternative embodiment of the
invention.
~ IG. 27 illustrates an alternative embodiment of the
invention.
FIG. 28 illustrates an alternative embodiment of the
invention.
~IG. 29 illustrates an alternative embodiment of the
invention.
FIG. 29A illustrates an alternative embodiment of the
invention.
FIG. 30A is a graph showing an enlarsed portion of a
tricoordinate graph illustrating measurements taken with a
network analyzer for a grid in combination with a suscep-
9 1 330066
tor using various distances of separation between the gridand the susceptor.
PIG. 30B is a graph showing power absorbed versus
grid/susceptor separation.
FIG. 30C is a graph showing calculated values for
power absorbed versus grid/susceptor separation.
FIG. 31 is a tricoordinate graph depicting measure-
ments mad~ with a network analyzer for a grid in
combination with a susceptor means comprising a microwave
magnetic absorbing material.
~IG. 32 shows a top view of a single opening in a
grid, in combination with a susceptor, used to develop an
equivalent circuit model.
FIG. 33 is a schematic diagram of an equivalent
circuit model for the grid/susceptor combination shown in
FIG. 32.
FIG~ 34 is a graph depicting relative percentage of
absorbed power as a function of hole diameter for various
values of susceptor resistivity, which are calculated
based upon an equivalent circuit model.
FIG. 35 i5 a graph comparing measured absorption
values from FIG. 8 with calculated absorption values from
FIG. 34.
FIG. 36 is a graph depicting relative power absorbed
as a function of hole diameter for various values of
susceptor resistivity, which are calculated based upon an
equivalent circuit model.
1 330066
--10--
~ IG. 37A is a copy of an image taken with an infrared
camera showing the heating pattern for the susceptor
alone.
S PIG. 37B is a copy of an image taken with an infrared
camera showing the heating pattern fo a susceptor and
grid in combination.
FIG. 38A shows a partially cut-away top view of a
grid having circular openings in a square lattice
configuration.
FIG. 38B shows a partially cut-away top view of a
grid having circular openings in an equilateral triangular
lattice configuration.
FIG. 38C shows a grid having square openings in a
square lattice configuration.
FIG. 38D shows a grid having square openings in an
equilateral triangular lattice configuration.
FIG. 39A shows a partially cut-away top view of a
grid having square openings.
FIG. 39B shows a partially cut-away top view of a
grid having circular openings.
FIG. 39C shows a partially cut-away top view of a
grid having triangular shaped openings.
FIG. 39D shows a partially cut-away top view of a
grid having hexagonal shaped openings arranged in an
equilateral triangular lattice.
1 330066
--11--
FIG. 39E shows a partially cut-away top view of a
grid having hexagonal shaped openings arranged in a square
lattice.
PIG. 39F shows a partially cut-away top view of a
grld having oval shaped openings.
PIG. 39G shows a partially cut-away top view of a
grid having rectangular openings.
FIG. 39H shows a partially cut-away top view of an
opening in a grid having a patch of conduc~ive material
located in the center of the opening.
~IG. 39I shows a partially cut-away top view of a
rectangular opening in a grid having a rectangular patch
of conductive material therein.
FIG. 39J shows a partially cut-away top view of a
grid having cross shaped openings.
FIG. 39K shows a partially cut-away top view of a
grid having crescent shaped openings.
FIG. 39L shows a partially cut-away top view of a
grid having U-shaped openings.
FIG. 39M shows a top view of a grid having square
shaped holes arranged in a differential geometry.
FIG. 39N shows a top view of a grid having circular
openings arranged in a differential geometry accordiny to
differential sizes.
1 330066
-12-
FIG. 390 shows a top view of a grid having circular
openings arranged in a differential geometry according to
hole spacing.
FIG. 39P shows a top view of a grid having U-shaped
openings arranged in an offset and interlocking
configuration.
FIG. 40A shows a cross-sectional side view of an
alternative embodiment of a grid and susceptor
combination.
PIG. 40B shows a cross-sectional side view of an
alternative embodiment of a grid and susceptor
combination.
FIG. 40C shows a cross-sectional side view of an
alternative embodiment of a grid and susceptor
combination.
FIG. 40D shows a cross-sectional side view of an
alternative embodiment of a grid and susceptor
combination.
FIG. 40E shows a cross-sectional side view of a grid
formed by strips of conductive material laid in a
checkerboard pattern.
FIG. 40F is a top view of the grid ànd susceptor
combination illustrated in FIG. 40E.
FIG. 41 is a graph depicting absorbed power versus
susceptor reactance for various grid/susceptor
combinations.
1 330066
-13-
FIG. 42 is a graph depicting absorbed power versus
susceptor reactance for various grid/susceptor
combinations.
FIG. 43 is a graph depicting absorbed power versus
susceptor reactance for varicls grid/susceptor
combinations.
FIG. 44 is a graph depicting absorbed power versus
susceptor reactance for various grid/susceptor
combinations.
FIG. 45 is a graph depicting absorbed power versus
susceptor reactance for various grid/susceptor
combinations.
FIG. 46 is a graph showing absorbance as a function
of susceptor surface reactance.
~IG. 47 is a qraph showing transmittance as a func-
tion of susceptor surface reactance.
FIG. 48 is a graph showing reflectance as a function
of susceptor surface reactance.
FIG. 49 is a front view of a network analyzer and
waveguide showing how reflectance, absorbance, and
transmissivity are measured.
FIG. 50 is a perspective view of a sample and
waveguide used in connection with the measuring equipment
illustrated in FIG. 46.
Some of ~he problems with prior art susceptors may be
best described with reference to FIG. 1. FIG. 1 is a
graph which depicts the reflected power, transmitted power
1 330066
-14-
and absorbed power for a susceptor, using a free space
model.
Problems With Conventional Susceptors
A typical susceptor is made by depositing a thin film
of metal upon a sheet of polyester. Thin film deposition
techni~ues, such as sputtering or vacuum deposition, are
typically used to deposit the metal film on the sheet of
polyester. The metallized polyester may then be adhe-
sively bonded to a sheet of paper, or paperboard if
rigidity is desired. When the susceptor is exposed to
microwave radiation, the susceptor can become relatively
hot. The heat produced oftentimes causes dimensional
changes, such as shrinkage, in the sheet of polyester.
Cracks often form in the metallized polyester layer.
These cracks are believed to cause conductivity breaks in
the metallized film. This crack formation process,
referred to as "breakup", is believed to be associated
with irreversible changes which occur in the performance
characteristics of the susceptor. The degree to which the
susceptor heats, and the percentage of microwave power
which is reflected, transmitted and absorbed, all change.
Prior to breakup, the curve for absorbed power for a
susceptor will appear like the curve identified with
reference numeral 10 in PIG. 1. In the curve depicted in
FIG. 1, the percentage of absorbed power peaks at 0.5, or
50%, for a susceptor having a surface resistivity of about
180-200 ohms per square. The transmitted power for such a
susceptor, according to this model, will generally follow
the curve identified with reference numeral 11 in PIG. 1.
Reflected power will follow the curve identified with
reference numeral 12.
-15- 1 330066
Por a give~ susceptor, the surface resistance may
increase during microwave radiation. Thus, the surface
resistance will move toward the right on the graph shown
in FIG. 1. The percentage of reflected power will
decrease, and the percentage of transmitted power will
increase.
It has been discovered that the electrical properties
of susceptors change when exposed to microwave radiation
durinq microwave cooking, and that such changes are gener-
ally deleterious to cooking performance. The metallized
layer of a conventional susceptor will tend to break up.
This creates a reactive component to the surface imped-
ance, where the surface impedance may be expressed as
ZS = RS ~ iXs, where ZS is the surface impedance, RS is
the surface resistance, and Xs is the surface reactance.
After breakup, the curve for absorbed power will tend to
shift as shown by the curve identified with reference
numeral 13 in FIG. 1. The transmitted power curve will
tend to take the form of curve 14 in FIG. 1. The curve
for reflected power will tend to take the form of the
curve identified with reference numeral 15. The surface
resistance will also shift to the right. Thus, it will be
seen that the percentage of power which is transmitted
through the susceptor greatly increases during microwave
cooking. The percentage of power reflected greatly
decreas~s. The percentage of power absorbed, and thus the
heating of the susceptor, decreases. This is a result of
the fact that the curve for the absorbed power changes
from curve 10 to curve 13, and also that the surface
resistance may increase sufficiently so that the susceptor
moves down the right side of the curve 13.
This problem with prior art susceptors is further
illustrate~ in FIG. 2. The graph shown in FIG. 2 depicts
the changing characteristics for various susceptors having
1 3300~6
-16-
initial surface resistivities of 17, 27, 59, 86, 175 and
435 ohms per square, respectively. Although the surface
resistivity changed after microwave heating, the plstted
graph only identifies the various samples according to
their initial resistivity for convenience of illustration.
It will be seen from FIG. 2 that a susceptor having a
surface resistivity of 17 ohms per square initially began
with over 90% reflected power, and only a few percent
transmitted power. After microwave heating, the percent-
age of reflected power dropped to less than 30%, and the
percentage of transmitted power was greater than 60%. The
percentage of absorbed power remained roughly the same.
Similar results were experienced for susceptors having
other resistivities.
This change in the performance characteristics of a
susceptor is undesirable in many applications. It would
be desirable to have some mechanism for positioning micro-
wave packaging material utilizing susceptor means at somepoint on the curve of PIG. 2, where the packaging material
could remain in substantially the same position on the
graph during microwave heating providing substantially
unchanged reflected, transmitted and absorbed power
characteristics. This has been substantially accomplished
usin~ the present invention.
~ IGS. 3 and 4 depict a preferred embodiment of the
present invention. The illustrated embodiment is particu-
larly useful for microwave cooking of pizza.
$he embodiment illustrated in ~IG. 4 includes a tray16 and top 17 enclosing a food product 18. In this
particular example, the food product 18 is a frozen pizza.
-17- 1 330066
To achieve desirable microwave cooking character-
istics of the pizza 18, a grid 19 and susceptor means 20
are provided in accordance with the present invention. As
shown in FIG. 4B, the susceptor means 20 ~ay include a
thin metal film 21 deposited upon a polyester substrate
22, which are adhesively bonded to a board or face 23.
The board 23 is preferably paper.
The grid 19 may serve at least two important
~0 functions.
~ irst, the grid 19 controls the microwave transmis-
siveness of the grid/susceptor combination. When micro-
wave radiation impinses upon the grid 19 and the susceptor
20, some of the microwave power will be transmitted
through the grid 19 and susceptor 20, some of the micro-
wave power will be absorbed by the susceptor 20, and some
of the microwave power will be reflected back.
The percentage, or fraction, of incident microwave
power which is transmitted through the grid/susceptor
combination is referred to as the transmittance or trans-
missiveness of the grid/susceptor system. The percentage,
or fraction, of microwave power which is reflected back
from the grid/susceptor combination is referred to as the
reflectance of the grid/susceptor system. The percentage,
or fraction, of microwave power which is absorbed by the
grid/susceptor combination is referred to as the
~bsorbance of the grid/susceptor system.
Second, the grid can serve the function of achieving
uniformity of heating. The grid 19 tends to spread the
heating effects of the microwave radiation to achieve a
more uniform heating of the susceptor means 20. Without
the grid 19, a conventional susceptor would tend to
develop hot spots and nonuniform heating. The grid 19 in
1 330066
-18-
combination with the susceptor means 20 minimizes or
eliminates the nonuniformity of heatins which has been a
significant problem in the past with microwave susceptors.
The grid 13 may also be referred to as a microwave
field spreading and transmissiveness control apparatus 19.
Por purposes of this invention, an array of elements may
~unction as a grid 19 when used to control transmissive-
ness of a susceptor system and to spread heating effects
of microwave radiation. Any generally planar array of
elements capable of redistribution of energy by mutual
lateral coupling between adjacent elements should be
essentially equivalent to the grid when used in the
environment of this invention. "Planar", as used here,
means a surface which does not necessarily have to be
flat. For example, a sheet defining a grid 19 may be
wrapped around a food item.
The susceptor means 20 may be a conventional
susceptor 20 comprising a metallized polyester sheet 21,
22, which ~ay optionally be adhesively bonded to a support
member 23. The susceptor means 20 heats when exposed to
microwave radiation. During microwave irradiation, the
susceptor means 20 heats to a relatively high temperature.
The heating effect of the susceptor means 20 can be used
to crispen or brown the surface of a food substance 18
immediately adjacent to the susceptor means 20.
The susceptor means 20 in the example illustrated in
FIG. 4 is shown more clearly in FIG. 4B. The susceptor
means 20 in this example comprised aluminum 21, vacuum
metallized onto 48 gauge polyester 22 which was adhesively
bonded to 16 point SBS paperboard 23. Initial surface
resistivity was measured at 50 to 70 ohms per square. The
susceptor 20 was obtained from James ~iver Corporation.
1 33~066
The grid 19 also serves as a diffusion means. When
microwave energy passes through openings 27 in the grid
19, the microwave energy tends to be spread across the
grid by coupling between adjacent holes ~elements) in the
S grid. For purposes of illustration, the effect of the
grid may be similar to that of light shining through a
frosted giass. Alternatively, an explanation of this
effect is that microwave energy tends to diffract when
passinq through the grid 19 in much the same manner that
light diffracts when passing through pin holes in an
opaque sheet. The reflectiveness of the grid 19 may be
adjusted in order to adjust or control the percentage of
power which is absorbed by the susceptor means 20. This,
in turn, provides a measure of control over the rate of
heating of the grid/susceptor combination which is used to
heat the food product 18.
The tray 16 is generally transparent to microwave
radiation, and is preferably composed of paperboard. The
top 17 electrically is conductive, and is preferably
composed of aluminum. An aluminum top 17 having a thick-
ness of about 6 mils has given satisfactory results in
practice.
The heating distribution of the pizza 18 may also be
adjusted by providing microwave admitting apertures or
openings 24. The size and position of the openings 24 may
be adjusted to assist in achieving uniformity of heating
of the food product 18. If more heating of the center
portion of the food product 18 is desired, larger openings
24 may be made in the center of the top 17 to allow more
microwave energy to reach the center of the food product
18. Conversely, lf more heating of the outer edges of the
pizza 18 is desired, then openings 24 could be provided
around the outer rim of top 17. In the particular illus-
tra ed example, it has been found that the provision of
1 330066
-- 20 --
openings in the center of the top 17 i~ desirable, as
shown in FIG. 3.
The susceptor and grid configuration of FIG. 4
generally results in performance characteristics which do
not ~ubstantially change during microwave heating. This
is shown more ~learly in FIG. 5. FIG. 5 depicts the shift
in performance characteristics before and after microwave
heating for susceptor and grid combination in accordance
with the present invention. ~he graph of FIG. 5 shows
that the performance characteristics do not substantially
change as a result of microwave heating. The superior
stability of the present invention may be best understood
by comparing the graph of FIG. 5, which represents the
present invention, with the graph of FIG. 2, which
represents conventional susceptor performance.
FIG. 6 is a graph depicting an enlarged ~iew of a
portion of FIG. 5. FIG. b shows the performance
characteristics of a grid and susceptor combination before
and after microwave heating. Susceptors having
resistivities of 17, 27, 59, 86, 175 and 435 ohms per
square were tested, both before and after heating.
Measurements were made with a network analyzer Hewlett
Packard Model No.8753A, where the grid was positioned
toward Part 1. The network analyzer test set up is
illustrated in FIG. 49 and FIG. 50.
FIG. 6, the point labelled 1, represents the
measurement of a microwave power reflection ~R~,
absorbtion ~ an~ transmi~sion ~T~ for a 17 ohms
susceptor prior to heating. The point labelled lA
represents the
6681b
1 330066
~ 20A -
R, A and T measurements for a 17 ohms susceptor after
heating. Similarly, the points 2 and 2A represent the R, A
and T measurements for a 27 ohms per square susceptor
before and after heating, respectively. The points 3 and
3A repre-
6681b
-21- 1 330066
sent the R, A and T measurements before and after heating,
respectively, for a 59 ohms per square susceptor. The
points 4 and 4A show the R, A and T measurements for an 86
ohms per square susceptor. The points labeled 5 and 5A,
and the points labeled 6 and 6A, represent the R, A and T
measurements for 175 and 435 ohms per square susceptors,
respectively.
FIGS. 5 and 6 show that the present invention results
in dramatic improvements during microwave heating in
stability of the performance characteristics of the
susceptor and grid combination in accordance with the
present invention.
Referring again to the preferred embodiment illus-
trated in FIGS. 3 and 4, the present invention solves the
problem of nonuniform heating which has plagued the prior
art. Attempts to heat a large pizza 18 with a susceptor
means 20 in the past have resulted in severe nonuniform
heating of the pizza 18. Typically, the outer edges of
the pizza 18 would be crispened or browned by the suscep-
tor means 20, but the center portion of the pizza 18 would
be soggy and undercooked. The crust of the center portion
18 would not be crispened. The grid 19 and susceptor
means 20 combination surprisingly results in uniform
heating of the pizza 18. Thus, the entire crust of the
pizza 18 may be browned, and the pizza 18 evenly cooked.
The apertures 24 are used to fine tune the uniformity
of heating uf the pizza 18. In the illustrated example,
the tray 16 is transparent to microwaves. Thus, some
microwave radiation leaks in through the outer edges 26 of
the tray 16. By providing apertures 24 generally concen-
trated at the center of the top piece 17, the dielectric
3; heating of the food 18 may be compensated by the openings
24 so that the heating is substantially uniform.
1 330066
-22-
When the package illustrated in FIG. 4 is placed in a
microwave oven for microwave cooking, the microwave radi-
ation is normally emitted into the oven cavity from a
magnetron typically located above the package. Conven-
tional microwave ovens have a microwave transparent shelfupon which the illustrated tray 16 rests. Below the
shelf, a reflective oven cavity wall is typically located.
Thus, microwave energy will enter the bottom of the tray
16 after being reflected from the bottom of the oven
cavity wall. Some microwave radiation will also enter
through the microwave transparent sides 26 of the tray 16,
and through the openings 24 in the top 17.
During operation, the grid l9/susceptor 20 comDina-
lS tion behaves substantially more like a conventional fryingpan as it is heated by microwave radiation, than was the
case with susceptors previously known to the art. One
purpose of a susceptor is to provide a degree of surface
heating, (to produce crispening or browning of the food
item), which would not otherwise occur. A serious draw-
back of susceptors previously known to the art is that
they became highly transparent during operation, as is
seen in FIG. 2. This process generally resulted in more
internal dielectric heating of the food item, and less
surface susceptor heating, than was desirable. In
contrast, the grid l9/susceptor 20 combination, by virtue
of the low degree of transmission which it maintains
throughout heating, provides a higher degree of surface
heat relative to internal dielectric heating. In this way
the grid l9/susceptor 20 functions substantially more like
a frying pan than did susceptors previously known in the
art. The metallized surface 21 substantially heats
responsive to microwave radiation, and the composite
grid/susceptor structure maintains a low percentage of
transmission of microwave energy.
-23- 1 330066
The heating effect of the susceptor 20 is substan-
t ally uniform due to the unique combination of a suscep-
tor 20 with a grid 19. This hot cooking surface 21, or
substantial "frying pan" effect, serves to uniformly brown
or crispen the crust of the pizza 18. The topping and the
crust of the pizza 18 are also cooked from a combination
of the heat emitted by the susceptor means 20 and
dielectric heating of the food 18 due to microwave
radiation which enters through the openings 24 in the top
17, and through the sides 26 and bottom of the tray 16.
This unique combination of a "frying pan" effect provided
by the susceptor means 20, and dielectric heating of the
food 18 by the transmitted microwave energy, significantly
reduces the total ccoking time for the pizza lB. For
example, in a conventional oven a pizza which might
require 30 ~inutes for proper cooking may be cooked using
the illustrated embodiment of the invention in about 11
minutes. The resultant pizza 18 will possess desirable
characteristics of molstness and amount of cooking,
together with a substantially uniform browned and
crispened crust.
The grid/susceptor combination achieves much more
uniform heating of the food substance 18, than would be
the case if a susceptor 20 was used without a grid 19. In
the case of a grid 19 having openings or holes 27, a
cert~in interaction between the openings occurs when
exposed to microwave radiation. The interaction is gener-
ally more pronounced as the distance between the openings
27 decreases. Conversely, the interaction between
openings 27 generally decreases as the openings 27 are
moved apart. Thus, it is desirable, up to a limit, to
place circular openings 27 in the grid 19 closer together
to increase interaction between the openings 27. If the
width of the strips 28, shown as "W" in FIG. 3A, is made
too thin, the mechanical integrity of the grid 19 may be
-24- 1 330066
adversely affected. Also, the electrical conductivity of
the strips 28 may be broken, and/or the electrical
resistance of the strips 28 may become too large, thus
adversely affecting the electrical integrity of the grid
S 19.
During operation, closely spaced openings 27 tend to
share fields during microwave radiation. A certain amount
of coupling occurs between adjacent openings 27. A high
field located in the proximity of a particular opening 27,
which wo~ld otherwise create a hot spot on the susceptor
20, will be partially coupled to adjacent openings 27 by
the action of the grid 19. This coupling or sharing of
'ields between adjacent openings creates a more uniform
heating effect on the food substance 18. The grid 19
creates a redistribution of energy by mutual lateral
coupling between adjacent elements 27 of the grid 19. In
a general sense, the term "elements" may be used to refer
to the openings 27, because equivalent structures may be
devised capable of accomplishing spreading of the heating
effect by mutual coupling between adjacent elements.
Thus, heating of the food substance 18 is made more
uniform as a spot on the susceptor means 20 located
coincident with an opening 27 in the grid 19 has its field
partially coupled to adjacent elements 27 of the grid.
The adjacent openings 27 share fields to some extent,
thereby distributing or spreading out the field to some
extent to achieve a more uniform heating effect.
Since the susceptor 20 absorbs predominantly the
component of the local electric field which is tangential
to the susceptor surface, hot spots Gn the susceptor means
20 may be at a point where the predominant field has a
particular local polarization. Most microwave ovens have
a mode stirrer. In operation, a mode stirrer may cause
1 330066
- 25 -
the instantaneous polarization at any particular point to
shift rapidly. What can be observed, in some cases, is a
polarization effect resulting from the average value of
the field at a qiven point. A particular local
polarization at a hot spot may have an effect upon Eharing
bet~een adjacent openings 27 and of the field at that
point. The orientation of columns and rows of openings 27
will, therefore, affect the sharing of the field. Local
polarization tends to affect the direction of sharing
between adjacent openings 27. This is shown by FIG. 14A
and FIG. 14B. FIG. 14A is grid having circular openings
27 with a square lattice orientation. FIG. 14A represents
a copy of an image taken with an infrared camera of a
grid/susceptor combination during microwave heating.
FIG. 148 is also a copy of an image taken with an infored
camera of a grid/susceptor combination auring heating in a
microwave oven. In FIG. 14B, the grid had circular
openings 27 with an eguilateral triangular lattice
configuration. In both cases, sharing of the field
between adjacent openings 27 i8 apparent, particularly
along certain columns.
The original images taken with an infrared camera,
from which FIGS. 14A and 14B were copies, were in color.
Black and white copies of these color images are used
herein for convenience, and to avoid difficulties and
unnecessary complications in printing this specification.
In the past, it has been necessary tc specially
formulate foods of a microwave oven application in order
to obtain improved results in a microwave oven. One
advantage of the present invention is that many products
prepared for cooking in a conventional oven may be cooked
in a package according to the present invention without
6681b
1 330066
-26-
changes in the composition of the product. Good results
are obtained using the present invention in combination
with foods prepared for cooking in a conventional oven.
The package illustrated in PIGS. 3 and 4 has been
successfully used to cook conventional dee? dish pizza
having a diameter of about 10-7/8 inches (27.6 cm), and a
total net weight of about 1.75 pounds, (about 796 grams).
The cooking time required was only 11 minutes. The
success achieved with the present invention was
surprising, because in the past it has been impossible to
satisfactorily cook a large pizza in a microwave oven.
In operation, the present invention overcomes
disadvantages experienced in the past during microwave
cooking where undesirable temperature differentials and
moisture movement has been experienced. In the present
invention, the "frying pan effect" achieved by the
susceptor 20 in combination with the grid 19 generates a
persistent and highly localized heating at the surface of
the pizza 18. This results in a temperature profile
through the thickness of the pizza 18 where the tempera-
ture of the surface of the pizza 18 is believed to be
significantly greater than the temperature of the interior
of the pizza 18. This encourages moisture movement oppo-
site that which had been experienced in microwave cooking
of such pizzas in the past, and which more closely
approximates the desired moisture movement which occurs in
a conventional oven. The moisture either escapes into the
oven atmosphere or migrates toward the interior of the
pizza 18. Thus, the moisture content of the bottom
surface of the pizza 18 is substantially reduced to
produce a taste sensation which is perceived by a consumer
as "crispness."
3;
-27- 1 330066
In an example in which an identical pizza was cooked
in a microwave oven for 11 minutes using only a conven-
tional susceptor, (no grid was used in combination with
the susceptor), and the pizza was not covered during
cooking, the results were unsatisfactory. The outer one
inch annular ring of the crust was browned, and was so
tough or hard that it was practically inedible. That
portion of the pizza crust was virtually impossible to
chew. The center portion of the pizza had barely been
warmed during microwave heating. No browning of the
center portion of the pizza crust was observed.
The present invention provides a means to achieve a
desired degree of surface heating of the food substance
18. This is oftentimes desirable because, with some food
substances, crisping or browning of the surface is
desired. In other food substances, a certain amount of
surface heating is desirable in order to maintain a
particular temperature profile or moisture migration
during heating. The present invention provides this
desirable degree of surface heating in combination with a
controlled degree of internal heating.
One problem with the use of conventional susceptors
without utilizing a grid 19 in accordance with the present
invention, was that conventional susceptors tend to break
down during microwave heating. ~hen conventional suscep-
tors break down during microwave heating, the susceptors
become highly transmissive and allow a large percentage of
the microwave energy to pass through the susceptor and
reach the food substance. The dielectric heating of the
food substance which :esults causes an undesirable degree
of internal heating. The present invention allows the
transmissiveness of the grid/susceptor combination to be
3~ controlled. mhe present invention also maintains rela-
tively stable performance characteristics during microwave
1 330065
-28-
cooking. Even though a susceptor is still believed to
break down to some extent when used in combination with a
grid, the grid/susceptor combination of the present inven-
tion appears to control break down so that the composite
characteristics of the grid/susceptor combination can be
substantially controlled. The present invention allows
the microwave transmission and absorption characteristics
of the grid/susceptor system to be controlled, changed,
and manipulated as desired in order to achieve a desired
degree of surface heating in combination with a desired
degree of internal heating. With a conventional susceptor
alone, it was very difficult and sometimes virtually
impossible to balance the degree of surface heating
against the degree of internal heating. The present
invention solves this problem.
In the illustrated embodiment, the grid 19 has a
diameter of 10.25 inches (26 cm). A cross-hatching of
metal strips 28 define a plurality of openings 27. The
openings 27 in the grid 19 are squares having a length and
width, shown as "D" in FIG. 3A, of about 1/2 inch ~1.27
cm). The illustrated hole geometry is square openings 27
arranged in a square lattice. The grid 19 was made from a
sheet of aluminum foil by cutting the openings 27 in the
foil. The metal strips 28 separating the openings 27 of
the grid 19 have a width "W" of about 3/16 inch (0.48 cm),
as shown in FIG. 3A. Thus, the spacing between holes is
about 3/16 inch. In the illustrated example, the grid 19
has an open area defined by the total area of the indi-
3~ vidual openings 27. The closed area, or microwave opaquearea, is defined by the area of the metal strips 28 of the
conductive grid 19. In the illustrated example, the ratio
of open area to microwave opaque area was about 52.9~. In
other words, the grid 19 had about 53% open area.
~5
1 330066
-29-
Preferably, the separation between the strips 2~ of
the grid 19 is sufficient to avoid arcing during microwave
cooking. The required spacing will depend somewhat upon
the load, which is a function, inter alia, of the amount
and composition of the food substance 18 and the thickness
of the food substance 18.
In practice, a metal thickness of about 1 to 4 mils
(0.0025 to 0.01 cm) for the strips 28 of the grid 19 has
given satisfactory results. Thickness as small as 0.275
mils (0.0007 cm) have also given satisfactory results in
practice.
Susceptors 20 having an initial resistivity of about
50 to 120 ohms per square have given satisfactory results
in practice.
FIG. 4~ shows a detailed view of the interface
between the top 17 and the tray 16.
Alternative Grid and SusceDtor Arranaements
~ IG. 7A schematically depicts the configuration of
the grid 19 and susceptor means 20 configuration for t~e
pizza container shown in FIGS. 3 and 4. In this example,
microwave power primarily impinges upon the grid/susceptor
combination from the direction indicated by the arrow 25.
Although some microwave radiation is admitted through the
openings 24 shown in ~IG~ 4, it is substantially
completely absorbed by the lossy pizza 18 in this
particular example.
FIG. 7~ illustrates an alternative embodiment of the
present invention having a grid means 19', and susceptor
means 20' in a configuration where microwave energy from
the direction indicated by the arrow 25' impinges upon the
_30_ 1 330066
grid 19' prior to the susceptor means 20'. Of course, the
susceptor means 20' may be composed of a thin film of
metal 21' ~eposited upon a polyester substrate 22', which
is adhesively bonded to paperboard 23'. The metallized
polyester 21', 22' is immediately adjacent to a food
substance 18'. In orde. to prevent microwave energy from
impinging from the left of FIG. 7B, a shielded food
container may be utilized in combination with the grid 19'
and susceptor 20'.
1~
PIG. 7C schematically depicts an alternative embodi-
ment of the present invention utilizing an unshielded food
container. Microwave energy impinges from two sides, as
generally indicated by the arrows 25". In this example, a
grid 19" and susceptor means 20" is provided similar to
the configuration shown in FIG. 7A.
FIG. 7D depicts an alternative embodiment of the
present invention utilizing an unshielded food container.
A grid and susceptor combination is utilized similar to
that shown in FIG. 7B, with the exception that microwave
energy impinges from two directions, as shown by arrows
25"'.
The paperboard 23 is not essential to the operation
of the present invention. The susceptor means 20 may
comprise a thin film of metal 21 deposited upon a poly-
ester substrate 22. ~his polyester plastic film may be
wrapped around a food product 18, and a grid 19 wrapped
around this composite structure. Alternately, the metal-
lized polyester 22 may be adhesively bonded to a grid 19.
Desian Factors
Performance of a microwave package constructed in
accordance with the present invention can be adjusted
-31- 1 330066
using at least four different factors. The size of the
holes 27 may be adjusted, the geometry of the holes may be
changed, the impedance characteristics, e.g., resistivity
and reactance, of the susceptor may be adjusted, and the
distance of the susceptor 20 from the grid 19 may be
adjusted. There are also at least four approaches to
analyzing the grid/susceptor combination: (l) analyzing
the performance characteristics utilizing a network
analyzer; (2) actual performance tests in a microwave
oven; (3) using an eqùivalent circuit model; and (4) using
mathematical models. Although different approaches have
been used for analysis, the agreement between all four
approaches has been surprising.
lS Hole Size
Adjusting the hole size may increase or decrease the
amount of power heating the food substance 18. Por a
given resistivity, increasing the hole size in the grid 19
will generally increase the percentage of power absorbed
by the susceptor 20 and will generally increase the
percentage of power transmitted through the grid/susceptor
combination, and vice versa.
The effect of the size of the holes 27 in the grid 19
is shown by the graph of FIG. 8. FIG. 8 represents a plot
of the size of the openings 27 in the grid l9, versus the
absorbance measured with a network analyzer. Each curve
represents a different resistivity for the susceptor means
20. Absorbed power was measured for susceptors having
resistivities of 12, 26, 72, 147 and 410 ohms per square.
The grid l9 had circular holes in this example. For
smaller hole sizes, the lower resistivity susceptors had
greater absorption. ~or intermediate hole sizes, for
example 3/4 inch (l.9 cm) , an intermediate range
susceptor havins a resistivity of 72 ohms per square had
-32- 1 3300~6
maximum absorption. The waveguide utilized with the
network analyzer that was used to make these particular
measurements did not permit measurements upon grids having
openin~s 27 larger than one inch. The data trend suggests
that if the curve for the larger resistivity susceptors is
extrapolated, e.g., the curve for the 147 ohms per square
susceptor, the larger resistivity susceptors will
eventually have greater absorption when the openings 27
are made sufficiently large.
FIG. 9 is a s.aph which depicts temperature measure-
ments made upon various size grids using three different
susceptors having resistivities of 17, 70 and 2000 ohms
per square. The temperature data was measured using an
infrared camera aimed at the underside of the susceptor,
i.e., the paperboard 23 side. Although the actual
temperature of the metallized film 21 was not actually
measured, the relative temperature measurements are
significant. This measurement technique was utilized
because attempts to measure the temperature of the thin
film 21 by aiming the infrared camera directly at the
metallized film 21 resulted in spurious reflected images.
The experimental data reflected in FIG. 9 is intended to
compare the relative temperatures. The absolute tempera-
tures are not critical for this particular experiment.
FIG. 9 shows the results for grids 19 having openingsas large as two inches (5.1 cm). In this experiment, or
hole diameters larger than 1.2 inches (3 cm), the higher
resistivity susceptors, (e.g., 2000 ohms per square), had
greater absorption than the lower resistivity susceptors.
For small size openings 27, e.g., 0.125 inch (0.32 cm~,
the low resistivity susceptor having 17 ohms per square
resistivity had greater absorption. For intermediate size
3; openings 27 in the grid 19, on the order of about O.S to
about 1.0 inch (1.27 cm to 2.5 cm), the inte-mediate range
1 330066
- 33 -
susceptor having a resistivity of about 70 ohms per sguare
had the qreatest amount of absorption.
FIG. 10 is a graph dipicting calculations based upon
a ~athematical model referrsd to herein as the Chen
model. The Chen model i8 more particularly described in
an article entitled ~Transmission of Microwave Through
Perforated Flat Plate of Finite Thickness~, by Chao-chun
Chen, published in IEEE Transactions on Microwave ~he~ry
~nd ~e~hniques, Vol. MTT-21, No. 1, pp.l-6, (January
1973). A comparison of reflection coefficiences
calculated using the Chen model, with reflection
co-efficients (for grids alone) measured using a network
analyser, is shown in FIG. lO. In FIG. 10, there is no
absorption. The Chen model i6 based upon a model of grid
alone, without a susceptor. The Chen model assumes
electrically thick grids made from good conductors. The
Chen model ~oes not include absorption in the mathematical
model. Therefore, the transmission is equal to 100% minus
the percentage of reflect~on. In this esample, the
reflection coeffecient decreases as the whole diameter
increases. Measurements made on grids with the network
analyser agree closely with the values predicted by the
Chen model.
A prefesed range of whole sizes for use in connection
with the present in~ention is between about 0.125 inch
(0.32 cm) and about 2 inches (5.1 cm). If a hole ~ize is
made too small, the amount of absorption in the susceptor
means will usually be insufficient. If the hole size is
made too large, the advantages of the present invention
relating to uniformity of heating and control of
reflectance will not be as apparent. The preferred range
of hole size is dependent upon the frequency of the
microwave rediation. The zbove preferred range of hole
sizes may alternatively be s~pressed as about 2.6% of the
6681b
-
_34_ 1 330066
wavelength to about 40% of the wavelength of the microwave
energy in free space. A less preferred range which may
give useful results in some applications, is about 0.65%
of the wavelength to about 1 wavelength. In the present
example, utilizing a microwave oven having a frequency of
about 2450 MH2, this range may be expressed as about 1/32
inch (0.03125 inch) (0.08 cm) to about 4.8 inches (12.2
cm). In this particular example, the term "wavelength"
refers to the free space wavelength of the microwave
energy, not the wavelength of the microwave radiation in
the food or packaging material. The wavelength in air is
essentially the same as the wavelength in free space. A
grid 19 with openings 27 having a size between about 1/8
incn and about 2.4 inches is more preferred. An even more
preferred range for the present invention is about 0.375
inch ~0.95 cm) to about 0.875 inch (2.2 cm) for the hole
size. This may be expressed as a range of about 7.8% of
the wavelength to 18.2% of the wavelength of the microwave
energy.
In the case of circular openings 27, the size is the
diameter of the hole 27. In the case of square openings
27, the size is the width. For a rectangle, the size is
the average of the length and width, i.e., the length plus
the width, all divided by two. For Qther shaped holes,
the size is the length of the major axis.
.
Susceptor Surface Impedance
For a given hole size, an optimum resistivity for the
susceptor element 20 may be selected for peak heating.
The size of the openings 27 and the resistivity of the
susceptor 20 may both be varied in order to adjust the
rate of heating of the food substance 18.
1 330066
-35-
FIGS. 17, 18 and 19 are graphs representing measure-
ments taken using a network analyzer on grid/susceptor
combinations having various resistivities for the suscep-
tor 20 and various hole 27 sizes for the grid 19. In this
experiment, the grids had circular openings 27. The
openings 27 were arranged in an equilateral triangular
lattice configuration. In each sample, the minimum width
of the strips 28 between adjacent openings or holes 27 was
maintained at about 1/8 inch (0.32 cm). The network
analyzer ~as used to measure the resistivity of the
various susceptors 20 employed in the experiment.
PIG. 17 is closely related to FIG. 8. 8Oth graphs
show the same experimental data. However, FIG. 17
includes the additional set of data points plotted to the
far righ. of the graph showing absorbed power in free
space for a susceptor 20 with no grid 19. In all cases,
susceptors 20 were used which had not been exposed to
micrvwave cooking.
As discussed above, as the size of the openings 27 is
increased, the percentage of microwave energy which is
absor~ed by the susceptor 20 increases. Also, the
percentage of microwave power which is transmitted through
the grid/susceptor combination also increases as the size
of the openings 27 is increased. This is shown by ~IG.
18-. ~IG. 19 shows that, as the size of the openings 27 is
increased, the percentage of microwave energy which is
reflected decreases. These figures further show that this
effect is, to some extent, a function of the resistivity
of the susceptor 20.
The impedance presentéd by a susceptor may, in
addition to having a surface resistance component, also
have a surface reactance component. Thus, the suscepto
1 330066
~urface reactance may be a factor to consider in the
design of a grid/susceptor system~
For a given grid geometry and grid~susceptor
separation, susceptor surface reactance may be adjusted to
predetermine susceptor power absorp~ion, for e~ample, to
ma~imize, minimize or attain any other desired degree of
absorption. As mentioned above, the susceptor impedence
ZS may be e~pres~ed as ZS ~ RS ~ iX~, where RS
is the surface S resistance 2nd Xs is the surface
reactance, both with units of ohms per square. The
surface reactance measured for susceptors before esposure
to a microwave energy in a microwave oven may be between
0 to about - 100 ohms per square, while after use in
microwave cooking without grids the surface reactance may
be in the range of about - 100 to about - 800 ohms
per sguare. The surface reactance is typically a negative
imaginary number. This is believed to arise from
capacitive nature of the breaks which typically occur in
the metallization layer 21 of a susceptor 20.
FIGS. 41, 42 and 43 are graphs showing a theoretical
relationship between power absorption and susceptor
surface reactance for a series of grid~susceptor
combination having various hole 27 sizes and the various
surface resistivities.
~ he theoretical relationship is based upon the
scattering matrix formulation for shunt elements and empty
waveguide from J.L. Altman, Microwave Circuits, pp.370-71
(1964~. Scattering transfer matrises may be derived from
these formulations, using techniques such as taught by
M. Sucher and J. Fox, Handbook of Microwave Measurements,
Chap. 4 (1963).
6681b
1 330066
-37-
Por each curve shown in these figures, the grid
openings 27 were circular and were arranged in an equi-
lateral triangular lattice. The minimum width of the
strips 28 between adjacent openings 27 was about 1/8 inch,
and the separation between the grid 19 and the susceptor
20 was about 0.0048 inch (0.0122 cm). In FIG . 41, the
holes 27 had a diameter of about 1/4 inch. In FIG. 42,
the holes 27 had a diameter of about one inch. IN FIG.
43, the holes 27 had a diameter of about 5/8 inch. The
curves in FIGS. 41, 42 and 43 show that susceptor absorb-
ance peaks at a value of sùsceptor surface reactance which
is dependent on the hole 27 size of grid l9.
FIG. 44 is a graph calculated for a grid identical to
that in FIG. 43. However, for this case, a separation
between the grid 19 and the susceptor 20 of about 0.048
inch (0.122 cm) was used. Comparison of ~IGS. 43 and 44
shows that the susceptor absorbance versus reactance curve
depends on the separation between the grid 19 and the
susceptor 20. Thus, for a given hole size and a given
grid and susceptor separation, the impedance of a suscep-
tor, including its surface reactance, may be adjusted to
increase (or maximize), or decrease, the amount of micro-
wave power absorbed by the grid/susceptor system. In
other words, the susceptor may be tuned to what is perhaps
a matched impedance, for maximum microwave power
absorbance.
The effect of surface reactance on suscepto~ power
absorption also depends on the geometry of the grid 19.
~IG. 45 is a graph showing a theoretical relationship
between power absorption and susceptor surface reactance
for a series of grid/susceptor combinations having various
surface resistivities. For each curve, the grid openings
27 were 5/8 inch squares arranged in an eguilateral
triangular lattice. The width of the strips 28 between
1 330066
-38-
adjacent openings 27 was about 1/8 inch, and the separa-
tion between the grid 19 and the susceptor 20 was about
0.0048 inch (0.0122 cm). Comparison of FIGS. ~3 and 45
shows that the susceptor absorbance versus reactance curve
depends on the geometry of the grid 19.
Por a given susceptor surface resistance, the grid 19
geometry, hole 27 size, and the separation between the
grid 19 and the susceptor 20 are believed to interact to
some degree. Accordingly, they must be iteratively
adjusted to optimize susceptor heating for each particular
case.
An experiment was performed to examine the effect of
susceptor reactance on the absorbance of the grid/suscep-
tor combination. The experiment was performed in WR-340
waveguide using a network analyzer. Susceptor reactance
was varied by making a series of cuts through the suscep-
tor metallization layer with a razor blade. The cuts were
paralle' to the long dimension of the waveguide cross-
section and extended entirely across the susceptor
surface. Susceptor reactance was measured without the
grid and increased with the number of cuts across the
susceptor surface. The susceptor 20 had an initial
surface resistance of about 15 ohms per square. The grid
19 used in this experiment had square openings 27 located
on a square lattice. The minimum width of the strips 28
between adjacent openings 27 was about 3/16 inch and the
separation between the grid 19 and the susceptor 20 was
about 0.0004B inch (0.00122 cm). The curves in FIG. 46,
FIG. 47 and FIG. 48 show the absorbance, transmittance,
and reflectance, respectively, of the grid/susceptor
combination and the susceptor 20 alone as the susceptor
reactance (negative) was increased by successive addition
or clts to the susceptor metallization laye~. FIG. 46
sho~ that the absorbance of this particular grid/suscep-
1 33Q066
- 39 -
combination was ma~imized when the surface reactance was
about - 50 to about - 150 reactive ohms per square and
that the absorbance change for the grid/suæceptor
combination was substantially larger than that observed
with for the susceptor alone. FIG. 47 and FIG. 48 ~how
that as surface reactance was added to the susceptor, the
grid/susceptor combination ~ubstantially retained
transmittance and high reflectance while the 6usceptcr
alone underwent major ~and in many products contests,
deleterious) changes in transmittance and reflectance.
It is believed that initial reactance of a susceptor
film can be affected by the amount of stress or ~train
introduced during susceptor manufacture, for e~ample, by
adjusting the film web ten~ion or through modifications in
the film-to-substrate bon~ing process. The capacitive
rea~tance of the susceptor may be adjusted making small
cuts in the surface of the susceptor.
The above discussion has primarily been directed to
the thin film susceptors which are resistive, as a
preferred embodiment of the present invention. However,
other lossy susceptor means 20 may be utilized. For
example, graphite may be used. Another e~ample of such a
~uitable susceptor means a is susceptor comprising
magnetic microwave absorbing material. A suitable
magnetic microwave absorbing material for use as the
~usceptor means 20 is disclosed in U.S. Patent
No. 4,266,108, issued May 5, 1981, to Anderson et al.,
entitled ~Microwave Heating Device and ~ethod~.
Magnetic microwsve absorbing materials include
materials having ferromagnetic or ferrimagnetic
properties, a Curie temperature and an ability to heat
when exposed or subjected to microwave radiation. Such
6681b
r'~
1 330066
- 40 -
materials include maqnetic oside materials that are known
as ferrites. These material~ tend to heat in response to
the magnetic component of the microwave energy field.
Magnetic microwave absorbing materials are
characterizea by their relative magnetic permeability '0
and their relative magnetic loss factor '. In the above
discussion of thin film susceptors, where resistivity was
used as a factor for de~igning a desired heating response,
magnetic permeability would be similarly utilized in the
case of magnetic microwave absorping materials.
FIG. 31 is a tricoordinate graph of esperimental data
measured with a network analyzer using a grid 19 in
combination with a susceptor means 20 comprising a
magnetic microwave absorbing material. The magnetic
microwave ~bsorbing material was removed from a Microwave
brand crisper~griddle Model PM 400/145, manufactured by
Anchor Hocking Corporation. The material was believed to
co position of Ba2 Mq2 Fe22 22 ~n a binder
material. A grid 19 having rec~angular openings in a
square lattice configuration was utilized. The openings
in each grid had a constant height of 1~2 inch ~1.27 cm)
for each grid. The width of the openings varied.
Grids were used with openings having widths of 0.25 inch
(0.63 cm.), 0.5 inch (1.27 cm.), 0.75 inch (1.9 cm.),
1.0 inch ~2.5 cm.) and 1.25 inch~s ~3.2 cm.). Because of
the polarization of the microwave energy in the
waveguide, only the ~imens~on of the width of the openings
in each grid was important for this esperiment.
For each grid having a particular size opening, two
measurements were made. One measurement was made looking
throuqh port 1 of the network analyzer. Another
measurement was made looking through port 2 of the network
analyser. Thus, the points plotted on the ~raph of FIG.
6681b
1 330066
-41-
31 labeled as points "1", "3", "5", "7" and "9" represent
measurements taken through port 1. The points plotted on
FIG. 31 and labeled "2", "4", "6", "8" and "10" represent
measurements taken through psrt 2 of the network analyzer.
The points plotted on FIG. 31 which are labeled "11" and
"12" represent measurements taken without any grid,
through ports 1 and 2, respectively. Points "1" and "2"
represent measurements for a grid having a width of 0.25
inch (0.63 cm). Points "3" and "4" correspond to a grid
having a width of 0.5 inch (1.27 cm), and so on.
From PIG. 31, it can be seen that a susceptor means
20 using magnetic microwave absorbing material can be used
to vary the percentage of power transmitted versus the
percentage of power reflected, within limits, while the
percentage of power absorbed remains relatively constant.
This effect is most apparent for grids having openings
larger than 0.75 inch (1.9 cm).
Combinations of susceptor means 20 may also be
utilized. For example, a thin film susceptor using
resistive heating may be combined with a susceptor using
magnetic microwave absorbing material. Also, a single
susceptor means 20 may be used employing both resistive
heating and magnetic microwave absorption in the same
material. A composite susceptor means 20 may be used
having a plurality of layers of either resistive heating
material or maqnetic microwave absorbing material.
Another heating mechanism which may be utilized for a
susceptor means in combination with a grid is an inedible
dielectric heater package element. Lossy dielectric heat-
ing package materials heat in response to the electric
component of the microwave energy. Such dielectric
materials are characterized by a relative dielectr_c
constant E' and a relative dielectric loss factor E".
~ 330066
-42-
The susceptor means 20 may be a planar package
element which heats responsive to microwave radiation. In
general, the susceptor means 20 may be a lossy microwave
energy absorber which is heated when exposed to microwave
radiation.
The susceptor means 20 may also be characteri2ed
according to the penetration depth of the susceptor.
Penetration depth is the distance over whiCh the microwave
power density diminishes to about 36.8~ of its original
value. For purposes of this disclosure, penetration depth
for the susceptor means 20 is calculated based upon the
following:
P = P0e-2xRe[y~
where P is the power density at a distance "x" within the
susceptor material, and P0 is the initial power density of
the microwave field. y is the propagation constant for
the susceptor material. Using Maxwell's equations for
plane waves, the real portion of the propagation constant
may be determined, for purposes of this invention, based
upon the f ol lowi ng:
Re[y] = Re ~ ~ E' - iE") (~
The values for E', E", ~' and ~" may be measured for
a particular susceptor material. Ao is the free space
wavelength at the microwave oven operating frequency. At
the oven operating frequency of 2.45 GHz used throughout
the Vnited States and elsewhere, ~0 is about 12.24 cm.
The power penetration depth may then be calculated, based
on the following relationship:
_5
_43_ 1 330066
d =
where d is the power penetration depth, and Re~y] is the
S real part of the propagation constant for the susceptor
material. The susceptor means 20 preferably has a
penetration depth less than 1.3 inches. An even more
preferred penetration depth for the susceptor means 20 is
a penetr~tion less than or equal to Q.65 inch. An even
more preferred penetration depth for the susceptor means
20 is a penetration depth less than or equal to 0.001
inch.
The susceptor means 20 preferably has a low thermal
mass. A preferred susceptor means should be operati~e to
heat quickly when exposed to microwave radiation. A
susceptor means having a large thermal mass could require
too long for heating. If a susceptor means having a large
thermal mass resulted in a long heating time for a food
item, much of the convenience associated with microwave
cooking would be lost.
The susceptor means 20 preferably is inexpensive and
disposable. It is undesirable for the susceptor me~ns to
cost more than the cost of the food item which is being
heated. A preferred susceptor means 20, when combined
with the grid 19 and other package components, should not
cost more than the cost of the food item lB. If the
e~tire package, including the grid/susceptor combination,
represents a significant portion of the total cost of the
packaged food item, the susceptor means will be impracti-
cal for most packaged food applications. This considera-
tion eliminates most ceramic materials and utensils as
desirable susceptor means 20, for most packaged food
applications. In such applications, the packaging is not
44 1 330066
intended for continuous reuse. Moreover, the packaging
should not require a separate preheatin~ step.
In this invention, the susceptor means heats to a
temperature of about 150 F (65.5 C) durin~ microwave
heating of the food. The susceptor means preferably heats
to a temperature of a~ least about 212 ~ (100 C). A
more preferred temperature range for the susceptor means
is 300 F (149 C) or greater. An especially preferred
temperature range for the susceptor means is 350 F
(177 C) or greater.
A thin film susceptor having a surface resistivity
between about 1 ohm per square and about 10,000 ohms per
square is preferred. A surface resistivity between about
5 ohms per square and about 5000 ohms per square is more
preferred. A surface resistivity for the susceptor
between about 30 ohms per square and about 800 ohms per
square is even more preferred. A surface resistivity
between about 50 ohms per square and about ? ohms per
square is especially preferred.
The grid 19 defines a relatively reflective first
region, which essentially forms a lattice structure
surrounding a plurality of second regions defined by the
openings 27 having susceptor material 20 exposed to
microwave radiation. The susceptor material 20 of the
second regions preferably has a microwave power transmit-
tance, when the susceptor material 20 is measured alone,
which is greater than 0.003 percent. The susceptor
material more preferably has a microwave power transmit-
tance greater than 0.07 percent. Even more preferably,
the susceptor material 20 has a microwave power transmit-
tance greater than 1.9 percent.
1 33006~
- 45 -
Hole Geometry
The geometry of the grid 19 may also be adjusted to
change the performance of the packase. For esample,
circular openings 27 may be utilized. Alternatively,
square openings 27 may be utilized. Also, circular
openings 27 may be arranged in an eguilateral triangular
lattice, as ~hown in FIG. 38B. Circular openings may
alternatively be arranged in a square lattice, as shown in
FIG. 38A. æquare openings 27 may be arranged in an
eguilateral triangular lattice, as shown in FIG. 38D.
Also, sguare openings 27 may be arranged in a square
lattice as shown in FIG. 38C. These four alternatives are
used aæ the basis for the curves plotted in FIG. 11.
The curves were plotted using values calculated by the
Chen model.
FIG. 11 is a graph depicting the percentage of
reflectance for these four different grid geometries, as a
function of the diameter of the openings 27. In the case
of sguare openings 27, the ~diameter~ i8 the length of the
sides of the ~guare opening 27.
FIG~ 11 shows that a square lattice geometry is
generally mure reflective than a triangular lattice
geometry, all other being equal. For circular openings
having a diameter equal to the height and width of a
~guare opening, the grid with such circular openings will
generally be more reflective than a grid with such sguare
openings, all other things being equal. This can only be
partially e~plained by the difference in percentage open
area between such grids. It should be appreciated that
each qeometry will have its own impedence. The variation
in reflectance, and impedence, is
6681b
1 330066
-46-
more than simply a function of percentage open area. A
similar conclusion is apparent for equilateral triangular
lattice grids having squ~re openings and having circular
openings.
In FIG. 11, case 1 represents circular openings with
an e~uilateral triangular lattice. Case 2 represents
circular openings with a square lattice. Case 3 repre-
sents square openings with an equilateral triangular
lattice. Case 4 represents square openings with a square
lattice. FIG. 11 represents calculations for a grid 19
considered alone, without an associated susceptor means
20.
Table I represents a comparison of Chen model calcu-
lations for various grid geometries using an opening size
of 0.5 inch (1.27 cm), and 0.1 inch (0.25 cm) spacing,
(i.e., the width of the strips 28 equals 0.1 inch (0.25
cm)). Table I compares data measured using a network
analyzer with the calculations produced by the Chen model.
Table II represents measurements made with a network
analyzer using a grid 19 and susceptor 20 in combination.
The susceptor 20 used in this example had a resistivity of
about lZ5 ohms per square. The term "stagger lattice" is
used in the tables as a shorthand expression for referring
to an equilateral triangular lattice.
~47~ 1 330066
COMPARISON OF CHEN MODEL GRID G~OMETRIES
0.5" (1.27 cm) HOLE SIZE, 0.1" (0.25 cm) SPACING
NET~ORK ANALYZER DATA AVERAGE OF TWO PORTS
TA~LE I
GRID ALONE
Measured Pre~icted
Hole Ty~e T.atti~ Alignment Reflectio~ ~Qfles~l~D
Case 1 Circle Stagger Lattice 0.958 0.973
Case 2 Circle Square Lattice 0.973 0.981
Case 3 Square Stagger Lattice 0.902 0.917
Case 4 Square Square Lattice 0.911 0.951
TABL~ II
GRTD AN~ SUSCEP~OR
Lattice Reflec- Trans-
~Ql~-Type Alig~ent tion Absor~tio~ mi$sio~
Case 1 Circle Stagger Lattice 0.779 0.187 0.034
Case 2 Circle Sq~are Lattice 0.847 0.130 0.024
Case 3 Square Stagger Lattice 0.696 0.249 0.056
35 Case 4 SquareSquare Lattice 0.715 0.229 0.057
Referring to FIG. 38A, the circular openings 27 have
a diameter "D" and a spacing between openings of "W". For
circular openings 27, the width of the strips 28, and the
spacing between openings 27, is considered to be the mini-
mum distance between openings 27, shown as the distance
"W" in FI~. 38A. The openings 27 have a center-to-center
spacing, shown in FIG. 38A as "X".
The circular openings 27 in an equilateral triangular
lattice have a diameter or hole size "D", and a center-
to-center separation of "X". In the equilateral triangu-
1 330066
-48-
lar lattice, the holes 27 have a center-to-center offset,
shown as "Y" in FIG. 38B. The separation between openings
27 is shown as "W" In FIG. 38B.
FIG. 38C shows square openings 27 arranged in a
square lattice. The openings 27 may also be rectangular
in shape, having a height and a width which are not equal.
The geometry shown in FIG. 38C corresponds with the grid
geometry shown in ~IG. 3A.
FIG. 38D shows a grid 19 having square openings 27
arranged in an equilateral triangular lattice configura-
tion. The openings 27 have a size "D", shown in FIG. 38D.
In the triangular lattice, the openings 27 have a center-
to-center offset shown as "Y" in FIG. 38D. The openings
27 chown in FIG. 38D may also be rectangular in shape.
The square openings 27 shown in FIG. 38D have an
edge-to-edge offset, identified as "Z".
Grid-Susceptor Separation
The adjustment of spacing between a grid 19 and a
susceptor 20 may be varied. The adjustment of grid-
susceptor spacing is a useful technique for varying thepercentage of power absorbed and the percentage of power
reflected by the grid/susceptor combination, within
limits, while the percentage of power transmitted remains
relatively constant. Grid-susceptor spacing can be better
understood by referring to FIG. 30A.
FIG. 30A is a graph in the form of a tricoordinate
plot. To better understand FIG. 30A, it should be appre-
ciated that the graph of FIG. 30A represents an enlarged
view of the extreme lower left-hand corner of a tricoordi-
nate plot, such as is shown in FIG. 5.
- \
1 330066
-49-
The measurements shown on FIG. 30A were made with a
network analyzer. The plotted points show measurements
alternatively through port l, and port 2 of the network
analyzer. Two points are plotted for each experiment.
Each experiment used a different separation between the
grid 19 and the susceptor 20. (One pair of points is
plotted for an example utilizing a susceptor alone, with
no grid.) The susceptor 20 used in these experiments had
a surface resistivity of 50 ohms per square. Measurements
were made without subjecting the susceptor 20 to microwave
heatins. The grid l9 utilized in these experiments had
1/2 inch (1.27 cm) squares arranged in a square lattice
configuration. The separation between the holes was about
1/8 inch.
As the separation between the grid 19 and the suscep-
tor 20 was increased, the distance between the points
measured through port l and port 2 of the network analyzer
increased. For example, with a separation between the
grid and susceptor of 0.032 inch (0.08 cm), the measure-
ment through port 1 resulted in a measurement of the
following parameters: absorption about 12%; reflectance
about 86%; and, transmission about 2%. For the same
separation, the followin~ parameters were measured looking
through port 2: absorption about 25%; reflectance about
73~; and, transmission about 2%.
By adjusting the distance between the grid l9 and the
susceptor 20, the relative percentage of power absorbed
and the relative percentage of power reflected may be
adjusted. The percentage of power transmitted remains
relatively constant.
From FIG. 30A, it can also be seen that as the grid
19 and the susceptor 20 are moved further and further
apart, the grid/susceptor combination, when viewed from
_50_ 133 006 6
the grid side, performs more and more like a grid alone.
The grid/susceptor combination, when viewed from the
susceptor side, performs more and more like the absorbance
of a susceptor alone, however, without the transmission of
microwave energy which would be characteristic of a
susceptor alone.
The percentage of power transmitted may be varied by
adjusting the hole size in the grid 19. For example, if
the hole size of the grid 19 were made larger, the family
of points plotted in FIG. 30A would then plot in an area
of the tricoordinate graph more to the right, and
upwardly, when viewed from the orientation of ~IG. 30A.
FIG. 30B is a graph representing the same data which
is shown in FIG. 30A. The vertical axis represents
percentage power absorbed, and the horizontal axis repre-
sents the separation distance between the grid and
susceptor. The lower line, identified with reference
numeral 97 in FIG. 30B, represents the measurements taken
through port 1. The graph line, identified with reference
numeral 98 in FIG. 30B, represents the data points taken
through port 2.
FIG. 30C is a graph, similar to FIG. 3QB, showing
percentage absorbed power versus the separation between
the grid and susceptor, expressed in inches. ~he line
indicated by reference numeral 98' represents the
percentage power absorbed when viewed from the susceptor
side. The line indicated in the graph by the reference
numeral 97' represents the percentage power absorbed when
viewed from the grid side.
The graph of FIG. 30C was calculated mathematically.
The sraph of FIG. 30C shows the trend of what happens as
1 3300~6
-51- .
the grid and susceptor are separated by distances up to
0.5 inch.
The mathematical model used to plot the graph of FIG.
30C was developed by taking the scatte ing matrix
formulation for 6hunt elements and empty waveguide from
J. L. Altman, Microwave Circuits, pp. 370-71 (1964).
Scattering transfer matrixes may be derived from these
formulations, using techniques such as are taught by
M. Sucher and J. Fox, Handbook of Microwave Measurements,
Chap. 4 (1963).
The percentage power reflected and the percentage
power transmitted from the combination of a grid a
susceptor may be computed for a variety of distances
separating the grid and susceptor. The percentage power
absorbed may be assumed to be one minus the sum of the
power reflected and the power transmitted for each
distance of separation. The results computed in this
manner were used to generate the graph of FIG. 30C.
The use of separation between a grid 19 and a
susceptor 20 can be more easily analyzed, and may be more
effective, when used in combination with a shielded
package, or "closed" system. That is, if the microwave
energy impinges upon the grid/susceptor combination and
food from only one direction, the separation between the
grid 19 and the susceptor 20 may be more easily utilized
as a factor for controlling the heating of the food lB.
A separation between the grid and susceptor less than
0.5 inch is preferred. A separation between the grid and
susceptor less than about 0.048 inch is more preferred. A
separation be~ween the grid and susceptor less than about
0.016 inch is even more preferred.
1 33~066
-52-
Interrelationship Between Hole Size,
Hole Geometry, Resistivity and Spacina
The interrelationship between hole size, resistivity,
spacing, and hole geometry may be effectively analyzed
using at least two approaches. First, an empirical tech-
nique is described below. Second, an equivalent circuit
model is developed and described.
By selecting a given hole geometry and grid-susceptor
spacing, the effect of variations in resistivity and
variations in the hole size in the grid may be effectively
analyzed by empirically observing the heating character-
istics of the system. Contour plots may be effectively
utilized to assist in visualizing the response of the
system.
FIG. 12 is a graph depicting a contour plot. The
horizontal axis of the graph represents the diameter of
the openings 27 in the grid 19. In the particular example
shown, the openings 27 are circular openings arranged in
an equilateral triangular lattice. The vertical axis
represents the log10 of the surface resistivity of the
susceptor 20. Each contour line shows a given absorption
value.
Using the contour plot of FIG. 12, for a qiven size
of opening 27 in the grid 19, the resistivity value of the
susceptor 20 may be optimized, for example, for maximum
absorption. Alternatively, a resistivity for the suscep-
tor 2~ may be selected which provides less than maximum
absorption where it is desirable to reduce the rate of
heating of the composite package. Of course, the reverse
approach may also be utilized. For a susceptor 20 having
a given resistivity, the diameter of the openings 27 in
1 330066
- 53 -
the grid 19 may be selected to achieve the desired rate of
heating.
A contour applied may also be applied for other grid
geometries. The contour plot of FIG. 12 was produced
e~perimentally by constructing a series of grids having
various sizes of openings. One-eighth inch (0.3175 cm.)
increments in hole sizes was utilize~ to produce the
contour plot in FIG. 12. For each grid 19 having openings
27 of a particular size, various susceptoræ 20 having
different surface resistivities are utilized. For each
grid 19 and susceptor 20 combination, the grid susceptor
com~ination is placed in a microwave oven and exposed to
microwave radiation. An infrared camera aimed at the back
of the susceptor 20 is used to measure the relative amount
of heating the susceptor/grid combination. Tn a preferred
testing method, low power microwave radiation is
utilized. An $nfrared camera measurement is made after a
short initial heating period, for e~ample, 10 seconds
after microwave heating has been initiated during the
microwave heating cycle. After the measurement is made,
microwave heating may then be discontinued. Using the
infrared camera temperature image analysis function, the
temperature of the back surface of the grid/susceptor
combination was averaged. Preferably, two identical
measurements are taken and then averaged to produce a
sinqle data point. The data from tbese measurements were
smoothed usin~ a full guadradic model. The guadradic
model was then used to plot the contour plot. In the
illustrated e~ample, the plot was made using a SAS~
Contour program produced by SAS Institute, Inc., of
Cary, North Carolina. Other contour plot programs could
also be utilized. The R squared value of the fit for the
data to the model was 0.91. In the illustrated e~ample,
log base ten of the susceptor surface resistivity is
utilized ~or the vertical ~is of the contour plot. Data
points
I'itiRl h
1 330066
-54-
representing the same final average temperature were then
connected and plotted as isotherms.
An equivalent circuit model can be helpful in
understanding the interrelationship between hole size and
resistivity of the susceptor means. The development of an
e~uivalent circuit model may best be understood by first
considering FIG~ 32.
FIG. 32 illustrates a single hole 27 in a grid 19.
For purposes of developing this equivalent circuit model,
only a single hole 27 will be considered. However, it
should be understood that the grid 19 contains a plurality
of openings 27. A susceptor means 20 is also included in
the circuit model. In the illustrated example, the
susceptor means 20 is coplanar with the grid 19. In the
view shown in ~IG. 32, a susceptor material 20 is located
so that it is visible through the opening 27. In the
illustrated example, the grid and susceptor combination is
shown illustrated by a view taken from the grid side.
FIG. 32 depicts arrows 86 which are representative of
currents which are believed to flow in the conductive grid
19 responsive tn the effects of microwave radiation which
impin~es upon the grid/susceptor combination. For
purposes o~ this circuit model, the currents represented
by arrows 86 may be assumed to exist. However, the
existence of such currents has been substantiated to some
extent by experiments with slots which indicate resistive
heating at locations corresponding to such current paths.
Voltage antinodes are believed to occur at about a
mid-point 87 in the sides of the opening 27. In a circu-
lar opening 27, the voltage antinodes 87 would occur at
opposite points around the circumference of the circular
opening.
1 330066
The direction of the currents 86 and the polarity of
the voltages 87 illustrated in FIG. 32 represent instanta-
neous currents and voltages. After having the benefit of
the teachings of this disclosure, it will now be appreci-
S ated by those skilled in the art that the currents 86 andvoltages 87 rapidly alternate in a sinusoidal fashion
responsive to the microwave radiation, and vary at the
same frequency as the microwave radiation.
The circuit model developed below also includes a
current 88 which flows in the susceptor 20 in response to
the microwave radiation. This current is indicated gener-
ally by the arrow illustrated in FIG. 32, referred to with
reference numeral 88. The illustrated direction of the
current 88 is also a representation of an instantaneous
current which will vary at the same frequency as the
microwave radiation. The presence of the current 88 has
been based in part upon experimental observations of the
location of heating effects on a susceptor in combination
with a grid or opening.
For purposes of the circuit model shown in FIG. 32,
the current 86 is treated as a current induced due to
inductance. The voltage 87 is treated as a charge which
is capacitively stored. The current 88 is treated as a
current through a resistive element.
This circuit may be expressed as an equivalent
circuit model which is illustrated in FIG. 33.
FIG. 33 shows a source of electromotive force
~"EMF"), illustrated as an equivalent source represented
by a Norton constant current generator 89. In the case of
a microwave oven, the EMP source 89 is typically the
magnetron of the oven. The grid/susceptor combination may
be characterized as having a capacitance "C" 90, an
1 3300~6
-56-
inductance "L" 91, and a resistance "R" 92. In the
circuit ~odel illustrated in FIG. 33, these are repre-
sented by lumped elements comprising capacitor 90,
inductor 91 and resistor 92 connected in parallel.
The equivalent circuit model in FIG. 33 also includes
a characteristic generator impedance Zc' identified with
reference numeral 94, associated with the source of EMF
89, and a downstream line impedance Z0, identified with
reference numeral 93. In this example, the grid/susceptor
combination is assumed to be in free space. Thus, Zc and
Z0 are equal to the characteristic impedance of free
space. In a microwave oven, these impedance values will
change depending upon the oven design, and the placement
of food in the oven.
The equivalent circuit shown in FIG. 33 is greatly
simplified. Lumped elements of capacitance 90 and
inductance 91 are used in developing this circuit model.
A more accurate representation of a grid/susceptor
combination might include distributed capacitance and
distributed inductance, especially in view of the
plurality of openings which are included in a typical
grid. However, as will be seen more fully below, the
simplified equivalent circuit illustrated in ~IG. 33 has
provided sufficiently accurate predictions to be useful in
connection with the present invention.
In developing the equivalent circuit analysis set
forth below, the opening 27 is assumed to be an inductor
having an inductance given by the equation:
1 330066
-57-
L = ~On2A
Q
where ~0 is the permeability of free space, equal to
S 4~ x 10 7 webers per ampere-meter; n is the number of
turns in the inductor, which is here assumed to be one
turn; A is the area within the turns of the inductor,
which is he:e assumed to be ~r2 for a circular opening 27;
and Q is the length of the electrical conductor defining
the inductor, which is here assumed to be ~D for a
circular opening 27. Of course, for the present example
assuming a circular opening, "D" is the diameter of the
circle and "r" is the radius of the circle.
In develcpment of the circuit model, the resistance
"R" 92 shown in FIG. 33 is assumed to be equal to the
surface resistivity of the susceptor 20. For example, the
resistance "R" 92 is assumed to have a value of 70 ohms
where the susceptor 20 has a resistivity of 70 ohms per
square.
Empirical measurements have shown that an opening 27
having a diameter of 2 inches (5.1 cm) is resonant in a
microwave oven. In developing the equivalent circuit
analysis set forth below, the natural frequency of the ~LC
circuit represented by the capacitor 90, inductor 91 and
resistor 92, having a 2 inch (5.1 cm) diameter hole, was
assumed to be 2.45 x 109 Hz.
Thus, the parallel admittances represented by the
lumped capacitive, inductive and resistive elements, 90,
91 and 92, respective'y, can be added. The admittance of
the reactive components represented by the capacitor 90
and the inductor 91 are freguency dependent. Thus, the
adrittance of the parallel circuit represen~ed by the
1 330066
-58-
capacitor 90, inductor 91 and resistor 92 can be expressed
as:
Y R + j~C + j~L
where ~ represents the frequency of the microwave
radiation.
The admittance represented by the line impedance ZL
93 and the generator impedance ZG 94 may also be added as
follows:
Y = i 1 + R + j~C ~ i ~L + j Z
At resonance, the reactive admittance due to the
capacitor 90 will cancel the reactive admittance due to
the inductor 91. Therefore, at resonance, the admittance
may be expressed as:
y = j 1 ,~, 1 + j 1
Thus, admittance at resonance may be alternatively
expressed as l/ZT, where ZT is the total impedance of the
circuit at resonance.
A quality factor "Q" for a parallel circuit may be
expressed as:
Q = 0
P ZT ~0CZT
1 330066
-59-
assuming a condition of resonance.
An admittance ratio, expressed as the ratio of the
admittance at the natural frequency of the equivalent
circuit as compared to the admittance at some other
frequency, may be expressed as:
Y o = ~1
Yw l + iQ(~ ~ -0)
where ~0 s the natural frequency of the equivaIent
circuit, and w is the frequency for which the admittance
Y is to be determined.
In order to rearrange this analysis to consider the
effect of changing the size of the hole 27 in the grid 19,
rather than expressing the frequency ~ as the variable,
the natural frequency ~0 can be assumed to be a variable
which is a function of hole size. In the case of a
microwave oven application, the frequency w does not vary,
but is the frequency of the microwave oven, i.e., 2.45
GHz. In order to do so, it can be assumed that the ratio
of the natural frequency as a function of hole size may be
expressed as:
wO = 2_1n~ x 2.45 x lO9 Hz
where D is the diameter of the opening 27, expressed in
inches, and wO is the natural frequency for that opening
27 having a diameter of D. This expression of the natural
frequency as a function of the size of the opening may
then be substituted into the above equation.
1 330066
-60-
The impedance ratio is a reclprocal of the admittance
ratio. From the above, an expression for the impedance Z~
at a given frequency w may be expressed as:
S z L~ ( (~ ) (~ )~ '
Referring again to FIG. 33, the voltage between
points "A" and "B" may be expressed as follows:
VAB = IGZ~
where VA8 is the voltage between points "A" and "B", and
IG is the current of the EMF source 89. It is desirable
to now solve for the current through the resistor "R",
identified with reference numeral 92 in FIG. 33. The
current IR through the resistor R may be expressed as:
I - AB _ GZw
R R R
The power dissipated in the resistor "R" 92 may be
expressed as the square of the current through the
resistor multiplied times the resistor, which may be
represented as:
Ps = IS2R
s ( R )
s R
For present purposes, the relative response of the
circuit model is of primary interest. A unit val~le for
-61- l 330066
the input current IG is assumed. Thus, Ps will be
proportional to the following relationship:
z 2
R
The expression for z~ may be substituted with the
above expression. The power dissipated in the susceptor
Ps may be expressed as being proportional to the following
expression:
~l ~ o )
The above discussion was developed for a circuit
model based upon a single opening. To approximate the
effect of an array of openings, the power dissipated in
the susceptor Ps is weighted by the fraction open area.
The relative power PR absorbed by the grid/susceptor
combination is then:
PR = PsFo
where Fo is the fraction open area for the grid. For a
grid having circular openings 27 with a radius r, and a
width of spacing 28 between holes, or margin, of m, the
fraction open area Fo is:
F = ~r
(2r ~m)2
The above mathematical model based upon this equiva-
lent circuit analysis was used to produced the graph
depicted in FIG. 34. The graph shown in FIG. 34 shows
relative power absorbed in the susceptor on the vertical
1 330066
-62-
axis, expressed as a percentage, versus hole diameter on
the horizontal axis. Various curves are plotted for
susceptors having resistivities of 12, 26, 72, 147, and
410 ohms per square. This mathematical model, based upon
the above equivalent circuit analysis, may be compared
with the results obtained empirically using measurements
taken with a network analyzer, which are depicted in the
graph of FIG. 8. The same trends as a function o~ hole
diameter and variations in resistivity can be seen in both
graphs shown in FIG. 34 and PIG. 8.
The empirical results obtained, and plotted in the
graph of FIG. 8, may be compared with the results obtained
using the mathematical model, and shown in the graph of
FIG. 34. A comparison of the relative power absorbed,
(represented by the vertical axis in the graphs in FIGS. 8
and 34), are shown in the graph of FIG. 35. The relative
power calculated, and shown in PIG. 34, is plotted on the
horizontal axis of FIG. 35. The relative power absorbed,
which was measured and shown by the vertical axis in FIG.
8, is plotted on the vertical axis of FIG. 35. If the
mathematical model perfectly predicted the measured
absorbance, all data points would fall upon a line 96
shown on FIG. 35 at a 45 angle on the graph. Line 96
represents the set of points having equal relative power
absorbed.
The points shown in PIG. 35 represent a close agree-
ment between the empirical measurements shown in FIG. 8
and the predicted values calculated with the mathematical
model which are graphed in PIG. 34. Statistically, the
agreement between the set of data points plotted in PIG.
35 has a linear regression correlation coefficient of
0.95.
~ ~ - ~
-63- 1 330066
Turning to FIG. 9, the measurements represented in
that graph may be compared with the values predicted by
the mathematical model, which are represented in the graph
of FIG. 34. The same trends of power absorbed by the
susceptor means, (which results in heating of the suscep-
tor means), may be observed for various hole sizes and
susceptor resistivity values.
FIG. 36 shows a ~raph of relative power absorption,
represented by the vertical axis, versus hole size,
represented by the horizontal axis. Various susceptor
resistivities were utilized to produce the curve shown in
PIG. 36. Each curve represents a different surface
resistivity "R" used in the circuit model. Resistivities
of 12, 26, 72, 147 and 410 ohms per square were used.
PIG. 36 plots values for hole diameters up to 2 inches.
The values calculated using the circuit model, and plotted
in PIG. 36, may be compared with the experimental measure-
ments whicn are plotted in FIG. 9.
The effects of adding a food load to the package may
be represented in the equivalent circuit shown schemati-
cally in FIG. 33 by an impedance ZF' identified with
reference numeral 95. The food which is contained in a
microwave package may affect the heating characteristics
of a grid in combination with either a magnetic, or
resistive, or dielectric heating susceptor means.
Adding a food load to the circuit model discussed
above will have several effects. First, the impedance ZF
added by the food load will tend to reduce the absorbance
in the susceptor. Addin~ a food load which is a dielec-
tric will affect the capacitance "C" of the circuit model.
This in turn will have an effect upon the "Q" of the
circuit model. Also, the natural frequency or the hole or
1 330066
-64-
opening in the grid will be changed due to the presence of
the dielectric represented by the food load.
In general, with the addition of a food load, the
optimum value of resistivity for the susceptor means will
be changed for a given hole size. The reflectior and
transmission of the total system represented by the
grid/susceptor combination and food load will also be
different from that of the grid/susceptor alone. The
empirical method discussed above, and contour plots, may
be utilized to account for changes introduced by adding a
food load.
Uniformity of Heat~
This invention greatly improves the uniformity of
heating of food, as compared to a conventional susceptor
utilized alone without a grid.
FIG. 14C is a copy of an image taken with an infrared
camera showing the heating pattern for a conventional
susceptor used alone without a grid. This image shows hot
spots which developed on the susceptor during microwave
heating. Such hot spots are typical of conventional
susceptors, when used alone, and result in uneven heating
and/or cooking of the food item. For example, a pizza can
be overheated along the outer perimeter, and can be heated
insufficiently in the center region of the pizza. In the
case of fish sticks, for example, the fish sticks on the
outside can be overheated and the fish sticks on the
inside can be neated insufficiently.
FIG. 14D is a copy of an image taken with an infrared
camera showing a heating for a susceptor in combination
with a grid. The uniformity of heating in the example
1 330066
- 65 -
shown in FIG. 14D may be compared with the hot spots shown
in FIG. 14C.
FIG. 14C snd FIG. 14D are black and white copies of
color images. Black and white copies are u~ed herein for
convenience, and are believed to be sufficient for
purposeæ of illustration.
A grid 19 eshibits a phenomenon where energy is
coupled between adjacent holes 27 in the grid 19. This
coupling between adjacent holes 27 has the effect of
making the heating of the su~ceptor 20 more un~form. This
phenomenon of couplin~ between holes 27 can best be
described by considering the effect of varying the spacing
between adjacent holes 27.
It is deæirable to have the openings 27 in the
grid 19 spaced ~ufficiently close ~o that the openings 27
effectively share fields. FIG. 15 is a graph plotting the
amount of heating of the susceptor 20 as a function of the
spacing of the openings 27 and the grid 19. In this
e~periment, heating occurrea at low power to minimize the
deterioration of the susceptor 20 as a result of heating.
The holes 27 and the grid 19 were circular holes having a
one inch (2.54 cm.~ diameter. In this e~periment, three
holes in a row were utilized for measurements.
Temperatures were measured utilizing an infrared camera
aimed at the backsiae of the susceptor 20. This method of
measurement does not necessarily provide the actual
temperature of the susceptor 20, but gives reliable
indications of relevant temperature.
In this e~periment, the temperature measurements were
affected by the orientation of grid/susceptor combination
in the oven. Therefore, ten measurements were made.
6681b
-`" 1 330066
-66-
For each measurement, the orientation of the three holes
in the grid 19 were rotated to a slightly different posi-
tion. The temperature measurements for the ten experi-
mental runs were then averaged to produce a single data
point, which is plotted on FIG. lS. This procedure was
repeated for each of five different spacings between the
holes.
The general trend shown by FIG. 15 is that the
temperature increases as the spacing between adjacent
holes decreases. As the spacing between holes decreases,
the sharing of adjacent fields between holes increases.
~here also seems to be an increase in the maximum
temperature reached when the holes were closely spaced.
FIG. 16 is a graph depicting the standard deviation
of temperature variation over a grid 19 as a function of
the spacing between the openings 27 in the grid 19. In
this experiment, a grid having circular openings of one
inch (2.54 cm) diameter with a square lattice
configuration was utilized. An infrared image of the
heating of the grid/susceptor combination was then taken
using an infrared camera, similar to that shown in FIG.
14A. Using a spot function programmed in commercially
available software provided in conjunction with the
infrared camera, the maximum temperature in each opening
27 was determined. The standard deviation of this
collection of maximum temperatures was then computed using
standard statistical techniques and that value is shown in
FIG. 16. Two trial runs were taken, and FIG. 16 shows the
average between the two trials.
As shown in ~IG. 16, the general trend is toward more
uniform heating of the grid/susceptor combination, (i.e.,
lower standard deviation), as the spacing between the
openings 27 decreases.
1 330066
A spacing between openings 27 less than or equal to
one inch will provide satisfactory results in practice. A
spacing between openings 27 of less than 1/2 inch (1.3 cm)
is preferred. The most preferred spacing is about 1/8
inch (0.32 cm). Spacings less than 1/8 inch (0.32 cm) are
difficult to achieve in practice because of limitations in
available materials and due to mechanical difficulties
experienced with such thin strips of metal 28.
Susceptors, (without using a grid), tend to heat
unevenly. Susceptors also tend to heat preferentially at
the edges, so that susceptors tend to heat more at the
edges than in the center. The problems of uneven heating
in susceptors is exacerbated by the circumstance of uneven
electric fields in the ovens. Many consumer microwave
ovens will have uneven field strengths. Where the fields
are stronger, the susceptors will tend to heat more.
Thus, the combination of all these factors tends to result
in a great deal of unevenness in the heating of a food
substance using a susceptor alone.
An experiment was performed to compare the difference
between a susceptor heated alone, and a susceptor/grid
combination which was also heated. Both ~xamples were
heated in a low power microwave until the same average
temperature was reached by each. An infrared camera was
used to make temperature measurements. FIG. 37A and FIG.
37B are copies of one set of infrared images taken with
the infrared camera during this experiment. Table III
shows the results of the measurements taken. Multiple
measurements were taken and averaged. Table III gives the
averaged measurements.
1 330066
-- 68 --
TABr~ III
~ ;ta~rd
i~ )!~ A~r~ P~vi~ti~
Susceptor 32.4~ C 46.5 C 37.2 C ~.00~ C
Susceptor/Grid 33.3 C 43.1 C 36.3~ C 1.76 C
The measurements set forth in Table III for the
susceptor alone represent averages for two e~perimental
runs. For the æusceptor, the average minimum temperature
was 32.9 C and the average masimum temperature was 46.5
C. The avera~e of the average temperatures for the
susceptor alone was 37.2 C. Using a statistical analysis
contained in the thermal imaging software package
accompanying the infrared camera system, a standard
deviation for the temperatures measured over the total
area of the suæceptor was computed. The average standard
deviation for the two runs was 3.0 C. The infrared
camera used here, and in other esamples in this
specification, was Agema Infrared Systems, Model
Thermovision 870 infrared camera. A thermal image
computer, Model TIC-8000 running CATS version 4 software
was used for the statistical analysis.
The numbered listed in Table III for the
susceptor/grid represent averages of three e~perimental
runs. In the case of the grid/susceptor combination, the
average minimum temperature was about 33.3 C and the
average ma~imum temperature was about 43.1 C. The
average of the average temperatures was 36.3 C. The
average standard deviation for the temperatures measured
was only 1.76 C.
The susceptor/grid combination had much more uniform
heating than was the case with a susceptor used alone.
1 330066
- 69 -
FIG. 37A and FIG. 37B are black and white copies of
~olor infrared images taken with an infored camera. Yor
convenience, black and white fi~ures have been used.
Referring to FIG. 37A, the masimum temperature was
reached at a relatively hot spot in the upper right hand
portion of the figure. The ma~imum temperature in this
measurement wa~ found to be about 45.4 C. The minimum
temperature ~hown in FIG. 37A 30.5 C. The minimum and
ma~imum temperatures shown in Table III are averages of
the those which were measured.
In FIG. 37B, the masimum temperature reached was
43.3 C. The minimum temperature reached was 35.5 C.
A comparison of FIG. 37A and FIG. 37B much more uniform
with the grid/susceptor combination, than with the
susceptor alone.
In this e~periment, the susceptors used had a
resistivity of about 70 ohms per square. The grid had an
equalateral triangular lattice geometry with circular
openings having a diameter of about 1~4 inch (0.64 cm.).
A spacing of 1/8 (0.23 cm.) between openings was u~ed.
The susceptor and grid were essentially in contact with
each other. The technique of taking temperature
meaæurements from the back side of the susceptor was used
in this e~periment, for the reasons described above. The
temperature measurements provide a relevant indication of
heating differences.
The above-discussed uniformity of heating due to the
grid/susceptor has also been observed when a food item 18
is included. A pizza was prepared a conventional oven, a
pizza was prepared using a susceptor
66alb
1 330066
-70-
alone, and a pizza was prepared using a grid/susceptor
combination in accordance with the present invention.
Measurements were made with an infrared camera to deter-
mine the uniformity of heating. Measurements of the
heating of the bottom of the pizza showed uniformity of
heating using the grid/susceptor combination which was as
good as or better than that achieved with a conventional
oven. Table IV shows a comparison of the measurements
taken.
TABLE IV
Standard
~im~m ~LaQ~Deviatio~
Susceptor 71.6 C 145 C 101 C15.0 C
Grid and Susceptor 89.7 C 130 C 116 C 6.3 C
20 Conventional Oven 84.9 C 117 C 102 C 6.6 C
The cooked crust was observed after the pizza had
been heated in each case. With the grid/susceptor combi-
nation, the browning of the crust was more uniform in this
experiment than the browning of the crust which was
achieved in a conventional oven. ~he browning of the
crust was significantly better than the bsowning which was
achieved using a susceptor alone. Using a susceptor
alone, only the outermost perimeter of the bottom of the
pizza was browned.
A grid and susceptor combination having a composite
microwave power transmittance, or transmissivity, less
than 50 percent, when the grid/susceptor combination is
measured alone, is preferred. A grid and susceptor
combination having a composite microwave power transmit-
tance, or transmissivity, less than 25 percent is more
preferred. A grid and susceptor combination having a
composite microwave power transmittance less than 10
1 330066
71- -
percent is even more preferred. A grid and susceptor
combination having a composite microwave power transmit-
tance less than 5 percent is especially preferred. A grid
and susceptor combination having a composite microwave
power transmittance less than 2 percent is more especially
preferred.
The grid ana susceptor combination must have a
composite microwave power transmittance greater than
3 x 10 4 percent.
It is desirable to have a heater for a microwave food
package which has a composite microwave power transmit-
tance which does not substantially change during microwave
cooking. For e~ample, in FIG. 6, the composite microwave
power transmittance of the examples shown changed less
than 3 percentage points after microwave heating.
A grid and susceptor combination preferably has a
composite microwave power transmittance which changes less
than 20 percentage points after microwave cooking. This
is measured by first measuring, using a network analyzer,
the composite microwave power transmittance before micro-
wave cooking, of the grid and susceptor combination alone.
This gives an initial transmittance Tl. The entire
package, including the food item, is then placed in a
microwave oven and heated for the predetermined heating
time determined by the food item. The grid and susceptor
combination is removed and measured alone in the network
analyzer to determine the composite microwave power trans-
mittance T2 after microwave cooking. The change TC is
preferably less than 0.20 (TC = Tl -T2), or 20 percentage
points. For example, if Tl is S percent, or 0.05, then T2
is preferably less than 25 percent, or 0.25.
1 330066
-72-
A more preferred grid and susceptor combination has a
composite microwave power transmittance which changes less
than 15 percentage points. An even more preferred grid
and susceptor combination has a composite microwave power
transmittance which changes less than 10 percentage
points. An especially preferred grid and susceptor combi-
nation has a composite microwave power transmittance whic.
changes less than 5 percentage points. A more especially
preferred grid and susceptor combination has a composite
microwave power transmittance which changes less than 4
percentage points as a result of microwave heating. An
even more especially preferred grid and susceptor combina-
tion has a composite microwave power transmittance which
changes less than 3 percentage points as a result of
microwave heating.
Grid Thickness
FIG. 13 is a graph representing a plot of the
reflectance of the grid 19 alone as a function of the
thickness of the grid 19. Thlckness ranges started with a
grid thickness of about 3/10000 of an inch (0.00076 cm).
This thickness was chosen because it is generally the
thinnest rolled foil which was considered practical. A
thickness as large as 0.003 (0.0076 cm) inch was utilized,
which represents a relatively thick aluminum foil for
packaging applications.
In FIG. 13, three different curves are shown. Each
curve represents the diameter of the openings 27 in the
grid 19. The grid geometry utilized was case l--circular
openings with an equilateral triangular lattice.
FIG. 13 shows that, for the ran~e of ~hicknesses
which are practical for the foil grid 19, the reflectance
is virtually unaffected by the thickness of the grid 19.
1 330066
-73-
If the thickness of the grid is made too thin, the
mechan cal integrity of the grid may be adversely
affected. Also, if the thickness of the grid is made too
thin, the electrical resistance of the strips 28 may
S become appreciable, and as a result, th~ strips 28 of the
grid may heat sufficiently to cause the conductivity of
the strips 28 to be broken. If this happens, it may
adversely affect the electrical integrity of the qrid. In
some applications, this may be undesirable.
Packaae Design Procedure
A preferred technique for designing a package for a
given food product involves an optimization procedure
which goes through iterations. ~o start, a food product
may be cooked in a microwave oven using nothing but a
conventional susceptor. In examples where the present
invention is most advantageous, the results of cooking the
food product with the susceptor alone will typically be
unsatisfactory. The results of this cooking test,
however, are used to evaluate a starting point for the
design of a package utilizing the present invention. The
product resulting from heating with a susceptor alone is
evaluated. If the interior of the food heats too much, or
is too tough, or other indications of overheating are
observed, shielding of the top of the package may be
indicated. If the edges of the food are overheated, too
tough, or otherwise show indications of absorbing too much
microwave energy, side shielding may be indicated. In
most applications, a starting point for a package design
may be a package having top and side shielding, with a
grid/susceptor combination forming the bottom of the
package. For many applications, it is useful to start
with a grid/susceptor combination having the following: a
~rid with s~uare openings in a square lattice configura-
tion, where the openings have a height and width equal to
1 330066
-74-
about 1/2 inch. The width of the strips of ~he grid,
i.e., the spacing between the holes, may be 3/16 inch. A
susceptor havin~ a surface resistivity of about 70 ohms
per square may be used initially. The susceptor may be
placed at the bottom of the package, with the grid in
contact with it and immediately above it. The food may
then be placed on top of the grid, so that the grid is
interposed between the susceptor and the food. The food
is then heated in a microwave oven using a package
according to this initial design. The resulting product
is evaluated.
The design factors discussed above may then be used
to optimize microwave heating of this package. If the
l; microwave heating time is too long, openings may be cut in
the shielding, or the size of the openings in the grid may
be increased. Once the heating time is within a desirably
short range of time, the package may be optimized or
fine-tuned.
~ or example, if the surface of the food is over-
heated, various design factors may be changed to compen-
sate. The hole size may be made smaller to decrease the
surface heating of the food. Alternatively, the
resistivity of the susceptor may be changed to move
surface resistivity to a less optimum point. If a contour
plot such as is shown in FIG. 12 is developed, adjustments
of hole size and resistivity may be made in accordance
with such a contour plot developed for the particular food
item in question. If the surface is underheated, opposite
steps may be taken.
If less reflection is desired, a triangular lattice
geometry may be used. If more uniformity is desired, the
spacing between openings may be reduced, and the size of
the holes may be reduced. Decreasing the size of the
1 33l:1066
holes may require the adjustment of other design factors,
such as the resistivity of the susceptor, in order to
compensate for the overall reduced heating which may
result.
If the susceptor is placed between the grid and the
food, the distance between the grid and the susceptor may
be increased to reduce the overall heating of the suscep-
tor. If the gr id is placed between the susceptor and the
food, increasing the separation between the grid and the
susceptor will increase the heating of the susceptor,
within limits. In such an example, however, separation
between the food and susceptor will also occur. Thus, the
overall heating of the food may be affected to the extent
that the food is not directly in contact with the suscep-
tor. ~s the spacing between the grid and the susceptor is
increased, the susceptor will tend to behave more and more
like a susceptor used alone without a grid.
For a given hole size and grid/susceptor spacing, the
impedance of the susceptor, including its surface
reactance, may be optimized or matched to increase the
absorbance, if desired.
In designing a package in accordance with the present
invention, the reflectance, absorbance and transmittance
of the grid/susceptor combination is considered indepen-
dently of the effect that the presence of food has upon
the parameters. Using the iterated process described
abo~e, the performance characteristics of the grid/suscep-
tor combination may be adjusted to optimize the package
without requiring a detailed analysis of the parameters
which result when the food is present.
In a preferred package design, the susceptor means
should not be allowed to overlap the area of the grid. If
-
1 330066
- 76 -
an outer edge of the susceptor means is esposed, it will
tend to severely overheat. The grid should be the ~ame
size as, or ~lightly larger than, the area of the
susceptor means.
ALTER~ATIV~ EMBODIMENTS
E~ample 1
FIG. 20 shows an alternative embodiment of the present
inven~ion. In this e~ample, sis Pizza Rolls* brand hot
snacks 29, made by The Pillsbury Company, were placed
inside a package comrpising two grids 30 connected by
shielded walls 31. ~he grids 30 and shielded walls 31
were conductive, and were made of aluminum foil in this
example. Two susceptors 32 were preovided on either side
of the grids 30 on the side remote from the hot snacks
29. In this esample and in the esamples discussed
hereinafter, the susceptors 32 and grids 30 are
functionally the same as the susceptor means 20 and grid
19 descrihed above. Also, in this particular esample, a
corrugated medium 33 made of paper supported the package.
In this esample, the grids 30 had openings 34 in the
shape of 1~2~ 2~ (1.27 cm) squres. The ~usceptors 32
were approsimately 70 ohms per squre. Complete aluminum
foil shielding 31 was provided around the ~ides of the
package.
In this esample, frozen snacks 29 were microwaved
~ 2 minutes on one ~ide. The entire package was flipped
over and heated an additional 1-1~2 minutes on the other
~ide. Six hot snacks ~90 grams) were cooked using this
method. This produced hot ~nacks 29 having a crisp
e~terior and a moist interior.
6681b
1 330066
- 77 -
E~ample 2
The ~ame package was used with Banguet~ brand
microwave chicken nuggets. Sis frozen chicken nuggets
were cooked 1 minute and 15 ~econds on one side, and for a
~imilar time period on the other side. Theis cooking
method produced chicken nuggets having a crisp e~terior
and a moict interior.
EsampL~ 3
FIG. 21 illustrates an alternative embodiment for
french fries 35. The french fries 35 were completely
enclosed in a shielded container 36, having a grid 37
along the bottom portion thereof. A ~usceptor 38, having
an oil coating on the polyester side, supported the french
fries 35, and was in direct contact therewith. The entire
package was supoorted on a corrugated medium 39. The grid
37 had about 70% open area. As shown in FIG. 21, the top
and sides of the package was completely shielaed with an
aluminum shield 36.
In this e~ample, frozen french fries 35 were heated
for 1-1/2 minutes. The top shield 36 was then removed to
flip the french fries 35 over. The shield 36 was
replaced, and the fries 35 were heated for an additional
1- V2 minutes. This produced browned, crispy fries with a
tender, moist interior. The crispness was similar to
pan-fried potatoes, the french fives 35 were crisper than
oven-baked french fries, although not necessarily as crisp
as freshly deep-fat-fried french fries. The french fries
used were partially cook~d frozen french fries 35.
6681b
-
1 330066
- 78 -
Exampl~ 4
FIG. 22 illustrates an e~ample of the invention used
in connection with fish ~ticks 40. In this esample, four
fish stick pieces 40 (100 grams) were prepared in a
package having aluminum shielded ~ides 41, with grids 42
directly adjacent the top an~ bottom of the fish sticks
40. Susceptors 43 were provided on the top and bottom of
the package immediately adjacent to the grids 42, located
on the side of the grids 42 remote from the fish æticks 40.
In this e~ample, the package of frozen fish sticks 40
was heated 1 minute and 15 seconds, and was then inver~ed
and heated an additional 1 minute and 15 seconds. The
fish sticks 40 heated in this manner had a tender interior
and a crisp e~terior. In the past, fish sticks heated in
a microwave oven on a standard susceptor typically
resulted in fish sticks where the end pieces of fish were
overcooked. The qrid~susceptor system shown in FIG. 22
eliminated nonuniform cookinq of the fish sticks 40. Van
De Kamp s* brand fish sticks were used in this e~ample.
~ampleL~
Using a package of the same construction as shown in
FIG. 22, frozen Van De Kamp s~ brand microwave fish
fillets were heated in a microwave oven. In ths e~ample,
best results were observed when the grids 42 above and
below the suscepors 43 were constructed havinq an open
area between 40% and 60%. Microwave cooking time was
between 6 and 8 minutes. Usinq a conventional susceptor,
only the bottom of these fish fillets is normally crisp.
The grid~susceptor system shown in FIG. 22 when used in
connection with ~uch fish fillets producted a crisp top
and bottom on the fish fillets, and eliminated toughening
o
6681b
~ 330066
-79-
the perimeter of the fish fillets which had been
previously experienced.
Example 6
s
FIG. 23 shows an alternative embodiment using a
package which is only partially shielded. Only the sides
44 of the package is shielded using aluminum shielding. A
grid 45 in combination with a susceptor 46 was provided at
the bottom of the package. A susceptor 47 was provided
alone at the top of the package. This package was used
for microwave cooking of fish fillets 48.
In this example, the grid 45 had an open area
percentage of 25%. The grid had openings 49 in the form
of 1/2" x 1/2" (1.27 cm) squares. The susceptor 46 had a
resistivity of approximately 70 ohms per square. A
similar susceptor 47 was used for the top of the package.
In this example, two pieces of fish fillet 48 (120
grams) were cooked for 1-1/2 minutes configured as shown
with the grid/susceptor system on the bottom. The package
was then flipped, and heated for an additional 1-1/2
minutes with the susceptor 47 on the bottom. This
produced fish fillets 48 having browned, crisp batter with
a tender fish interior.
Exam~le 7
FIG. 24 illustrates an embodiment used for cooking
bread 50 in a microwave oven. Baking bread in a microwave
oven is a challenge. The baking time must be slow enough
for the bread to rise and establish a good cell structure.
Crust formation and brownin~ are highly desirable in
baking breaà. Conventional microwave cooking offered no
means for slowing the heating rate. Conventional
,
1 330066
- 80 -
microwave cooking would result in coarse irregular cell
structure in the bread because steam would be generated
too rapidly for the bread structure to contain it. A
package constructed in accordance with the present
invention slowed the heating rate and producea some crust
browning during microwave cooking.
In this e~ample, Rhoaes~ brand frozen yeast bread was
heated in a microwave oven using tow aluminum bread pans
51 and 52. One bread pan 51 was in~erted and formed the
top of the package. Both bread pans 51 and 52 had 1/2~ ~
1/2~ (1.27 cm) square holes cut in the pans. This formed
a grid 53 completely surrounding the bread 50. The
grid 53 was fully lined on the inside with su~ceptors 54.
In this e~ample, the bread dough was defrosted and
proofed at ambeint temperature. The package and bread 40
were then placed in a microwave oven and heated for
22 minutes. In this e~ample, about 454 grams of bread 50
was coo~ed. This cooking time compares with a cooking
time of about 35 to 40 minutes in a conventional oven.
The volume of bread loaf 50 appeared to be good, and cell
structure was observed to be regular. The bottom and
sides of the bread SO were browned. ~ome browning
occurred on the top of the bread 50, but the bread was not
uniformly browned.
E~ample 8
FIG. 25 illustrates an e~ample used to heat caramel
rolls 55. Eight caramel rolls 55 were prepared by heating
between two susceptors, a first suscpetor 56 on top having
a resistivity of about 1000 ohms per square, a second
susceptor 57 on the bottom having a resistivity of about
70 ohms per ~quare. The bottom susceptor 57 was placed
coplanar with, and immediately above, a grid 58 made by
6681b
-
1 330066
- 81 -
cutting openings in the bottom of an aluminum pan. The
sides 59 of the aluminum pan provided ~hielding. A
mi~utre of melted butter and caramel topping 60 was placed
upon the bottom susceptor 57. Heating time for the eight
rolls 55 was about sis minutes. In this esample, the
caramel on the bottom turned out ~ery well, and a golden
brown top for the rolls 55 was produced. The rollæ 55
were not tough.
In this e~ample, different resi~tivity susceptors 57
and 56 were used to achieve ma~imum browning on the top of
the rolls 55, and better caramelization on the bottom.
When this esample was attempted with two susceptors both
having a resistivity of about 70 ohms per square, no
caramelization occurred before the bread was unacceptably
toughened. An attempt to use gridæ 58 over both the top
susceptor 56 and the bottom susceptor 57 slowed the cookig
of the bread, and the bottom browned but the top did not
brown. Remo~ing ony the top grid and staying with 70 ohms
per sguare susceptor resulted in too much heating of the
bread. Top browning occurred, but the bread was
unacceptably tough. The best product was achieved when a
high resistivity suceptor 56 of appro~imately 1000 ohms
per square was used without a grid on top of the rolls 55.
Esanple 9
FIG. 26 illustrates another e~ample in which
Pillsbury~ ~1869~ brand refrigerated biscuits 61 were
prepared. Four biscuits 61 were placed in a package
between a top susceptor 62 and a bottom susceptor 63. The
susceptors 62 and 63 both had a resistivity of about 70
ohms per squre. A top grid 64 a~d a bottom grid 65 were
placed coplanar with, and immediately adjacent to, the
susceptors 62 and 63. The grids 64 and 65 were placed
6681b
.... .
1 330066
-82-
next to the susceptors 62 and 63, respectively, on the
sides remote from the biscuits 61. Shielding 66 was
provided on the sides of the package.
In this example, the four biscuits 61 were heated in
a microwave oven for about four minutes. The tops and
bottoms of the biscuits 61 were browned. The cell struc-
ture of the bread 61 was somewhat dense, but no excess
toughening from overcooking was noticed.
Example 10
FIG. ~7 illustrates an example where an unsupported
susceptor film 67 was used in connection with a flexible
foil grid 68. The susceptor 67 comprised a flexible sheet
of polyester, containing a metallized coating deposited
thereon. The metallized polyester 67 was wrapped around
five fish sticks 69.
The alu~inum foil grid 68 was folded and crimped
along the three open edges, taking care not to have any
loose edges that might form a critical gap and arc. Slits
were cut in the polyester film 67 along the edges of each
fish stick 69 on both the top and bottom for venting
purposes. This assembly was then placed upon a corrugated
paper pad 70. The package was placed in a microwave oven
and exposed to microwave radiation for two minutes. The
package was then flipped over and exposed to microwave
radiation for another two minutes.
After ~icrowave heatinq, considerable moisture
condensation was observed on the polyester film 67. The
susceptor film 67 had a slightly melted look around the
edges of each square in the grid 68, but was otherwise
intact. Overall crispness of the breading of the f ish
sticks 69 was believed to be equal to that achievable on a
1 330066
-83-
standard susceptor. Both the top and the bottom of the
fish sticks 69 were crisp. The fish sticks 69 were more
evenly cooked than fish sticks which had been cooked alone
or on a standard susceptor.
Example 11
~ IG. 28 illustrates an example where cookies 71 were
heated in a microwave oven. In this example, a top
susceptor 72 and a bottom susceptor 73 were provided on
the top and bottom of the package, respectively. Immedi-
ately adjacent to each susceptor 72 and 73 was a top grid
74 and a bottom grid 75, respectively.
In this example, the ~rid/susceptor combination 72,
74 and 73, 75 do not contact the food 71. Instead, the
susceptors 72 and 73 heat the air inside the package,
baking the cookies 71 much like a conventional oven.
Thus, the present invention can be used to simulate a
baking environment in a conventional oven, when it is
desirable to do so.
The grid 75 was made by cutting circular holes in the
bottom of an aluminum foil pan 76. The holes in the grid
75 were 3/4 inch (1.9 cm) in diameter with 1/8 inch
(0.3175 cm) spacing between the openings. Circular
openings with an equilateral triangular lattice structure
were utilized. An aluminum foil sheet 77 generally
bisected the package and provided a support for the
cookies 71. The top of the package was similarly formed
from an inverted aluminum foil pan 78. The grid 74 cut in
the top of the inverted pan 78 had a similar geometry.
The aluminum foil sheet 77 was made from one mil thick
aluminum foil. The edges where the bottom pan 76 and the
top pan 78 join together were carefully sealed with the
1 33006~
-84-
foil 77 to prevent leakage of microwave energy. Thus, the
sides of the pans 76 and 77 formed shielding 79.
The top susceptor 7~ and the bottom susceptor 73 each
had a resistivity of about 70 ohms per square. Six
cookies 71 were placed in the package, each having a net
weight of about lS grams. A Pillsbury brand refrigerated
chocolate chip cookie dough was used to form the cookies
71.
The resulting cookies 71 which were heated in a
microwave oven turned out like cookies baked in a conven-
tional oven. The cookies spread during cooking to a
generally uniform thickness. The surface appearance of
lS the cookies 71 was typical of a conventional oven baked
cookie. Light even browning of the surface of the cookies
71 was achieved. The cookies were heated in a microwave
oven for six minutes to achieve these results.
Prior attempts to make a cookie in a microwave oven
using just a susceptor were unsatisfactory. The cookie
prepared in this manner would not spread adequately.
Browning of the surface of the cookie was unsatisfactory.
The top surface would not brown, and the bottom surface
would be overly browned.
Example 12
FIG. 29 illustrates another example for heating
biscuits 80 in a microwave oven. This particular package
is suitable for refrigerated dough products in general.
The biscuits 80 were placed upon a conventional aluminum
foil tray 81. Unlike the above-described examples, the
foil tray ~1 did not have any openings cut in the bottom
of the tray 81. Immediately below the tray 81, a suscep-
tor 82 was placed in contact with, and supporting the
1 330066
-85-
bottom of, the tray 81. Immediately below the susceptor
B2, a grid 83 was provided.
A grid 84 was provided on the top of the packase.
The grid 84 was secured in sealing engagement with the
tray 81 to prevent microwave leakage around the grid 84.
A top susceptor 85 was also provided on the side of the
grid 84 remote from the biscuits 80. The susceptors 82
and 85 both had a resistivity of about 70 ohms per square.
In this example, uniform browning on the bottom of
the biscuits 80 was achieved. The biscuits 80 r~se
adequately and had a good cell structure. The top of the
biscuits B0 browned where they contacted the top
grid/susceptor combination 84, 85. The combination of the
grid 83 and the susceptor 82 on the bottom of the package
provided a very uniform heating of the bottom of the foil
tray 81.
Example 13
FIG. 29A shows an example utilizing the invention
having a completely unshielded package. In this example,
a Pillsbury brand Microwave French Bread Pizza was
utilized. The package was modified to include the
insertion of a grid 100 cut from aluminum foil. The grid
had 1/2 inch sguare holes cut in an equilateral triangular
lattice configuration. The spacing between the holes was
1/8 inch,
The commercially available french bread pizza had a
susceptor tray 99 which rested upon a corrugated paper pad
102. The french bread pizza 101 was removed from the tray
99, and the grid 100 inserted between the french bread
pizza 101 and the susceptor tray 99.
1 330066
86-
The cooking time in this example was increased by 15
seconds over the heating time for the commercially
available product. The results were improved over the
commercially available product in that the crispening of
the crust of the french bread pizza 101 was more uniform.
Example 14
A pizza package was constructed in accordance with
FIGS. 3 and 4, except that the susceptor 20 was made from
microwave magnetic absorbing material. The microwave
magnetic absorbing material was removed from a commer-
cially available microwave browning utensil, i.e., a
Microware brand crisper/griddle Model PM 400/145, by
Anchor Hocking Corporation. When heated in a microwave
oven for the same period of time, the pizza 18 was
satisfactory.
Exam~le 15
In this example, various hole sizes were tested for
temperature variation in susceptor heating. Square holes
were used in all instances where a grid was used. A grid
was used in all instances, except two control cases, where
a susceptor was heated alone.
In case 1, a single 10 inch wide square hole was
used. The "grid" was a 1/4 inch wide conductive strip
around the outside edges of a square susceptor, slightly
larger than 10 inches in width. In case 2, four square
openings having a width of about 5 inches were used. In
case 3, 16 square openings having a width of about 2.5
inches were used. In case 4, 64 square openings having a
width cf about 1.25 inches were used.
- 1330066
-87-
Temperature measurements were made with an infrared
camera, and the minimum, maximum, average temperature and
standard deviation measurements were taken in the manner
described herein for such measurements.
TABLE Y
Standard
~lnim~m ~a~imYm AYera~e Dçyiation
lG Contr~l 1 28.0 C 61.2~ C 31.2 C 3.1D C
Controi 2 28.3~ C 49.8 C 31.1 C 2.2 C
Case 1 28.0 C 35.6 C 30.~ C 1.5 C
Case 2 27.6 C 38.6 C 32.0 C 2.0 C
Case 3 27.3 C 37.2 C 30.3 C 1.9 C
20 Case 4 27.1 C 38.1 C 29.6 C 1.9 C
The holes 27 may have various shapes. The holes may
be circular (see FIG. 39B), square (see FIG. 39A and FIG.
39M), triangular (see FIG. 39C), hexagonal (see FIG. 39D
and 39E), "U" shaped ~see FIG. 39L and FIG. 39P),
rectangular (see FIG. 39G), cross-shaped (see FIG. 39J),
and oval (see FIG. 39F). The holes 27 may include various
shaped reflective patches 103 within the hole 27, such as
a reflective circle 103 within a circular shaped hole 27,
as shown in FIG. 39H. Alternatively, a reflective square
or rectangle 104 within a square or rectangular hole 27
may be used, as shown in FIG. 39I. Crescent shaped holes
27 may be used, as shown in FIG. 39K.
~he geometry of the holes may take various forms. In
addition to the square lattice and equilateral triangular
lattice described above, the geometry may be radial. In
addition, differential geometry may be used where
different spacing is provided between holes 27 in the grid
19, as shown in FIG. 390, or different size holes may be
used in various regions of the grid 19, as shown in FIG.
1 330066
-88-
39M and FIG. 39N. In addition, different shaped holes may
be used in various locations of the grid.
The susceptor means 20 is preferably a thin film of
S aluminum m~tallized on a polyester substrate. The heater
or susceptor 20 may alternatively be another type of thin
film susceptor, a dielectric material having a dielectric
loss factor E" greater than 2, a microwave magnetic
absorbing material, graphite, or combinations of such
materials, or a composite structure composed of different
layers or dispersed portions of such materials.
The package 17, 16 enclosing the food item 18 may be
partially shielded, as shown in FIG. 3 and FIG. 4.
Alternatively, the package enclosing the food item may be
totally shielded (except for the grid/susceptor combina-
tion), totally unshielded (except for the grid/susceptor
combination), or partially shielded. Alternatively, a
grid/susceptor combination may be wrapped around a food
item, and may itself comprise the package in which the
food item is heated in a microwave oven.
The grid is preferably an aluminum foil lattice
structure. However, the grid may also be hot stamped
metal, metallized films such as aluminum, stainless steel,
copper or steel, or the grid may be a wire mesh. The grid
may comprise interwoven str_ps of metal or overlaid strips
of metal 105, as shown in FIG. 40F and FIG. 40E. The grid
may be formed from a sheet of metal having holes punched
33 therein. The grid may comprise expanded metal, punched
and upset metal sheets. The grid could alternatively be
formed from a lattice structure formed from overlappina
spiral and radially cut strips.
The preferred susceptor and grid configuration is a
planar grid in contact with, or spaced less than 0.048
1 330066
-89-
inch (0.122 cm) from, a planar susceptor. Alternative
arrangements may include susceptor material 20 filling the
holes 27 in the grid 19, as shown in FIG. 40A and PIG.
40B. Optionally, a sheet of backing material 106, such as
paper, may be provided. In FIG. 40A, the holes 27 in the
grid 19 are completely filled with susceptor material 20.
In ~IG. 40B, the holes 27 are not completely filled. The
holes 27 may be circular, square, or some other shape,
leaving an annular opening space around the susceptor
material 2~. Referring to FIG. 40D, susceptor material 20
may be placed behind the grid 19 in the form of patches,
circles or squares of susceptor material which preferably
are ge~metric shape as the holes 27, and slightly larger
than the size of the holes 27 so that the susceptor means
20 overlaps the hole 27. Other arrangements include a
grid lattice 19 embedded or encased in a susceptor medium
20, as shown in FIG. 40C. In addition, the grid 19 may be
constructed so that it is sandwiched between susceptor
sheets.
The grid preferably has a microwave power reflectance
more than 40 percent when the grid is measured alone.
More preferably, the grid has a microwave power
reflectance which is more than 85 percent, when measured
alone. A microwave power reflectance more than 95 percent
for the grid, when measured alone, is even more preferred.
A spacing 28 between adjacent openings 27 in the grid
19 which is less than one inch is preferred. A spacing
between openings 27~1ess than 1/2 inch is more preferred.
A spacing less than or equal to about 3/6 inch between
openings 27 is even more preferred. Openings having a
spacing which is less than or equal to about 1/8 inch is
especially preferred.
1 330066
--so--
Measurement Procedures
In the above descriptions, measurements of
resistivity, reflectance, transmission, absorbance, etc.,
were all taken at room temperature ~21 C) unless
otherwise specified.
In the abcve descriptions, measurements taken with a
network analyzer all involved the procedure described
below, unless otherwise specified. The procedure may best
be understood with reference to FIG. 46 and PIG. 47. A
Hewlett Packard Model No. 8753A network analyzer 107 in
combination with a Hewlett Packard Model No. 85046A S-
parameter test set were used. A11 measurements were made
at the microwave oven operating frequency of 2.45 ~Hz.
All measurements were made at room temperature, unless
otherwise specified. All measurements are made using WR-
284 waveguide 108, unless otherwise specified. The
measurements graphed in FIG. 30A and in FIG. 30B were made
using a WR-340 waveguide. Measurements of reflectance,
transmission and absorption for a grid/susceptor
combination, in the above discussion of design factors,
should be made without the presence of a food item.
Measurements are preferably made by placing a sample
109 to be measured between two adjoining pieces of
waveguide 108. Conductive silver paint 110 is preferably
placed around the outer edges of a sample sheet which is
cut slightly larger than the cross-sectional opening 111
of the waveguide. Colloidal silver paint ~10 made by Ted
Pella, Inc. has given satisfactory results in practice.
The sample 109 is preferably cut so that it has an overlap
of about 50/1000 inch (0.127 cm) around the edge. The
waveguide is calibrated according to procedures specified
and published by Hewlett Packard, the manufacturer of the
network analyzer.
1 330066
-91-
Scattering parameters, Sll, S12, S21 and 22'
measured directly by the network analyzer. These measured
parameters are then used to calculate the microwave power
reflectance, power transmittance, and power absorbance.
The reflectance looking into port 1 is the magnitude
of S11 squared. The reflectance into port 2 is the
magnitude of S22 s~uared. The transmittance looking into
port 1 is the magnitude of S21 squared. The transmittance
looking into port 2 is the magnitude of S12 s~uared. The
power absorbance, looking into either port 1 or port 2, is
equal to one minus the sum of the power reflectance and
the power transmittance into that port.
The complex surface impedance of an electrically thin
sheet is obtained from the measured scattering parameters
using formulas presented in "Properties of Thin Metal
~ilms at Microwave ~requencies", by R. L. Ramey and T. S.
Lewis, published in the Journal of Applied Physics, ~ol.
39, No. 1, pp. 3883-84 (July 196~), along with the
information in J. Altman, Microwave Circuits, pp. 370 71
(1964). ~or unused susceptor material, the impedance is
essentially all resistive. For highly conductive grids,
the impedance is entirely reactive.
Conclusion
The present invention is particular concerned with
microwave environments of the type encountered in micro-
wave ovens. The present invention is particularlyapplicable in microwave environments were the average rms
electrical field strength is greater than 1 v/cm. In a
typical application involving the present invention, the
food package including a grid/susceptor combination is
intended for use in the enclosed cavity of a microwave
1 330066
- 92 -
oven having a power input of at least 10 watts, more
typically in escess of 400 watts.
The above disclosure has been directed to a preferred
embodiment of the preæent invention. The invention may be
embodied in a number of alternative embodiments other than
those illustrated and descr~bed a~ove. A person silled in
the art will be able to conceive of anumber of
modifications to the above describd embodiment6 after
having the benefit of the above diæclosure and having the
benefit of the teachings herein. ~he full æcope of the
invention shall be determined by a proper interpretation
of the claims, and shall not be unnecesæarily limited to
the specific embodiments described above.
--~=trade-mark--.
6681b
