Note: Descriptions are shown in the official language in which they were submitted.
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Improved microwave heating
Field of the Invention
______________________
This invention relates to improvements in the heating
of substances in a microwave oven. While the substances
most commonly heated will be foodstuffs, and the examples
below will therefore relate to foodstuffs, the present
invention is not limited in this respect and can be used
for heating other substances.
Background of the Invention
In normal microwave transparent containers, the micro-
wave energy can enter through the top, bottom and sides of
the container. This is similar to the situation
encountered during conventional oven cooking (or heating).
With normal microwave foil containers the microwave energy
15 can only enter through the top (food surface).
Prepared foods are commonly reheated in a cooking
utensil on a stove top. One characteristic of this type
of reheating is that the hea~ enters the food through the
bottom of the container/utensil.
Heating the food from the bottom offers some
advantages, as a result of the heat transfer mechanisms
that take place. The food in contact with the base heats
and becomes less dense. This provides a driving force for
~'
13~S~g
convective transport, the warm food rising and being
replaced by cooler food from nearer the surface. The
extent of thiS Convection depends on the viscosity of the
food. At a later stage of heating, bubbles of steam
nucleate at or near the base and rise through the food.
ThiS transfers heat throughout the food as well as
agitating the product.
Prior_Art
With a view to simulating this type of "bottom heating"
in a microwave oven, there has been proposed in R. Nakanaga
U.S. patent 4,661,672 issued April 28, l9B7, a rectangular
container having a microwave energy shielding layer extend-
ing over the top and down at least the upper portions of
the side walls, while the remainder, and particularly the
lS bottom of the container, was made of a microwave trans-
parent material. In this way, the microwave energy is
caused to enter the container through its bottom, and
possibly to some extent through the lower parts of the side
walls. This prior patent also discloses the feature of
elevating the container so that its bottom is spaced above
the ~loor of the microwave oven. A similar arrangement is
disclosed in K. Sugisawa et al European patent application
0,185,488 published June 25, 1986, although in this case
the top of the container is only shielded at its edges,
whereby to avoid excessive heatin~ of the upper surface of
the material located at the sides of the container.
For food loads that have low viscosity and hence allow
substantial heat transfer by convection, conventional
containers will often perform satisfactorily. ~o~ever, in
many cases the characteristic non-uniform heating that
results from the dominant fundamental energy distribution
will not be sufficiently equalised by convective heat
transfer, and an unsatisfactory product will result. In
particular, there will tend to be excessive heating at the
edges and insufficient heating in the middle of the body
~30~iS~9
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of food. Viscous products such as meat stews or
casseroles, lasagna, macaroni cheese, thick soups and
chowders are particularly difficult in this respect.
Summary of the Invention
The object of the present invention is to minimize
this difficulty, and in particular, to provide an arrange-
ment in which the food product ~or other substance) is not
only heated at its undersurface (although not necessarily
only at its undersurface), but is also heated more rapidly
and/or more uniformly across its lateral dimensions.
To this end, the present invention provides a stand
for use with a container having at least one ~ortion
transparent to microwave energy including a bottom on
which there is supported an undersurface of a substance to
be heated. The stand comprises means for modifying the
microwave field pattern to which the container is exposed,
and means for supporting the container spaced above such
modifying means so that the undersurface of the substance
is maintained at a predetermined distance from such
modifying means.
In the preferred form of the invention, the modifying
means takes the form of means for generating a modified
microwave field pattern having at least one mode of
microwave energy of hi~her order than the fundamental
modes of SUCIl energy.
Migher order mode generating means are known per se.
see, Eor example, R. Keefer Canadian patent application
Serial No. 485,142 filed June 25, 1985, now Canadian
patent No. 1,239,999 issued August 2, 1988 (u.s. patent
application Serial No. 878171 filed June 25, 1986, now
U.S. patent No. 4,866,234 issued September 12, 1989 and
European patent application No. 86304880 filed June 24,
1986 and published December 30, 1986). Such higher order
mode generating means may take the form of one or more
electrically conductive plates (or apertures in an
electrically conductive sheet) arranged in a symmetrical
planar array. Examples of such structures are discussed
below.
l3a~s~s
The term "mode" is used in the specification and claims
in its art-recognized sense, as meaning one of several
states of electromagnetic wave oscillation that may be
sustained in a given resonant system, each such state or
type of vibration (i.e., each mode) being characterized by
its own particular electric and magnetiC field configur-
ations or patterns. The fundamental modes of the
container and body are characterized by an electric field
pattern (power distribution) confined or concentrated
around the edge of the container (as viewed in a
horizontal plane), these fundamental modes predominating
in a system that does not include any higher order mode-
generating means. The fundamental modes are defined by
the geometry of the container and the contained body of
material to be heated.
A mode of a higher order than that of the fundamental
modes is a mode for which the electric field pattern
(again, for convenience of description, considered as
viewed in a horizontal plane) is concentrated around the
periphery of an area smaller than that circumscribed by
the electric field pattern of the fundamental modes.
Each such electric field pattern may be visualized, with
some simplification but nevertheless usefully, as
corresponding to a closed loop in the horizontal plane.
Alternatively, or additionally, the modifying means
may take the form of means for enhancing the coupling of
microwave energy into the undersurface of the substance to
be heated. Such coupling enhancement means are more fully
described below.
In addition to its microwave transparent portion or
portions, the container may have some portions that are
reflective of microwave energy. When bottom heating is to
be the dominant mode of heating, the transparent portions
will be principally constituted by the bottom Of the
container. Thus, in one embodiment, the lid and side
1 3 ~ 6 ~ ~ 9
walls of the container can be reflective of the microwave
energy, while the bottom is transparent to such energy, so
that all the energy enters the food by its undersurface.
There may, however, be instances where it will be
convenient to allow some of the microwave energy to enter
the food through areas other than the undersurface, and
such an arrangement is not excluded by the present
invention. For example, some foods such as baked goods,
or those having a surface layer needing to be melted, e.g.
a cheese layer on lasagna, or a potato layer on shepherd's
pie, are ideally heated at the top and the bottom
simultaneously. In this case the container lid can be
microwave transparent, or can simply be removed during the
heating process. Nevertheless, in the preferred embodi-
ments of the invention, the majority of the miCrowave
energy Will enter through the undersurface Of the Container
to maximize the bottom heating effect, the advantages of
which have been discussed above.
In addition, the invention does not exclude the
possibility that may be desirable in some instances, namely
that parts of the bottom of the container, for example the
peripheral edge of such bottom, can be shielded, so as to
concentrate the microwave energy in the central portion of
the undersurface of the substance being heated.
Finally, it should be mentioned that, in those examples
where it is desired to avoid microwave energy ente~ing the
food at its lateral edyes, the stand can include upwardly
projecting metallic parts that shield these lateral edges,
thus avoiding the need for the container itself to have
reflective side walls.
In one preferred form of the invention, ~he container
will have a flat bottom, the field modifying means will be
planar, and the supporting means will be so dimensioned as
to support this flat bottom in a plane parallel to the
field modifying means. As a result a predetermined spacing
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between the container bottom and the field modifying means
is maintained ~niform throughout the lateral dimensions of
the container.
The invention also consists of the combination of a
stand as described above and a container for holding the
substance to be heated. This combination can consist of
two separate elements that are brought together for use,
with the stand being available ~or reuse with the same or
another container. Alternatively, these two elements can
be joined together and sold as a single assembly, either
for single or multiple use. For multiple use the
combination will constitute a permanent cooking vessel.
The shape of the container may be that of a
conventional tray in which frozen food is commonly sold,
;.e. a relatively shallow, rectangular or round tray with
a flat bottom, side walls and a flat removable lid.
However, one of the advantages of bottom heating, is that
the normal limitations on product depth are much less
important. Since other heat transfer mechanisms
(convection, steam bubbles) are being encouraged, deeper
loads (similar to those which would be used in a stove top
saucepan) can be satisfactorily dealt with. ThiS repre-
sents a real advanta~e. It iS also worth noting that
microwave heating from the bottom will be better than
normal stove top heating, because the penetration of the
microwave heating obviates the need to stir the product.
In normal stove top cooking, the heat energy is transferred
to the food through the base by conduction. Rapid heating
of the food normally requires the temperature of the base
of the utensil to be raised to a high temperature. To
avoid burning the food in contact with the base, low power
settings can be used ~which extend the heating time) or,
alternatively, the food must be stirred frequently (for
viscous foods).
65~9
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In a method aspeCt, the invention Can be def ined as
the provision in a method of heating a substance by micro-
wave energy, of the steps of conf ining such sub5tance in a
partia]ly shielded container that is of such a nature that
at least some and preferably the ma~ority of the energy
enters the substance through it~ undersurface, while
modifying the microwave field pattern by means located a
predermined distance below such undersurface to enhance
the coupling of microwave energy into such undersurface
and/or to improve the uniformity of heating in the lateral
dimensions.
srief Description of the Drawings
_______________________________ _
Fig. 1 shows a vertical central section of an assembly
of a container and a stand therefor, according to an
embodiment of the invention;
Fig. 2 is a section on II-II in Fig. l;
Fig. 2A is a modification of Fig. 2;
Fig. 3 is a vertical central section of a modified
stand according to a further embodiment of the invention;
Fig. 4 is a view on IV-IV in Fig. 3;
Fig. 4A is a modification of Fig. 4;
Fig. S shows a view similar to Fig. 1 of an
alternative embodiment;
Fig. 6 is a similar view of yet another embodiment;
Fig. 7 is a side view of an alternative form of stand;
Fig. 8 is a side view of a still further alternative
form of stand;
Figs. 9A and 9B are diayrams illustrating the positions
of temperature sensors used in the tests illustrated in
Figs. 10 and 11;
Figs. lO(a) and (b), and ll(a) and (b) are comparative
graphs comparing the performance of different arrangements;
Fig. 12 is a vertical central section illustrating an
application of the invention to a multi-compartment
container;
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Figs. 1~ to 15 each show alternative stand
constructions;
Fig. 15A is a graph related to Fig. 15; and
Figs. 16 to 18 each show still further alternative
stand constructions.
Detailed Description of the Preferred Embodiments
____________..____________________________________
Figs. 1 and 2 show a container 10 having a bottom 12
of a suitable microwave transparent material, e.g. fibre-
board or a plastic material, side walls 14 of metal foil
or of a laminate containing metal foil 15, and a lid 16
also of metal foil or of a fibreboard laminate including
metal ~oil 17, the lid being held in place by a fold down
rim 18. The design of the lid and rim is such that there
îs no possibility of arcing. A food load 20 is supported
in the container with its undersurface 21 on the bottom
12. This container 10 can be circular, rectangular, or
any other convenient shape in plan view. In Figs. 1 and
2, it has been assumed that the container 10 is circular.
Fig. 2A shows a rectangular stand for a rectangular
container.
The cooking assembly includes a stand 22 on which the
container 10 is designed to be seated, such stand 22
consisting of a base 24, side walls 26 and a rim 28 with
an inwardly sloping portion 30, all made of a microwave
transparent material. The base 24 is formed with either a
continuous peripheral depression or a series of such
depressions forming feet 25 that serve to elevate the base
24. Centrally of the base 24, there is a plate 32 of
conducting material, e.g. aluminum, that will serve to
modify the microwave field pattern and generate the higher
order modes. In Fig. 2, the plate 32 is circular; in Fig.
2A it is rectangular. The dimensions of the stand 22 are
such that the spacing S between the undersurface 21 of the
food load 20 and the upper surface of the plate 32 is set
at an optimum value for the conditions. The choice of the
value for this spacin~ S is discussed below. Since the
undersurface of the food into which the microwave energy
is being propagated lies in continuous contact with the
~3~65~9
bottom 12 of the container, this spacing ,S is uniform
across the lateral dimensions X and Y of the container.
The stand 22 may be a reusable kitchen appliance that
is constructed of a sturdy plastic or glass, or it may be
a more cheaply made disposable element that is sold with
the container 10 either as a separate item to be assembled
in the oven or as a fi~ture secured to the bottom of the
container 10.
The size and arrangement of the plate 32 centrally of
the base 24 in Figs. 1 and 2, is similar to arrange~ents
of conducting plates shown in the Keefer patent application
referred to above. If it is preferred to generate still
higher order modes of microwave energy at the bottom 12 of
the container, an array of a larger number of smaller
plates 34 can be provided on the base 24' of a modif ied
stand 22' shown in Figs. 3 and 4 and designed for use with
a rectangular container, thiS array o~ plates 34 being
generally similar to that shown mounted on a container lid
in Fig. 10B of said Keefer patent. This latter arrangement
is well suited to the heating of relatively shallow food
loads, since the higher order modes may not penetrate as
far into the food load as the fundamental modes. On the
other hand, they achieve enhanced unifoemity of heating
across the lateral dimensions of the container.
As explained in the Keefer patent application, an
array of plates, such as the plates 34, can be replaced by
an array of apertures in a metallic sheet that otherwise
covers the surface. Fig. 4A shows a suitable array of
apertures 36 in a conductive plate 38 on the base of a
stand 22", or the whole stand may be conductive, e.g. made
of aluminum.
Fig. 5 shows a further modification in which a stand
22 made of aluminum has upwardly extended sloping end
and side walls 27, and a base 39 containing apertures 36.
A container 11 with a food load 20 has end walls 13 that
nest snuggly within the walls 27 to support the container
~3(16509
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with its bottom 12 and hence the undersurface 21 of the
food load a predetermined distance S above the base 39.
The container ll has a lid 16. In this arrangement the
metallic wa]ls 27 of the stand provide lateral shielding
for the food load~ so that the container 11 can be made
entirely of a microwave transparent material. The lid 16
may be meta]]ic, if top shielding is required, or micro-
wave transparent, if such shielding is not required, or
some combination thereof, if partial shielding is required.
Fig. 6 shows an application of a somewhat similar
construction, as applied to a reusable cooking vessel 41
made of glass with a metallised outer surface layer 43
having apertures 45 in the portion 47 thereof that extends
across the bottom surface of the bottom portion 49. This
bottom portîon 49 of the utensil 41 is relatively thick
compared to its sides whereby to provide the necessary
spacing S' between its upper bottom surface that supports
the undersurface of the food load (not shown) and its
bottom surface 47.
Fig. 7 shows an alternative arrangement in which a
stand 40 consists of a flat base 42 supporting four posts
44 on which the container lO is placed. Conductive plates
46 are located on the upper surface of the base 42 for
generating the higher order modes.
Fig. 8 shows another construction of stand 50 made of
a solid plastic or glass slab 52 on the upper surface of
which the container lO will be placed. Legs 54 hold the
slab 52 above the oven floor, and conductive plates 56 are
secured to the underside of the slab 52.
Figs. 9A and 9B show how the tests reproduced in Figs.
lO and ll were conducted. As shown in Fig. 9A, four
temperature probes A, B, C, D were inserted into the food
load 20, approximately centrally of both lateral dimensions
of the container, and at varying depths, probe A being
nearest the undersurface of the food and probe D nearest
the top surface. Fig. 9B shows the locations of four
~3`~5¢9
temperature probes C, E, F, G inserted into the food load,
all at the same depth, i.e. at approximately one quarter
depth, and respectively located at approximately the
centre, the left end, the right end and the side (located
at the back when placed in the microwave oven) of the
container.
Fig. lO(a) shows the temperatures measured by probes
A-D when heating a load of about 680 grams of canned beef
and vegetable stew for 15 minutes in a 700 watt microwave
oven in a conventional circular foil container, i.e. one
having the following dimensions: outside top diameter 181
mm; inside top diameter 171 mm; bottom diameter 140 mm;
slant depth 38 mm; and capacity 796 ml.
Fig. lO(b) shows the same experiment when conducted in
a similar container modified to make the lid and sides
microwave reflective and the bottom microwave transparent,
and mounted on a stand as shown in Fig. 2 having a single
circular aluminum plate 32 with a diameter of 55 mm.
The results illustrate dramatically how the more
uniform heating of the invention enables all levels in the
food to assume an acceptable temperature, i,e. at least
80C, ~ithin 6 minutes, in contrast to the 15 minutes of
Fig. 10 (a) .
Figs, 11 (a) and (b) respectively show the readings
obtained from probes C, E, F and G in a rectangular
container having the following dimensions: outside top
146 x 121 mm; inside top 130 x 105 mm; bottom 115 x 89 mm;
slant depth 38 mm; and capacity 455 ml. The first test
was conducted with a microwave transparent base, but no
higher order mode generating stand (Fig. ll(a)), and then
with such stand (Fig. ll(b)~. The load was about 400
grams of a frozen Chili-con-Carne product. Fig. ll(a)
shows that the outer regions of the product had thawed and
heated to an acceptable temperature (60C) in nine - ten
minutes, while the central region was still Çrozen until
- 12 -
after about e]even minutes had elapsed. Acceptable
temperatures were not achieved in the central region until
after about lS minutes. It should be noted that, at this
time, some regions around the edge of the container had
been boiling for about five minutes, which is undesirable.
~he erratic temperature variations during the rapid heating
part of the curves are indicative of turbulence caused by
bubbles of steam rising through the product.
In the equivalent container used in conjunction with a
higher order mode generating stand (Fig. lO(b)), the
heating behaviour obtained is very different. The mode
generating device in this case was a single ~oil block as
shown in Fig. 2A, the block being rectangular, 55 x 30 mm.
In this case it is notic~able that the centre region thawed
and heated in a much shorter time than before. Further-
more, the ov~rall heating behaviour is noticeably more
uniform. Thus the fastest region to heat was only boiling
for about one minute before all the measured temperatures
had reached an acceptable temperature t60C).
In another test (not illustrated) when using a
standard Container, the initial weight of a load of
Chinese style chicken fried rice that had been pre-cooked
and frozen was 330.8 grams and its final weight was 239~5
grams, for a weight loss of 91.3 grams, i.e. 27.6%, over
a ten minutes heating time. In a corresponding test when
the container was placed on a stand as shown in Fig. 4,
the initial weight was 329.5 grams and the final weight
318.8 grams, for a weight loss of 10.7 grams, or 3.2%,
over a seven minutes heating time which was all that was
necessary. This reduced weight loss is a further advantage
of the present invention.
Fig 12 illustrates how a multi-compartment container
60 having two different food loads 62, 64 can be mounted
on a common stand 66. Depending on the different natures
of the two food loads and the amount of microwave energy
13~6S~P~
that it is desired they should each absorb, the conditions
can be adjusted appropriately. For example, the portion
68 of the stand 66 situated below the food load 6~ may
employ a single higher order mode generating conductive
plate 70t while the portion 72 situated below the food
load 64 may emp]oy multiple plates 74. AlternatiYely, in
an example not i]lustrated, one of the portions of the
stand 66 may not include any means for generating higher
order modes and the food load associated with such portion
may be entirely shielded from the microwave energy. This
latter arrangement would be especially appropriate if the
fully shielded food load is required to remain cold.
As ~ar as spacing is concerned, there will be a
requ;rement for a certa;n minimum spac;ng between the
conducting plates (or foil surround, in the case of
apertures) and the metal of the oven floor, in order to
avoid arcing. It is for this reason that the embodiment
of Fig . 1, and many of the other embodiments, are provided
with feet 25. However, if the oven has a sufficiently
thick glass tray on its floor, or a separate microwave
transparent rack is used, such feet can be dispensed with,
e.g. the vessel of Fig. 6. Such arcing-avoidance spacing
will typically be required to be at least 3 mm. It should
also be mentioned that, in a case where the stand is not
provided with ~eet and is placed directly on a glass tray
on the oven floor, i.e. with mainly glass and little air
between the conducting material and the oven floor, the
array of plates or apertures may require dimensional
modification to take into account the dielectric constant
of the glass.
The following considerations should be taken into
account when selecting the preferred value for the spacing
between the undersurface of the food and the field modify-
ing means, i.e. the spacing S when in air IFig. 1 or 5) or
S' when in a plastic or glass material (Fig. 6 or 8).
13~5(JY~
- 14 -
The opti~um spacing will depend in part on the
properties of the foodstuff (for example, the dielectric
properties will change the phase shift which occurs on
reflection). A possible range for the spacing S in air is
from about 3 to 30 mm. A spacing S of 15 mm (with air
separating the foil structure from the container base) has
been successfully used in practice. ~s indicated, this
spacing will depend on the dielectric constant of the
material between the foil array and the bottom of the food
load. The following table gives examples of modifications
to the 15 mm spacing that would be appropriate if materials
of di~ferent dielectric constant were present between the
bottom of the food and the foil array structure.
Specifically, the table shows that the spacing S' for a
medium other than air separating the foil structure from
the container base is the corresponding spacing S for air
divided by the square root of the dielectric constant of
the medium.
Material Dielectric Constant Spacing S or S'
(Relative Permittivity)
~ir 1.0 15 mm
Silica Glass 3.78 7.72 mm
Polyethylene 2.25 10 mm
Plexiglass 2.6 9.3 mm
Tests have also been carried out to measure the effect
of the invention on total power absorption. A rectangular
container (with a microwave transparent base) and a stand
with the 9-block foil array structure as in Fig. 4 was
used. Power measurements were made using water as the
load.
Test 1 - Container placed directly on the oven glass
plate.
Measured power - 271.5 watts
Test 2 - Container raised 30 mm above the glass plate
(no foil array)
Measured power - 268.2 watts
13~65~
Test 3 - Container raised 30 mm above the glass ~late
(with the 9-block arra~ as in Fig. 4 located
midway, i.e. 15 mm from the tray and 15 mm
from the food undersurface).
Measured power - 307.2 watts
This corresponds to an improvement in power absorption
of approximately 13%. Increased power absorption is useful
(reduced cooking time), in addition to the improvement in
heating uniformity that many of the embodiments of the
present invention provide.
In the examples described so far it has been assumed
that the stand will have a flat bottom. It is, however,
within the scope of the invention to employ a stand embody-
ing higher order mode generating means incorporating a
stepped structure, e.g. a stepped structure of one of the
types disclosed in R. Keefer Canadian patent applications
Serial Nos. ~08,812 filed May 9, 1986; 536,589 filed May 7,
1987; and 544,007 filed August 7, 1987 (U.S. patent appli-
cations Serial Nos. 943,563 filed December 18, 1986 and
04~,588 filed ~pril 30, 1987 and European patent appli-
cations Nos. 87304120.6 filed May 8, 19~7 and published
November 19, 1987 under No. 246041 and 87309398.3 filed
October 23, 1987 and published June 22, 1988 under No.
271981). Some of the patent applications just referred to
also disclose a container having a wall (e.g. a bottom
wall) having a modified portion that has a different
electrical thickness from that of adjacent portions of the
wall, the electrical thickness being defined as a function
of the actual spatial thickness of the wall and the
dielectric constant of the wall material. Such a wall
structure comprising appropriately arranged contiguous
wall portions of respectively different electrical
thicknesses can serve to generate at least one mode of a
higher order than the fundmental modes. In the present
invention, higher order mode generating means located in
13(~6509
-- 16 --
the stand can utilize such an arrangement of various
portions of dif~ering electrical thickness instead of the
foil plates or apertures described above.
Fig. 13 shows a stand with such a structure based on
portions 75, 76 of different physical thic~ness, while
Fig. 14 shows a structure in which portions 77, 78 have
the same physical thickness, but a different electrical
thickness by virtue of having different dielectric
constants, respectively designated L and H for low and
high.
Fig. 15 shows a structure in which apertures 65 are
formed in a conducting base 67 supported by non-conducting
supports 69, a central aperture 65a being formed in a
raised portion 67a of the base, whereby its distance S2
from the undersurface of a food load (not shown) in a
container 10 is less than the distance Sl of the remainder
of the base 67. Fig. 15A shows the effect on the power P
conveyed to the load as a function of S. Curve 61 is for
larger apertures 65, while curve 63 is for smaller
apertureS.
A plan view of Fig. 13, 14 or 15 would show the
portions 75, 76 or 77, 78, or the apertures 65, forming
a nine block array similar to Fi~. 4, although this array
can be modified as required.
As a further alternative~ the higher order mode
generating means employed in a stand according to the
present invention can take the form shown used on a
container in R. Keefer U.S. patent application Serial No.
051078 filed May 15, 1987, now U.S. patent No. 4,814,568
issued March 21, 1989 tCanadian application Serial No.
566,653 filed May 12, 1988). This alternative is
illustrated by the plan view of a circular stand in Fig.
16 where the portion 79 is a shaped piece of foil on a
microwave transparent base 80.
Higher order modes of microwave energy can also be
generated by a stepwise discontinuity of lossiness between
~3~S(~9
~ 17 -
a pair of regions of a susceptor. Such a susceptor, which
may constitute a separate element or may form a wall
component of a container, is disclosed in R. Keefer
Canadian patent application Se~ial No. 552,110 filed
November 18, 1987 (European ap~licaton 88310658.5
published May 24, 1989 under No. 317023). In accordance
with the present invention such a susceptor structure can
be used in the stand to p~ovide higher or~er mode
generating means, as well as to generate heat that can be
conveyed to the container and the food or other material
therein. Such a structure is shown in Fig. 17, where the
portions 81 and 82 have different lossiness. A plan view
of Fig. 17 could show the portions 81, 82 as a single
block array, similar to Fig. 2 or 2A, or the portions 81,
82 could be strips extending fully across a rectangular
container.
An arrangement for retaining and concentrating micro-
wave energy in a container, i.e. enhancing the coupling oE
such energy into the container, is described in R. Keefer
Canadian patent No. 1,228,126 issued October 13, 1987
~U.S. patent 4,656,325 issued April 7, 1987). A similar
arrangement can be embodied in a stand in accordance with
the present invention, as illustrated, for example in Fig.
18 which shows a stand with a substrate 83 of a dielectric
material having a relatively low dielectric loss factor,
e.g. polyethylene polyester film. On this substrate 83
there is an array of conductive plates or islands 84, e.g.
aluminum foil. The total surface area o~ the metallic
islands should preferably be between 50 and 80% of the
surface area of the substrate. Fig. 18 shows the substrate
83 on a stand having a foot portion 85 and a rim 86 for
supporting a Container. The dielectric substrate 83 and
the array of conductive plates should cooperatively
provide a dielectric ConStant greater than 10, and the
spacing between sùch array and the undersurface of the
substance to be heated in the Container (not shown) should
be between one-fifteenth and one-sixth of the wavelength
~3~65(~9
- 18 -
Of the miCrowave energy, whiCh is approximately between 8
and 20 mm in air. This arrangemellt may al80 serve at the
same time to generate some higher order modes of microwave
energy. However, in view of the relatively large number
of plates 84 used in the 20-block array shown in Fig. 18,
the height of the higher order modes will be greater than
that of the modes generated by the single and nine-block
arrays illustrated in other views. These very high order
modes will penetrate a shorter distance into the food, and
hence the advantage of the Fig. 18 embodiment flows more
from the increased coupling of energy into the food than
from higher order mode generation, although the latter
phenomenon will contribute to some extent to the overall
improvement in performance.
