Microwave oven shelf

A metal rod rack shelf for a microwave oven wherein extension support rods are substantially prevented from being excited with high electric currents. The rack is embodied with quarter wavelength choking structures in the regions where the extension support rods connect to the rack mainframe. Accordingly, the effective short circuits in those regions are transformed to open circuits at a distance of one-quarter wavelength along the support rods. As a result, the support rods do not rise to high temperatures or radiate intense microwave fields that could cause melting of or arcing to microwave transparent brackets which mount to walls of the oven cavity and engage the rods.

BACKGROUND OF THE INVENTION 
In a microwave oven, it is desirable to elevate the food a short distance 
from the floor of the cavity. This spaces the food from the electric 
component null on the conductive surface of the floor. Also, it permits 
microwave energy to enter the food from all sides. In the prior art, food 
has been elevated by a relatively thin microwave transparent tray 
supported at the edges of the conductive floor which is recessed 
downwardly in the central region. 
While positioning a shelf at some level above the bottom tray may have some 
advantage in particular microwave cooking applications by increasing the 
total food supporting area, the use of an elevated shelf is of great 
importance in a thermal oven where the heat should be conducted into the 
food from all sides, including the bottom. Accordingly, in a cavity used 
for both microwave and thermal heating, it is most desirable to have both 
the bottom transparent tray and an elevated shelf thereabove. 
In a prior art combination microwave and convection heat oven, a shelf 
defined by a metal rod rack was used. The open spaces in the rack 
permitted circulation of the hot air around the cavity. With this 
construction, it was desirable to use metal for the rods to provide 
sufficient strength. It was desirable to be able to change the height of 
the rack or remove it altogether for various cooking applications. Also, 
it was desirable to be able to slide the rack forward to assist in 
handling of the food dish. Accordingly, pairs of grooved microwave 
transparent shelf support brackets were screwed in vertical alignment on 
opposing sides of the conductive cavity. The side perimeter rods of the 
rack were inserted into respective horizontal grooves in the brackets 
thereby facilitating forward and backward movement. It was found that 
under unusual circumstances, the support brackets would melt or deform due 
to overheating. Also, there were isolated instances of arcing between the 
metal rack and the support bracket screws or between the metal rack and 
the dielectric material of the support bracket. Such arcing may occur to 
the surface of insulating material at microwave frequencies because the 
displacement current in the dielectric completes the circuit. 
SUMMARY OF THE INVENTION 
The invention defines a metal rod rack which is adapted for functioning as 
a horizontal shelf for supporting food in the cavity of a combination 
microwave and thermal oven, comprising first, second and third parallelly 
spaced rods connected perpendicularly to a fourth rod, the second rod 
being between the first and third rods, the first and third rods bending 
in opposite right angles at a distance of approximately one-quarter 
wavelength from their connections to the fourth rod, and the second rod 
extending greater than one-quarter wavelength from its connection to the 
fourth rod for supporting the rack in the oven. Although the microwave 
energy could be any frequency within the microwave spectrum, 915 megahertz 
or 2450 megahertz are universally used because cooking is allocated to 
narrow bands around them. The thermal energy may preferably be provided by 
a hot air convection system. It may be preferable that the first and third 
rods be spaced approximately 0.2 inches from parallel portions of the 
second rod. Also, the sections of the first and third rods on the sides of 
the bends opposite their connections to the fourth rod may preferably 
define perimeter portions of the rack. Also, the second rod may extend 
inwardly past its connection to the fourth rod and be connected 
perpendicularly to a fifth rod. 
The invention may also define a shelf adapted for supporting food in an 
elevated position in the cavity of a microwave oven, comprising a metal 
rod rack having metal rods extending outwardly from opposing sides of the 
rack for supporting the shelf in an elevated horizontal position within 
the cavity, the rack compising means for suppressing the excitation of 
electric currents in the outwardly extending rods, and a microwave 
transparent plate supported by the rack. It may be preferable that the 
suppressing means comprise a one-quarter wavelength choke which may be 
defined to be a shorted transmission path which transforms a high 
impedance one-quarter wavelength away back the path. Preferably, the 
preventing means may comprise effective short circuits between the 
outwardly extending rods and parallelly spaced rods, the short circuits 
being transformed to a high impedance one-quarter wavelength away on the 
outwardly extending rods. 
The invention may also be practiced by a microwave oven comprising a 
conductive cavity having parallel side walls, means for energizing the 
cavity with microwave energy, microwave transparent brackets connected to 
the side walls of the cavity, a metal rod rack for providing a shelf in 
the cavity, the rack having metal rods extending outwardly for supporting 
the rack in a horizontal position from the brackets, and the rack 
comprising means for substantially preventing electric currents from being 
excited in the outwardly extending rods by the microwave energy. The 
transparent brackets may preferably define horizontal grooves. The 
invention also defines a microwave oven comprising a conductive cavity 
having first and second parallel walls, means for energizing the cavity 
with microwave energy, first and second elongated microwave transparent 
brackets respectively mounted to the first and second walls of the oven, 
the brackets having grooves defining a horizontal plane elevated above the 
floor of the cavity, a metal rod rack having rods extending outwardly for 
supporting the bracket by engagement with the grooves, the microwave 
energy exciting electrical currents in the rack and the rack comprising 
means for suppressing the flow of the currents in the outwardly extending 
rods. The brackets may be fabricated of Ryton. It may be desirable to use 
a plurality of pairs of the first and second brackets so that the rack may 
be positioned at different heights.

DESCRIPTION OF THE PEFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a microwave and hot air convection oven 
10. The oven includes a common cavity 12 where food may be heated by 
microwave energy, convected hot air, or a combination of the two. Cavity 
12 is defined by side walls 14 and 16, back wall 18, ceiling 20, floor 22 
and door 24 in its closed position. These defining surfaces are all 
conductive. Door 24 may have a plurality of small perforations for 
providing a view of the interior of the cavity. Door 24 is also provided 
with a suitable microwave seal to prevent the escape of microwave energy 
from the cavity when the door is closed. 
Still referring to FIG. 1 and also to FIG. 2, microwave energy, typically 
at a frequency of 2450 megahertz, is generated by magnetron 26 and 
propagated down waveguide 28 to a region under the center of floor 22. The 
microwave energy excites antenna probe 30 and is coupled through a small 
aperture 31 in floor 22 to directional antenna 32. Directional antenna 32 
consists of a transmission line conductor 34, conductive vertical legs 36 
and radiating elements 38. Transmission line conductor 34 is spaced from 
the floor 22 by apertured disks 40 which preferably are Teflon. A 
substantial percentage of the radiation from directional antenna 32 is 
from radiating elements 38 because they are spaced approximately 
one-quarter wavelength from floor 22 which functions as a ground plane to 
provide maximum radiation in a direction normal to the floor. There is 
very little radiation from transmission line conductor 34 because it is in 
close proximity to floor 22. The directive patterns of radiating elements 
38 are substantially independent because elements 38 are orthogonal to 
each other; accordingly, the vectors are also orthogonal minimizing 
additive or subtractive characteristics. Motor 42 rotates probe 30 which 
rotates directional antenna 32. Accordingly, the patterns of the radiating 
elements 38 move in circles concentric to the axis of probe 30. A more 
detailed description of the microwave feed system is provided in U.S. Pat. 
No. 4,284,686 to Simpson, dated Aug. 11, 1981, which is hereby 
incorporated by reference. 
As shown, floor 22 has a central downwardly recess 44. The top entrance to 
recess 44 may preferably define a circle having a diameter of 12.75 
inches. The bottom of recess 44 may preferably define a circle having a 
diameter of 8.75 inches. The height of recess 44 may preferably be 1 inch. 
Directional antenna 32 is aligned in the recess as shown and radiating 
elements 38 are different distances from the center of rotation. A layer 
46 of microwave transparent material such as Pyroceram covers the bottom 
of the cavity. Layer 46 which must be resistent to the temperatures to 
which cavity 12 is heated, provides a surface for supporting food and 
keeps recess 44 free from spills and deposits. Layer 46 has no appreciable 
affect on the microwave characteristics of cavity 12. 
Heat for hot air convection is provided by electric heating element 50 
which may, for example, be a Calrod. Heating element 50 is connected to an 
electric source in conventional manner. In operation, air is recirculated 
past heating element 50 by blower 52. More specifically, blower 52 forces 
air up duct 54 into chamber 56 which has ceiling 20 as one of its forming 
surfaces. The air flows from chamber 56 to cavity 12 by way of 
perforations 58 in ceiling 20. Perforations 58 are positioned in a pattern 
substantially conforming to the shape of heating element 50 so that the 
forced air entering cavity 12 is heated by element 50 in the convection 
mode. The input air for blower 52 is drawn from cavity 12 through 
perforations 60 and plenum 62 into duct 54. Accordingly, in the hot air 
convection mode, air is recirculated past heating element 50. Shelf 65, 
which will be described in much more detail later herein, is substantially 
open so that the recirculation flow of air thereby is not significantly 
restricted. 
Blower 64 shares a common shaft 66 with blower 52. The shaft is connected 
to blower drive motor 68. Blower 64 intakes surrounding air and forces it 
down channel 70 to cool magnetron 26 as is conventionally done. The air 
from the magnetron is then directed outside the oven according to 
well-known practice. 
Control panel 72 controls the operation of the cooking modes and will not 
be described in detail because it is the same control panel that has been 
used commercially in the prior art on the model RMC-20B microwave oven by 
Amana Refrigeration, Inc. of Amana, Iowa. Dial 74 is connected to a timer 
(not shown) used for timing cooking operations. Buttons 76, 78 and 80 
respectively control whether the cooking is by microwave alone, a 
combination of microwave and hot air convection, or hot air convection 
alone. Control 82 can be used to set a holding temperature for the food. 
Slide control 84 sets the microwave power level of the oven. Bottons 86 
and 88 respectively control the starting and stopping of the oven. Button 
89 turns the cavity light on and off. 
Referring to FIG. 3, a top view of cavity 12 taken along line 3--3 of FIG. 
1 is shown. Door hinge 90 is shown but door 24 has been removed from the 
drawing to simplify the illustration. Decorative chrome casting 92 
surrounds the access opening that door 24 closes. As described earlier 
herein, side walls 14 and 16 and back wall 18 partially define cavity 12. 
The cavity is enclosed by outer casing 94. Still referring to FIG. 3 and 
also to FIGS. 1 and 2, horizontal pairs of elongated shelf support 
brackets 96 are mounted in vertical alignment on side walls 14 and 16. 
Although only two pairs are shown, more pairs could be used to provide 
more possible positions for shelf 65. Referring to FIG. 4, an expanded 
view of a shelf support bracket 96 is shown. The ends of each shelf 
support bracket 96 are mounted to respective side walls 14 and 16 by a 
suitable fastener such as, for example, a bolt 102 and nut 104; screws 
could also be used. Each shelf support bracket 96 has an elongated 
horizontal groove 106 or track which acts as a guide so that shelf 65 may 
be slid in and out. As will be described in detail later herein, grooves 
106 have openings 108 for insertion of extension rods 111 and 112 into 
groove 106. By fabricating openings 108 in the front and back, brackets 96 
may be used for either the right or left side of cavity 12. Although other 
suitable materials can be used, brackets 96 are fabricated of Ryton 
because, not only is it transparent to microwave energy, but it is also 
resistent to the temperatures to which cavity 12 is elevated in the 
convection mode. For example, the cavity may be heated to approximately 
325.degree. F. and Ryton doesn't start to soften until approximately 
500.degree. F. Although bracket 96 could be a conductive material, a 
microwave transparent material has advantage because there may be arcing 
across air gaps that are inherently formed between loosely fitting metal 
parts. 
Shelf 65 includes metal rack 113 which is formed by metal rods 114 such as 
plated steel which are connected together by suitable means such as 
welding. The metal provides the strength required for rack 113 which is 
mostly open space so as not to restrict convection air flow. An expanded 
view of part of rack 113 is shown in FIG. 5. Shelf 65 also includes flat 
plate 116 fabricated of a microwave transparent material such as glass 
ceramic. As is shown best in FIG. 3, plate 116 is supported in a central 
region of rack 113 which is void of metal rods 114. Accordingly, plate 116 
can be used to support a food body 117 into which microwave energy can 
enter from the bottom without microwave pattern interference from the 
metal rods 114. Plate 116 is supported by rods 114a-d which are welded to 
the undersides of rods 114e-h. Accordingly, the horizontal position of 
plate 116 is secured. 
In accordance with the invention, outer perimeter rods 114i and 114j are 
respectively bent inwardly at right angles and are connected to inner rods 
114k and 114m. Legs 110 of rods 114i and 114j are one-quarter wavelength 
long. Also, U-shaped rods 114n and 114p are connected between legs 110 and 
have legs 115 which are parallel to legs 110 and also have a length of 
approximately one-quarter wavelength. Between respective legs 110 and 115, 
extension rods 111 and 112 extend outwardly to engage grooves 106 to 
support shelf 65. In addition to connecting to rods 114k and 114m, 
extension rods 111 and 112 also connect respectively to rods 114r and 114s 
to provide structural strength. Perimeter rod 114j is spaced from back 
wall 18 by microwave transparent bumpers 118 which prevent arcing 
therebetween. As can be seen best in FIG. 3, shelf 65 can be pulled 
outwardly by rod 114i with the ends 120 of extension rods 111 and 112 
providing support while sliding in grooves 106. Central weighting on shelf 
65 is predominantly supported by front extension rods 111 while back 
extension rods 112 maintained shelf 65 in a horizontal alignment. When 
front extension rods 111 reach front openings 108, the front of shelf 65 
can be raised as it pivots about back extension rods 112 in grooves 106. 
Shelf 65 can then be pulled futher forward until rods 112 reach front 
openings 108 wherein, shelf 65 can be removed from the cavity. 
It has been determined that the preferred embodiment fulfills the objective 
of preventing high currents from being induced in metal parts that contact 
the microwave transparent shelf support brackets 96. In the prior art, 
high currents heated the rod rack which caused melting or deformation of 
the shelf support brackets 96 under certain unusual circumstances. Also, 
there were rare instances of arcing between the metal rack and the 
dielectric material of the bracket or its fastening screws. More 
specifically, it was found if the rack configuration were such that the 
outer perimeter rods of the rack were inserted in grooves 106, standing 
waves set up on the rods could damage the brackets. Although, under most 
circumstances, the hot spots in the fields of these standing waves were 
not strong enough to melt or scorch the microwave transparent brackets, 
there were circumstances where failures did occur. The failures were 
generally associated with an unloaded cavity and/or the rack being in a 
particular undesirable position where the hot spots were maximized and 
adjacent to metal fasteners. More specifically, the wavelength of a 
standing wave is, in part, a function of the dielectric surrounding the 
conductor. Accordingly, by moving the rack slightly forward or backward, 
the characteristic of the standing wave on the rod within groove 106 would 
be altered as a result of the longitudinal relationship of the two parts. 
Occasionally, a standing wave would be created that would be strong enough 
to heat the rod above the melting point of the bracket. In accordance with 
the invention, high current is prevented in the support rods that engage 
grooves 106 of the Ryton shelf support brackets 96. As a result, the 
problem of potential melting or arcing has been solved. 
In theory, the operation of the invention can be explained with reference 
to a shorted transmission line or a quarter wavelength choke. More 
specifically, currents excited in rods 114i, 114j, 114n, and 114p would 
see short circuits to respective extension rods 111 and 112 at the 
respective connections to rods 114k and 114m. These short circuits are 
transformed to open circuits one-quarter wavelength away back at the right 
angle bends. As a result, the regions around extension rods 111 and 112 
are choked and, accordingly, high currents are not excited in the 
extension rods. In terms of an equivalent circuit, the shelf support 
bracket 96 can be considered as capacitance between the metal wall and an 
extension rod 111 and 112. The above-described quarter wavelength choke is 
put in series with the capacitance such that the very high impedance at 
the entrance to the choke is near the shelf support bracket 96. The 
voltage between rack 113 and oven wall 14 or 16 is finite and determined 
by the microwave fields in the cavity. Since the high impedance of the 
choke is in series with the capacitance from end 120 of the rod 111 or 112 
to the wall 14 or 16, most of the voltage appears across the choke. 
Accordingly, the bracket is subjected to low voltage and low current. The 
spacings from extension rods 111 and 112 to respective legs 110 and 115 
may preferably be approximately 0.2 inches. From the outer perimeter of 
rack 113, as defined by rods 114i and 114j, extension rods 111 and 112 
preferably extend less than one-quater wavelength. 
Rack 113 has been described in combination with microwave transparent 
support brackets. By suppressing the currents in extension rods 111 and 
112 in accordance with the invention, if rack 113 were supported instead 
by metal brackets, there would be less, if any, arcing than if a prior art 
metal rod rack were used. 
Referring to FIGS. 6A and 6B, partial top and side views are shown of metal 
rack 121 which is an alternate embodiment of metal rack 113 shown and 
described with reference to FIGS. 1, 2, 3 and 5. In general, the 
embodiment of FIGS. 6A and 6B uses a shorted coaxial line to suppress 
currents. More specifically, a metallic cylinder 123 is connected to the 
outer two rods 124 and 126 of rack 121. Connected to an end 122 of 
cylinder 123 is a coaxial rod 128 which extends outwardly past the rod 124 
for engagement in groove 106 of a support bracket such as that shown and 
described with reference to FIG. 4. The operation of cylinder 123 and rod 
128 in suppressing currents in rod 128 can be explained in terms of a 
shorted coaxial line which transforms an open circuit one-quarter 
wavelength away on rod 128. Accordingly, there are only very small 
currents induced in rod 128 and, as a result, the problems of melting the 
support bracket or arcing to fasteners or to the support bracket are 
solved. 
Referring to FIG. 7, there is shown shelf support bracket 130 which is an 
alternate embodiment to shelf support bracket 96. Bracket 130 would be 
connected to side walls 14 and 16 of cavity 12 by suitable means such as, 
for example, screws (not shown) directed horizontally through the bottoms 
of grooves 132 into the side walls. The embodiments of racks 113 and 121 
can be used in conjunction with shelf support bracket 130 as shown in FIG. 
7. The advantage of the embodiment of bracket 96 as shown in FIG. 4 over 
bracket 126 as shown in FIG. 7 is that the operator is prevented from 
inadvertently pulling the rack forward to a position where the front end 
is not supported. Also, the fasteners such as bolts 102 of bracket 96 are 
completely isolated from surfaces of the rack. 
Referring to FIG. 8, there is shown a partial view of metal rack 134 which 
is an alternate embodiment of metal rack 113. Rack 134 will not work in 
conjunction with shelf support bracket 96 as shown in FIG. 4. Accordingly, 
a different support such as bracket 130 as shown in FIG. 7 is required. 
The operation of rack 134 is protecting the microwave transparent support 
brackets from intense fields is similar to racks 113 and 121 previously 
described. More specifically, rods 136 and 138 extend outwardly and are 
joined by cross rod 140. High currents in the structure formed by rods 
136, 138 and 140 are prevented by the high impedance reflected from the 
short circuit at connections to rod 142. 
Referring to FIG. 9, there is shown a choke design for microwave 
transparent bumpers 144 which serve the same function as bumpers 118 
described earlier herein. More specifically, high currents in rods 146 and 
148 are prevented by respective short circuits at rods 150 and 152 which 
are respectively reflected as open circuits. 
This completes the description of the preferred embodiment. However, those 
of ordinary skill in the art will recognize there are many alterations or 
modifications which are possible without departing from the spirit and 
scope of the invention. Accordingly, it is intended that the scope of the 
invention be limited by the appended claims rather than by the specific 
embodiments.