Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 9~6 sD-RG-15057
BACKGROUND OF THE INVENTION
The present invention relates to a microwave cooking oven and
more specifically to an improved excitation system for such an oven which
enhances the time-averaged uni~ormity of energy distribution within the
cooking cavity.
A contlnuing problem in the design of microwave cooking ovens is to
eliminate hot and cold spots in the cooking cavity resulting from the non-
unifonm spatial distribution of energy in the cavity. Such non-uniform energy
distribu~ion is often explained to be the result of the establish~ent of ,L
electromagnetic standing wave patterns, known as "modes," within the cooking
cavi~y. ~hen such standing wave patterns exist in the cavity, the intensity
of the electric and magnetic fields vary greatly with position. The
particular mode patterns which may be established in the cavity are depen-
dent upon many variables including the frequency of the microwave energy
used to excite the cavity and the dimensions of the cavity.
A number of different approaches to enhance unifonm energy dis-
tribution by a1tering the stand~ng wave patterns in the cavity have been
tried. One common approach involves ~he use of a so-called "mode stirrer"
which typically resembles a fan with metal blades. This stirrer is nor-
mally located near the point where energy is coupled into the cookingcavity, such as in the cooking cavity itself or in the waveguide, coupling
energy from the magentron to the cavity near an exit port of the wave-
guide. In any case, the mode stirring approach is an attempt to randomize
energy reflections in the cavity by introducing time varying scattering
of the microwave energy by reflection from the stirrer blades as the
microwave energy enters the cavity. While mode stirring has been found
to provide some improvement in energy distribution uniformity, side-to-
side and front-to-back field strength variations are not entire1y elimi-
nated.
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9D-RG-15057
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An~ther ~Ipproach has involved the use of a rDtating antenna in
the cavity. Prior art relating to such use of rotating antenna may be
found in U.S. Patent 4,D~8,521 to Uyeda et al; 49284,868 to Simpson, and
4,316,069 to Fi~amayer, for example. Even though rotating antennas tend
S to improve uniformity of energy distribution in the cavity, typical
antenna configurations tend to leave cold spots. For centrally mounted -~
antenna, such cold spots tend to occur near the center of rotation of
the antenna. Rotating antenna arrangements also have a significant
assembly disadvantage in that energy coupling efficiency and impedance
matching are extremely sensitive tb assembly tolerances. For example,
coupling efficienGy is extremely sensitive to the depth of penetration
of the antenna probe into the waveguide; also, antenna impedance is
extremely sensitive to the spacing between ~he antenna arms and the
ground plane; i,e., the adjacent cav1ty wall. In addition, antennas
lS generally protrude into the cooking cavity, reducing the usable cavity
space.
The use of radiating slots is also known in the art. U.S.
Patents 4,019,009 to Kusunoki et al; U.S. Patent 2,804,802 to Blass et
al; and U.S. Patent 3,810,248 to Risman et al provide examples of station-
ary radiating s1Ots arranged beneath the food load to be heated. U.S.Patent 3,210,5}1 to Smith provides single diametrically opposed slots on
the top and bottom walls of the cooking cavity oriented at right angles
~o each other to producè circularly polarized radiation in the cavity.
U.S. Patent 4,327,266 to Austin et al combines a rotating
antenna and slot to provide a coaxially fed bilaterally symmetrical
rotating plate antenna disposed near the bottom wall of the cavity and
having radiating wings at its periphery and a substantially tangential
radiating slot closely adjacent its axis of rotation, which purportedly
results in uniform microwave heating of food items in the cavity due to
a balance between aperture radiation and wing radiation. This antenna
sD-RG-15~57
configuration appears to protrude into the cavity to an undesirable
extent.
U.S. Patent 3,746,823 seeks to provide improved energy dis-
tribution uniformity by providing a rotating disk having formed therein
several elongated radiating apertures sequentially oriented transverse
to the longitudinal waveguide axis, each aperture being oriented and
positioned such that when transverse to the longitudinal waveguide axis
the apertures appear electricalty at integral multiples of half-wave ~-
points from the magnetron to achieve maximum en~rgy transfer through the
transverse apertures, while allowing only a minimum amount of energy to
be transferred into the cavity when the apertures are aligned parallel
to the longitudinal wavegui~e axis, thus appearing to produce a radiation
system permitting maximum power transfer to the cavity. However, such
an arrangement is believed to be limited as to time-averaged uniformity
lS of energ~ distribution in the cav~ty.
While each of the approaches mentioned herein appears to
provide s~me improvement in the attempt to overcome the energy non-
uniformity problem in microwave ovens, a need remains for a relatively
simple, efficient low profile energiza~ion system which provides good
uniformity of energy distribution in the cooking cavity without extending
obtrusively into the cavity so as to maximize the space available in the
cavity to reoeive items to be heated.
It is therefore an object of the present invention to provide
a re!atively simple, efficient excitation system for a microwave oven
which enhances the time-averaged uni~ormity of energy distribution
within the cavity employing an extremely low profile radiating me~ber
which projects only minimally into the cooking cavity.
SUMMARY OF THE INVENTION
In accordance with the present invention, a microwave oven
having a cooking cavity of the resonant type comprising a generally
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rectangular enclosure defined by conductive walls is provided with an
excitation system which employs a low profile rotating radia~ing member
to enhance time-averaged unifonmity of energy distribution within the
cavity. A rectangular waveguide extending generally centrally along the
upper wall of the cavi~y couples energy from the magnetron to the GQoking
cavity A circular opening is formed in a common wall between the
waveguide and the cooking cavity, which opening is essentially blocked
by a rotatably mounted metallic circular disk which overlaps the opening
on the cavity side of the wall. A radiating aperture is fon~d on the
disk for coupling energy from the waveguide to the cooking cavity. The
aperture is in the form of an elongated slot extending generally transverse
to the radius of the disk. The axis of rotation of the disk is longitudi-
nally spaced an odd integral mult~p1e of quarter gu~de wavelengths from'
~h~ short circuit tenmination of the waveguide, and the longitudinal
ax~s o~ the radlating slot is radially spaced approximately one quarter
guide wavelength from 'the axis of rotation of the disk. As the disk
rotates, the slot is alternately oriented as a series slot and a shunt
s'tot with each quarter revolution of the disk. This locaticn of the
disk relative to the waveguide' tenmination and the radial spacing of the
slot relative to the axis of r~tation of the disk assures that the slot
will be at a maximum current point when oriented as a series slot and at
a maximum fleld point when oriented as a shunt slot. Thus, during each
complete rotation of the disk the slot passes through four maximu~
energy coupling positions with less optimum coupling positions inter-
2~ spersed therebetween. Thus, by periodically varying the radiation inten-
sity of the slot and lts position in the cavity during each rotation of
the disk, the time-averaged energy distribution in the cavity is signifi-
cantly enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
While the novel functions of the invention are set forth with
particularity in the appended claims, the invention both as to organization'
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12C396~6 9D-RG-15057
and content will be better understood and appreciated from the following
detai1ed description taken in conjunction with the drawings in which:
FIG. 1 is a perspecti~e view of a microwave oven illustratively
embodying the excitation system of the invention;
FIG. 2 is a front schematic sectional view of the microwave
oven of FIG. 1 taken along lines 2-2;
FIG. 3 is a schematic side view, partially in section, of the
microwave oven of FIG. 1 with portions removed to illustrate structural
detail 5;
FIG. 4 is a partial top view of the oven of FIG. 1 with portions
removed to show structural detai~s of the waveguide and slotted disk mount-
ing and driving arrangement; and
FIG. 5 is a schematic view of the disk showing the various maxi-
mum co~ ling positions assumed by the radiating slot during each rotation
of the disk in relation to the standing wave ~ield and current patterns
in the wa~eguide.
DETAILED DESCRIPTION
Referring now to FIGS. 1-4, there is shown a microwave oven
designated generally 10. The outer cabinet comprises six cabinet walls
including upper and lower walls 12 and 14, and rear wall 16, two side
walls 18 and 20 and a front wall partly formed by hingedly supported
door 22 and partly by control panel 23. The space inside the outer
cabinet is divided generally into a cooking cavity 24 and a controls
compartment 26. The cooking cavity includes top wall 28, a bottom wall
30, side walls 32 and 34, the rear cavity wall be:ing cabinet wall 16 and
the front cav~ty wall being defined by the inner ~aoe 36 of door 22.
Nominal dimensions of cavity 24 are 16 inches wide by 8 inches high by
11 inches deep.
Controls compartment 26 has mounted therein a magnetron 40
which is adapted to produce microwave energy ~having a center frequency
of approximately 2450 MHz at output probe 42 thereof when coupled to a
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suitable source of power (not shown) such as the 120 volt AC power
supply typically provided at domestic wall receptacles. A cooling air
plenum hav~ng a lower portion 43a substantially enclosing magnetron 40
and an upper portion 43b substantially enclosing a portion of the feed
waveguide is formed by a bottom wall 44 extending beneath magnetron 40
between cavity side wall 32 and cabinet side wall 18, opposing side
wa11s 45 and 46 extending upwardly ~rom bottom wall to upper cabinet
wall lZ and an end wall 47 extending front to back between upper cavity
wall 28 and upper cabinet wall 12. Each of plenum walls 44, 45, 46 and
47 is secured along the ad?acent cavi~y wall edge to ~he adjacent cavity
wall by suitable means such as by welding. A flange 48 is ~ormed along
the opposite edges of these plenum walls adjacen~ the cabinet wall. A
strip 49 of gasket material is sandwiched between the flanged edge 48
and tlle cablrlet walls to provide an airtight seal therebetween. A
' 15 blower for magnetron cooling designated generally 50, compr~sing a fan51 driven by electric motor 52, is mounted in a circular opening 53 in
rear partition 46. An annular shroud 54 surrounds fan 51. Blower 50
draws in cooling air from outside the outer cabinet through perforations
55 in rear cabinet wall 16. The air enters plenum portion 43a where it
passes over the magnetron cooling fins 56. A portion of this air enters
the cooking cavity 24 through ventilation holes 57 in cavity side wall
~` 32. The balance enters upper plenum 43b to provide air flow for rotating
~the slotted disk radiator of the present invention in a manner to be
described in greater detail hereinafter. Openings 58 and 59 formed in
partitions 44 and 45, respectively, are provided to prevent the buildup
of back pressure in plenum 43.
The front facing opening of controls compartment 26 is enclosed
by control panel 23. It will be understood that nu~erous other components
are required in a complete microwave oven but for clarity of illustration
`` 30 and description only those elements believed essential for a proper
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understanding of the present invention are shown and described. Such
other elements may all be conventional and, as such, are well known to
those skilled in the art.
The structure of the excitation system in accordance with the
present invention as illustratively embodied in microwave oven I0 will
now be described. The source of microwave energy is Inagnetron 40.
Microwave energy from magnetron ~utput probe 42 of magnetron 40 is
coupled tn the cooking cavity 24 via rectangular feed waveguide 68 which
extends generally centrally along the upper cavity wall 28. Waveguide
68 is of generally rectangular cross section being formed by member 70
of generally U-shaped cross section and a portion of top cavity wall 28
which forms a common wall for waveguide 68 and cavity 24. Conductive
end wall 72 provides a short circuit termination for waveguide 68 remote
from magnetron 40. Member 70 is suitably flanged as at 74 for attachment
to top cavity wall 28 by suitable means such as welding. ~aveguide 68
is dimens1Oned to support a TElo propagating mode. Specifica11y, the
width (the dimensions running front to rear of the cavity) is more than
one-half but less than one guide wavelength, and the height is less than
one^half waveguide wavelength. As used herein, the guide wavelength
refers to the wavelength of microwave energy propagating within the
waveguide. In the illustrative embodiment, the height of waveguide 68
is nominally .75 inches and the width is nominally 3.66 1nchesO
A microwave energy launching area 76 for energy radiated from
magnetron probe 42 is provided by an extension of waveguide member 70
which encloses probe 42 on top and sides. Support flange 77 encloses
the bottom of the launch area. Conductive end wall 78 is spaced approxi-
mately 3/4 inch from probe 42 to provide a launch area short circuit
waveguide termination. The spacing is in accordance with magnetron
manufacturer recommendation for proper power output and operating charac-
. 30 teristics. Launching area 76 is of the same width as waveguide 68 but
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3LZ0~;46 9DL RG-15057-Miller
of height on the order of 2 inches, with the open end facing curved step
79 formed at the intersection of cavity side wall 32 and top walt 28.
Curved step ~ (radius of curvature nominally .64") provides the desired
sending impedance for satisfactory impedance ~atching.
Microwave energy from waveguide 68 is radiated into cooking
cavity 24 by a radiating aperture in the form of elongated slot 80
formed in a circular disk 82 extending generally transversely to the
radius of the disk 82. Disk 82 extends within cavity 24 and is mounted
for rotation in a plane parallel to and in close proximity to upper
cavity wall 28. A circular opening 84 to accommodate disk 82 is for~ed
in that pôrtion of upper cavity wall 28 in common with waveguide 68
having a diameter slightly less than the diameter of disk 82. A plastic
cover 86 for supporting disk 82 adjacent opening 84 and enclosing opening
84 and disk 82, attaches to upper cavity wal1 28 by resit~ent tabs 88
which project through sma11 slots 89 in wall 28~ annularly distributed
about opening 84 for this purpose. A plastic shaft member 90 is formed
integrally with cover 86 projecting upwardly from cover 86 to rotatably
support disk 82, the longitudinal axis 91 of shaft 90 defining the axis
of rotation for disk 82. For reasons to be explained in greater detail
hereinafter, cover 86 is mounted to top wall 28 with shaft 90 centered
relative to cooking cavity 24 and at a point located approximately 3
quarter guide wavelengths from waveguide end wall 72.
~` Disk 82 is carried by an integrally molded plastic member
`~ designated genera11y`92, comprising a circular support and spacer disk
94 which is co-extensive with disk 82, a vertically extending cylindrical
central portion 96, and a plurality of vanes 90 projecting radially from
the central portion 96. Disk 82 is secured to support disk 94 by three
polypropylene snap buttons 97. An aperture is formed in support disk 94
co-extensive with slot 80 in disk 82.
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In addition to supporting disk 82, disk 94 also aets as an
insulating spacer separating disk 82 from cavity wall 28. Since the
radially outenmost portion of metallic disk 82 overlaps that portion of
waveguide wall 28 surrounding the circular opening 84 formed herein,
capcitive coupling exists between the disk edge and the adjacent waveguide
wall. The dielestric spacer provided by support disk 94 increases the
capacitance between wall 28 and disk 82 so as to minimize the resultant
impedance. In addition, the high voltage breakdown in the region of
overlap is increased so as to avoid arcing between disk 82 and cavity
wall 28. The thickness of the spacer employed in the illustrative
embodiment is approximately .~60 inches.
While the dielec~ric spacer in the illustrative embodiment is
a ~ull disk which completely covers metallic disk 82, an annular ring of
d~electric material which covers the disk 82 in the region of overlap
between disk 82 and wall 28 could be used as well.
The vertically extending cylindrical portion 96 fonmed at the
center of support disk 94 has formed therein a downwardly facing blind
bore 101 which receives shaft 90 to rotatably support plastic member 92
and disk 82 on shaft 90. Vertical spacing between disk 82 and wall 28
~ 20 is on the order of .090 inches, .060 being spacer thickness and .030
`~ ~ being air.
Means for rotating disk 82 comprises radially extending vanes
98. Yanes 98 rotate about shaft 90 in response to air moving down
waveguide 68. To this end, vanes 98 project through opening 84 in wall
` 25 28 into the interior of waveguide 68. Air for rotating disk 82 enters
waveguide 68 through openings 104;formed for that purpose in end wall 78
and in the waveguide side walls in the vicinity of probe 42~ This air
travels down waveguide 68 to impinge on vanes 98 and then exits waveguide
68 through exit holes 106 formed in end wall 72. A diverting wall 108 30 is formed in waveguide 68 of microwave pervious material. Diverting
wall 108 extends the full height of waveguide 68 and projects at an
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12(J~ 6 9D-RG-15057-~iller
angle from the rear side of waveguide 68, stopping short of the front
wall, leaving a gap 114 therebetween. Air forced down waveguide 68 by
blower 50 is thereby channeled through gap 114 to impinge on the front-
wardly extending vanes 98 to cause rotation of the plastic member 92 and
disk 82 carried thereon in a clockwise direction, as viewed in FIG. 4.
While the illustrative embodiment described herein employs an air driven
disk arrangement, it will be apparent that other means for rotating the
disk, such as a motor driven arrangement, could be similarly employed.
Cover 86, shaft 90 and plastic member 92 are pre'ferably made
of a plastic material having high heat tolerance and low dielectric loss
characteristics. A material particularly sui`table for this purpose is
the synthetic flouride resin sold under the trademark of Teflon, which
in addition to the desired heat resistance and low dielectric losses
also provtdes low frictional losses durin~ rotation of the disk.
In the discussion to follow, the rotating disk and slst con-
figuration is described in more specific geometric and dimensional
detail with particular reference to FIGS. 4 and 5. It is to be empha-
sized, however, that the specific dimensions of the illustrative embodi-
` ment herein described do not necessarily represent limits of useful
values or limitations on the full scope of the invention but, rather,
are intended to provide direction to those skilled in the art. Similarly,
` the accompanying explanation of the present understanding of the theory
`~ of operation of thi; invention is provided for the benefit of workers in
the art and should not be viewed as limiting the invention described
` 25 herein to a precise theory of operation.
In the illustrative embodiment,'disk 82 is fonmed of sheet
metal 0.032 inches thick and having a diameter of 4.0 inches. Aper~ure
80 is a substantially arcuate elongated slot having a width-to-length
ratio less than 0.2. This slot length relationship provides a slot
which is the dual of a wire line dipole'antenna, thus having a sinusoidal
electric field distribution along the slot length. In the illustrative
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embodiment, arcuate slot length is approximately 2.5 inches, and slot
width is approximately 0.375 inches.
The orientation and radial spacing of slot 80 relative to the
axis of rotation 116 of the disk 82 and longitudinal spacing of the axis
of rotation 116 relative to the short circuit termination at end wall 72
pf waveguide 68 are critical for efficient energy coupling. In the
description to follow, the spacing dimensions are given in terms of
guide wavelengths, ~ 9~ The term guide wavelength is used herein to
specify the wavelength of the standing wave in the waveguide which is a
well known function of the free space wavelength and waveguide dimensions.
In accordance with the invention, the radial distance between the axis of
rotation 116 of disk 82 and the longitudinal center line 118 of slot 80
is approximately one-quar~er guide wavelength ( A 9/4). The longitudinal
dis~ance from the short circuit termination provided by end wall 72 of
waYeguide 68 and the axis of rotation 116 of disk 82 is an odd integral
multiple of quarter guide wavelengths. For the frequency and waveguide
s~ructure employed in the illustrative embodiment of FIGS. 1-4, the
guide wavelength is approximately 6.4 inches; the radial dimension
between axis of rotation 116 and slot center line 117 is approximately
1.6 inches; and the distance from end wall 72 to the axis of ro~ation
116 is approxima~ely 4.8 inches corresponding to three quarter guide wave-
lenths.
Orientation of elongated slot 80 substantially transverse to a
radial line extending from the axis of rotation 116 through the longitudi-
~5 nal midpoint of the slot is important in that for efficient energy
coupling the slot should be oriented substantially transverse to the
longitudinal axis of the waveguide at distances from the short circuit
termination which are integral multiples of hatf guide wavelengths and
be oriented substantially parallel to the longitudinal axis of the
waveguide at distances from the short circuit termination which are odd
multiples of quarter guide wavelengths. For the arcuate slot of the
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1211~ 6 9D-RG 15057-Miller
illustrative embodiment or alternatively for a straight elongate slot,
these conditions will be satisfied by spacing of the axis of rotation of
the disk an odd multiple of quarter guide wavelengths from the waveguide
short circuit termination and by orienting the slot to extend substantially
transverse to a radial line extending from the axis of rotation to the
longitudinal midpoint of the slot and spacing the slot such that th~
length of the radial line is approximately one quarter guide wavelength.
The significance of the slot orientation and spacing dimensions
will now be described with reference particularly to FIG. 5. hs is well
known, microwave energy propagates in short circuit terminated rectangular
waveguides such as waveguide 68 with a standing wave characterized by an
electric field which varies in intensity and direction sinusoidally
along the length of the waveguide, with zero field points at the short
circuit term~nation and half gu~de wavelength intervals therefrom, and
max~mum F~eld po~nts occurring at interva1s along the length of the
waveguide which are odd multiples of quarter guide wavelengths from the
short circuit termination. Wall currents established in waveguide walls
also vary sinusoidatly along the length of the waveguide but 90 out of
phase with the electric field in the waveguide. Thus, maximum wall
current points are present at the short circuit termination and at half
guide wavelength intervals therefrom.
In accordance with established slotted waveguide theory, slots
transverse to the direction of propagation, i.e., the longitudinal axis
of the waveguide, can be characterized as series slots, and slots parallel
2~ to the direction of propagation can be characterized as shunt slots.
Maximum coupling, that is, maximum power transfer for series slots, is
obtained by centering such slots at the minimum field, maximum current
points, i.e., at distances which are integral multiples of half guide
wavelengths from the short circuit termination. Conversely, maximum
coupling for shunt slots is achieved by positioning the slot in shunt
orientation at the maximum field, minimum wall current point, i.e., at
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9D-RG-15057-Miller
~2~964.E;
distances which are odd multiples of quarter guide wavelengths1 and
offset laterally from the waveguide longitudinal center line.
It will be apparent from FIG. 5 that by positioning the disk
82 and orienting slot 80 in accordance with the invention, that is, with
the axis of rotation of the disk positioned an odd number of quarter guide
wavelengths from the short circuit termination of the waveguide, and with
the slot radially p~sitioned a quarter guide wavelength from the axis of
rotation, during each complete rotation of the disk the slot passes through
four maximum coupling positions. The waveforms illustrated in FIG. 5
qualitatively represent the electric field and wall current magnitudes
for the standinQ wave supported in the waveguide as a function of distance
(expressed in guide wavelengths~ from the short circuit waveguide tenmi-
nation 72, with field intensity represented in full and current represented
in phantom.
During each rotation of the disk, the slot sequentially passes
through the four positions designated a, b, c and d, positions b, c and
d being illustrated in phantom. In positions a and c, slot 80 is oriented
as a series slot generally transverse to the longitudinal waveguide axis
and centered at a minimum electric field, maximum current point along
the waveguide for maximum coupling. When at positions b and d, slot 80
is oriented as a shunt slot generally parallel to the longitudinal
waveguide axis and laterally offset from the center line of the waveguide
and centered longitudinally relative to the waveguide at a maximum
elec~tric field, minimum current point for maximum shunt slot coupling.
Thus, with each quarter revolution o~ the disk, the slot is alternately
oriented relative to the waveguide as a series slot and a shunt slot
with the spacing of slots relative to the short circuit termination
being such that maximum coupling of energy ~rom the waveguide to the
cooking cavity via the slot is accomplished. When passing between these
four positions, the slot functions as a hybrid series shunt slot with
varying coupling efficiency. ~onsequently, the radiation ~rom the slot
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9D-RG-15057-Miller
a~;~
varies in intensity during each rotation with four positions a3 b, c and
d of relative maximum intensity angularly spaced apart by gO.
Thus~ by this arrangement~ efficient coupling of energy from
the waveguide 68 to the cooking cavity 24 is achieved in a manner which
enhances time-averaged energy distribution in the cooking cavity.
While in the illustrative embodiment herein to be described an
arcuate slot is used, it is to be understood that a straight elongated
slot similarly oriented could also be used. The arcuate slot is used in
the illustrative embodiment in order to obtain satisfactory slot length
within the size constraints imposed by the maximum width of the waveguide
which limits the maximum diameter of circular opening 84 in wall 28,
which in turn limits the maximum straight slot length which can be
provided for a straight slot centered A 9/4 from the axis of rotation
without ex~ending beyond opening 84 in wall 28. However, it is to be
l~ understood that where spacing permits, an elongated straight slot in
disk 80 could be satisfactorily employed, provided that its longitudinal
axis or center line is oriented to be substantially perpendicular to a
radial line extendi~g from the axis of rotation of dîsk 82 and intersecting
the midpoint of the longitudinal center line of the slot. This insures
`20 that as the disk rotates the sl~t is substantially transverse to the
longitudinal axis of the waveguide at integral multiples of half guide
wavelengths and substàntially parallel to the longitudinal axis of the
~ waveguide at odd multiples of quarter guide wavelengths.
`~ While a specific embodiment of the invention has been illus-2~ ~rated and described herein, it is realized that numerous modifications
and changes will occur to those skilled in the art. It is therefore to
be understood that the appended claims are intended to cover all such
modi~ications and changes as fall within the true spirit and scope of
the invention.
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