Microwave phase shifting device

An improved phase shifting device for varying the phase of the standing wave in a hollow rectangular waveguide is provided which is particularly applicable to microwave cooking appliances. A metallic septum is constructed at the end of the waveguide remote from the microwave source which extends inwardly into the waveguide from the adjacent waveguide end wall parallel to the narrow walls of the waveguide and electrically connects the broad walls of the waveguide, thereby dividing the waveguide into two sub-waveguides, each of which exhibits a cut-off characteristic at the operating frequency. The leading edge of the septum provides a short circuit termination reference point for the waveguide. The moving parts comprise a pair of dielectric plugs, each of which is received in a respective one of the sub-waveguides for selective movement in tandem from a reference position completely within the sub-waveguides to one or more phase shifting positions in which the plugs extend forward of the septum leading edge toward the microwave source. The shift in the phase of the standing wave varies linearly with the extent of forward displacement of the plugs relative to the septum leading edge. The plugs are selectively moved in tandem relative to the reference position in the sub-waveguides to provide the desired phase shift.

BACKGROUND OF THE INVENTION 
This invention relates generally to devices for shifting the phase of 
microwave energy propagating in waveguides, and more particularly to such 
devices applicable to microwave cooking appliances. 
Non-uniform spatial energy distribution of microwave energy in the cooking 
cavity of microwave ovens is a problem of long standing for such 
appliances. 
One approach to this problem has been to employ phase shifting devices in 
the feed waveguides. One example of such an approach is described in 
commonly-assigned U.S. Pat. No. 4,301,347 in which a phase shifter is used 
in combination with a circular polarizing element to radiate microwave 
energy into the cooking cavity with rotating elliptical polarization. The 
phase shifter described therein is a mechanical phase shifter comprising a 
resonant loop secured to a shaft journalled in the narrow walls of the 
waveguide, which shaft is rotated by magnetron cooling air. Reference is 
also made to the use of conventional electronic phase shifters, either 
solid state or ferrite. 
In commonly-assigned, copending U.S. patent application, Ser. No. 411,153, 
filed Aug. 25, 1982 by Bakanowski et al, an array of slots is arranged 
along the waveguide to support a substantially stationary first radiating 
pattern when a first phase relationship for the standing wave exists in 
the waveguide, and a second radiating pattern when a second phase 
relationship exists in the waveguide. Phase shifting means is employed to 
periodically change the phase relationship to switch radiating patterns in 
the cooking cavity. The mechanical phase shifting means used in Bakanowski 
et al includes a solenoid actuated plunger positioned a quarter wavelength 
from the waveguide termination, which is inserted into the waveguide to 
physically shift the short circuit termination from the end wall to the 
plunger position, and a rotatable planar conductive vane which when 
oriented parallel to the broad walls has a minimal effect on the phase of 
the standing wave, but when oriented transverse to the broad walls 
provides a short circuit termination. 
Mechanical phase shifters, such as employed in the Bakanowski system, 
provide the desired phase shift but include some less desirable features. 
The physical movement of metallic probes or vanes involve metal touching 
or closely approaching the waveguide walls, presenting the possibility of 
current arcing and contact wear. 
The use of longitudinally movable metallic termination devices which in 
effect move the conductive end wall of the waveguide can be used to 
selectively vary the phase shift of the standing wave in the waveguide. 
However, arcing problems are severe at the interface of the waveguide side 
walls, particularly in typical microwave oven waveguide configurations 
where the height-to-width ratio for the guide is small, resulting in a 
relatively high voltage gradient per unit height. In addition, the 
metal-on-metal movement must overcome a relatively high coefficient of 
friction and is subject to considerable wear. Finally, the amount of shift 
introduced is equal to the longitudinal displacement of the metallic 
termination device; thus, to introduce a quarter wave phase shift, the 
device must move a distance equal to a quarter guide wavelength. 
Frequently, such displacement requires complex moving means and may 
require more space to accommodate the means for moving the device than 
would be preferred. 
The insertion of dielectric material into a waveguide to change the phase 
of the standing wave in the guide is a well known technique. However, the 
phase shift in regions of the waveguide relatively remote from the 
material depends upon the presence or absence of the material in the 
guide, but not the relative longitudinal position of the material in the 
waveguide. Thus, longitudinal movement of a dielectric slab in a typical 
waveguide will not appreciably change the phase of the standing wave in 
regions of the guide relatively remote from the slab. Thus, while a 
dielectric slab avoids the arcing problem and the friction and mechanical 
wear problems inherent with movable conductive metal parts, use of 
dielectric slabs in conventional fashion does not provide the capability 
to selectively vary the phase of the standing wave throughout the 
waveguide by movement of the slabs in the waveguide. 
In view of the advantageous use of phase shifters in relieving the problems 
of non-uniform energy distribution in the microwave cooking appliances and 
in view of the drawbacks of known mechanical phase shifting devices for 
such purposes, a mechanical phase shifter comprising a low-cost, 
non-metallic moving part to selectively shift the phase of the standing 
wave in the waveguide would be highly desirable. 
It is therefore an object of the present invention to provide a phase 
shifting device applicable to microwave cooking appliances which 
incorporates low-cost, non-conductive moving parts which provide the 
desired phase shift, while substantially reducing the occurrence of high 
current arcing and high voltage breakdown in the waveguide of the 
appliance. 
SUMMARY OF THE INVENTION 
An improved phase shifting device for varying the phase of the standing 
wave in a hollow rectangular waveguide is provided which is particularly 
applicable to microwave cooking appliances. A metallic septum is 
constructed at the end of the waveguide remote from the microwave source 
which extends inwardly into the waveguide from the adjacent waveguide end 
wall parallel to the narrow walls of the waveguide and electrically 
connects the broad walls of the waveguide, thereby dividing the waveguide 
into two sub-waveguides, each of which exhibits a cut-off characteristic 
at the operating frequency. The leadin edge of the septum provides a short 
circuit termination reference point for the waveguide. The moving parts 
comprise a pair of dielectric plugs, each of which is received in a 
respective one of the sub-waveguides for selective movement in tandem from 
a reference position completely within the sub-waveguides to one or more 
phase shifting positions in which the plugs extend forward of the septum 
leading edge toward the microwave source. The shift in the phase of the 
standing wave varies linearly with the extent of forward displacement of 
the plugs relative to the septum leading edge. Means are provided to 
selectively move the plugs in tandem relative to the reference position in 
the sub-waveguides to provide the desired phase shift.

DETAILED DESCRIPTION 
In the description to follow, the phase shifting apparatus of the present 
invention is illustratively incorporated in the excitation system of a 
microwave cooking appliance, an application which makes particularly 
advantageous use of the invention. It is not intended by this manner of 
illustration to suggest that the usefulness of the apparatus is limited to 
such applications. 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, a 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 control compartment 26. 
The cooking cavity 24 includes a conductive top wall 28, a conductive 
bottom wall 30, conductive side walls 32 and 34, conductive rear wall, 
which wall is the cabinet wall 16, and the front wall defined by the inner 
face of door 22. Nominal dimensions of cavity 24 are 16 inches wide by 
13.67 inches high by 13.38 inches deep. 
A support plate 37 of microwave pervious dielectric material such as that 
available commercially under the trademark "Pyroceram" or "Neoceram" is 
disposed in the lower region of cavity 24 substantially parallel to bottom 
cabinet wall 14. Support plate 37 provides the means for supporting food 
objects to be heated in the cavity 24, and defines a plane hereinafter 
referred to as the cooking plane. Plate 37 is supported from a support 
strip 38 which circumscribes cavity 24. Strip 38 is secured front to back 
along cavity side walls 32 and 34 and side to side from bottom wall 30 by 
expandable tabs (not shown) which project through small holes (not shown) 
spaced along front and back edges of bottom wall 30 and side walls 32 and 
34. 
The source of microwave energy for cavity 24 is magnetron 40 which is 
mounted in control compartment 26. Magnetron 40 has a center frequency of 
approximately 2450 MHz at its output probe 42 when coupled to a suitable 
source of power (not shown) such as the 120 volts AC power supply 
typically available in domestic wall receptacles. In connection with the 
magnetron, a blower (not shown) provides cooling air flow over the 
magnetron cooling fins 44. The front facing opening of the controls 
compartment 26 is enclosed by control panel 23. It will be understood that 
numerous other components are required in a complete microwave oven, but 
for clarity of illustration and description, only those elements believed 
essential for a prope 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. 
Microwave energy is fed from magnetron 40 to the oven cavity 24 through a 
waveguide having a horizontally extending top feed branch or section 46, a 
vertically oriented side branch or section 48, and a bottom feed branch 50 
comprising a horizontally extending bottom section 51 which extends across 
the bottom of cooking cavity 24 and a vertically extending terminating 
section 52 which extends partially up the far side wall 34. 
Waveguide sections 46, 48 and 50 are conventionally dimensioned to 
propagate 2450 MHz microwave energy in the TE.sub.01 mode. This is 
accomplished preferably by choosing the width of the section (the 
dimension running front to rear of the oven) to be more than one-half 
wavelength but less than one full wavelength and the height of the section 
(the dimension extruding perpendicular to the adjacent cavity wall) to be 
less than one-half wavelength. In the illustrative embodiment, the height 
of sections 46, 48 and 50 are nominally 0.75 inches and the width is 
nominally 3.66 inches. 
The upper waveguide branch 46 runs centrally of upper wall 28 of the 
cooking cavity and, as shown, is formed by elongated member 54 having a 
generally U-shaped cross section which is attached by suitable means such 
as welding to the top wall 28 of cooking cavity 24. Waveguide branch 46 
includes two coupling apertures 56 located in wall 28, through which 
microwave energy is transmitted into the upper region of the cooking 
cavity 24. The slots 56 extend parallel to the longitudinal dimension of 
guide 46. 
Waveguide section 46 also includes portions 58 and 60 which extend beyond 
cavity 24 in the direction of the magnetron 40 to enclose an area 61 which 
serves as a launching area for microwave energy originating at probe 42. 
Conductive wall 60 serves as a short circuit waveguide termination for 
area 61 and is conventionally spaced approximately one-sixth guide 
wavelength from probe 42. 
The side waveguide branch 48 runs in a vertical direction centrally of 
cooking cavity side wall 32 and serves to couple the microwave energy from 
magnetron 40 to bottom feed waveguide branch 50. Waveguide branch 48 is 
formed generally by the cavity side wall 32 and an elongated member 62 
having a generally U-shaped cross section and suitable flanges for 
attachment to the side wall 32. A right angle bend is formed by wall 
portion 49 at the lower end of section 48 to efficiently couple energy 
from section 48 to section 50. 
Microwave energy from launch area 61 in the vicinity of probe 42 of 
magnetron 40 is split between section 46 and section 48 by bifurcator 80 
which operates to provide a stable power split between these sections. 
Bifurcator 80 is positioned at the junction of three waveguide sections 
comprising guide sections 46, 48 and launch area 61. The upper portion of 
bifurcator 80, comprising upper face 81 of horizontally extending divider 
82 and step 83, functions as a quarter wave transformer to efficiently 
match the impedance of guide section 46 to launch area 61 for maximum 
power transfer. To this end the horizontal length for uppe face 81 is a 
quarter guide wavelength. The height of step portion 83 is chosen as a 
function of the height of guide sections 46 and launch area 61 in 
accordance with conventional quarter wave transformer design. The lower 
portion of bifurcator 80 provides a conventional mitered corner at 84 for 
proper impedance matching with side waveguide section 48. 
Horizontally extending section 51 of bottom feed waveguide section 50 runs 
horizontally across the center of bottom wall 30 of cavity 24 
approximately underneath upper waveguide section 46 and terminates in 
vertically extending end portion 52 which extends part way up side wall 34 
approximately across from side waveguide section 48. 
Bottom waveguide section 51 is made up of a U-shaped cross section member 
68 attached to the flat central section 70 of bottom wall 30 of cooking 
cavity 24. The U-shaped member 68 includes an upper wall 72 which together 
with flat section 70 of bottom wall 30 provides oppose parallel broad 
walls and integral side walls 74 extending downwardly toward the bottom 
wall 30 of cooking cavity 24 which provide opposed parallel narrow walls 
joining broad walls 72 and 70. Side walls 74 have suitable flanges 76 to 
facilitate attachment to the bottom wall 30 in a conventional manner, such 
as by welding. Open end 64 of section 51 is in communication with side 
branch 48 to receive microwave energy therefrom. At the opposite end of 
section 51, a right angle bend is formed by wall portion 66 to efficiently 
couple energy to the vertically extending end portion 52. 
As best seen in FIG. 3, the upper wall 72 of guide section 50 has formed 
therein an array of radiating apertures designated generally 88. Apertures 
88 are arranged to provide different substantially stationary radiating 
patterns in cooking cavity 24, depending upon the phase relationship of 
the standing wave of the electric field established in the waveguide 
section. Phase shifting apparatus in accordance with the present invention 
is illustratively employed to vary the phase relationship of the standing 
wave in waveguide 50, thereby varying the radiating pattern from waveguide 
50 at the cooking plane. 
As discussed briefly in the Background, insertion of a single dielectric 
slab in the waveguide would change the phase of the standing wave 
propagating therein. However, once inserted, movement of a slab in the 
waveguide would not vary the phase of the standing wave in the region of 
the waveguide relatively remote, i.e., more than about a half guide 
wavelength from the dielectric slab. However, it has been discovered that 
by terminating the waveguide with a conductive septum which divides the 
end portion of the guide into two sub-waveguides, thereby acting 
essentially as a modal filter to block the primary propagating mode in the 
waveguide, and by inserting a pair of dielectric plugs in each of the 
sub-waveguides, a phase shift can be introduced which varies linearly with 
the forward displacement of the plugs relative to the leading edge of the 
septum when the plugs are moved in tandem longitudinally in the waveguide, 
and with a proportionality constant which is significantly less than one. 
Such phase shifting apparatus in accordance with the present invention is 
illustratively embodied in section 52 of waveguide section 50. Section 52 
is terminated by a metallic septum or divider wall 90 which extends 
inwardly from end wall 92 of section 52 generally parallel to the narrow 
waveguide walls 94 and 96 to divide the end portion of waveguide section 
52 into two sub-waveguides 98 and 100. Septum 90 is connected by suitable 
low resistance contacting means, such as welding, to opposed broad walls 
102 and 104, which is a portion of wall 34 and end wall 92 of waveguide 
section 52 to provide a low resistance electrical connection therebetween. 
As hereinbefore described, the width of the waveguide sections 46, 48 and 
50 are chosen to propagate the basic TE.sub.01 mode. The widths of 
sub-waveguides 98 and 100 formed by septum 90 are too narrow to progagate 
the TE.sub.01 mode and thus exhibit cut-off characteristics at the 2450 
MHz operating frequency. 
The low impedance leading edge 106 of septum 90, that is the edge nearest 
the magnetron 40 in the waveguide path, provides a short circuit 
termination reference point for waveguide section 50. It has been 
empirically determined that satisfactory results are achieved with septum 
length, measured from end wall 92 to leading edge 106, in the range of 
one-quarter to one-half guide wavelength. 
A pair of dielectric plugs or blocks 108 and 110 are movably mounted in 
sub-waveguides 98 and 100, respectively, for tandem longitudinal movement 
in wave guide section 52. In the illustrative embodiment, the plugs are 
formed of tetrafluoroethylene ("Teflon"). Alternatively, other types of 
conventional non-conductive materials could readily be used, provided they 
have a dielectric constant of 4.2 or greater. The plugs are configured 
such that when fully recessed in their respective sub-waveguides, plugs 
108 and 110 substantially fill the sub-waveguides with just enough 
clearance to permit the plugs to slide easily. When so recessed, exposed 
surfaces 112 and 114 of plugs 108 and 110, respectively, are substantially 
flush with leading edge 106 of septum 90. In the position of the plugs 
illustrated in FIG. 5A, referred to hereinafter as the first position, the 
plugs have essentially no phase shifting effect on the standing wave in 
waveguide section 50, and the phase of the standing wave in the waveguide 
is determined by the physical location of the septum leading edge 106. 
As mentioned briefly hereinbefore, the phase of the standing wave in the 
waveguide shifts has been found to vary linearly with the forward 
displacement of the plugs relative to leading edge 106 with a 
proportionality constant which is less than one. In the illustrative 
embodiment, the proportionality constant was found to be on the order of 
0.3-0.4. This somewhat surprising result provides significant advantages, 
particularly in structures where spacing is cramped since the reduction in 
required displacement allows for the us of shorter strokes when using 
solenoid actuators or smaller cams when using cam drive arrangements. 
As used herein, forward displacement refers to positioning of the plugs 
such that surfaces 112 and 114 of plugs 108 and 110, respectively, are 
positioned forward of leading edge 106; i.e., closer to magnetron 40 in 
the waveguide path than leading edge 106. Thus, the phase relationship of 
the standing wave in waveguide section 50 in accordance with the present 
invention can be selectively varied by appropriate forward displacement of 
the dielectric plugs relative to leading edge 106. In the illustrative 
embodiment, the waveguide 50 is slotted to support one radiating pattern 
with zero phase shift and a second pattern when the phase is shifted by 
one quarter guide wavelength. To provide the desired quarter guide 
wavelength phase shift, a second predetermined position for plugs 98 and 
100 is provided in which the plugs are sufficiently forwardly displaced 
from leading edge 106 to introduce the quarter guide wavelength phase 
shift. In the illustrative embodiment, a displacement of 0.6 inches has 
been determined to be sufficient to provide the desired quarter guide 
wavelength (1.6 inches) phase shift. This second position for plugs 98 and 
100 is illustrated in FIG. 5B. 
The means employed in the illustrative embodiment for selectively or 
periodically moving plugs 108 and 110 to selectively vary the phase of the 
standing wave in waveguide section 50 will now be described. The prime 
mover is an electric timer motor 116 which is supported from the outer 
surface of cavity side wall 34 by motor mounting bracket 118. Mounting 
bracket 118 is suitably secured to wall 34 such as by welding. Motor 116 
is secured to bracket 118 by screws 120. Motor drive shaft 122 is 
drivingly linked to driveshaft 124 of an eccentric cam 126 by a 
conventional gear train (not shown) enclosed within gear housing 128. Plug 
drive rods 130 and 132 are integrally formed with a tie bar 134. Rods 130 
and 132 project through apertures 136 and 138, respectively, in end wall 
92 and are suitably secured in holes 140 and 142 bored in plugs 108 and 
110, respectively, such as by gluing. Tie bar 134 linking plug drive rods 
130 and 132 is biased into cam-following engagement with eccentric cam 126 
by a pair of compression springs 144, each of which encircles one of plug 
drive rods 130 and 132, and is sandwiched between end wall 92 of waveguide 
section 52 and tie bar 134. Cam 126 is contoured to provide the desired 
pattern of movement for the plugs. The contour illustrated enables the 
plugs to dwell for relatively long periods in the first and second 
positions illustrated in FIGS. 5A and 5B, respectively, and to move 
relatively quickly therebetween when the cam is rotated at a constant 
rate. Motor 116 may be continuously energized to continuously move plugs 
108 and 110 between the first and second positions or intermittently 
energized to allow a desired amount of dwell time at each extreme position 
or at positions in between. Predetermined dwell times at various positions 
can also be provided by use of appropriate dead time gear linkages. 
Use of a timer motor allows considerable flexibility to make advantageous 
use of the linear change of phase with displacement of the plugs. However, 
many other means for moving the plugs could be used. For example, if only 
movement between two discrete positions is desired, a solenoid plunger 
could readily be used for selectively positioning the plugs. 
To more fully appreciate the utility of the present invention as 
illustratively embodied in microwave oven 10, the radiating aperture 
arrangement of waveguide section 50 will now be described in greater 
detail. It will be recalled that an electric field is supported in 
waveguide 50 between the top and bottom walls of guide section 50, which 
field is characterized as a standing wave. This standing wave has a 
certain phase relationship in the guide which can be defined in terms of 
either the location of the nodes of the standing wave or the maximum field 
points, relative to a reference point in the waveguide. In the 
illustrative embodiment, this reference point is the short circuit 
reference point provided by leading edge 106 of septum 90. One effect of 
the short circuit termination for the bottom feed wave guide section 50 
provided by leading edge 106 is to establish a standing wave node or 
minimum field point at leading edge 106. This defines a first phase 
relationship for the standing wave in guide 50. When this relationship 
exists in the waveguide, a particular combination of slots in guide 50 is 
excited to radiate a first radiating pattern in cooking cavity 24. 
Shifting of the phase of the standing wave changes the phase relationship. 
Shifting the phase by a quarter guide wavelength establishes a second 
phase relationship in the waveguide. When this second phase relationship 
exists in the guide section 50 a different combination of slots is excited 
to radiate the second radiating pattern in cooking cavity 24. 
Referring again to FIG. 3, the arrangements for the radiating apertures 90 
to provide the two different radiation patterns will now be described. 
Each of apertures 88 in the illustrated embodiment is constructed as a 
series slot; that is, the longitudinal axis of the slot is oriented 
transverse to the direction of wave propagation in guide section 50. The 
dimensions of the slots are chosen with a view to evenly distributing the 
energy along the radiating chamber and to provide the desired impedance 
matching. Specifically, slot lengths were chosen at substantially less 
than one-half a waveguide wavelength so as to provide non-resonant slots. 
This assures that energy is relatively evenly distributed along the length 
of guide section 50 rather than radiating primarily from those slots 
nearest the entrance to section 50. 
Slots 88 are arranged in two staggered rows, designated generally A and B. 
Within each row the lateral spacing between the slots is one-quarter guide 
wavelength. Slot A-1 is located one guide wavelength from leading edge 
106. Thus, all the slots of Row A are centered an integral multiple of 
quarter guide wavelengths from leading edge 106. When guide 50 is 
terminated by a short circuit at leading edge 106, i.e., plugs 108 and 110 
in the first position, slots A-1, A-3, A-5 and A-7 are centered at minimum 
field or standing wave node points which correspond to maximum power 
coupling points for series slots, while slots A-2, A-4 and A-6 are at 
maximum field points corresponding to minimum power coupling points for 
series slots. When the phase of the standing wave in guide 50 is shifted 
by a quarter guide wavelength, this situation is reversed with slots A-2, 
A-4 and A-6 being centered at maximum power coupling points and slots A-1, 
A-3, A-5 and A-7 being at minimum coupling points. 
Slot B-1 is centered seven-eighth guide wavelengths from leading edge 106. 
Consequently, slots B-1-B-7 are each centered at odd integral multiples of 
eighth guide wavelengths from end wall 65. Thus, each of slots B-1-B-7 is 
centered at a half power coupling point, i.e., midway between adjacent 
maximum and minimum power coupling points when either the first or second 
phase relationship exists in guide section 50. 
FIGS. 6-8 are sketches of representative energy distribution patterns at 
the cooking plane for the oven of the illustrative embodiment, with FIGS. 
6 and 7 representing the energy distribution at the cooking plane from 
waveguide 50 for the first and second phase relationships, respectively. 
The cross-hatched regions in each FIGURE represent regions of relatively 
high energy density. These radiation patterns at the cooking plane are the 
result of the interference of radiation from the slots of Row B with those 
slots of Row A centered at the maximum coupling points. More specifically, 
the radiation from each maximum power point slot in Row A constructively 
interferes with the radiation from its immediately adjacent half power 
point slots of Row B to form a region of high energy density at the 
cooking plane over each three slot clusters. 
FIG. 6 shows the basic radiation pattern when plugs 108 and 110 are in the 
first position (FIG. 5A). Region 0-1 is formed by radiation from slot A-1 
and B-2; region 0-2 is formed by radiation from slots A-3, B-3 and B-4; 
region 0-3 is formed by radiation from slots A-5, B-5 and B-6; and region 
0-4 is formed by radiation from slots A-7 and B-7. FIG. 7 shows the basic 
radiation pattern when plugs 108 and 110 are in the second position (FIG. 
5B). The phase of the standing wave is shifted by a quarter guide 
wavelength and, consequently, high intensity region S-1 is formed by 
radiation from slot B-1; region S-2 is formed by radiation from slots A-2, 
B-2 and B-3; region S-3 is formed by radiation from slots A-4, B-4 and 
B-5; and region S-4 is formed by radiation from slots A-6, B-6 and B-7. By 
periodically moving plugs 98 and 100 between the first and second 
positions, the radiation pattern at the cooking plane is switched to first 
and second patterns. 
Although in the illustrative example the slot array is arranged primarily 
to provide two radiating patterns, it should be noted that since the phase 
shifting device of the present invention provides a linear variation of 
the phase between the two extreme positions, slots B-1-B-7 would be 
positioned at maximum power coupling points when the phase is shifted by 
one-eighth guide wavelength, i.e., when the plugs are at the midpoint 
between the first and second positions. Thus, the displacement of the 
plugs could be controlled by appropriate intermittent energization of 
motor 116 to provide a pause at each of three positions, the first and 
second positions previously described and a third position midway between 
these two positions. In this third position, the plugs would establish a 
third phase relationship in the waveguide during the pause at the third 
position, resulting in a third radiation pattern with alternate B slots as 
primary radiators and the adjacent A slots being positioned at the 
half-power coupling points. 
It will be apparent that the capability to control the phase of the 
standing wave over a continuous range of phase angles as provided by the 
phase shifting apparatus of the present invention allows much greater 
flexibility in arranging arrays of slots for selective excitation as a 
function of the phase of the standing wave in the waveguide, than is 
possible with conventional mechanical phase shifting devices which 
typically provide a discrete quarter guide wavelength phase shift by 
switching between an open circuit and a closed circuit termination. 
While the specific embodiment illustrated and described herein incorporates 
the phase shifting apparatus of the present invention in a microwave 
cooking oven, it is understood that the apparatus could be readily adapted 
to other applications requiring means for providing a linear phase 
shifting capability for a standing wave in a rectangular waveguide. In 
addition, 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 modifications and 
changes as fall within the true spirit and scope of the invention.