U.S. patent number 3,701,162 [Application Number 04/354,341] was granted by the patent office on 1972-10-24 for planar antenna array.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Arthur F. Seaton.
United States Patent |
3,701,162 |
Seaton |
October 24, 1972 |
PLANAR ANTENNA ARRAY
Abstract
A circularly polarized planar array of slot antenna elements
provide a pencil-beam pattern substantially perpendicular to the
plane of the array.
Inventors: |
Seaton; Arthur F. (Palos Verdes
Estates, CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
23392875 |
Appl.
No.: |
04/354,341 |
Filed: |
March 24, 1964 |
Current U.S.
Class: |
343/771;
342/365 |
Current CPC
Class: |
H01Q
21/005 (20130101); H01Q 13/20 (20130101); H01Q
21/24 (20130101) |
Current International
Class: |
H01Q
21/00 (20060101); H01Q 21/24 (20060101); H01Q
13/20 (20060101); H01q 013/10 () |
Field of
Search: |
;343/771,100.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Benjamin A.
Assistant Examiner: Berger; Richard E.
Claims
What is claimed is:
1. A circularly polarized antenna array comprising:
a. means defining at least one waveguide having a broad wall, said
waveguide being capable of supporting a traveling wave TE.sub.10
mode of propagation when excited;
b. said waveguide including means defining a slow-wave structure
providing a reduction in guide wavelength;
c. said broad wall having at least two pairs of crossed slots
orthogonal to each other, said pairs of crossed slots being
longitudinally separated from each other by substantially one guide
wavelength, said crossed slots being disposed laterally on one side
of the centerline at the locations at which the wall currents are
circularly polarized when said waveguide is excited;
d. and said broad wall having at least two nonintersecting slots
orthogonal to each other, said nonintersecting slots being
longitudinally separated from each other by substantially half a
guide wavelength, said nonintersecting slots being disposed
laterally on the other side of said centerline from said crossed
slots and being positioned in relation to said crossed slots such
that radiation from said nonintersecting slots is in phase with
radiation from said crossed slots.
2. A circularly polarized antenna array comprising:
a. means defining at least one waveguide having a broad wall, said
waveguide being capable of supporting a traveling wave TE.sub.10
mode of propagation when excited;
b. said waveguide including means defining a slow-wave structure
providing a reduction in guide wavelength;
c. said broad wall having at least two pairs of crossed slots
orthogonal to each other and at an angle of substantially
45.degree. to the longitudinal centerline of said waveguide, said
pairs of crossed slots being longitudinally separated from each
other by substantially one guide wavelength, said crossed slots
being disposed laterally on one side of said centerline at the
locations at which the wall currents are circularly polarized when
said waveguide is excited;
d. and said broad wall having at least two nonintersecting slots
orthogonal to each other and at an angle of substantially
45.degree. to the longitudinal centerline of said waveguide, said
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the other side of
said centerline from said crossed slots and being centered
longitudinally between said crossed slots, the ends of said
nonintersecting slots that are proximate to said centerline being
farther apart than the other ends thereof.
3. A circularly polarized linear antenna array comprising:
a. a rectangular waveguide having first and second opposing broad
walls, said waveguide being capable of supporting a traveling wave
TE.sub.10 mode of propagation when excited;
b. said first broad wall being corrugated to define a slow-wave
structure providing 25 percent reduction in guide wavelength;
c. said second broad wall having two pairs of crossed nonresonant
slots orthogonal to each other and at an angle or 45.degree. to the
longitudinal centerline of said waveguide, said pairs of crossed
slots being longitudinally separated from each other by
substantially one guide wavelength, said crossed slots being
disposed laterally on one side of said centerline at the locations
at which the wall currents are circularly polarized when said
waveguide is excited;
d. and said second broad wall having two nonintersecting
nonresonant slots orthogonal to each other and at an angle of
45.degree. to the longitudinal centerline of said waveguide, said
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the other side of
said centerline from said crossed slots and being centered
longitudinally between said crossed slots, the ends of said
nonintersecting slots that are farthest apart being closest to said
centerline.
4. A circularly polarized linear antenna array comprising:
a. a rectangular waveguide having first and second opposing broad
walls, said waveguide being capable of supporting a traveling wave
TE.sub.10 mode propagation when excited;
b. said first broad wall being corrugated to define a slow-wave
structure providing substantially 25 percent reduction in guide
wavelength;
c. said second broad wall having a plurality of pairs of
intersecting nonresonant slots, the two slots of each of said pairs
of intersecting slots being substantially orthogonal to each other
and at an angle of substantially 45.degree. to the longitudinal
centerline of said waveguide, the centers of said pairs of
intersecting slots being longitudinally separated from each other
by substantially one guide wavelength, said intersecting slots
being disposed laterally on one side of said centerline at the
locations at which the wall currents are circularly polarized when
said waveguide is excited;
d. and said second broad wall having a plurality of pairs of
nonintersecting nonresonant slots, the two slots of each of said
pairs of nonintersecting slots being orthogonal to each other and
at an angle of 45.degree. to the longitudinal centerline of said
waveguide, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the opposite side
of said centerline from said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of crossed slots, the ends of the two slots of each of said pairs
of nonintersecting slots that are proximate to said centerline
being farther apart than the ends that are distant therefrom.
5. A circularly polarized antenna array comprising:
a. means defining at least one rectangular waveguide having a broad
wall, said waveguide being capable of supporting a traveling wave
TE.sub.10 mode of propagation when excited;
b. said waveguide including means defining a slow-wave structure
providing a reduction in guide wavelength;
c. said broad wall having a plurality of pairs of intersecting
slots, the two slots of each of said pairs of intersecting slots
being substantially orthogonal to each other and at an angle of
substantially 45.degree. to the longitudinal centerline of said
waveguide, the centers of said pairs of intersecting slots being
longitudinally separated from each other by substantially one guide
wavelength, said intersecting slots being disposed laterally on one
side of said centerline at the locations at which the wall currents
are circularly polarized when said waveguide is excited;
d. and said broad wall having a plurality of pairs of
nonintersecting slots, the two slots of each of said pairs of
nonintersecting slots being orthogonal to each other and at an
angle of 45.degree. to the longitudinal centerline of said
waveguide, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the opposite side
of said centerline from said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots, the ends of the two slots of each of said
pairs of nonintersecting slots that are proximate to said
centerline being farther apart than the ends that are distant
therefrom.
6. A circularly polarized antenna array comprising:
a. means defining at least one pair of rectangular waveguides each
having a broad wall, said waveguides being parallel and adjacent
and each being capable of supporting a traveling wave TE.sub.10
mode of propagation when excited;
b. means coupled to said waveguides for feeding said waveguides
180.degree. out of phase;
c. said waveguides including means defining slow-wave structure
providing a reduction in guide wavelength;
d. each of said broad walls having a plurality of pairs of
intersecting slots, the two slots of each of said pairs of
intersecting slots being substantially orthogonal to each other and
at an angle of substantially 45.degree. to the longitudinal
centerline of said waveguides, the centers of said pairs of
intersecting slots being longitudinally separated from each other
by substantially one guide wavelength, said intersecting slots
being disposed laterally on one side of said centerline at the
locations at which the wall currents are circularly polarized when
said waveguides are excited;
e. each of said broad walls having a plurality of pairs of
nonintersecting slots, the two slots of each of said pairs of
nonintersecting slots being orthogonal to each other and at an
angle of 45.degree. to the longitudinal centerline of said
waveguides, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the opposite side
of said centerline from said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots, the ends of the two slots of each of said
pairs of nonintersecting slots that are proximate to said
centerline being farther apart than the ends that are distant
therefrom;
f. and the slot pattern of one of said waveguides being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern of the other of said waveguides.
7. A circularly polarized planar antenna array comprising:
a. a pair of rectangular waveguides each having first and second
opposing broad walls, said waveguides being parallel and adjacent
and each being capable of supporting a traveling wave TE.sub.10
mode of propagation when excited;
b. means coupled to said waveguides for feeding said waveguides
180.degree. out of phase;
c. each of said first broad walls being corrugated to define
slow-wave structure providing substantially 25 percent reduction in
guide wavelength;
d. each of said second broad walls having a plurality of pairs of
intersecting nonresonant slots, the two slots of each of said pairs
of intersecting slots being substantially orthogonal to each other
and at an angle of substantially 45.degree. to the longitudinal
centerline of said waveguides, the centers of said pairs of
intersecting slots being longitudinally separated from each other
by substantially one guide wavelength, said intersecting slots
being disposed laterally one one side of said centerline at the
locations at which the wall currents are circularly polarized when
said waveguides are excited;
e. each of said second broad walls having a plurality of pairs of
nonintersecting nonresonant slots, the two slots of each of said
pairs of nonintersecting slots being orthogonal to each other and
at an angle of 45.degree. to the longitudinal centerline of said
waveguides, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the opposite side
of said centerline from said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots, the ends of the two slots of each of said
pairs of nonintersecting slots that are proximate to said
centerline being farther apart than the ends that are distant
therefrom;
f. and the slot pattern of one of said waveguides being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern of the other of said waveguides.
8. A circularly polarized planar antenna array comprising:
a. a plurality of rectangular waveguides each having first and
second opposing broad walls, said waveguides being parallel and
adjacent and each being capable of supporting a traveling wave
TE.sub.10 mode of propagation when excited;
b. means coupled to said waveguides for feeding said waveguides
with alternate ones thereof 180.degree. out of phase;
c. each of said first broad walls being corrugated to define
slow-wave structure providing substantially 25 percent reduction in
guide wavelength;
d. each of said second broad walls having a plurality of pairs of
intersecting nonresonant slots, the two slots of each of said pairs
of intersecting slots being substantially orthogonal to each other
and at an angle of substantially 45.degree. to the longitudinal
centerline of said waveguides, the centers of said pairs of
intersecting slots being longitudinally separated from each other
by substantially one guide wavelength, said intersecting slots
being disposed laterally on one side of said centerline at the
locations at which the wall currents are circularly polarized when
said waveguides are excited;
e. each of said second broad walls having a plurality of pairs of
nonintersecting nonresonant slots, the two slots of each of said
pairs of nonintersecting slots being orthogonal to each other and
at an angle of 45.degree. to the longitudinal centerline of said
waveguides, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally on the opposite side
of said centerline from said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots, the ends of the two slots of each of said
pairs of nonintersecting slots that are proximate to said
centerline being farther apart than the ends that are distant
therefrom;
f. and the slot pattern of alternate ones of said waveguides being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern of the other of said waveguides.
9. A circularly polarized antenna array comprising:
a. means defining a waveguide having first and second opposing
broad walls, said waveguide being capable of supporting a plurality
of traveling wave TE.sub.10 modes of propagation along parallel
longitudinal axes when excited;
b. means coupled to said waveguide for feeding said waveguide with
waves which are 180.degree. out of phase along alternate ones of
said axes;
c. said first broad wall being corrugated to define slow-wave
structure providing substantially 25 percent reduction in guide
wavelength;
d. said second broad wall having a plurality of pairs of
intersecting nonresonant slots and nonintersecting nonresonant
slots disposed in longitudinal rows on alternate sides of each of
said axes, the two slots of each of said pairs of intersecting
slots being substantially orthogonal to each other and at an angle
of substantially 45.degree. to said axes, the centers of said pairs
of intersecting slots being longitudinally separated from each
other by substantially one guide wavelength, said intersecting
slots being disposed laterally from said axes at the locations at
which the wall currents are circularly polarized when said
waveguide is excited;
e. the two slots of each of said pairs of nonintersecting slots
being orthogonal to each other and at an angle of substantially
45.degree. to said axes, the centers of the two slots of each of
said pairs of nonintersecting slots being longitudinally separated
from each other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally from said axes the
same distance as said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots, the ends of the two slots of each of said
pairs of nonintersecting slots that are proximate to one of said
axes being farther apart than the ends that are distant
therefrom;
f. and the slot pattern along alternate ones of said axes being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern along the other of said axes.
10. A circularly polarized antenna array comprising:
a. means defining a waveguide having a broad wall, said waveguide
being capable of supporting a plurality of traveling wave TE.sub.10
modes of propagation along parallel longitudinal axes when
excited;
b. means coupled to said waveguide for feeding said waveguide with
waves which are 180.degree. out of phase along alternate ones of
said axes;
c. said waveguide including means defining slow-wave structure
providing a reduction in guide wavelength;
d. said broad wall having a plurality of nonresonant slots arranged
in longitudinal rows parallel to said axes, a row of intersecting
pairs of slots being on one side of each of said axes and a row of
nonintersecting pairs of slots being on the other side of each of
said axes, the two slots of each of said pairs of slots being
substantially orthogonal to each other and at an angle of
substantially 45.degree. to said axes, the centers of said pairs of
intersecting slots being longitudinally separated from each other
by substantially one guide wavelength, said intersecting slots
being disposed laterally from said axes at the locations at which
the wall currents are circularly polarized when said waveguide is
excited, the centers of the two slots of each of said pairs of
nonintersecting slots being longitudinally separated from each
other by substantially half a guide wavelength, said
nonintersecting slots being disposed laterally from said axes the
same distance as said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots on the opposite side of the proximate one of
said axes, the ends of the two slots of each of said pairs of
nonintersecting slots that are proximate to one of said axes being
farther apart than the ends that are distant therefrom;
e. and the slot pattern along alternate ones of said axes being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern along the other of said axes.
11. A circularly polarized antenna array comprising:
a. means defining a waveguide having a broad wall, said waveguide
being capable of supporting a plurality of traveling wave TE.sub.10
modes of propagation along parallel longitudinal axes when
excited;
b. a plurality of mode suppressors disposed in said waveguide for
suppressing modes of propagation other than said plurality of
TE.sub.10 modes;
c. means coupled to said waveguide for feeding said waveguide with
waves which are 180.degree. out of phase along alternate ones of
said axes;
d. said waveguide including means defining slow-wave structure
providing a reduction in guide wavelength;
e. said broad wall having a plurality of nonresonant slots arranged
in pairs staggered across said axes at substantially half a guide
wavelength spacing intersecting pairs of slots being on one side of
each of said axes and nonintersecting pairs of slots being on the
other side of each of said axes, the two slots of each of said
pairs of slots being substantially orthogonal to each other and at
an angle of substantially 45.degree. to said axes, the centers of
said pairs of intersecting slots being longitudinally separated
from each other by substantially one guide wavelength, said
intersecting slots being disposed laterally from said axes at the
locations at which the wall currents are circularly polarized when
said waveguide is excited, the centers of the two slots of each of
said pairs of nonintersecting s slots being longitudinally
separated from each other by substantially half a guide wavelength,
said nonintersecting slots being disposed laterally from said axes
the same distance as said intersecting slots, each pair of said
nonintersecting slots being centered longitudinally between pairs
of intersecting slots on the opposite side of the proximate one of
said axes, the ends of the two slots of each of said pairs of
nonintersecting slots that are proximate to one of said axes being
farther apart than the ends that are distant therefrom;
f. and the slot pattern along alternate ones of said axes being
longitudinally displaced substantially half a guide wavelength with
respect to the slot pattern along the other of said axes.
Description
The invention described herein was made in the performance of work
under a NASA contract and is subject of the provisions of section
305 of the National Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 U.S.C. 2457).
Planar arrays are used in applications requiring high gain and high
aperture efficiency. However, conventional planar arrays are
ordinarily linearly polarized. Where circular polarization is a
requirement, a parabolic antenna, rather than a planar array of
slot elements, is usually employed.
However, parabolic antennas have a lower gain and lower aperture
efficiency than a planar array and, in addition, are larger and
heavier. Hence, attempts have been made to adapt planar arrays to
circular polarization.
The so-called "leaky pipe" waveguide planar array, although usually
linearly polarized, can be made circularly polarized but it does
not provide a "broadside" beam, that is, a beam perpendicular to
the plane of the array. The mode of operation is essentially
"end-fire," that is, with the beam substantially parallel to the
plane of the array (sometimes called the zero order beam). The
leaky-pipe array cannot be made to operate in the broadside mode,
and tilts the beam at least 40.degree. off broadside. Aperture
efficiency is low because the effective aperture is reduced by the
cosine of the beam-tilt angle.
The annular slot waveguide planar array also can be made to provide
circular polarization but it suffers from the necessity of spacing
the slots a full guide wavelength apart. This results in the
generation of secondary maxima, that is, in addition to the
broadside lobe, the array generates other large lobes at large
angles off broadside. Consequently, the annular slot antenna has
low efficiency.
Circularly polarized traveling wave linear arrays providing a
broadside beam have been constructed employing nonresonant crossed
slots on the broad wall of a waveguide operating in the TE.sub.10
mode. These linear arrays may be combined to form planar arrays but
such an array also has an extremely low aperture efficiency
because, again, a large slot spacing is required. This large slot
spacing is necessitated by the fact that crossed slots can be used
only on one side of the centerline of the waveguide for a given
sense of circular polarization (i.e., right-hand or left-hand
circular polarization). Crossed slots on the other side of the
centerline radiate a circularly polarized wave of the opposite
sense. Broadside operation of the array thus requires that the
slots be spaced a full guide wavelength apart. This spacing, which
is approximately equal to 1.4 times a free-space wavelength, allows
the generation of secondary maxima which seriously detract from the
principal maximum, or main lobe.
Accordingly, it is an object of the present invention to provide a
circularly polarized planar antenna array having a narrow beam
pattern substantially perpendicular to the plane of the array and
having substantially no secondary lobes.
Another object of the invention is the provision of a circularly
polarized planar antenna array having a high efficiency.
Yet another object of the present invention is to provide an
efficient circularly polarized high-gain antenna which is light in
weight and which has a thin, flat configuration.
In accordance with these and other objects of the invention, there
is provided a thin rectangular enclosure having one broad surface
slotted to form radiating elements and the opposing broad surface
corrugated to form a slow-wave structure. The rectangular enclosure
forms a wide rectangular waveguide operating in a multimode fashion
to effectively produce a plurality of virtual waveguide linear
arrays extending side-by-side and operating in the traveling wave
mode. Circular polarization is obtained by using nonresonant
crossed slots on one side of the centerline of each virtual
waveguide and pairs of separated nonresonant slots on the other
side of the centerline. Each pair of separated slots is centered
between the crossed slots to obtain the correct sense of rotation
and phase. Alternate linear arrays are displaced by half a guide
wavelength to account for the phase reversal of the alternate half
cycles of the multimode feed arrangement. The antenna is excited
from feed slots in the bottom of the first corrugation trough of
each linear array. The feed slots are shunt-type slots in a feed
waveguide extending tranversely across the ends of the linear
arrays. The feed waveguide operates as a standing wave array and is
centerfed by means of a magnetic loop coax-to-waveguide transition.
Conductive strips serving as mode suppressors extend from the
corrugations to the aperture plate along the line of the virtual
waveguide walls.
The following specification and the accompanying drawings describe
and illustrate an exemplary embodiment of the present invention.
Consideration of the specification and the drawings will provide an
understanding of the invention, including the novel features and
objects thereof. Like reference characters denote like parts
throughout the figures of the drawings.
FIG. 1 is a plan view of an embodiment of a planar antenna array in
accordance with the invention, with a portion of the aperture plate
broken away to show the interior;
FIG. 2 is a perspective view, partly broken away, of a portion of
the planar array of FIG. 1;
FIG. 3 is a schematic diagram indicating the current distribution
in a portion of the planar array of FIGS. 1 and 2; and
FIG. 4 is a vector diagram indicating the relative exciting
voltages for three different instants of time of the slots shown in
schematic diagram of FIG. 4.
Circularly polarized arrays can be constructed by using crossed
slots on the broadwall of a waveguide at a point of circular
polarization of the magnetic fields within the waveguide. However,
efficient aperture illumination is difficult to obtain using these
slots along since they must be spaced a full guide wavelength apart
in order to keep the radiated fields in phase. This results in a
free space distance between elements of approximately 1.4
multiplied by a free space wavelength. This large spacing is
undesirable because it results in a loss of aperture efficiency in
the generation of secondary maxima (also known as second order
beams or grating lobes).
In a linearly polarized broadwall shunt slot array it is
conventional to place consecutive slots at a half guide wavelength
spacing but on opposite sides of the centerline, thereby keeping
all slots in phase. This technique will not work with crossed slots
because only the transverse components of current on the waveguide
walls are reversed on the opposite side of the centerline. The
longitudinal components are in phase at all points across the guide
at any given transverse plane. This situation results in circular
polarization of the opposite sense being radiated from crossed
slots placed on the opposite side of the guide.
To solve these problems, the present invention employs additional
slots in a new arrangement which together with a slow wave
structure produce adequate interelement spacings. These additional
slots are also of the nonresonant type, and may be considered to be
the two orthogonal portions of a crossed slot separated by a
spacing of half a guide wavelength. The separated slots are located
on the waveguide relative to the position of the normal crossed
slot such that at broadside both the normal crossed slot and the
separated crossed slot radiate a circularly polarized wave in phase
and of the same sense.
In FIG. 1 there is illustrated an exemplary embodiment of a
circularly polarized planar array 10 constructed in accordance with
the present invention. There is provided a thin, rectangular
enclosure 11 made of metal or other conductive material. The
rectangular enclosure 11 forms a wide rectangular waveguide
suitable for excitation in a multimode manner. In the present
example, the enclosure 11 is 38.5 inches square and 1 inch thick.
One broad surface 12 of the enclosure 11 is slotted to form
radiating elements. The other broad surface 13 of the enclosure 11
is corrugated to form a slow-wave structure.
When the enclosure 11 is excited, it operates as a plurality of
rectangular waveguide linear arrays extending side-by-side. In the
present example, none virtual waveguides are formed. No waveguide
walls are necessary to separate the virtual waveguides because the
electromagnetic fields operate as if there were solid walls between
the virtual waveguides. However, due to the lack of mirror image
symmetry in the slots on each side of the virtual walls, mode
suppressors 14 are employed. The mode suppressors 14 are elongated
metal strips extending along the virtual walls between the aperture
surface 12 and the crest of the corrugated surface 13, and form
partial waveguide walls. The mode suppressors 14 suppress
undesirable propagation modes in the virtual waveguides.
The first virtual waveguide is indicated by the bracket 15
extending across the broad dimension thereof at the feed end. On
one side of the waveguide centerline, there is a row of
crossed-slot elements 16. Each crossed-slot element 16 is formed of
two intersecting orthogonal slots 17, 18. The slots 17, 18 are
disposed at an angle of 45.degree. with respect to the sides of the
enclosure 11 so that they can be elongated without extending past
the waveguide edge. The crossed-slot elements 16 are located at the
point in the waveguide where the wall currents are circularly
polarized and are spaced approximately one guide wavelength apart.
Actually, the spacing is somewhat less than one wavelength, as will
be discussed hereinafter.
On the other side of the waveguide centerline, there is a row of
separated slot-pair elements 20. Each separated slot-pair element
20 is formed of two nonintersecting orthogonal slots 17', 18'.
These slots 17', 18' are also oriented at an angle of 45.degree.
with respect to the sides of the enclosure 11, and at an angle of
90.degree. with respect to each other. Each slot designated 17' of
a separated slot-pair element 20 is parallel to a slot designated
17 of a crossed-slot element 16. Similarly, each slot designated
18' of a separated slot-pair element 20 is parallel to a slot
designated 18 of a crossed-slot element 16. The centers of the two
slots 17', 18' of each separated slot-pair element 20 are spaced
nominally half a guide wavelength apart, as modified by spacing
factors to be discussed. Each separated slot-pair element 20 is
centered longitudinally along the guide between two crossed-slot
elements 16. With this slot pattern, the crossed-slot elements 16
and the separated slot-pair elements 20 radiate a
circularly-polarized wave in-phase and of the same sense.
The slot pattern for each of the other waveguides is the same
except that the slot pattern is displaced by half a guide
wavelength in alternate waveguides to account for the phase
reversal of the alternate half cycles of the multimode excitation.
The planar array 10 may be considered to be nine linear arrays,
each having slot-pair elements 16, 20 staggered across the
centerline to obtain in-phase, circularly polarized radiation from
an interelement spacing of half a guide wavelength.
FIG. 3 schematically shows the currents for the TE.sub.10 mode on
the broadwall of a slotted rectangular waveguide representing one
of the slotted virtual waveguides of the planar array 10 of FIGS. 1
and 2. By noting the direction of the current lines exciting the
crossed slots 17, 18 as the wave progresses down the waveguide, it
can be determined that the radiated field is left-hand circularly
polarized for a wave coming out of the page. The field is
circularly polarized rather than elliptically polarized because the
slots 17, 18 are displaced laterally from the centerline to the
location where the magnitude of the current remains constant as the
wave propagates or, in other words, where the magnitudes of the
longitudinal and transverse currents (which are 90.degree. in
time-phase) are equal.
The separated slots 17', 18' are disposed on the opposite side of
the centerline and centered between the crossed slots 17, 18 to
provide the correct sense of rotation and phase with respect to
that of the crossed slots 17, 18. The lateral displacement of the
separated slots 17', 18' from the centerline is identical to that
of the crossed slots 17, 18, because the same criterion holds. FIG.
4 vectorially illustrates the relative exciting voltages in the
slots 17, 17', 18, 18' at three successive instants of time
differing from each other by 45.degree. in phase of the wave
traveling in the direction of propagation shown in FIG. 3. The
summation of the vectors, indicated as E.sub.t in FIG. 4,
represents the addition that takes place in the direction of the
main beam. As may be seen from FIG. 4, the total electric field
propagated in the direction of the main beam from the two separated
slots 17', 18' is always equal in amplitude, orientation and phase
to the electric field propagated in that direction from the crossed
slots 17, 18.
The planar array 10 may also be considered to be two interlaced
linearly polarized planar arrays orthogonal to each other and fed
in time quadrature. That is, all of the slots oriented in the
direction of slots 17 and 17' may be viewed as one linear array
arranged in rows of slots, with the slots closely spaced more or
less end-to-end, and the rows widely spaced. The rows extend at an
angle to the sides of the enclosure 11 along lines through the
centers of the slots, such as the row of slots indicated by the
line in the direction of the arrows 19--19 shown in FIG. 1.
Similarly, all of the slots oriented in the direction of slots 18
and 18' may be viewed as rows of slots forming the second linearly
polarized array orthogonal to the first.
To adequately suppress secondary maxima (second order beams or
lobes) in order to obtain high aperture efficiencies, the interrow
spacing has been reduced to less than a free-space wavelength. This
is accomplished by means of the corrugated surface 13 which forms a
slow-wave structure that provides a 25 percent reduction in guide
wavelength. When the array 10 is considered as two interlaced
linearly polarized planar arrays, it is apparent that the
separation between slots in the same row, such as the row indicated
by the arrows 19--19 in FIG. 1, is much less than a free-space
wavelength. By use of the corrugated surface 13 as a slow-wave
structure, the orthogonal separation between rows is reduced to 0.8
of a free-space wavelength. In this manner, an aperture efficiency
of 92 percent was obtained.
To obtain maximum gain, the planar array 10 is uniformly
illuminated. Accordingly, because this is a traveling wave array
and not a standing wave array, the coupling coefficient of each
succeeding slot 17, 17', 18, 18' is made progressively larger as
the distance down each virtual waveguide from the feed point
becomes larger. The coupling coefficient is increased by increasing
the length of the slots which makes them more nearly resonant. In
this manner, each pair of slots radiates the same amount of power.
The last slot is adjusted to couple out as much of the remaining
power as possible. To compensate for phase differences thus
introduced, the spacing between the slots is gradually decreased as
the distance from the feed point becomes larger. The end of the
enclosure 11 farthest from the feed end is closed or shorted in the
present example, although it may also be left open if desired.
To obtain a predetermined space coverage and good impedance match,
the beam is tilted 6.degree. back towards the feed end by
decreasing the slot spacing an additional amount. The result of
this reduction of slot spacing is to reduce the row spacing by some
fraction of the percentage of reduction in slot spacing. This
reduction in slot spacing is also highly desireable from the
standpoint of input impedance. A separation of one guide wavelength
between the crossed-slot elements 16 results in a separation of a
half guide wavelength between the separated slots 17', 18'. The
disadvantage of exactly half a guide wavelength spacing is that
reflections from the slots would all add in-phase, thereby
producing a high voltage-standing-wave-ratio at the input to the
linear arrays.
The linear arrays are fed from a rectangular feed waveguide 25
(best seen in FIG. 2) operating as a standing wave array and
located underneath the radiation virtual waveguides and at one end
of the array 10. Coupling from the feed waveguide 25 to the linear
arrays is accomplished by nonresonant longitudinal shunt slots 26
located in the broad wall of the feed waveguide 25 and opening into
the first trough in the corrugated surface 13. The coupling or feed
slots 26 are spaced apart by half a free space wavelength and are
disposed on the same side of the centerline of the feed waveguide
25 to provide 180.degree. phase shift between each of the feed
slots 26. The feed waveguide 25 is terminated in shortcircuits at
each end, one-quarter guide wavelength beyond the last feed slots
26. The reactive component of the admittance is tuned out at each
feed slot 26 by an inductive post 27 disposed at each feed slot 26.
A coaxial connector 28 couples to the feed waveguide 25 by means of
a coupling loop 30 located opposite the central feed slot 26.
For operation at a frequency of 2,295 megacycles per second, the
broad dimension of each virtual waveguide is 4.267 inches and the
narrow dimension is 0.991 inch. The corrugated surface 13 has a
dimension of 0.491 inch from the crests to the troughs, and the
corrugations are 0.1 inch in width and are separated by 0.4 inch.
By constructing the array 10 of thin aluminum (on the order of from
0.003 - 0.010 inch thick, for example), the total weight of the
array 10 is very small, 81/2 pounds for example.
An embodiment of an array constructed in accordance with the
present invention provided an over all efficiency of 70 percent,
aperture illumination efficiency of 92 percent and a gain of 27.0
decibels. The measured ellipticity of the polarization was 1.0
decibel.
Thus, there has been described an efficient, circularly polarized
planar antenna array providing a pencil-beam pattern substantially
perpendicular to the plane of the array by virtue of employing
crossed slots and separated slots in a new arrangement, in
conjunction with a slow-wave structure.
While only one embodiment of the invention has been shown and
described, variations may be made, and it is intended that the
foregoing disclosure shall be considered only as illustrative of
the principles of the invention and not construed in a limiting
sense.
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