Note: Descriptions are shown in the official language in which they were submitted.
1147851
Backqround of the Invention
This invention pertains generally to antennas for radio
frequency energy and particularly to antennas wherein à planar
array of slotted waveguides is used.
It has been common practice in the art of designing radar
antennas for seekers in guided missiles to use a so-called
resonant slot array. According to known practice, such an
array is formed by mounting a plurality of similarly dimensioned
slotted rectangular waveguides in proximity with one another to
cover a predetermined aperture. An electrical short circuit
is formed across one end of each waveguide to make a resonant
structure wherein standing waves may exist to optimize the
energization of the slots. A corporate feed of conventional
design then is connected to the second ends of the waveguide to
allow operation of the resonant slot array either as a trans-
mitting antenna or a receiving antenna, such as a monopulse
antenna.
Ordinarily, when a resonant slot array is to be used in a
guided missile, it is necessary that: (1) a broadside pencil
beam be formed so that antenna gain is maximized, with sidelobe
levels a~ low as possible; and (2) the energy in the beam be
linearly polarized, with cross-polarization effects minimized.
In order to achieve the foregoing in the limited space available
in the cylindrical body of a guided missile, the aperture of the
usual slot array is circular in shape, the array itself is
-4'~q~l,5l1f mounted so as to be steerable in y~ and a~lmu~h and the orien-
S119 tation of all of the slots with respect to the longitudinal axes
~1/1s of the waveguides is maintained constant. Further, if the slot
array is to be operated as a monopulse antenna, the number of
waveguides and disposition of slots is such that an equal number
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8~1
of slots is located in each quadrant of the aperture. In
addition, the constraints on any slot array which must be met
to avoid grating lobes or reduction in efficiency must be
observed. That is to say, for a given frequency of operation,
proper attention must be given to the dimensions of the wave-
guides, the spacing between slots and the position of the
electrical short circuit in each one of the waveguides. Thus,
in a typical application wherein the aperture of an antenna in
a guided missile may have a diameter of S", a slot array for
X-band may have a maximum of 20 slots when known techniques are
used to design such an array. Because the antenna gain of any
slot array is directly related to the number of slots, the
antenna gain of the array is limited.
Another problem is encountered with the conventional slot
array wherein similarly dimensioned waveguides are used.
Because the positions of the slots in each waveguide (along the
length of such guide) are fixed, it is difficult to produce a
symmetrical pencil beam. As a result, the quality of performance
of the conventional slot array varies, depending upon the direc-
tion of a target from boresight.
785~L
Summary of the Invention
In view of the foregoing background of this invention, it is a pri-
mary object hereof to provide a slot array wherein, with a circular aperture,
the number of slots may be increased beyond the largest number possible when
known techniques are applied in the design of such an array.
Another object of this invention is to provide a slot array which
incorporates a larger number of slots than heretofore deemed possible in a cir-
cular aperture and at the same time may be used as a monopulse antenna.
Still another object of this invention is to provide an improved
slot array wherein the spacing between slots may be optimized to produce a
symmetrical pencil beam.
The foregoing and other objects of this invention are attained gen-
erally by providing, in a resonant slot array having a circular aperture, a
plurality of differently dimensioned slotted rectangular waveguides disposed
in proximity with one another to form the circular aperture, the spacing be-
tween the slots and the position of the requisite electrical short circuit in
each one of such waveguides being determined by a selected dimension of each
wavegulde.
In accordance wlth the present invention, there is provided a linear-
ly polarlzed slot array antenna having a substantially circular aperture, the
ratio between the diameter of such aperture and the wavelength of radio fre-
quency energy at the design frequency of such antenna being in the order of
5:1, such antenna comprising: (a2 a first plurality of rectangular waveguide
resonators dimensioned, when ~ux~aposed with narrow walls abutting, substan-
tially to cover a first half of the circular aperture, the width of the broad
wall of each successive one of such waveguide resonators decreasing outwardly
from the centrally located one of such waveguides; (b) a second like plurality
of rectangular waveguide re~onator~ covering the second half of the circular
aperture; and (c? a plurality of radiating slots formed through the broad wall
of each one of the rectangular waveguide resonators, such slots being parallel
one to another with the center of each different slot lying in a plane of
maximum electric field within its corresponding rectangular waveguide resonator,
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~7~5~
the distances of such slots in each such resonator from the center-line thereof
increasing from the center to each end of such resonator to provide amplitude
taper along the length of such resonators.
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~4785~ `
srief Description of the Drawings
For a more complete understanding of this invention,
reference is now made to the following description of the
accompanying drawing wherein the single Figure is a plan
view of an antenna according to this invention.
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Description of the Preferred Embodiment
Referring now to the FIGURE, the disposition of slots
(numbered below) in waveguides lOu, 12u, 14u, 141, 121 and 101
of an exemplary resonant slot array (intended for use at X-band
in a monopulse radar in a guided missile where a largest allow-
able aperture 11 is circular in shape with a diameter of 5") is
shown. Because the slots in waveguides 101, 121 and 141 are
disposed, respectively, in the same way as the slots in wave-
guides lOu, 12u and 14u, and because the dimensions of similarly
numbered waveguides are the same, only the latter waveguides
will be described. Also, the thickness of each wall in each
waveguide and the conventional corporate feed needed have not
been shown.
Starting with waveguide 14u, the width W14 (meaning the
inslde dimension of the broad wall of a rectangular waveguide)
, i9 cho~en so that the cutoff frequency of the dominant TEol
mode in such guide here is near the low end of X-band. The
thickness (not shown but meaning the inside dimension of the
narrow wall of a rectangular waveguide) is chosen so that the
cutoff frequency of the next higher mode (the TElo or TE20
- mode) is above the highest frequency in the X-band. Here a
conventional rectangular waveguide (having inside dimensions
of 0.900" x 0.400") for use at X-band is employed.
It will be recognized that the waveguide wavelength of
- X-band energy within waveguide 14u is longer than the wave-
length of such energy in free space and that the slots must
be spaced at distances determined by the waveguide wavelength.
Even so, with an aperture 5" in diameter, with shunt slots it is
,~ still possible here to have six slots spaced at half-wavelengths
~- 30 (measured in the waveguide along the axis 14u) and an electrical
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short circuit spaced one-quarter wavelength from the last slot
(say slot 14u(3')).
Slots 14u(3), 14u(1) and 14u(2') are spaced from each other
at intervals of one wavelength along the axis 14u. Similarly,
slots 14u(2), 14u(1') and 14u(3') are spaced from each other at
intervals of one wavelength from each other and are interleaved
at one-half wavelength intervals from slots 14u(3), 14u(1) and
14u(2'). In addition, slots 14u(1) and 14u(1') are equally
spaced (measured along the YAW AXIS, not numbered) from the
axis 14u; slots 14u(2) and 14u(2') are also similarly spaced;
and finally slots 14u(3) and 14u(3') are also similarly spaced.
It will now be recognized that the slots 14u(1) through 14u(3')
constitute a linear array with amplitude taper along the PITCH
AXIS, with the centerline of the beam produced by such array
broadslde to the waveguide 14u (meaning orthogonal to the plane
defined by the PITCH AXIS and the YAW AXIS). Further, the first
sidelobe (measured along the axis 14u) is determined in accord-
ance with the selected amplitude taper.
Waveguide 12u here is dimensioned so that its width is less
than the width of waveguide 14u. Therefore, the wavelength of
energy in waveguide 12u is greater than that in waveguide 14u.
In consequence, then, the spacing (measured along axis 12u)
between the various slots 12u(1), 12u(2), 12u(1'), 12u(2') is
greater than the spacing of the correspondi.ng slots in wavegui.de
14u. It should be noted here that if energy is fed to the same
end of waveguide 12u as waveguide 14u, the electrical short
circuit (not shown) in the former would be disposed one-quarter
wavelength from slot 12u(2') so the sense of the electric field
at each pair of corresponding slots in the two waveguides would
; 30 be appropriate along the YAW AXIS.
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~147851
The spacing (measured along the YAW AXIS) of each one of
the slots 12u(1), 12u(2), 12u(1'), 12u(2') is determined to
allow an amplitude taper along the YAW AXIS (without affecting
polarization). Therefore, each slot in waveguide 12u is at a
greater distance from the axis 12u' than the distance of the
corresponding slot in waveguide 14u from the axis 14u'.
Waveguide lOu is dimensioned so that at least two slots
lOu(l), lOu(l') may be fitted in the zone defined by the free
side of waveguide 12u and the line defining the largest allow-
able aperture 11. Here the width of waveguide lOu is .740 inches.
As would be expected, because the wavelength of energy within
the waveguide lOu is greater than the wavelength within wave-
guide 14u or 12u, slots lOu(l) and lOu(l') are further apart
than slots 12u(1) and 12u(1') or 14u(1) and 14u(1'). Also,
the position of the electrical short circuit (not shown) near
slot lOu(l') is determined b~ the wavelength of the energy in
the waveguide lOu and the senses of the electric fields at
slots lOu(l) and lOu(l') are, respectively, the same as the
senses of the electric fields at slots 14u(1) and 14u(1').
In order to provide a desired amplitude taper along the
YAW AXIS, the amount of energy fed into each one of the wave-
guides 14u, 12u, lOu, 141, 121, 101 is adjusted in any con-
venient manner in the corporate feed (not shown). In addition,
the positions (measured along the YAW AXIS) of slots lOu(l),
lOu(l') are changed to contribute to the desired amplitude
taper so that the shape of the beam along the YAW AXIS is
optimized and sidelobes are reduced to a minimum.
It will now be evident to one of skill in the art that,
for a slot antenna with a small (meaning in the order of 5"
diameter) circular aperture when designed for X-band, the
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flexibility in design offered by using waveguides of different
widths is advantageous. That is to say, because the relative
positions of corresponding slots in the different waveguides
may be adjusted without changing the orientations of such
slotsr antenna gain may be maximized for a pencil beam with
relatively small sidelobes and without affecting the polariza-
tion of such beam. In this connection, it should be noted
that the length of each slot also may be adjusted to modify
the phase distribution across the aperture. With an a priori
knowledge of the effect of changing the len~th of a resonant
slot, conventional empirical techniques may be followed to
adjust the phase distribution across the aperture for any
particular case. Thus, as here where the width of the beam
measured along the yaw axis is to be the same as the width of
the beam along the pitch axis, the lengths of the slots may be
changed to optimize the phase distri.buti.ons along such axes.
It is felt, therefore, that this invention should not be
restricted to its disclosed embodiment, but rather should be
imited only by the spirit and scope of the appended claims.
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