U.S. patent number 3,683,387 [Application Number 05/101,645] was granted by the patent office on 1972-08-08 for compact scanning radar antenna.
This patent grant is currently assigned to The United States of America as represented by the. Invention is credited to James M. Meek.
United States Patent |
3,683,387 |
|
August 8, 1972 |
COMPACT SCANNING RADAR ANTENNA
Abstract
Disclosed are radar sectoral horn antennas of the rolled and
folded paral plate type having two internal reflectors which makes
possible a more compact construction. The double fold or double
reflector construction is applicable to sectoral horns having an
f/d ratio greater than one and the reflectors may be straight or
curved. The horns may be provided with a dielectric, geodesic or
wave guide lens and by combining two orthogonally related antennas,
three dimensional scanning is possible.
Inventors: |
James M. Meek (Silver Spring,
MD) |
Assignee: |
The United States of America as
represented by the (N/A)
|
Family
ID: |
22285710 |
Appl.
No.: |
05/101,645 |
Filed: |
December 28, 1970 |
Current U.S.
Class: |
343/761; 343/786;
343/840; 343/914 |
Current CPC
Class: |
H01Q
19/19 (20130101) |
Current International
Class: |
H01Q
19/10 (20060101); H01Q 19/19 (20060101); H01q
013/00 () |
Field of
Search: |
;343/781,786,912,915,761,840,914 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eli Lieberman
Attorney, Agent or Firm: Harry M. Saragovitz Edward J. Kelly
Herbert Berl Saul Elbaum
Claims
1. A sectoral horn antenna having rolled and folded paralleled
spaced plates comprising: an inlet aperture formed by a rolled
portion of the plates; a first internal linear reflector formed by
a folded strip joining the edges of the rolled portion of the
plates, a second internal substantially parabolic reflector formed
by a folded strip joining the edges of the flat portion of the
plates; and
2. The structure of claim 1 wherein the antenna has a focal length
identified as f, and wherein the width of the outlet aperture is
identified as d, the ratio f/d being greater than one.
Description
This invention relates to a double fold or double internal
reflector antenna which by incorporating a pair of internal
reflectors make it possible to construct a much more compact
antenna which retains the desirable features of larger physical
constructions. The antenna finds particular utility as a radar
antenna useful in tracking range and/or angular position of
airplanes, missiles, satellites, or other air or space borne
bodies.
Radar antennas have been used for many years in fire-control
systems and also in other applications requiring range and angle
detection of space objects. Early radar antennas utilized motion of
the entire antenna to achieve volume coverage. Subsequently,
constructions appeared that provide line scanning utilizing
oscillatory feed motion. Some of these employed large, trapezoidal,
parallel plate sectoral horns which were quite large and cumbersome
and as a consequence were difficult to manufacture and operate.
In U.S. Pat. No. 2,585,562, to W. D. Lewis, there is disclosed a
sectoral horn antenna which employs a much more mechanically
desirable nonoscillatory or rotary feed system and one in which the
sectoral horn is bent or folded into a much more compact
construction. This antenna operates in the transverse electric and
magnetic (TEM) wave mode and the input end of the trapezoidal horn
is formed annularly so that a small feed horn can be continuously
rotated at the annular orifice. Rotation of the feed horn at the
input orifice results in a repetitive angular displacement of the
beam emanating from the large output aperture which scans a
sectoral volume of space. In operation, the angular position of a
target is determined as a function of the angular position of the
horn, the horn angle being proportional to the beam swept angle.
That is, electromagnetic energy is reflected (or emitted) from the
target, received by the antenna, and focused at a point in the
annular orifice where it is picked up by or enters the feed horn
each time it passes the point during rotation. Usually, electronic
tracking circuitry is employed to measure the angle as a function
of elapsed time between a scan start position pulse provided by a
motion transducer on the scanner and the "center" of the energy
reflected from the target. Range is determined by the time required
for the microwave pulses to propagate to the tracked object and
back to the receiver. In order to collimate the outgoing energy, a
dielectric lens (similar to an optical lens) is placed in the
sectoral horn near the exit aperture.
The present invention is directed to a scanning radar antenna of
generally similar parallel plate construction utilizing a
trapezoidal sectoral horn but one which is of improved and more
compact construction. The present invention is based on the fact
that it has been found that by increasing the ratio of sectoral
horn focal length to exit aperture width, it is possible to
incorporate an additional fold or additional internal reflector
into the sectoral horn, thus reducing the flat plate area of the
horn before rolling by as much as 50 percent or more compared to
the uninterrupted flat plate area. The double fold construction of
the present invention retains the desirable continuous rotary feed
features of prior single fold antennas but, in addition, provides a
longer focal length in design volumes, improved radar resolution,
less severe lens contours, new configurations which permit easier
fabrication and dismantling for inspection and repair, and makes
possible a compact construction capable of scanning a sectoral
volume of space. In addition, by incorporating curved reflectors,
it is possible in some instances to eliminate the need for a lens
near the exit aperture of the horn.
It is therefore one object of the present invention to provide an
improved scanning radar antenna.
Another object of the present invention is to provide a scanning
radar antenna that is more compact and therefore more easily
manufactured and operated.
Another object of the present invention is to provide a radar
scanning antenna having a pair of internal reflectors.
Another object of the present invention is to provide a compact
radar scanning antenna which makes possible longer focal lengths in
a predetermined volume.
Another object of the present invention is to provide an improved
compact scanning radar antenna incorporating one or more curved
reflectors.
Another object of the present invention is to provide a radar
antenna construction which makes possible volume or line scanning
of space utilizing a continuously rotating feed horn.
These and further objects and advantages of the invention will be
more apparent upon reference to the following specification,
claims, and appended drawings, wherein:
FIG. 1 is a plan view of a rolled parallel plate double reflector
antenna constructed in accordance with the present invention;
FIG. 2 is an elevational view of the antenna of FIG. 1;
FIG. 3 shows the annular feed aperture of the antenna of FIGS. 1
and 2;
FIG. 4 is a plan view of the antennas of FIG. 1-3 before rolling
and folding and illustrates the two internal antenna
reflectors;
FIG. 5 is a development view similar to that of FIG. 4 showing an
oblique double reflector antenna with a pair of straight reflectors
and a rear scanner;
FIG. 6 is a similar development view of an antenna incorporating a
pair of inner reflectors having a front scanner;
FIG. 7 is a development view of a further modification showing an
antenna with a pair of curved reflectors;
FIG. 7A shows the inlet aperture of FIG. 7 after rolling;
Referring to the drawings, FIGS. 1 and 2 illustrate an antenna,
generally indicated at 10, constructed in accordance with the
present invention. In this embodiment, the antenna takes the form
of a rolled parallel plate antenna having a linear internal
reflector 12 and a parabolic internal reflector 14. The antenna is
formed from a pair of parallel conductive metal plates 16 and 18
spaced to define an air gap between them equal to one-half the
operating wave length or less. The antenna, depending upon the
dimensions, may be operated at any suitable microwave or radar
frequency, such as K-band, X-band, S-band, L-band, or the like. A
typical spacing between parallel plates may be on the order of
one-fourth inch to 1 inch, but for TEM mode operation not to exceed
one-half wave length. The antenna comprises wave guide feed line 20
adapted to be electrically coupled to a conventional
transmitter-receiver for transmitting electromagnetic energy from
the antenna and receiving reflections from a target in a well-known
manner. Wave guide 20 is connected to a rotating scan head 22 by
way of a rotary joint 24. Energy from the wave guide 20 passes
through the scan head 22 into the sectoral horn, generally
indicated at 26, formed by the spaced parallel plates 16 and 18. A
portion of the horn adjacent the feed head is rolled into a cone,
as illustrated at 28. The plates are joined along their edges 12
and 14 to form a strip so that the microwave energy is reflected
first from edge 12 and then from edge 14 as it passes through the
sectoral horn. The parallel plates are illustrated as cylindrically
curved or bent, as at 30, and terminate in an outwardly flared edge
or end 32 forming an outlet aperture for electromagnetic energy
transmitted from the feed line through the antenna. In the
preferred construction, the energy exiting from the antenna (and
similarly reflected energy from the target) is collimated along the
longer dimension of the aperture by a curved external reflector 34.
Energy emanating from the antenna is indicated by the arrows
36.
FIG. 3 illustrates the feed arrangement of the antenna of FIGS. 1
and 2 in more detail and shows the rotary joint 24 coupled to feed
horn 38 which is continuously rotated, as illustrated by the arrow
40 in FIG. 3, in the conventional manner. Feed horn 38 is located
within the scan head 22 of FIG. 1 and includes a 90.degree.
cylindrical bend (not shown) so that energy emanating from the feed
horn enters the annular inlet aperture 42 between the portions of
plates 16 and 18 forming feed cone 28.
FIG. 4 is a development view of the parallel plates of the antenna
of FIGS. 1-3 before the parallel plates are bent and rolled. The
feed horn 38, which if desired FIG. 4, be made slightly asymmetric
to compensate for illumination asymmetry, scans along the inlet
aperture 42. In development, the edge of the inlet aperture is of
arcuate configuration forming the arc of a circle which when rolled
into the shape of a cone, as illustrated in FIGS. 1-3, lies in a
plane adjacent the rotary feed horn. The center of the
electromagnetic energy from the feed horn, as illustrated by the
ray line 44, impinges upon electrically conductive edge 12 where it
is reflected and propagates between the plates to impinge upon the
electrically conductive edge 14. The electromagnetic energy is
again reflected from edge 14 and passes out of the antenna through
output aperture 32. In development, that is before bending, edge 12
forms a linear secondary reflector and edge 14 forms a parabolic
primary reflector. After bending and rolling, the propagation of
the microwave energy through the antenna is in a sense the same,
that is, it is reflected from both linear reflector 12 and
parabolic reflector 14. The scan angle is illustrated at 46 in FIG.
4. Lines 48 and 50 indicate approximately the angle of the scanned
sector. By incorporating a curved internal reflector, such as the
parabolic reflector 14 in FIGS. 4, it is not necessary to provide a
lens in the antenna of FIGS. 1-3 since focusing is achieved by the
parabolic reflector 14 across the narrower dimension of the output
energy. While FIGS. 1-4 have been described in conjunction with
energy transmitted outwardly from the antenna, it is understood
that reflected energy from the target passes through the antenna in
the same manner but in the opposite direction from outlet aperture
32 to feed aperture 42. In order to retain the precise
electromagnetic characteristics of the development of FIG. 4, it is
necessary that the sectoral horn be bent, curved or rolled into
developable geometric shapes, such as cones and cylinders, without
distorting the input and output edges or stretching the median
surface between conducting sheets.
FIG. 5 is a development view of a modified antenna constructed in
accordance with the present invention. It is understood that the
antenna of FIG. 5 in its final configuration is rolled and folded
in the manner of the antenna of FIGS. 1-3 so as to be compact and
to have an annular input aperture. The antenna of FIG. 5 is an
oblique double reflector parallel plate antenna with rear scanner
and incorporates a pair of linear reflectors.
The antenna of FIG. 5 again comprises a pair of parallel conductive
plates, one of which is indicated at 54, and the antenna 52 has an
arcuate inlet aperture 56 which, when rolled into a cone, presents
a planar aperture scanned by a continuously rotating feed horn 58,
in all respects similar to the rotatable feed horn previously
described. In FIG. 5, the central ray line 60 passes through the
center of the input aperture 56 and impinges on a secondary
reflector 62 which is a straight reflector formed by one edge of
the antenna. The central ray then passes between the parallel
plates where it is again reflected off a second straight reflector
or edge 64, forming the primary reflector of the sectoral horn.
From here the central ray 60 passes through an output aperture 66
which includes a microwave lens area 68. Since the reflectors 62
and 64 in this embodiment are straight, they provide no focusing
and it is desirable to provide a lens 68, which may be formed of a
plastic or dielectric lens, metal plate, but is preferably in the
form of a geodesic lens. This may consist of the conventional
parabolic-bulge or the semi-paraboloid-of-revolution lenses, for
examples.
The limits corresponding to the extreme sweep positions of the feed
horn 58 are illustrated at 70 and 72 in FIG. 5. The scan center for
feed horn motion is preferably located at the geometric or optical
center of the lens 68.
All the double reflector horns of the present invention may be
thought of as originally derived from a trapezoidal or sectoral
parallel plate horn, as indicated by dashed lines at 74 in FIG. 5.
The horn may be further thought of as this trapezoid twice folded
with the overlapping or coextensive portions of the original
trapezoid omitted. Thus, as seen from FIG. 5, the overall length
and volume of the sectoral horn is substantially reduced by at
least half without sacrificing the original focal length given by
the dimension f in FIG. 5 which extends from the center of the
entrance aperture to the center of the lens if a lens is employed.
The angle of scan is determined by the circular or arcuate
curvature of the input aperture. The longer dimension of the output
aperture is given by the dimension d in FIG. 5. It has been found
that for sectoral horns having a ratio f/d greater than 1, that two
internal reflectors may be employed to substantially decrease the
size of the antenna and increase its compactness. In the present
invention, the preferred range for the f/d ratio is on the order of
1.2 to 1.7.
FIG. 6 is a development view of a modified double reflector antenna
constructed in accordance with this invention. The antenna
illustrated in FIG. 6 is an oblique double reflector plate antenna
with two straight reflectors similar to that shown in FIG. 5, but
provided with a front instead of the rear scanner illustrated in
FIG. 5. In FIG. 6, the antenna is generally indicated at 76 and
comprises an arcuately curved inlet aperture 78, rotary feed horn
80, straight secondary reflector (second fold) 82, a primary
straight reflector (first fold) 84, output aperture 86, and lens
area 88. As before, inlet aperture 78 forms a part of a rolled cone
so that the arcuate aperture when rolled lies in a single plane.
The central ray line 90 in FIG. 6 is first reflected from reflector
82 and then reflected from reflector 84 to pass out through the
center of the lens 88.
FIG. 7 shows a further modified embodiment and is a development
view of a "half-cassegrain" 2-reflector parallel plate antenna in
which both reflectors are curved. The antenna comprises a
concavely, rather than convexly, curved input aperture 94 forming a
part of the antenna 92 and the annular nature of this aperture
after rolling into a cone is illustrated in FIG. 7A. A first
reflector is formed by edge 96 curved into the segment of hyperbola
and a second reflector is formed by edge 98 having a parabolic
curvature. The output aperture is illustrated at 100. Central ray
102 these reflectors and is emitted through the center of the
outlet aperture 100. In this embodiment, the feed horn acts as a
point source feed at the hyperbola reflector focus and the focus of
the parabola 98 is at the center of the virtual scan arc as
illustrated at 104 (common focus for the parabola and hyperbola).
The second hyperbola focus is at the center of arc 94. It is seen
that as a result of the geometric relationships involved in this
embodiment, a more compact scanner is achieved in addition to other
advantages which are common to the various designs. Since size of
the scan head is a limiting factor in determining scan rate, higher
rates are realizable with this arrangement.
The focal lengths of the antennas may be for example 10 or 20 feet
or more and, depending on size and shape, the antennas may be used
over a wide range of microwave frequencies. Scan repetition rates
of up to 30 Hz or greater can be obtained with peak power
transmission in the megawatt region. Since the output beam is line
scanned, two dimensions are measurable with a single antenna within
the space sector. For example, range and azimuth angle. To make
cross angle measurements, such as elevation, a second antenna may
be used. The antenna beam may be either a pencil beam or may be the
characteristic fan beam shape. Lenses of the geodesic, metal plate
or dielectric types may be employed to provide the collimation in
the narrow angle dimension of the fan or pencil beam and external
reflectors may be used to provide the desired amount of collimation
in the broad angle dimension. The horn may be rolled up or curved
with discretion into developable geometric shapes, such as cones
and cylinders, so long as the input and output edges are not
distorted and so long as stretching of the median plane is not
involved.
The invention may be embodied in other specific forms without
departing from the spirit and essential characteristics thereof.
The present embodiments are therefore to be considered in all
respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are therefore
intended to be embraced therein.
* * * * *