U.S. patent number 3,713,163 [Application Number 05/200,717] was granted by the patent office on 1973-01-23 for plural beam antenna.
Invention is credited to Louis R. Dod, George C. Keller, Marvin S. Maxwell.
United States Patent |
3,713,163 |
Keller , et al. |
January 23, 1973 |
PLURAL BEAM ANTENNA
Abstract
An antenna capable of deriving a plurality of beams about a
single boresight axis includes a focusing means, such as a
parabolic reflector or a lens. Plural arrays, each including plural
radially aligned antenna elements extending from the boresight
axis, are located in proximity to a focal point for the focusing
means. The elements of the plural arrays are independently excited.
Each of the arrays is independently rotated about the boresight
axis.
Inventors: |
Keller; George C. (Hyattsville,
MD), Maxwell; Marvin S. (Silver Spring, MD), Dod; Louis
R. (Silver Spring, MD) |
Family
ID: |
22742890 |
Appl.
No.: |
05/200,717 |
Filed: |
November 22, 1971 |
Current U.S.
Class: |
343/754; 342/365;
343/839; 342/374; 343/895 |
Current CPC
Class: |
H01Q
3/245 (20130101); H01Q 3/18 (20130101); H01Q
25/007 (20130101) |
Current International
Class: |
H01Q
25/00 (20060101); H01Q 3/24 (20060101); H01Q
3/18 (20060101); H01Q 3/00 (20060101); H01q
003/26 () |
Field of
Search: |
;343/754,761,839,854,895 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
We claim:
1. An antenna capable of deriving a plurality of energy beams
centered about a single boresight axis having a focal point for the
beams comprising plural arrays of plural antenna elements in
proximity to the focal point, means for focusing the beams on the
antenna elements, said elements of each array being aligned and
spaced by a different distance from the boresight axis, means for
independently exciting elements of the plural arrays, and means for
independently rotating each of the plural arrays about the
boresight axis.
2. The antenna of claim 1 wherein the elements are positioned so
that radiation beams derived from adjacent elements overlap.
3. The antenna of claim 1 wherein a pair of said arrays is
provided, one of said arrays being positioned on one side of the
focal point, the other of said arrays being positioned on the other
side of the focal point.
4. The antenna of claim 1 wherein the focusing means includes a
reflector.
5. The antenna of claim 1 wherein the focusing means includes a
lens.
6. The antenna of claim 1 wherein one of the arrays is displaced
from the other arrays along the boresight axis, said one array
being positioned between the focusing means and said other arrays,
said one array including an element having its center on the
boresight axis.
7. The antenna of claim 1 wherein the means for exciting includes
switch means selectively connecting each of the elements in circuit
with an active device.
8. The antenna of claim 1 wherein different arrays are located on
either side of the focal point.
Description
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United
States Government and may be manufactured and used by or for the
Government for governmental purposes without the payment of any
royalties thereon or therefor.
FIELD OF INVENTION
The present invention relates generally to antennas and more
particularly to an antenna including a plurality of independently
rotatable arrays, each including plural radially aligned antenna
elements extending from a boresight axis.
BACKGROUND OF THE INVENTION
In many tracking and communications applications with moving
targets, it is desirable simultaneously to form plural independent,
steerable beams with an antenna having a single aperture. Generally
in the past, plural antenna apertures, formed by plural movable
parabolic reflectors or the like, have been used for simultaneously
tracking and communicating with plural moving targets, with one
aperture being assigned to each target. Tracking and communicating
with plural targets has also been performed by utilizing multibeam
phased arrays, including many antenna elements, or focusing
structures including phased array feeds. It has also been proposed
to employ a rotating linear feed in combination with an
articulating parabolic reflector to achieve simultaneous tracking
and communications with moving targets.
Each of the proposed prior art structures suffers from certain
disadvantages. The plural reflector approach, in addition to being
costly, requires precise movement and control of relatively large
masses. If these massive reflectors are mounted on a spacecraft,
problems in controlling the spacecraft position result in response
to movement of the reflectors. The multibeam phased arrays, in
actuality, are low efficiency devices because each of the many
switches and/or hybrids employed has a finite loss which becomes
substantial when many of them are interconnected in a cascade
relationship to form a complete array. Also, control circuits for
the phased arrays are frequently in the form of a complex
electronic network. The rotating linear feed suffers from some of
the same problems as the plural reflector systems because of the
requirement to position accurately the relatively massive
articulated reflectors utilized with the feed. In addition, the
reflector must frequently be articulated to a different position to
track more than one target, whereby simultaneous derivation of
plural different beams, for a plurality of different targets, is
often not possible.
SUMMARY OF THE PRESENT INVENTION
In accordance with the present invention, there is provided a
single fixed aperture antenna capable of simultaneously deriving
plural beams for tracking and communicating with plural targets.
The antenna is provided with a plurality of antenna arrays, each
including plural antenna elements on a straight or curved arm one
end of which is mounted to pivot about the boresight axis, and a
focusing structure, such as a parabolic reflector or a lens, for
focusing the beams of the antenna arrays. Each of the arrays, which
are positioned in proximity to the focal point, is independently
rotatable and the elements of the several arrays are independent
activated by a simple switching network, whereby one or more of the
elements of each array is excited at a time. The antenna elements
in each array are spaced closely enough together so that the beams
derived by adjacent antenna elements overlap substantially so that
significant nulls do not occur in the overall antenna pattern. As
the plural arrays are rotated about the boresight axis and the
several elements of each array are activated, complete coverage of
the antenna aperture field of view is provided for several targets,
at least equal to the number of arrays provided. Thereby,
simultaneous tracking and communication with each target is
possible.
It is, accordingly, an object of the present invention to provide a
new and improved antenna capable of simultaneously tracking and
communicating with plural targets.
Another object of the present invention is to provide an antenna
having a fixed, single aperture capable of simultaneously tracking
and/or communicating with plural moving targets.
A further object of the invention is to provide a new and improved
antenna system capable of simultaneously tracking and/or
communicating with plural moving targets, which antenna system
employs a relatively simple and high efficiency network for
selectively exciting a plurality of antenna elements.
Still another object of the present invention is to provide a new
and improved antenna system particularly adapted for use on
spacecraft and capable of tracking and/or communicating with plural
targets without having an effect on position control of the
spacecraft.
The above and still further objects, features and advantages of the
present invention will become apparent upon consideration of the
following detailed description of several specific embodiments
thereof, especially when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a perspective view of one preferred embodiment of the
present invention;
FIG. 2 is a pair of arms, as viewed from the vertex of the
reflector, included in the antenna of FIG. 1;
FIG. 3 is a diagram of the circuit utilized to excite the elements
of one of the arrays of the antenna system illustrated by FIG.
1;
FIG. 4 is a view illustrating the overlap in antenna patterns from
an array of the antenna system of FIGS. 1 and 2; and
FIG. 5 is a modification of the system of FIGS. 1 and 2,
illustrating the use of a lens for focusing the beams.
DETAILED DESCRIPTION OF THE FIGURES
Reference is now made to FIGS. 1 and 2 of the drawing wherein there
is illustrated a beam focusing structure comprising parabolic
reflector 11 having a boresight axis 12 on which lies reflector
focal point 13. Positioned in proximity to and on either side of
focal point 13 are curved or straight arms 14 and 15 which extend
radially from boresight axis 12. Arms 14 and 15 are independently
driven by concentric shafts 16 and 17, which are coaxial with
boresight axis 12. Shafts 16 and 17 are driven independently by a
pair of motors 18 and 19 which rotate the shafts and arms 14 and 15
either at the same or different predetermined rotational velocities
or in response to tracking command signals. The entire assembly
including arms 14 and 15, shafts 16 and 17 and motors 18 and 19, is
supported by any suitable means, such as struts 20 which are
fixedly mounted to the edges of reflector 11.
Each of arms 14 and 15 carries a plurality of antenna elements
positioned with respect to reflector 11 to enable a number of
relatively narrow width secondary beams (FIG. 4) to be derived by
the reflector in response to excitation of the elements. The plural
elements mounted on each of arms 14 and 15 extend away from the
boresight axis 12 and are positioned a different radial distance
from the boresight axis. Arms 14 and 15 can be planar or curved. If
the arms are curved, the curvature is selected to give the best fit
for the focusing plane, as described in Korvin et al. U.S. Pat. No.
3,569,976. Arm 14, the closer of the two arms to reflector 11, can
be provided with an antenna element 22 centered on the boresight
axis 12. Excitation of antenna element 22 centered on boresight
axis 12 results in a far field pattern of reflector 11 coincident
with the boresight axis. Arms 14 and 15 are provided with an equal
plurality of off axis antenna elements 23 and 24 to enable beams
having centers displaced from boresight axis 12 to be derived. The
centers of the off axis beams are determined by the angular
positions of arms 13 and 15 relative to boresight axis 12, as well
as the radial positions of the off axis excited elements. Adjacent
ones of elements 23 and 24 of each of arms 14 and 15 are positioned
in close enough proximity to each other whereby the 3 db beam
widths resulting from excitation of the different elements have a
substantial overlap, as indicated by the far field patterns of FIG.
4. Thereby, complete coverage of the desired field of view of the
aperture of parabolic reflector 11 is attained by rotation of the
arms and excitation of the elements. Simultaneous tracking and
communication with two targets is achieved through rotation of both
of arms 14 and 15 independently by motors 18 and 19. If a pair of
targets simultaneously approaches the boresight axis and comes
within the field of view of the beam formed by the antenna element
in alignment with boresight axis 12, a communication and/or
tracking link is established only with centrally located antenna
element 22 on arm 14 by deactivating elements 23 and 24. Similarly,
when the arms, in their rotation, cross over each other, the
communication and/or tracking link is established only with arm 14
until the arms separate sufficiently for each to resume
communication and/or tracking separate targets. If simultaneous
tracking and/or communication with more than two targets is
desired, the number of arms is increased accordingly so that one
arm is provided for each target.
The antenna elements 22, 23 and 24 on arms 14 and 15 are preferably
cavity backed spirals of the type well known to those skilled in
the art. The spirals can be employed with cavities having air
dielectrics, but are preferably employed with cavities having solid
dielectrics with dielectric constants greater than one, whereby the
depth of the cavity can be decreased so that the emitting surfaces
of the spirals can be placed in greater proximity to the focal
surface which includes focal point 13 and is normal to boresight
axis 12 to effect maximum gain of the antenna so that the responses
for excited elements on arms 14 and 15 having the same radial
displacement from boresight axis 12 are approximately equal. The
emitting surfaces of the spirals are approximately equidistant from
this plane. Cavity backed spirals are preferably employed because
of the circularly polarized beams derived thereby. The spirals can
be of the two-armed type if communication only is desired;
four-armed spirals are employed, as disclosed in U.S. Pat. No.
3,344,425, when the antenna system is utilized for tracking. While
spirals are utilized as the antenna elements in FIG. 2, it is to be
understood that other antenna elements, such as crossed dipoles,
linear dipoles, horns and optical systems, may be utilized
depending on the function desired and frequency employed.
Positioned on the rear faces of arms 14 and 15, that is, the faces
of the arms away from reflector 11, are switching networks and
active devices, such as transmitters, receivers or transceivers.
The switching and active elements may be located on arms 14 and 15
in flat boxes 25 and 26, respectively, and thereby rotate with the
arms to obviate the need for crossing a rotating interface, such as
a slip ring, at low signal levels. Box 25 is as flat as possible so
that it does not contact the front face of arm 15 or displace the
radiating elements too far from the plane including focal point
13.
A typical circuit for exciting elements 24 on arm 15 is illustrated
in FIG. 3 wherein an active device 31, such as a transceiver, is
selectively connected to one or more of elements 24 at a time.
Connected in series between transceiver 31 and each of antenna
elements 24 is a separate PIN diode 32 which is normally back
biased to a nonconducting state in response to D.C. blocking
voltage applied thereto through RF choke 33 by control circuit 34.
To excite an antenna element 24, the diode 32 connected in series
with the element to be excited is activated to a forward biased
condition in response to a positive voltage being applied to its
anode by a positive voltage being derived at an output terminal of
control circuit 34. When it is desired to scan an area, the several
output terminals of control circuit 34 are supplied in sequence
with positive voltages so that at any time a positive voltage is
applied to only one of the diodes 32. Switching diodes 32 to a
forward biased condition is at a much faster rate than rotation of
arm 15. Simultaneously, arm 14 is being rotated and the several
active elements mounted thereon excited in response to the diode
switches in box 25 being forward biased. The active devices 31 in
boxes 25 and 26 operate in the same band, for example S band
(between 2.2 and 2.3 GHz). The frequency of each active device is,
however, different to enable tracking and communication with
targets having different frequency characteristics.
For tracking applications, control circuit 34 responds to signals
derived from the excited antenna element 24, or predetermined
commands, to forward bias the diode 32 connected to the active
element having a radial position corresponding with the off axis
position of the tracked target. Also, the angular position of arm
15 about boresight axis 12 is controlled in response to signals
derived from the active antenna element 24 feeding error signals
from the element or from predetermined command signals to the motor
19. In response to the signals derived from the excited antenna
element 24 exceeding a predetermined level, or from a command,
motor 19 is rotated or excitation of the antenna elements 24 is
switched to an adjacent element.
Reference is now made to FIG. 5 of the drawing wherein there is
illustrated a modification of the system illustrated by FIG. 1. In
FIG. 5, the beam focusing structure, illustrated in FIG. 1 as
parabolic reflector 11, comprises focusing lens 41, having its
major axis approximately perpendicular to the faces of arms 14 and
15. The focal point 43 of the lens 41 lies on the pivots of arms 14
and 15. The system of FIG. 5 functions in a similar manner as the
antenna system of FIG. 1 to derive simultaneously a plurality of
beams having independently controlled positions about boresight
axis 12 as a function of radius and angular position.
While there have been described and illustrated several specific
embodiments of the invention, it will be clear that variations in
the details of the embodiments specifically illustrated and
described may be made without departing from the true spirit and
scope of the invention as defined in the appended claims.
* * * * *