U.S. patent number 3,795,004 [Application Number 05/335,877] was granted by the patent office on 1974-02-26 for cassegrain radar antenna with selectable acquisition and track modes.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Army. Invention is credited to Whilden G. Heinard, James M. Meek, Clarence F. Ravilious.
United States Patent |
3,795,004 |
Meek , et al. |
February 26, 1974 |
CASSEGRAIN RADAR ANTENNA WITH SELECTABLE ACQUISITION AND TRACK
MODES
Abstract
A Schwarzschild Antenna cooperates with an organ pipe scanner to
achieve e angle sectoral scanning of a high gain pencil beam. A
rotatable mirror switches the antenna to conical scanning whereby
microwave energy communicates between a nutating horn and
reflectors of the Schwarzschild Antenna. During conical scanning,
the organ pipe scanner remains unenergized. The mode of operation
is selectable by the operator and the system is designed for rapid
switching.
Inventors: |
Meek; James M. (Silver Spring,
MD), Ravilious; Clarence F. (Rockville, MD), Heinard;
Whilden G. (Bethesda, MD) |
Assignee: |
The United States of America as
represented by the Secretary of the Army (Washington,
DC)
|
Family
ID: |
23313601 |
Appl.
No.: |
05/335,877 |
Filed: |
February 26, 1973 |
Current U.S.
Class: |
343/761; 343/777;
343/779; 343/781R; 343/835; 343/837; 343/839; 343/781CA |
Current CPC
Class: |
H01Q
19/191 (20130101); H01Q 19/18 (20130101); H01Q
3/18 (20130101); H01Q 3/245 (20130101) |
Current International
Class: |
H01Q
19/10 (20060101); H01Q 19/18 (20060101); H01Q
19/19 (20060101); H01Q 3/24 (20060101); H01Q
3/18 (20060101); H01Q 3/00 (20060101); H01g
003/18 (); H01g 003/20 (); H01g 003/26 () |
Field of
Search: |
;343/777,779,781,761,757,758,835,837,839 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3696432 |
October 1972 |
Anderson et al. |
|
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Nussbaum; Marvin
Attorney, Agent or Firm: Kelly; Edward J. Berl; Herbert
Elbaum; Saul
Claims
Wherefore we claim the following:
1. A Schwarzschild antenna system comprising:
reflector means for reflecting microwave energy that impinges
thereon;
scanner means adjacent the reflector means and communicating with
the reflector means for producing a wide angle unidirectional
sectoral scan;
horn means disposed in adjacent spaced relationship to the
reflector means and communicating with the reflector means for
producing a narrow angle scan; and
microwave switching means mounted adjacent to the reflector means
to selectively complete microwave communication between the
reflector means and either the scanner means or the horn means;
whereby the switching means enables rapid selection of the sectoral
scan or the narrow angle scan.
2. The structure of claim 1 wherein the scanner means comprises a
plurality of adjacently spaced feed horns communicating with
respective waveguide pipes to form an organ pipe scanner which
produces a unidirectional scan across the outlet ends of the
feedhorns.
3. The structure of claim 1 wherein the horn means comprises at
least a single nutating horn for conical scan tracking.
4. The subject matter of claim 1 wherein the horn means is a
multi-port assembly comprising a plurality of adjacently positioned
stationary horns for operating in a steady track mode.
5. The structure of claim 2 wherein the switching means comprises a
movably mounted flat mirror reflector positioned in intermediate
relation between the scanner means and the horn means for
selectably communicating microwave energy between the reflector
means and either the scanner means or the horn means.
6. The subject matter of claim 2 wherein each of the feed horns has
a pyramidal shape.
7. The structure of claim 3 wherein each of the horn means has a
pyramidal shape.
8. The subject matter of claim 2 and further wherein the horn means
comprises at least a single nutating horn for conical scan
tracking.
9. The subject matter of claim 5 wherein the mirror is
longitudinally mounted to a shaft to permit selectable rapid
rotation of the mirror between two angular positions.
10. The subject matter of claim 9 wherein the shaft is connected to
a torquer that selectably drives the shaft between the two angular
positions in response to electrical energization of the torquer.
Description
The invention described herein may be manufactured, used, and
licensed by or for the United States Government for governmental
purposes without the payment to us of any royalty thereon.
FIELD OF THE INVENTION
The present invention relates to a Cassegrain Antenna, and more
particularly to such an antenna with selectable acquisition and
track modes.
BRIEF DESCRIPTION OF THE PRIOR ART
The prior art relating to microwave antennas includes a structure
known as a Cassegrain Antenna which is comprised of coaxial
reflectors. The Cassegrain has met with wide acceptance because its
structure eliminates the need for mounting a heavy feed radiator
far in front of the main reflector of the antenna. An improvement
of the Cassegrain came with the discovery of an antenna structure
known as the Schwarzschild Antenna which is basically a modified
Cassegrain with reflectors shaped to form an aplanatic system. As
those of skill in the art know, the aplanatic Schwarzschild meets
the Abbe sine condition and evidences superior off-axis microwave
focusing capability, when compared with the older, conventional
Cassegrain. Although the Schwarzschild Antenna has been designed to
operate in the conical scanning mode, there has not been a
satisfactory design heretofore capable of effecting rapid switching
between this mode and a sectoral scan mode in one antenna
assembly.
Therefore, in conventional radar systems where relatively wide
angle sectoral scanning is required along with conical, or steady
tracking, a relatively complicated antenna structure becomes
necessitated. A result of this complexity is that there is a
decrease in performance characteristics and flexibility.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
The present invention is directed to a Schwarzschild Antenna which
cooperates with an organ pipe scanner for relatively wide angle
unidirectional sectoral scanning. A structurally simple rotatable
mirror acts as a microwave energy switch to deactivate the organ
pipe scanner, and instead, complete an energy path between nutating
horns and the reflectors of the antenna. The switch to the latter
described mode effects conical scanning. The combination of the
Schwarzschild Antenna with an organ pipe scanner is novel. The
further addition of a switching capability to switch the antenna
from the sector scan mode to a conical scan mode lends further
uniqueness to the present invention.
The resultant structure of the present invention provides an
improvement in tracking radar antenna systems related to scanning
capabilities, multiple operating modes, and beam pattern
optimization.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a side elevational view of the present Schwarzschild
Antenna illustrating a pivotally mounted mirror which switches in
either an organ pipe scanner or a conical scan horn.
FIG. 2 is a front sectional view taken along a plane passing
through section line 2 -- 2 of FIG. 1.
FIG. 3 is a rear sectional view taken along a plane passing through
section line 3 -- 3 of FIG. 1.
This view illustrates the common termination of all pipes in the
organ pipe scanner.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the figures and more particularly FIG. 1 thereof, a
side elevational view of the present Schwarzschild Antenna and
associated scanners are illustrated.
A sub-reflector 10 is shown in radially spaced parallel relation to
a main reflector 12. The reflectors constitute a Schwarzschild
system. Although the projections of the reflectors 10 and 12 are
circular, the contours of the reflectors satisfy the design
criterion for aplanatic reflector systems meeting the Abbe sine
condition. Parallel rays 14 and 16, for example, are seen to
communicate between the lower portion of the main reflector 12 and
the far field, while parallel rays 18 and 20, for example, are seen
to communicate between the upper portion of the main reflector 12
and the far field. An axis 21 intersects the center of both the
sub-reflector 10 and the main reflector 12. A rectangle aperture 26
is formed around the intersection of the axis 21 and the main
reflector 12.
When receiving microwave energy in the sectoral scan mode, ray 16
impinges upon the main reflector 12 at point 22. Ray 16 is
illustrated to represent the lower limit of impinging microwave
energy. Thereafter, the ray is reflected to the sub-reflector 10
where incidence occurs at point 24. Subsequent reflection from
point 24 occurs so that the ray passes through the central aperture
26 for formation of a virtual image at focal point 28.
An image at point 28 never develops because of the interceding
disposition of a pivotally mounted flat mirror 30. While operating
in a relatively wide angle sectoral scan mode, the mirror, which
may be rectangularly shaped, assumes the solid position indicated
by A. The mirror 30 is positioned to reflect the microwave energy
after it passes through the aperture 26. Therefore, ray 16 is
reflected from the lower part of the mirror 30 until it is focused
at focal point 34. A pyramid-shaped horn (flare) 36 surrounds the
focal point 34 so that the horn 36 can feed microwave energy to (or
from) connected waveguides. Upper ray 18 travels a similar route.
After reflection from the main reflector at point 37, the ray
impinges upon the sub-reflector 10 where it is further reflected at
point 39 through the aperture 26. After passing through the
aperture 26, the ray 18 impinges upon the upper portion of mirror
30 and is reflected therefrom to the focal point 34. Thus, the
energy associated with the upper beam 18 is likewise fed to the
horn 36.
As clearly shown in FIG. 2, a plurality of adjacently-spaced
pyramid horns 36 and their connected pipes 40 form an assembly of
pipes 38 that is conventionally referred to as an organ pipe
scanner. The horn portions 36 of the organ pipe scanner 38 have
outward ends referred to as output flares. These flares lie along a
circular arc 41. The edges of the horns 36 are positioned as shown,
perpendicular to the arc 41.
Each horn 36 has a rectangular cross section and pyramidal or
tapered interior shape.
The inward end of each horn communicates with its own pipe such as
40. The lower end portion of all pipes such as 40 may be twisted
90.degree. as shown by the rearwardly extending section 42 so as to
provide the desired direction of linear polarization in the far
field. The latter mentioned section 42 articulates to a further
perpendicularly disposed pipe section 44 that has its outer end 46
(FIG. 3) communicating with a circular chamber generally indicated
by reference numeral 48 in FIG. 3. As indicated by FIG. 3, the
openings 46 of the various pipes perpendicularly intersect the
circumference of the circular chamber 48 and are arranged so that
the microwave E planes coincide (coplanar). Otherwise stated, the
pipes are radially positioned with respect to the center of the
circular chamber 48.
A single horn 50 flared in the microwave E plane is mounted about
an axis 54 that is perpendicular to the plane of the circular
chamber 48. The axis intersects the center of the circular chamber
48. The outer, outwardly flared end of the horn 50 communicates
with several pipe openings 46 (stacked in the narrow E plane
dimension as stated) at a given instant of time as the horn 50
rotates either clockwise or counterclockwise. This is due to the
relatively large opening 52 of the horn 50 as compared with a much
smaller opening in the end 46 of the radially positioned pipes. As
illustrated in FIG. 3, the axis about which the feed horn 50
rotates is indicated by reference numeral 54. The inward end of
horn 50 communicates with a waveguide fitting 56 that is connected
to waveguides (not shown) which deliver microwave energy from the
antenna to a remote transmitter-receiver.
As the horn 50 rotates in a circular manner, energy is sequentially
collected from the ends 46 of the pipes. At some point along the
circumference of the circular chamber 46, one of the pipes will
represent the left most horn 36 in FIG. 2, while an adjacent pipe
along the circumference 48 in FIG. 3 represents the right most horn
in FIG. 2. Accordingly, as the horn 50 rotates circularly, an
arcuate scan is produced in a unitary directional manner at the
horns 36. This unidirectional scan is indicated by the direction
arrow 58 in FIG. 2. The result of this unidirectional scan is a
wide angle unidirectional sector scan by the radar.
Proper illumination of the sub-reflector is accomplished to a large
extent by providing a primary beam width corresponding to the
circular extent of the sub-reflector. Generally, a narrower flare
(36) transverse to the direction of scan provides a broader beam
and vice versa. In the direction of scan, the total width of the
several horns illuminated at a given instant is essentially the
governing factor. The phasing of output waves at these horns should
preferably be controlled by adjusting waveguide electrical path
lengths between feed and output, so that the output wave is
directed toward the center of the subreflector at all scan
angles.
It should be understood that the antenna is a reciprocal device and
the ray paths illustrated apply for both transmission and
reception.
As previously mentioned, the central object of the present
invention is to be able to selectably switch from the organ pipe
sector scan to a steady monopulse or conical scan. This is done by
rotating shaft 64 that mounts mirror 30. This rotation is achieved
by a mirror drive torquer (motor) 65 shown in FIG. 2. The torquer
65 achieves rapid 90.degree. rotation of the mirror 30 to position
B shown in FIG. 1. As the mirror is shifted to this new position,
the microwave channel to the pipe organ scanner is disconnected and
becomes inoperative. For precise mirror positioning, a
zero-backlash, motor driven cam mechanism may be employed in lieu
of the torquer mentioned above.
Referring once again to FIG. 1, reference numeral 60 denotes a
nutating pyramid horn that is positioned above the mirror reflector
which is now assumed to be in the dotted position B. Reference
numeral 20 is seen to denote the geometric upper limit of received
mircowave energy which impinges upon the main reflector at point
62. From there, the beam 20 is reflected at point 39 on the
sub-reflector 10. Thereafter, the beam passes through the opening
26 where it impinges upon the front surface of mirror 30.
Reflection from the mirror takes place and the beam 20 intersects
the focal point 65. In a similar manner, the geometric opposite ray
14 reflects off the main reflector 12 at point 67 for subsequent
impingement upon the sub-reflector 10 at point 24. Thereafter, the
ray 14 is directed through the aperture 26 until it impinges upon a
lower portion of the mirror 30. The ray 14 is reflected from this
lower portion so that it intersects the focal point 65. The
nutating horn 60 centered at the focus collects these rays, as well
as the rays that are present in between the edge rays 20 and 14. By
virtue of the nutating motion, conical scanning is achieved.
During transmit operation of the antenna, the microwave signal flow
is oppositely directed as compared to during receive operation.
Thus far, sectoral scan and conical scan have been described. The
beam is a high gain pencil or fan beam. The present invention is
equally applicable to steady tracking or monopulse radar operation.
To achieve this monopulse operation, four or more ports are
employed. For example, the ports are characterized by pyramid horns
including the aforementioned horn 60, horns 66 (FIG. 1), 68 and 70
(FIG. 2). These four fixed horns form approximately a square
pattern and transmit and receive microwave energy in the
conventional manner.
Thus described, the present Schwarzschild Antenna is seen to
operate with selectable acquisition and track modes. These modes
include unidirectional organ pipe; wide-angle-sector scan;
monopulse steady track; and conical scan. The beam may be generally
described as either pencil beam or oval (fan) beam.
Although the present invention has been discussed in a manner
indicating only two extreme positions of mirror 30 (position A and
B), it should be appreciated that the mirror 30 can be varied in
position about these extreme mirror locations to achieve within
certain angular limitations bi-level or raster scanning patterns of
the far-field pencil beam. This is achieved, for example, by
adjusting the mirror 30 so that it steps to a new scanning plane at
the termination of a unidirectional sector scan. This is easily
accomplished with state of the art techniques. Instead of a
two-position torquer, a stepping motor or Geneva movement can be
employed, for example.
An additional design consideration for the present invention is
directed to the utilization of a twistflector for the main
reflector, and a transflector for the sub-reflector. As those of
ordinary skill in the art know, the twistflector has a grating for
changing linear polarization (90.degree.) during reflection. The
transflector allows energy to be propagated or reflected depending
on the polarization of incident energy. In considering the
microwave energy path between the reflectors, the twistflector
changes the polarization of the energy impinging thereupon and
effects reflection to the sub-reflector on receive. The
sub-reflector then directs the microwave energy through the
aperture 26. As is well known in the art, the use of a
twistflector-transflector combination has the advantages of
minimizing side lobes, maximizing gain, ang generally producing an
improved pattern due to decreased blockage.
When designing the antenna of the present invention, certain
parameters are specified. Thus, for a particular application one
must choose or specify for example frequency of operation, antenna
size, scan angle, gain, beam width, and maximum side lobe
level.
In order to convert these given parameters to the antenna component
measurements, ray tracings are performed. This is a conventional
technique which gives shapes of Schwarzschild reflectors. After the
tracings have been made, a computer program is employed to
determine beam pattern parameters for design optimization. That is,
the best compromise must be made between the desired radiation
characteristics and physical size limitation of antenna geometry.
The ray tracing-beam pattern parameters may include:
Specification of a Schwarzschild or a regular Cassegrain antenna
design.
Antenna diameter to operating wavelength ratio.
Antenna/feed aperture illumination functions.
Sector scan angle.
Focal length.
Magnification factor.
Diameter of sub-reflector for opaque reflectors (blockage).
Size of aperture in main reflector when using twisttransflector
(blockage).
The desired scanner is designed by laying out, as scale drawings,
various configurations of waveguide convolutions, fitting scanner
feed arc to reflector focal arc (from ray tracings). Also feed
apertures are inserted in the design drawings and test models to
give proper feed beam width and directivity to illuminate the
sub-reflector.
The following will provide references, in the literature, to the
above-discussed design considerations.
BEAM PATTERN PROGRAM
One may use a computer program developed by the Rome Air
Development Center (Griffiss Air Force Base New York) and TRG
Company (Nihen and Kay). Reference RADC TR-66-582, November 1966,
authored by Hildebrand. The reference is entitled "Design and
Evaluation of Two-Reflector Antenna Systems." This program is
modified to permit the use of quasi-parabolic primary illumination
functions and for patterns in scan direction and orthogonal
thereto.
RAY TRACING PROGRAM
Ray tracing equations may be derived from the basic Schwarzschild
reflector equations given in an Airborne Instrument Laboratory
report found in the records of the IRE Convention of 3-20-62,
Topics in Electronics, Volume IV, 1963, authors W. White and L.
DeSize. The title is "Scanning Characteristics of Two-Reflector
Antenna Systems." Also reference Radio Engineering and Electronics
Physics, USSR, Volume VI, 1961, authored by N. G. Ponomarev. The
title is "Graphical Methods of Designing Aplanatic Antennas."
OPTIMIZATION OF TWIST-TRANSFLECTOR (GRATING) SYSTEM
Wheeler Labs (Great Neck, New York), Hazeltine Corporation,
Greenlawn, New York. The report is denoted as 666 and is entitled
"Design of Twistreflector Having Wideband and Wideangle
Performance." The report is dated 4-7-55 and authored by Peter W.
Hannan.
ANTENNA APERTURE BLOCKAGE
IRE Transactions on Antennas and Propagation, 3-12-60. The report
is entitled "Microwave Antennas Derived from the Cassegrain
Telescope." The article was authored by Peter. W. Hannan.
ORGAN PIPE SCANNERS
Naval Research Lab Report 3842, dated Aug. 1, 1951. The authors
were K. Kelleher and H. Hibbs. The report is entitled "An Organ
Pipe Scanner."
RING FEED SCANNERS
Georgia Tech Research Institute Report 212-168; AB No. 17816; 1953.
The report is entitled "Two Beam Scanning Antenna."
We wish it to be understood that we do not desire to be limited to
the exact details of construction shown and described for obvious
modifications can be made by a person skilled in the art.
* * * * *