US 3803619 A
A plurality of radial waveguides communicate microwave energy to respective circumferentially positioned horns. A stationary feed horn is disposed in the center of the waveguides to sequentially feed microwave energy to centrally located inlet openings in the waveguides. The waveguides rotate as a wheel while the circumferential horns generate a unidirectional sector scan beam. The scanner is installed in a Schwarzschild reflector system to provide a high gain, scanning, pencil beam in the far field. In addition, a conical scan mode is provided. A switchable mirror permits the operator to select either mode of operation.
Description (OCR text may contain errors)
United States Patent 119 Meek et a1.
[ MULTI-HORN WHEEL SCANNER WITH 1 STATIONARY FEED PIPE FOR SCHWARZSCl-IILD RADAR ANTENNA  Inventors: James M. Meek, Silver Spring;
' Clarence F. Ravilious, Rockville;
Whilden G. Heinard, Bethesda, all
 Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.
22 Filed: Feb. 26, 1973 21 Appl. No.: 335,879
521 US. Cl 343/761, 343/779, 343/781,
1 343/836, 343/839 51 1111.01. H01q 3/12,l-l01q2l/00  Field ofSearch- 343/754, 756, 761,779,
 References Cited UNITED STATES PATENTS 3,546,699 l2/l970 Smith ..343/762 1 11 3,803,619 145]- Apr. 9, 1974 3,680,141 Karikomi 343/781 Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Saul Elbaum  ABSTRACT A plurality of radial waveguides communicate microwave energy to respective circumferentially positioned horns. A stationary feed horn is disposed in the center of the waveguides to sequentially feed microwave energy to centrally located inlet openings in the waveguides. The waveguides rotate as a wheel while the circumferential horns generate a unidirectionalsector v scan beam. The scanner is installed in a Schwarzs'child reflector system to provide a high gain, scanning, pen- 'cil beam in the far field. In addition, a conical scan mode is provided. A switchable mirror permits the operator to select either mode of operation.
11 Claims, 3 Drawing Figures GOVERNMENT RIGHTS The invention described hrein may be manufactured, used, and licensed by or for the United States Govern! ment for governmental purposes without the payment to use of any royalty thereon.
. F QEQT HE ififii i- The present invention relates to a Schwarzchild Antenna, and more particularly to such an antenna that can be selectively switched from a sector-scan or acquisition mode to a tracking mode.
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 paraboloid-hyperboloid reflectors. The Cassegrain has met with the wide acceptance because its structure eliminates the need for mounting a heavy feed radiator for-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 unidirectional sector-scan mode in one antenna assembly. Therefore, in conventional radar systems where relatively wide angle sectoral scanning is required along with conical scanning or steady tracking, a relatively complicated antenna structure becomes necessary. 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 has a wheel-like assembly comprising a number of waveguide spokes that communicate at a central point with a stationary feed horn. The waveguides rotate as an assembly so that the stationary feed horn communicates microwave energy to the waveguides in a sequential manner. The sequential energization of the waveguides produces a unidirectional sector scanning effect that is reflected to the reflectors of the antenna system. A high gain pencil, oval, or fan shaped scanning beam is generated in the far field.
Microwave power to the sectoral scanning mechanism can be turned off and instead a nutating horn can be energized to reflect a conical scan to the antenna system reflectors.
The present invention offers another mode of operation by virtue of its mechanical design. Specifically, a bi-level or multi-level scan can be generated reliably. by offseting the rotating waveguides in two or more adjacent parallel planes. Thus, the present invention offers unidirectional sector scanning, multi-level scanning, and conical scanning modes.
BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a side elevational view of the present invention illustrating the disposition of the scanner behind the main and sub-reflectors of the antenna.
FIG. 2 is a partial side elevational view of a scanner such as shown in FIG. 1, with minor modifications thereto. I
FIG. 3 is a partial side elevational view of the scanner illustrated in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION Referring to the FIGS., and more particularly FIG. 1, the Schwarzschild Antenna reflectors are shown. Reference numeral 10 denotes the sub-reflector while reference 12 indicates the main reflector. A rectangular aperture 14 is centrally formed in the main reflector 12. To the left of the aperture 14 is a pivotally mounted flat rectangular mirror 16 which can be rotationally switched between two positions. The first position is indicated in solid lines while the second position of the mirror is indicated in dotted lines. The relevance of these positions will be discussed hereinafter.
A conical scan is produced from a pyramidal or conical feed born 18 that is connected to a rotatable joint 20. A waveguide 22 feeds microwave energy to the nutating horn 18 during conical scanning. With the mirror positioned in the dashed orientation, the conical scan beam will be reflected from the mirror 16 and passed through aperture 14 for deflection from the main and subreflectors (12, 10) to the far field. A conventional double throw waveguide switch may be used to energize the appropriate waveguide channel for sector or conical scanning. Rapid rotational positioning of the mirror may be provided, for example by a zero backlash, motor driven cam mechanism (not shown).
When it is desired for the antenna system to operate in the unidirectional sector scan mode, the nutating horn 18 is deenergized and the mirror 16 is positioned in the orientation indicated by solid lines. A multi-horn wheel scanner is generally. indicated by reference numeral 24. During transmission or reception, a waveguide 26 is connected to a transmitter-receiver (not shown). A bracket 28 supports the waveguide and the waveguide 26 extends to an elbow portion 30. This latter mentioned elbow portion terminates outwardly in a pyramidal flared feed horn 31.
FIG. 2 illustrates the structure of the scanner in greater 'detail. As will be seen in this FIG., the feed horn 31 is radially positioned. The radial waveguides constitute a wheel-like structure rotatable as a rigid assembly. Considering one of the waveguide spokes generally indicated by reference numeral 32, the radial portion is indicated by 34. The radially inward end of this waveguide spoke extends toward the central area of the wheel so that the waveguide can communicate with the feed horn 31 when the waveguide is disposed in registry with the born 31. The spoke 32 has a straight radial length 34 that communicates outwardly into a right angle bend or miter 36. A circumferential length of waveguide 37 extends from the right angle bend or miter 36 toward the center of the wheel. The pyramidal horn 40 is connected to the circumferential waveguide portion 37 through a second right angle bend or miter 38. (Polarization twist transitions may be installed in each waveguide, if desired, to obtain a particular orientation of linear polarization of the far-field microwave energy.)
Proceeding in a clockwise direction, the next spoke" is indicated by reference numeral 42. If one were to proceed in a counter-clockwise direction from the first mentioned spoke 32, the spoke 44 would be reached. As will be seen from studying FIG. 2, the wheel scanner is comprised by a number of spokes that have their output flares, such as 40, disposed along an inner circumference. In the design of an antenna system, this circumference should be arranged to match or nearly coincide with the focal arc 56 of the reflector system to provide good focusing.
During operation of the device, the spoke waveguides in the upper semi-circular portion of 46 become sequentially energized. Thereafter, the spoke waveguides in the lower semi-circular portion become sequentially energized. FIG. 2 illustrates a transition region where the feed horn 31 communicates with the last spoke" waveguide 32 of the bottom semi-circular portion, simultaneous with the first spoke waveguide 44 of theupper semi-circular portion. During sequential energization of the waveguides in the upper semicircular portion, a unidirectional sector scan is created. When the lower semi-circular portion becomes energized, the scan jumps back to an original position and again undergoes a unidirectional scan. This is achieved by discreet geometrical positioning of the horns at the end of the spokes. Thus, as will be noted, the spoke" waveguide 50 extends in a clockwise direction along an intermediate waveguide length 52 for termination radially in an output flare. On the other hand, the intermediate portion 54 of waveguide spoke 44 extends circumferentially counter-clockwise, in an opposite direction from the arcuate portion 52 of spoke" 50. The number of horns simultaneously energized and the combined aperture size determine the primary beam width for efficiently illuminating the subreflector and providing acceptable far-field patterns. In terms of the unidirectional scanning action, it may be observed that while the wheel-horns are rotating clockwise, for example, the center of the microwave energy will move clockwise along arc 56 at a relatively slower angular speed (in the example of FIG. 2), the wheel rotating nearly one-half revolution while the energy traverses are 56. Furthermore, relative to a coordinate frame fixed in and rotating with the wheel, the energy will be observed to rotate counterclockwise at the difference" speed.
The net result is equivalent to a directional point source aimed at the sub-reflector and moving repetetively along the focal arc in one direction to produce far field sector scanning.
An important design consideration is to have several waveguides communicate with horn 31 at one giveninstant. This provides for proper primary beam width as previously explained and minimizes cogging" of the scanning beam.
Reference numeral 56 generally indicates the Schwarzschild focal arc which denotes the portion of on time" that energy is communicated between the stationary horn 31 and the spoke waveguides. The focal arc is the conventional Schwarzschild focal arc ,but reflected off axis to location 56 by mirror 16. The arc actually represents that physical are where microwave energy is emitted from the several horns rotating with the wheel.
It should be recognized that in order to achieve proper electrical path lengths (see FIG. 2), the circumferential pipe lengths, e.g., length 37, may for convenience or of necessity, be increased or adjusted by installing bends, miters, or other convolutions not shown. The adjacent path lengths should preferably be adjusted so that the waves emanating from corresponding output horns will recombine to form a phase front directed toward the center of the Schwarzchild subreflector, in order to properly illuminate the subreflector and minimize spillover. The locations of the input horn 31 and output horns 40, FIGS. 2 and 3 will nevertheless be disposed substantially as shown.
Referring back to FIG. 1, it should be mentioned that the scanner 24 can be interpreted as corresponding to that indicated in FIG. 2 with the exception that the waveguides in FIG. 2 do not incorporate the previously mentioned polarization rotation transition.
In FIG. 3, rather than the previously discussed straight radial disposition of the wheel waveguides, a variation is shown wherein the individual waveguides are bent or bowed in the plane of horn 31 in order to insure that the output flares, such as, 40, traverse through the Schwarzschild focal arc 56 and yet provide correct path lengths. In other respects, the structure and function of the apparatus shown in FIGS. 1 and 3 are the same as previously discussed in connection with FIG. 2. In all variations it is important to emphasize, the correct electrical lengths of the waveguides must be selected or provided so that wave fronts emanating from adjacent horns are properly phased.
Referring again to FIG. 1, in order to transmit a relatively wide angle sector scan to the far field, the mirror 16 is switched to the position illustrated by solid lines. The horns, such as horn 40, undergo rotation through the Schwarzschild focal arc and cause a unidirectional sector scan beam to be reflected from the mirror 16. The mirror 16 causes an additional reflection of this beam through the rectangular aperture 14 and to the far field, via the main and sub-reflectors (12, 10).,
To minimize sub-reflector blockage, the antenna may incorporate polarization twist (main) and transreflector design. Alternate means for achieving conical scanning may be effected by providing a nutating subreflector deflecting the beam conically, the-beam itself emanating from the wheel scanner while stationary.
Although the previous discussion was concerned with the transmit mode, it will be appreciated that the system will operate in the receive mode. In this latter mode, the microwave energy path is the reciprocal 'of that just discussed for the transmit mode.
The present invention has excellent application in bilevel scanning. Referring to FIG. 2, if the horns in the upper semi-circular portion 46 are for example located in a plane offset relative to the plane of the lower semicircular horns 48, the first set of horns will undergo a unidirectional sector scan along the first plane. Thereafter, when microwave energy is directed to the lower horns, a second sector scan will occur in a plane displaced from the first mentioned plane. Thus, a true bilevel scan can be effected as the wheel scanner continues to rotate. Accordingly, as will be appreciated, a simple mechanical offset can achieve the bi-level scan. Numerous variations of this principle may be employed to achieve unidirectional scans having tri-level, zig-zag, and other multi-level scan patterns.
Although the previous discussion included no specific means for driving the wheel scanner, it will be apparent to those of ordinary skill in the art that a ring gear could be suitably attached to the wheel and driven by a pinion gear. An alternate method would be a pully system that entrains the scanner wheel. Specific mechanical means for rotating the wheel have been left out from the figures to increase their clarity with regard to the novel aspects of the invention.
It should be understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art.
.wlrererore, we clairnth ll w g a l. A Schwarzschild antenna having main and subreflectors, the antenna comprising:
a rotatable wheel having spoke"-like waveguides extending outwardly from a central point, the inward ends of the waveguides sequentially communicating with a stationary feed horn as the wheel rotates;
output horns connected to the outward ends of respective waveguides along a circumference for directing a unidirectional sector scan to a predetermined plane; and
reflector means located at the plane for reflecting the sector scan to the far field, during transmission, via the main and subreflectors.
2. The subject matter of claim 1 wherein the wheel is divided into first and second semi-circular portions that create two unidirectional sectoral scans during each revolution of the wheel.
3. The structure recited in claim 1 wherein the inward ends of the waveguides are smaller than the output flare of the feed horn thus permitting simultaneous communication between several waveguides and the feed horn;
thereby creating a desired beam pattern.
4. The subject matter set forth in claim 1 wherein the feed horn is pyramidal shaped.
5. The subject matter of claim 1 wherein the output horns are pyramidal shaped.
6. The system as defined in claim 1 together with means for generating a conical scan, said means located adjacent the reflector means, the reflector means transmitting the conical scan to the reflector means when the reflector means is repositioned in a second orientation.
7. The subject matter of claim 1 wherein the reflector means is a rotatable mirror.
8. The system of claim 2 wherein the inward ends of the waveguides are smaller than the output flare of the feed horn thus permitting simultaneous communication between several waveguides and the feed horn:
thereby creating a desired beam pattern.
9. The subject matter of claim 2 whereby the output horns of the rotatable wheel are disposed in various parallel planes of rotation to effect multi-level or zigzag scanning.
10. The subject matter set forth in claim 8 together with means for generating a conical scan, said means located adjacent the reflector means, the reflector means transmitting the conical scan to the reflector means is repositioned in a second angular orientation.
11. The subject matter set forth in claim 10 wherein the conical scan means is comprised of a feed horn which transmits microwave energy therefrom while the horn undergoes nutating motion.