Schwarzschild Radar Antenna Operable In Sector Scan And Conical Scan Modes With Anti-blockage Reflector

Abstract

A Schwarzschild radar antenna has an organ-pipe scanner for producing relvely wide angle unidirectional sector scans of a high-gain pencil beam. When conical scanning is desired, energy from selected pipes of the organ-pipe scanner is directed to a nutating mirror that reflects the resulting conical scan to the reflectors of the antenna that reflect conical scan energy to the far field. A rotatable disc behind the main reflector controls the size of the aperture therein and provides decreased blockage when the antenna operates in the conical scanning mode.

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 microwave Schwarzschild antenna that has an adjustable main reflector aperture that becomes smaller during conical scanning. This reduces propagation blockage. Conical scanning is provided by a nutating reflector located behind the main reflector.

BRIEF DESCRIPTION OF THE PRIOR ART

The prior art relating to microwave antennas includes a structure known as a Cassegrain antenna which is principally comprised of coaxial reflectors. The Cassegrain has met with wide acceptance because its structure eliminates the need for mounting heavy feed radiators 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 Cassegrain with reflectors that are modified to provide 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 ability, 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 sectoral scan mode in one radar antenna assembly.

Therefore, in conventional radar systems where relatively wide angle sector scanning is required along with conical scanning, 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 cooperates with an organ-pipe scanner for producing relatively wide angle sector scanning. A unidirectional-scan beam from the organ-pipe scanner is reflected from a mirror which causes subsequent reflection to twistflector-transflector reflectors. The result of this structure is a unidirectional relatively wide angle scan of a high gain pencil beam in the far field. When conical scanning is desired, the organ-pipe scanner feed is locked in a position where only a few selected pipes in the scanner emit a fixed flow of microwave energy. A mirror, which is mounted at an angular position relative to a drive motor shaft is rotated so that the mirror nutates. As a result, the energy reflected from the nutating mirror is a nutating or conical scan. The conical scan is reflected to the transflector-twistflector reflectors for reflection to the far field.

Propagation blockage control means are provided so that the aperture in the twistflector is decreased during conical scanning. This changes the far field diffraction pattern (beam pattern) and results in a significant reduction in side lobes and a small increase in gain of the main beam during conical scanning.

By virtue of the present invention, a single Schwarzschild antenna may be rapidly switched from unidirectional sector scanning to conical scanning. By including an anti-blockage device, the scanning capabilities in both modes can be optimized. By virtue of the system to be described hereinafter, it will be seen that the present invention provides an improvement in tracking radar antenna systems related to scanning capabilities, multiple operating modes, rapidity of mode switching, and side lobe and main beam optimization.

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.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side elevational view of the present antenna system illustrating the reflectors of the antenna as well as the scanning mechanisms behind the main reflector.

FIG. 2 is a schematic illustration of electro-mechanical components that position various elements of the scanners in preselected positions to effect proper sector scanning or conical scanning.

FIG. 3 is a partial elevational view taken along a plane passing through section line 3 -- 3 in FIG. 1.

FIG. 4 is a diagrammatic illustration of the anti-blockage system as positioned during sector scanning.

FIG. 5 is a diagrammatic view similar to FIG. 4 with the anti-blockage system as positioned during conical scanning.

FIG. 6 is an electrical schematic diagram of circuitry employed to position the anti-blockage device in one of the orientations illustrated in FIGS. 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures and more particularly FIG. 1 thereof, reference numeral 10 illustrates a subreflector. The main reflector 12 is positioned in conventional spaced relationship from the subreflector 10 so that the Schwarzschild antenna combination is formed. Wide angle scanning requires a relatively large subreflector. To decrease blockage and side lobe generation, the main reflector is preferably a twistflector while the subreflector 10 is a transflector. A rectangular aperture 14 is formed in the main reflector 12 to allow the generated beam to pass from the scanning systems behind the main reflector to the far field. As will be discussed hereinafter, the aperture 14 is adjustable so that its size is decreased during a conical scan mode thereby minimizing aperture blockage.

The direction of unidirectional sector scan, FIG. 1, is perpendicular to the page, requiring the long dimension of aperture 14 to be in the same alignment during this scan mode.

A supporting structure 16 is fastened to the rear side of the main reflector 12. This support structure mounts an organ-pipe scanner 18 that is known in the art. Operation of the organ-pipe scanner is fully discussed in our co-pending application Ser. No. 335,877, filed Feb. 26, 1973. In essence, transmitter energy can be introduced at the input waveguide 17. The energy is then delivered to a radially oriented, rotatable horn 63 as shown in FIG. 3.

With reference to FIG. 3, input microwave energy is delivered to a radially disposed flared horn 68 that communicates with the ends 19 of waveguide organ pipes 70. The pipes 70 are located along a circumference. However, the horn 68 communicates with several pipe ends 19 at a given time. As the horn 68 rotates, energy is delivered to the pipe ends 19 in circular sequence. Each pipe 70 (FIG. 2) is bent to form a section 21 (FIG. 1) that extends to a curved portion 23 (FIG. 1). This curved portion extends to a further section 25 that terminates outwardly in a pyramid horn 34. Each waveguide or organ pipe of the scanner 18 terminates in a pyramid horn 34. When viewing FIG. 1, bear in mind that the output flares of all pyramid horns 34 in the organ-pipe scanner are arranged along an arc which is the Schwarzschild focal arc. In operation of the scanner, as the rotatable joint 36 (FIG. 3) and the connected horn 68 rotate, a unidirectional sector scan is generated at the output flares of the horns 34 (FIG. 1).

A conical scan assembly is generally indicated by reference numeral 20 in FIG. 1. The assembly includes a flat plate mirror 22 that is mounted at an angle with respect to the reflector axis 24. In one embodiment a wedge block 26 connects the mirror 22 to a counterbalance plate 28 that is attached to the shaft 30 of motor 32. This arrangement produces an oval scan pattern due to the fact that the reflected beam angle moves at twice the angular rate of the mirror and the component of mirror angle relative to the feed axis varies 2/1 in each half revolution.

If a true circular conical scan is desired, the block 26 may be replaced by a motor driven cam-follower which compensates for the 2/1 effect. In this arrangement the mirror and counter weight discs oscillate symmetrically about pivots adjacent to cam 26 at 2 OSC per revolution. Such cam arrangements are well known in the art. Symmetrical, oppositely phased oscillation of the two masses will result in cancellation of vibrating forces.

As will be explained hereinafter, for conical scanning, the organ-pipe scanner 18 is operated so that it delivers a fixed microwave energy flow, rather than a sector scan. This fixed energy flow from certain horns 34 in the scanner 18, preferably, the central horns, reflects from the mirror 22 while the motor 32 is energized. Because of the angular orientation of the mirror 22, nutating motion is achieved and a conical scan is transmitted via the twistflector-transflector reflectors (12, 10) or opaque reflectors to the far field.

The following will further describe the operation of the system during unidirectional sector scanning.

Referring to the upper portion of FIG. 2, drive motor 38 causes the rotation of output shaft 40 which in turn mounts a first clutch plate 42. A slideable key arrangement at 44 permits the axial movement of the clutch plate 42 with respect to the shaft 40. A second clutch plate 46 is journaled to the shaft 40 but keyed to a second, larger shaft 52. A key 54 insures interlock relation between clutch plate 46 and the shaft 52. Journaling of the shaft 40 at its end 48, within the shaft 52 is provided by bore 50. The clutch plates 42 and 46 are fabricated from a ferro-magnetic material so that they can be electromagnetically forced into engagement upon the energization of coil 56. Clutch assemblies of this type are well known in the art. The coil 56 is connected to power leads 58 and 60, in parallel with the power terminals of motor 38. Thus, when the antenna system is to operate in a sector scan mode, the coil 56 is energized thereby creating positive clutch action between the clutch plates 42 and 46. The motor 38 will drive the clutch assembly and the connected shaft 52. The rotatable joint 36 is connected to the outward end of shaft 52 so that the rotatable joint can rotate with this shaft. As a result, horn 68 undergoes circular motion and when energy is introduced into waveguide 17, it is communicated through the rotatable joint 36 to the horn 68. As the horn 68 rotates with energy emanating therefrom, the pipes 70 in the organ-pipe scanner are sequentially energized. Then, their respective output horns 34 (FIG. 1) create a unidirectional sector scan that is directed to the mirror 22 (FIG. 1). Reflection from this mirror to the main and subreflectors (12, 10) in FIG. 1 result in the generation of a wide angle sector scan in the far field.

When the operator wishes to switch from sector scanning to conical scanning, it is necessary to bring the horn 68 to a fixed position as indicated in FIG. 3. With the horn in a fixed position, a steady flow of microwave energy will be delivered to only those pipes 70 which directly communicate with the output flare of the horn 68. In actuality, the relative position between horn 68 and the pipes 70 is such that a few of the most centrally located pipes become energized. As a result, the centrally located horns 34 will impinge energy upon mirror 22 which is positioned at an angle of approximately 45.degree. with respect to the axis 24. Braking means are provided for insuring that the horn 68 assumes the position shown in FIG. 3 each time the conical scan mode is selected. Likewise, braking means are provided to insure that mirror 22 assumes its illustrated angular position. The braking means for fixing the position of horn 68 will now be discussed with reference to the upper portion of FIG. 2.

Circular recesses 62 and 64 are formed in clutch plates 42 and 46. The recesses confront one another and are concentric about the shaft 40. A spring 66 is axially mounted on shaft 40 and is contained within the recesses. When the motor selector switch 112 is turned off, power no longer energizes coil 56. Thus, the spring 66 has no counter-force to overcome and as a result, the spring biases the clutch plates 42 and 46 outwardly into disengagement. This causes the freewheeling rotation of shaft 52. At the same time, the motor selector switch 112 turns power off to the coil 56 (and motor 38)., a solenoid 74 is energized by switching means discussed hereinafter. This energization causes a plunger 76 to extend into engagement with detent 80 of an adjacent braking wheel 78. If the momentum of the wheel and connected parts is small (such as in cases of low scan rates or lightweight components), the shock associated with suddenly stopping the wheel will not be excessive. In cases of high scan rates, numerous variations may be provided by those skilled in the art to slow the wheel assembly and provide positive locking reliably and without causing large stresses. For example, instead of the clutch arrangement illustrated, an electromagnetic brake system may be employed.

For the arrangement of FIG. 2, the plunger 76 is forced into recess 80 by solenoid 74, thereby arresting motion and locking horn 68 in the central position. A normally opened switching relay 82 connects power lines 84 to the terminals of the solenoid 74 when braking is to occur. A mode selector switch 85 is ganged to the motor selector switch 112 so that when the switch is turned to a position that de-energizes lines 58 and 60, it causes the switching relay to energize the solenoid 74. The disposition of the solenoid 74 relative to the braking wheel 78 may better be appreciated by viewing FIG. 3.

Referring to FIG. 1, the aperture 14 must be large enough to allow passage of energy from the relatively long organ pipe array positioned along the scan focal arc. However, during conical scanning, the aperture need not be as large. As a matter of fact, it would be advantageous to decrease the size of the reflector aperture during conical scan so that aperture blockage caused by this opening can be minimized. The present invention includes means for accomplishing the decrease in aperture size. The drive for closing the aperture 14 is generally indicated by reference numeral 86. Reference numeral 88 illustrates a (Schwarzschild) contoured aperture plate that has a central opening that is the same size as the aperture within the main reflector 12. While the antenna is operating in the sector scan mode, these apertures are in registry as shown in FIG. 4. Reference numeral 90 denotes the rectangular aperture formed in the contoured aperture plate. When the system is switched to a conical scan position, the aperture plate 88 is caused to rotate 90.degree. so that the previous large rectangular opening is reduced, as shown in FIG. 5, to a smaller, squared opening. The means for electrically accomplishing these ends is illustrated in FIG. 6 whereat motor 94 is energized which causes the rotation of its output shaft and a pinion gear 96 attached to the outward end thereof. These components are seen in FIGS. 1, 4, and 5. The pinion gear 96 drives a mating ring gear 98 that is attached to the periphery of the aperture plate 88. Energization of the motor 94 is permitted by the previously mentioned mode selector switch 85. This selector switch is a double throw switch for purposes to be discussed hereinafter. When conical scan is to be effected, the switch 85 completes a power path between power lines and the motor 94. As a result, the aperture plate 88 rotates 90.degree.. After a 90.degree. rotation, a projection 99, extending from the rear surface of the aperture plate 88, moves into a position that causes the projection 99 to depress an actuator 100 of the normally closed microswitch 102. Prior to reaching the 90.degree. mark, a series path is maintained between the mode selector switch 85 and the microswitch 102 to the motor 94. However, when the normally closed microswitch 102 is depressed, the energization path to the motor 94 is interrupted and the motor stops turning. As a result, the pinion gear 96 no longer causes rotation of the mating ring gear 98. The aperture plate 88 comes to a stop and the orientation of the main reflector aperture 14 and the aperture 90 of plate 88 is seen in FIG. 5. Because of the relatively low mass of the aperture plate, it comes to rest relatively quickly so that separate braking means is not required. However, such means may be used.

When the operator wishes to switch this sytem back to a unidirectional sector scan mode, the mode selector switch 85 and the ganged motor selector switch 112 (FIG. 2) are manually switched. In order to terminate conical scanning and perform sector scanning, rotation of the mirror 22 must be terminated and the mirror must be set at its angular position as shown in FIG. 1. Further, the main reflector aperture must be enlarged to the original condition shown in FIG. 4. To do so, the aperture plate 88 must be rotated back to its original position. (It will not be a serious problem if the large aperture persists momentarily after switching to con-scan; in switching to sector scan the large aperture should be provided during or before starting this scan mode.)

The reverse rotation of the aperture plate is achieved when the mode selector switch 85 is switched to the sector scan mode. As shown in FIG. 6, power is fed through the switch 85 and the serially connected, normally closed microswitch 108, to the reverse terminals of the reversible motor 94. When this terminal is energized, the pinion gear 96 and mating ring gear 98 rotate in opposite senses than they did previously, which results in the return of the aperture plate 88 to its original position. As the original position is approached, the projection 99 engages the actuator 110 of the microswitch 108 and the switch contacts are opened. This terminates current flow through leads 104 and 106 that lead to the reverse terminals of motor 94. When de-energization occurs, the pinion gear 96 and mating ring gear 98 come to rest and the aperture plate 88 takes its original position as shown in FIG. 4.

As will be remembered, during sector scanning, the mirror 22 must be brought to rest at the angular position shown in FIG. 1. This is accomplished when the motor selector switch 112 cuts off power to motor 32 through the interconnecting leads 114, as shown in FIG. 2. The gears 118 and 120 slow down. Simultaneous with this change, the mode selector switch 85 switches the state of the switching relay 82 so that the coil 144 is de-energized. The coil 144 cooperates with clutch plates 146 and 148 of the clutch assembly 130, that normally complete a transmission path to rotate mirror 22. These clutch plates operate in the same manner as previously discussed in connection with the clutch plates 42 and 48, in the upper portion of FIG. 2. Specifically, a key arrangement 126 connects the clutch plate 146 to the shaft 124. This permits the axial movement of the clutch plate 146. A bore 128 is formed in an enlarged shaft 129 that mounts the clutch plate 148. The outward end of shaft 124 is journaled within the bore 128. When de-energization of coil 144 occurs, the clutch plates 146 and 148 separate under the influence of the axially mounted spring 147.

Simultaneous with this occurrence, the leads 134 are energized when a control signal on lead 132 actuates the switching relay 82, and as a result, solenoid 136 becomes energized. Plunger 138 extends outwardly to engage a detent recess 140 in a wheel 142. The braking action between plunger 138 and the detent recess 140 is identical to that previously described with respect to the detent recess 80 and solenoid plunger 76. Thus, when the shaft 129 undergoes freewheeling motion, the braking wheel 142 is arrested and then locked by the plunger 139 engaging the detent recess 140 in shaft 129. When this occurs, the mirror 22, attached to the shaft 129, is stopped in the position shown in FIG. 1. At this point, the system is operating properly for sector scanning.

As a further design note, the front surface of aperture reflector-plate 88 should be positioned 1/2 wavelength (or multiple) behind the confronting surface of the main reflector 12 to obtain proper phasing of the farfield energy. Also, a twistflector, consisting of opaque reflector and spaced grid, may be formed in aperture plate 88 so that the reflector and grid are contiguous (during con scan) between the aperture plate 88 and those of the main reflector 12. As an example, for vertical far-field polarization, the grid wires in the subreflector would be horizontally positioned and those in the main reflector positioned 45.degree. to the horizontal (assuming the antenna is looking horizontally). The grid wires in the plate 88 should be aligned, during con scan, with those of the main reflector. (This grid will be obscured behind the main reflector in the plate orientation corresponding to sector scan.)

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.

Claims

Wherefore we claim the following:

1. A Schwarzschild antenna comprising:

a subreflector;

a main reflector having a central aperture therein;

means positioned behind the main reflector for selectively producing a sector scan;

separate means positioned behind the main reflector for alternately producing a conical scan; and

means adjacent the main reflector responsive to selection of the conical scan for reducing the size of the aperture;

whereby aperture blockage is minimized during conical scan.

2. The subject matter of claim 1 wherein the sector scan is generated by an organ pipe scanner comprising:

a plurality of pipes each having a fixed horn at a first end thereof, the fixed horns being positioned along an arc;

each pipe having an opposite end positioned along a circle;

rotatable horn means for sequentially communicating microwave energy to the pipe ends along the circle; and

mirror means positioned near the fixed horns for reflecting sector scan energy between the fixed horns and the main and subreflectors of the antenna.

3. The subject matter of claim 2 wherein conical scan is achieved by means comprising:

means for positioning the rotatable means at a stationary position relative to preselected pipe ends along the circle for effecting steady microwave energy transmission between the mirror and the horns of preselected pipes; and

means for nutating the mirror to generate the conical scan.

4. The subject matter of claim 2 wherein the mirror is rotatably mounted, and further wherein lock brake means are provided to lock the mirror in a stationary position while the antenna system operates in a sector scan mode.

5. The system of claim 3 together with lock brake means for insuring the return of the rotatable horn to the stationary position when the system operates in a conical scan mode.

6. The structure of claim 1 wherein the means for reducing the size of the aperture comprises:

a contoured circular rotatable plate havng an aperture centrally formed therein which is disposed in registry with the aperture in the main reflector while the system operates in a sector scan mode;

whereby the rotatable plate is rotated in response to the conical scan mode so that the apertures are located perpendicularly to form an effective aperture of decreased size.

7. In the system of claim 6 together with a ring gear mounted along the circumference of the rotatable plate, and reversible means for reversibly driving the gear.

8. The subject matter of claim 7 together with switching means connected to the driving means, the switching means cooperating with the rotatable plate for de-energizing the driving means when the rotatable plate reaches a position where the apertures are located perpendicular to one another.

9. The system of claim 8 together with second switching means connected to the driving means, the second switching means cooperating with the rotatable plate for de-energizing the driving means when the driving means is reversed to return the rotatable plate to a preselected position for sector scanning.

10. The structure of claim 1 wherein the main reflector and the means for decreasing the aperture are twistflectors and the subreflector is a transflector.