Schwarzschild Radar Antenna With A Unidirectional Turnstile Scanner

Abstract

A Schwarzschild Antenna includes feed horns respectively mounted on a turile waveguide switch. During transmission, as each horn passes a particular arc of rotation, microwave energy is emitted from the horn onto an adjacent mirror that reflects the energy to the antenna reflectors. The result is a unidirectional scan in the far field. A tracking mode of operation is also provided. Either of the two modes of operation may be selected by the operator.

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 Cassegrain Antennas having a unidirectional scanner, and more particularly to the Schwarzschild Antenna with rotating feed horns to produce the unidirectional scan for reflection to the far field via a mirror reflector.

BACKGROUND OF THE INVENTION

As discussed in our co-pending application Ser. No. 335,877, filed Feb. 26, 1973, the recent prior art has introduced the Schwarzschild Antenna which is an aplanatic system with reflectors shaped to satisfy the Abbe sine condition. Our co-pending application is primarily directed to a Schwarzschild Antenna with a switching means that allows the antenna to operate in a tracking mode (this is monopulse or con scan) or a relatively wide angle unidirectional sector scan.

The combination of scan modes and the ability to switch rapidly between these modes of operation are novel advantages. In addition, utilization of a Schwarzschild antenna results in superior off-axis focusing capabilities.

A detailed discussion of preliminary design considerations for Schwarzschild Antenna is included in the referenced co-pending application. The antenna of the co-pending application employs an organ-pipe scanner for producing a unidirectional scan. Although this operates satisfactorily, it requires a relatively large physical space.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a unidirectional scanner for a Schwarzschild Antenna which is relatively compact when compared to the structure of the invention, disclosed in the referenced co-pending application. A high-gain pencil or oval beam sweeps the far field during scanning.

In a first embodiment of the invention, curved turnstile microwave conduits traverse an adjacently positioned mirror that reflects microwave energy, during transmission, to a pair of reflectors. The reflectors in turn reflect the moving beam to the far field producing a high gain unidirectional scanning beam.

By utilizing a metallic or a dielectric lens in this embodiment, superior directivity is rendered the beam between feed and subreflector.

A second embodiment of the invention discloses the utilization of a simplified turnstile switch and feed horn assembly cooperating with an arcuate section of an organ-pipe scanner. This combination achieves the relatively wide angle unidirectional sector scan that is desired without the inclusion of a metal or dielectric lens. Due to the use of the turnstile switch, the organ pipe transition is markedly simplified when compared to conventional organ pipe scanners.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side elevational view of a first embodiment of the present invention.

FIG. 2 is a front sectional view taken along a plane passing through section line 2--2 in FIG. 1.

FIG. 3 is a side elevational view of a modified form of the present invention.

FIG. 4 is a perspective view of the components shown in FIG. 3 whereby a lens is shown in its spatial relationship to the feed horns of a turnstile scanner.

FIG. 5 is a rear elevational view illustrating the combination of a turnstile feed horn assembly with an organ-pipe transition in a second embodiment of the invention.

FIG. 6 is a side elevational view of the structure shown in FIG. 5 and taken along a plane passing through section line 6--6 of FIG. 5.

FIG. 7 is an end view of a second type of microwave lens for use with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures, and more particularly FIG. 1 thereof, the subreflector of the antenna system is indicated by reference numeral 10. The main reflector 12 is disposed in spaced relation from the subreflector. In a preferred embodiment of the invention, the subreflector is a transflector while the main reflector is a twistflector. These reflectors form the basic Schwarzschild antenna, the reflectors forming an aplanatic system which meets the Abbe sine condition. A rectangular aperture 13 is centrally formed in the main reflector.

To the left of the aperture 13 is an angularly oriented flat mirror 14 which reflects energy that is transmitted from a pyramid horn 16. The horn 16 is a transmitter output of a turnstile scanner having four legs, each leg terminating in a horn such as 16. This assembly is more visually apparent in FIG. 2.

Considering a single leg of the turnstile scanner assembly, the output horn 16 is connected to and fed energy by a waveguide section 18 which then extends to a radially disposed waveguide section 20. In a similar manner, FIG. 1 illustrates a second radially disposed leg that has a radial waveguide section 22 that curves around to a final output pyramid horn 24. FIG. 2 illustrates the disposition of the remaining horns 26 and 28, of the turnstile scanner. The inward arms of the turnstile scanner assembly are mounted in a turnstile R.F. switch generally indicated by reference numeral 50. This type of switch is conventional and is disclosed in the textbook MIT Radiation Laboratory Series, Vol. 26, Radar Scanners and Radomes, by Cady, Karelitz and Turner. The text is published by McGraw-Hill, 1948. See page 58.

The turnstile switch 50 includes a central cylindrical body 30 that is stationarily mounted. This body is mounted to a rotatable hub 40 which receives the radially inward ends of the waveguide pipes in the turnstile scanner assembly, as indicated at reference numeral 32. It is to be emphasized that the turnstile R.F. switch allows passage of microwave energy to only one turnstile leg at a time in the scanner assembly. In FIGS. 1 and 2, only the lower leg becomes energized as it traverses the "on" time arc illustrated in FIG. 2. Therefore, as each leg of the turnstile assembly approaches this arc, it becomes energized. After the leg traverses the arc, microwave energy ceases to flow through it. The R.F. switch has a waveguide extension 34 which permits the connection of the switch to a waveguide 36. The outward end of the waveguide 36 is connected to a transmitter-receiver (not shown).

Referring to FIG. 2 of the drawings, an explanation of the generation of a unidirectional sector scan will now be made.

If it is assumed that the rotation of horn 16 is in the counterclockwise direction, as indicated in the figure, "on" time for each feed begins at the point indicated by reference numeral 54. The feed horn 16 then traverses the arc 58 which may be considered as the "on" time arc. Traversal of this arc by the feed horn 16 is generally indicated by reference numeral 52. As the feed horn 16 traverses the arc 58, transmitter energy flows from waveguide 36 (FIG. 1) to the R.F. switch 50. A stationary window (not shown) in the hub portion 40 of the R.F. switch allows the microwave energy to pass therethrough and into the microwave pipe 20 for the eventual delivery to the horn 16. Energy emanates from the horn 16 and impinges against a confronting surface of the mirror 14. The mirror 14 is normally disposed at the nominal 45.degree. angle shown. Reflection from the mirror results, and energy is directed through the aperture 13 to the inward side of the subreflector 10. Thereafter, the energy is reflected from the main reflector to the far field. To illustrate the reflection path, the travel of an edge ray is illustrated.

While the horn 16 travels the "on" time arc 58, microwave energy is continuously emitted from the horn 16 and is relatively constant. When the horn 16 reaches point 56, the energy is cut off from the pipe 20. At this time, the next horn 26 will be entering the "on" time arc 58 and it will direct energy to the mirror in the same manner as horn 16 did. As each horn traverses the "on" time arc 58, the angle of reflection from the mirror 14 varies and as a result, the reflected energy becomes a relatively wide angle sector scan. As the system is reciprocal, it will operate similarly in the receive mode to the transmit mode as described.

FIGS. 3 and 4 illustrate a modification of the embodiment just discussed. A first change is directed to the previously discussed mirror 14. Instead of the rectangular shape shown in FIGS. 1 and 2, the embodiment of FIGS. 3 and 4 show a generally oblong, beveled mirror edge. This shape is desirable for compactness and also for compatibility with the beam pattern handled by the antenna system.

A second change from the embodiment of FIGS. 1 and 2 is directed to a metal plate lens generally indicated by reference numeral 62. The lens is fabricated from a series of adjacently positioned metal plates 64. The plate spacings 66 are in effect waveguide channels of which the spacings or heights correspond to the microwave E-plane. The propagation velocity in each channel may be varied by varying the height of the channel. (This is equivalent to varying the dielectric constant or the index of refraction within the channel). By choosing spacings in adjacent channels which cause the combined phase front of the wavelets emanating from the channels to assume a direction, for all horn positions, toward the center of the subreflector, optimum directivity may be achieved and good focusing maintained. In another variation of the lens, instead of having the gaps vary, the length of the various individual plates can be made to vary by bending the channels in either E- or H-plane, serpentine fashion. The purpose of the lens is to center the energy sweep at the center of the subreflector during the entire scan. Otherwise stated, the particular advantage of the lens 62 is to improve directivity of the beam. The directivity is centered on 68 at the center of the subreflector. The disposition of the lens 62 is shown to be between a horn traversing the "on time" arc and the mirror 60.

Instead of a metal plate lens 62, a dielectric lens (FIG. 7) 69 can be employed. This means that the various plates or segments of the lens are fabricated from a dielectric material. The individual plates of the dielectric lens may have constant thickness but have lengths that vary. In another variation, the lengths may be constant and the index of refraction may vary from channel to channel. The individual plates of the dielectric lens are denoted by reference numeral 70.

FIGS. 3 and 4 illustrate the exterior geometry of each of the horns in the turnstile scanner assembly. In FIG. 4, it will be seen that each horn of the turnstile scanner as typified by 72, is outwardly flared in a pyramid fashion when viewing the horn 72 in one plane. However, when viewing the horn 72 from an orthogonal plane, the same end is seen to be straight (FIG. 3). The lens section may then be flared 63 in the orthogonal direction to provide the desired primary aperture in both E- and H-planes. (The E- and H-plane apertures will generally provide the appropriate primary beamwidth which is essentially the angle subtended by the outer circumference of the subreflector.)

Up to this point, the present invention has been described in terms of a relatively wide angle unidirectional sector scan. However, the system may be employed for generating a conical scanning or steady tracking beam. Thus, as shown in FIG. 4, a nutating horn 74 is positioned adjacent to the principal focus 75 of the system for the alternate rotatable mirror position. Energy that is transmitted from the nutating horn 74 is reflected by the mirror 60 and generates a conical scan that is reflected from the main and subreflectors (10,12) to the far field.

FIGS. 5 and 6 illustrate another embodiment of the present invention. This embodiment utilizes a turnstile scanner. However, rather than using the curved turnstile legs illustrated and discussed in connection with FIGS. 1-4, the turnstile scanner illustrated utilizes a R.F. turnstile switch 50' for commutating microwave energy to four radially disposed horns 76, 78, 80 and 82. As clearly shown in FIG. 6, these horns and their connecting pipes are straight radially disposed members rather than the curved members shown in connection with the previous figures. In order to obtain the circular-motion source of microwave energy from the turnstile scanner, as was accomplished by the curved microwave pipe sections of the previously described turnstile scanners, a simplified organ-pipe scanner transition generally indicated by reference numeral 84 is used. In this case, the organ-pipe transition occupies an arcuate portion of 90.degree.. The construction of an organ-pipe scanner is well known in the art, and is fully discussed in our co-pending application Ser. No. 335,877. Basically, the R.F. turnstile switch 50' will "turn on" a horn that communicates with the organ-pipe scanner. This is indicated by horn 80 in FIG. 5. Thus, during transmitter operation, with microwave energy flowing out from the R.F. switch 50', the feed horn 80 will communicate this microwave energy to the ends 86 of the organ-pipes such as 87 until the microwave energy leaves the feed horn 88. As the horn 80 traverses the 90.degree. arc, it will sequentially energize the pipes 87 of the organ pipe scanner. As a result, the respective horns 88 will be sequentially energized. The sequential energization forms a microwave scan which is reflected from the mirror 92 that is disposed at the approximate 45.degree. disposition shown in FIG. 6. As the various horns 86 become sequentially energized, the angle of incidence upon the mirror 92 will vary which results in a scanning beam being reflected from the mirror and transmitted to the reflectors 10 and 12 of the antenna system. Accordingly, a relatively wide angle sector scan is transmitted to the far field. Viewing FIG. 5, if we assume that the rotation of the R.F. switch 50' is counterclockwise, the first horn to be energized is the horn specifically indicated by reference numeral 89. The last horn to be energized will be that specifically indicated by reference numeral 90. However, it should be noted that the organ-pipe scanner includes waveguide ends such as 86 which are arranged along an arc which is preferably slightly more than 90.degree.. However, the feed "output" horns such as 88 are arranged to fit the focal arc of the reflector system. As the turnstile horn 80 traverses the feed arc, it reaches the waveguide connected to horn 90 and thereafter becomes inoperative. As horn 80 reaches the cut off position indicated by 90, the next horn 82 enters the "on time" arc and begins generating the sector scan until it in turn passes the "on time" arc.

The aforementioned discussion is directed to the generation of a relatively wide angle unidirectional sector scan. The beam may be pencil, oval, or fan shaped. FIG. 6 illustrates the inclusion of a nutating horn 94 that will generate a conical scan when the mirror 92 is repositioned to the position shown by dashed lines. The horn 94 is connected to a rotary joint 96 that allows nutating motion. A waveguide 98 is connected to the rotary joint 96 and provides a transmission medium from a transmitter-receiver. As will be noted from FIG. 6, the axis of horn 94 is slightly offset from the center of the mirror 92 so that the nutating motion of horn 94 can produce a conical scan.

If it is desired to utilize a different type of tracking mode, instead of conical scan, four or more horns such as 94 may be stationarily positioned to form four corners of a rectangle. In this instance, the stationary horns will provide usual sum and difference patterns, as necessary for a monopulse mode of operation.

Fast mode switching is needed to be able to acquire in the con scan when switching from the sector scan mode. This requires that the flat mirror (e.g. 92 in FIG. 6) be rotated and positioned rapidly and accurately. The mirror assembly should be designed for high rigidity, low inertia, zero backlash, minimum rebound, and should have a rapid, reversible motor drive. Magnification effects in the reflector system have the beneficial effect of substantially reducing errors in far field beam position caused by any residual error in positioning the flat mirror.

Suitable microwave energy absorbing material may be installed behind the main reflector near the aperture, around the perimeter of the antenna forward of the main reflector, and elsewhere to absorb primary feed sidelobe energy and other spurious reflections.

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.

Claims

Wherefore, we claim the following:

1. A Schwarzschild antenna system having main and subreflectors, the system comprising:

a plurality of waveguide pipes extending radially from a central rotating hub;

window means formed in the hub for insuring passage of energy through only one pipe at a time;

a rotatable mirror positioned adjacent a central aperture formed in the main reflector for transmitting microwave energy to and from the aperture;

and means disposed adjacent the rotatable mirror for directing microwave energy in the pipes along a direction coincident with the mirror;

whereby microwave energy flowing through the hub during transmission will sequentially energize each pipe, as each pipe passes through a preselected arc of rotation, the sequential energization of the pipes resulting in the reflection of a unidirectional sector scan from the mirror, the subreflector, and the main reflector to the far field.

2. The subject matter set forth in claim 1 wherein the plurality of waveguide pipes are mounted 90.degree. apart, to form a turnstile scanner, each pipe curving along an outward end portion and further wherein the microwave energy directing means comprises a feed horn connected to an outward end of each pipe.

3. The subject matter of claim 1 wherein the plurality of waveguide pipes are radially mounted 90.degree. apart to form a turnstile scanner, each pipe terminating outwardly in a horn, and further wherein the microwave energy directing means comprises an organ-pipe transition.

4. The subject matter of claim 2 together with a microwave lens positioned between a feed horn traversing the arc and the mirror; whereby the lens optimizes directivity of a beam reflected from the rotatable mirror.

5. The structure of claim 4 wherein the lens may be fabricated from a plurality of adjacently spaced metal plates having varying channel lengths therebetween.

6. The structure of claim 4 wherein the lens may be fabricated from a plurality of adjacently spaced metal plates having varying gaps therebetween.

7. The subject matter of claim 4 wherein the lens may be fabricated from a plurality of adjacently spaced dielectric plates that have varying lengths.

8. The subject matter of claim 4 wherein the lens may be fabricated from a plurality of adjacently spaced dielectric plates that have varying indices of refraction.

9. The structure of claim 1 wherein the hub forms a portion of an R.F. turnstile switch, the switch being connected through a rotatable joint to a transmitter-receiver.

10. The structure set forth in claim 1 wherein the main reflector is a twistflector.

11. The structure of claim 1 wherein the subreflector is a transflector.

12. The system defined in claim 1 wherein the rotatable mirror has beveled tapering edges to render the rotatable mirror more compact for rapid switching.

13. The subject matter of claim 3 together with means for generating a conical scan, said means located adjacent the rotatable mirror, the mirror transmitting the conical scan to the subreflector and to the main reflector when the mirror is repositioned in a second angular orientation.