US3793637A

US3793637A - Radar antenna with mirror-wheel scanner for sector and conical scanning

Info

Publication number: US3793637A
Application number: US00335874A
Authority: US
Inventors: J Meek
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1973-02-26
Filing date: 1973-02-26
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

A quasi-spherical multi-reflector radar antenna has a subreflector and a main reflector with an aperture formed centrally therein. A mirror wheel is positioned behind the aperture while a fixed microwave energy beam is directed axially or radially against the wheel for subsequent reflection to the far field. As the mirror wheel rotates, the microwave energy reflected from the mirror wheel reflects at varying angles to effect unidirectional sector scanning of a high gain pencil or oval beam in the far field.

Description

United States Patent [191 Meek [451 Feb. 19, 1974 RADAR ANTENNA WITH MIRROR-WHEEL SCANNER FOR SECTOR AND CONICAL SCANNING [75] Inventor: James M. Meek, Silver Spring, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Feb. 26, 1973 [21] Appl. No.: 335,874

[52] US. Cl 343/761, 343/781, 343/837, 343/839 [51] Int. Cl H0lq 3/18, HOlq 3/20, HOlq 15/14 [58] Field of Search... 333/781, 758, 761, 766, 837, 333/839, 763

[56] References Cited UNITED STATES PATENTS Cook et al. 343/765 Primary ExaminerEli Lieberman Assistant Examiner-Marvin Nussbaum Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Saul Elbaum [57] ABSTRACT A quasi-spherical multi-reflector radar antenna has a sub-reflector and a main reflector with an aperture formed centrally therein. A mirror wheel is positioned behind the aperture while a fixed microwave energy beam is directed axially or radially against the wheel for subsequent reflection to the far field. As the mirror wheel rotates, the microwave energy reflected from the mirror wheel reflects at varying angles to effect unidirectional sector scanning of a high gain pencil or oval beam in the far field 10 Claims, 5 Drawing Figures PATENTE FEB 1 9 m4 SHEET 1 0? 2 RADAR ANTENNA WITH MIRROR-WHEEL SCANNER FOR SECTOR AND CONICAL SCANNING The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

FIELD OF THE INVENTION The present invention relates to a multi-reflector antenna which produces a sector scan by utilizing fixed microwave energy reflection from a rotating mirror wheel. The wheel has a number of flat mirrors mounted thereon. As the mirrors rotate, the fixed microwave energy beam reflects at varying angles to produce a unidirectional (pencil or oval beam) sector scan. A figured quasi-spherical main reflector is employed to collimate the beam in the far field.

BRIEF DESCRIPTION OF THE PRIOR ART The prior art relating to microwave antennas includes spherical reflector antennas. In the visible optical spectrum, telescopes have evolved, attributable to Schmidt and Bouwers involving multiple reflectors having one or more spherical reflectors. See ANTENNA ENGINEERING HANDBOOK, 1961 Edition (1st Edition) Henry Jasik, pg. -12, McGraw Hill Book Co., New York; and ACHIEVEMENTS IN OPTICS, Albert Bouwers (Delft, Holland) Elsevier Publishing Co., New York, AmsterdamQIn the case of spherical antennas the majority of examples have employed a primary feed located in front of the reflector. This has limited the size and complexity of the feed and associated scanning mechanisms and only relatively simple scanning operations have heretofore been achieved. Multiple scan modes such as the conical/unidirectional sector combination effecting rapid switching between modes has therefore not previously been accomplished using spherical, area-aperture reflectors in one assembly. Employment of a bulky feed-scanner in front of the main reflector results in large blockage and attendant degradation of the far field beam pattern. Performance of the system in terms of angle and range acquisition and tracking suffers correspondingly and usefulness for fire control or other applications diminishes correspondingly. In the'accompanying description, bulky feed scanners are located behind the main reflector and many of the aforementioned problems are eliminated.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention is directed to a multi-reflector radar antenna which uses a wheel comprising flat mirror components to generate a sector scan as a fixed beam of microwave energy impinges against the rotating wheel.

In another embodiment of the present invention, the wheel can be fixed so that a nutating or conical scan can impinge against it thereby generating a conical scan in the far field. The use of the wheel scanner is believed to be novel. Also, the capability of rapid switching between a sector scan mode and 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 rapid scanning capabilities, multiple operating modes, rapid mode switching, and conversion of polarization or scan direction.

BRIEF DESCRIPTION OF THE FIGURES FIG.. 1 is a side elevational view of one embodiment of the present multi-reflector antenna illustrating the disposition of a mirror wheel relative to a fixed horn that directs radially propagating microwave energy against the rotating mirror wheel.

FIG. 2 is a partial end view of the mirror wheel scanner taken along a plane passing through section line 2 2 of FIG. 1.

FIG. 3 is a section view of another embodiment of the present invention which illustrates a feed horn that may remain fixed while the mirror wheel rotates to effect unidirectional sector scanning. With the wheel fixed and the horn nutating, a conical scan in the far field can be achieved.

FIG. 4 is a partial end view of the mirror wheel taken along the plane passing through section line 4 4 of FIG. 3.

FIG. 5 is a partial end view of the mirror wheel to illustrate the principle of unidirectional scanners.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings and more particularly FIG. 1 thereof, reference numerals l0 and 12 indicate the sub-reflector and main reflector, respectively, of the antenna. As indicated by reference numeral 14, a rectangular aperture is formed in the central portion of the main reflector 12. The long dimension of the rectangle is aligned parallel to the direction of scan; in FIGS. 1 and 3 the plane of scan motion of the far-field beam is perpendicular to the page. In FIGS. 2 and 4 the scan plane is left-right.

Behind the main reflector 12 is a pyramidal feed horn 16, FIG. 1. This horn is radially disposed with respect to a rotating drum or wheel having flat internal mirrors. The wheel is generally indicated by reference numeral 18 and is seen to have a hexagonal cross section as shown in FIG. 2. However, it is to be emphasized that the number of flat internal mirrors is indicated as six for purposes of illustration only and is not intended to be a limitation of the invention. The individual flat internal mirrors are indicated in FIG. 2 by

reference numerals

20, 22, 24, 26, 28 and 30.

During transmission, microwave energy is fed from the stationary feed horn 16 in a generally radially outward direction as indicated by ray 32. This ray impinges against the flat mirror 24 and reflection occurs as indicated by ray 341A separate flat mirror 36 is angularly disposed with respect to the feed horn 16. More specifically, an opening is formed in the central portion of mirror 36 to allow the feed horn 16 to protrude therefrom. With mirror 36 in the position shown,-the reflected ray 34 impinges against this mirror 36 at point 38 for further reflection, as indicated by 40, for impingement-at point 42 of the sub-reflector l0. Thereafter, the ray is reflected from the rearward surface of sub-reflector 10 as indicated by 44. The ray 44 is reflected from the forward surface of the main reflector 12 at point 46 for final transmission to the far field, as boundary ray 48. Considering'an opposite boundary ray, reference numeral 54 illustrates another ray impinging against the flat internal mirror 24 of the mirror wheel 18. This ray is reflected as ray 56 which intercepts the lower portion 58 of the angularly oriented stationary flat mirror 38. Reflection results and ray 60 falls incident to point 62 on the rearward surface of the sub-reflector 10. Ray 64 is reflected therefrom for impingement against the forward surface of the main reflector 12. After final reflection, ray 66 is then transmitted after refraction to the far field as a lower ray boundary relative to the parallel disposed ray 48 after refraction.

Rays

66 and 48 are refracted by lens 92 and may be regarded thereafter as parallel boundary rays corresponding to the far field pencil beam.

Considering the microwave energy feed, the stationary feed horn 16 curves around an elbow waveguide portion 68 for communication with a horizontal waveguide portion 70 that is rigidly connected to a waveguide portion 72. This latter mentioned waveguide portion is finally curved to a waveguide portion 74 that terminates in an RF. input 76 via a coupling 78. The microwave energy of course is bi-directional. That is to say, the input 76 can be connected to a transmitterreceiver so that the disclosed antenna system can operate in either the transmit or receive mode.

The following discussion will relate to the drive means for rotating the mirror wheel 18.

A hub 80 is suitably attached to the left end of the wheel 18. The hub 80 extends to an intermediate shaft portion that is properly journaled by bearings 82. An outer end portion 84 serves as a mounting shaft for a gearing arrangement including driven gear 86 and a driving pinion gear 88 that is suitably attached to the output shaft ofa drive motor 90. When the drive motor 90 is energized, the

gearing

88, 86 causes rotation of the shaft 84 which is transmitted to hub 80 thereby causing rotation of the mirror wheel 18.

To providesatisfactory focusing for all scan angles, the main reflector may be shaped or figured to deviate from exactly spherical; hence the name quasi-spherical. In addition a dielectric lens may be installed at 92 if desired. t

Linear polarization of the far field radiation may be effected by use of the linearly polarized feed 16 and either opaque surfaces at both reflectors and 12 or transflector at 10, twistflector at 12. The direction of sector scan may be chosen, e.g. horizontal or vertical, simply by rotating the antenna system around the bore sight axis to the appropriate orientation. Conversion of the plane of linear polarization, e.g. vertical to horizontal, may be effected by rotating the feed born about its long axis to the appropriate orientation. (When the twistflector-transflector design is employed, the wire grids in the subreflector and mainreflector must be oriented so as to correspond to the feed polarization. Details of correct transflector-twistflector design may be found in the literature.) The relative ease of selecting or changing polarization independently of scan direction (i.e. by rotating the feed) is a unique advantage of the mirror scanner.

The described embodiment of the present invention is most practical for operating in the unidirectional sector scan mode. FIGS. 3 and 4 illustrate a modification to the described invention which allows the antenna system to be utilized in the wide sector scan mode or the conical scan mode. Otherwise stated, in a single antenna system of the Schwarzschild type, the system can be switched between the sectoral scanning mode and the conical scanning mode.

Considering the embodiment illustred in FIG. 3, flat sub-reflector 90, quasi-spherical main reflector 92, and lens 145, constitute the antenna configuration. A rectangular aperture 94 is centrally formed in the reflector 92. Behind the aperture 94 is a mirror wheel generally indicated by 96 that is a bit different from the previously described mirror wheel 18 of FIG. 1. As clearly illustrated in FIG. 4, the mirror wheel 96 is comprised of a set of pyramidally disposed, flat, internal mirrors 98 that form the circumference of the wheel 96. Reference numeral 98 typifies a single mirror component in the wheel.

A pyramidal or conical feed horn 100, FIG. 3, is mounted in axially spaced relationship from the axis of the wheel 96. To achieve sector scanning, the feed horn 100 remains stationary while the wheel 96 rotates. When the wheel rotates, a ray 102 emanating from the horn during transmission impinges upon mirror 98 at point 104. The ray is reflected as illustrated by 106 to a vertically downward direction. An angularly oriented fixed mirror 108 intercepts the ray at an upper portion 110. The point of incidence is indicated by reference numeral 112. The incident ray is reflected as ray 114. The ray 114 impinges, at point 116, on the rearward contoured surface of the subreflector 90. After reflection, the ray 118 impinges at point 120 against the forward surface of the main reflector 92. A final reflection occurs as indicated by ray 122 which characterizes the upper limit of a beam transmitted to the far field. The second limit of such a beam is initially generated by ray 124 that emanates from the horn 100. The ray hits the mirror 98 and is reflected at 126 as ray 128. The mirror 108 intercepts this ray at the point 130 whereat the ray is reflected (132) for impingement against the rearward surface of the sub-reflector 90, at point 134. Ray 136 is reflected from the sub-reflector until it impinges upon the lower point 138 of the main reflector 92. A final reflected ray 140 results for transmission to the far field. Ray 140 passes through lens and represents the lower ray limit of the transmitted beam in a unidirectional sector scan, scanning perpendicular to the plane of the page.

As indicated in FIG. 3, each component or individual mirror of the wheel has a wedge 142 attached to the rearward surface thereof. This wedge serves as a means for mounting its respective mirror to a bearing assembly 14.

To further illustrate the principle involved in the operation of the unidirectional scanners, reference is made to FIG. 5. A portion of a scanner wheel 1 is shown rotating schematically about center 2 which is coincident with the mouth of a stationary feed horn. The microwave energy emanating fromsaid horn has a principal ray 2 3 intersecting a mirror on wheel 1 at point 3. Wheel 1 is in continuous clockwise motion, direction 4, so that the reflected principal ray will sweep through an are 5 5 centered approximately at point 3. To explore this action in more detail, consider the motion of the reflected principal ray starting with one corner of the wheel at arbitrary position 6 and rotating to position 6. The reflected ray will occupy successively positions 3 7, 3 5, 3 5, and 3 7. The intervals between these positions represent a portion of a scan, a scan retrace (or flyback), and another portion of a scan, successively. The total scan angle is represented by are 5 5'. The scan retrace of flyback transition occurs more or less instantaneously as the ray 2 3 intercepts corner 6 in passing toward position 6'. In actuality, the principal ray represents the center of a relatively narrow feed beam and the transition can be considered complete when the high intensity beam has passed from one mirror to the next. During transition, the beam is momentarily split so that part of the energy is propagated in the direction of ray 3 5' and the residual along 3 5. Thus a beam decay is occurring at 3 5 while a buildup is occurring at 3 5, the time interval being only a small fraction of the scan interval. The operating principle may now be applied to the embodiments of FIGS. 1 through 4, wherein the scanning beam is further collimated, as described, and directed to the far field as a scanning pencil beam by the main and sub-reflectors as shown. At the end of each unidirectional scan, the far field beam will exhibit the intensity decay (at end of scan) and simultaneous buildup (at begin of scan) as a result of the beam splitting effect within the scanner.

Conventional means, such as gearing, pulleys, or the like (not shown) cause rotation of the mirror wheel.

The above-described embodiment dealing with FIGS. 3 and 4 were directed to the production of a relatively wide angle unidirectional sector scan. However, the invention can be employed to produce a conical scan or steady track mode. To accomplish this, rotation of the mirror wheel 96 is stopped. Instead, the feed horn 100, is mounted to that its principal ray axis is tilted slightly with respect to the asis of a journal supporting the horn assembly and rotationally driven by a motor (not shown). A microwave rotary joint permits horn rotation. The tilt angle of the feed combined with rotation provides conical scanning in the far field, the beam collimation being provided in the same manner as in the sector scan mode. If monopulse tracking would be desired instead of a conical scan. then the nutating horn 100 could be replaced by four or more adjacently placed fixed horns that define the corners of a rectangle. Thus, with no mechanical motion occurring in the system, monopulse range and angle detection or sequential lobing may be effected in a conventional manner.

FIG. 4 illustrates the inclusion of ten flat mirrors in the mirror wheel. However, as in the embodiment disclosed in connection with FIGS. 1 and 2, this number of mirrors is merely exemplary and is not intended to be a limitation. Actually, for both embodiments, the number of mirrors is determined by the scan angle desired during unidirectional sector scanning as well as other desired beam characteristics. For example, to obtain a relatively small transition or flyback" time period compared to scan period, the feed beam illumination should cover a small percentage of a particular mirror in a wheel. This requires either a narrow feed beam width or broad mirrors. The feed beam width affects the far-field radiation pattern (gain, side lobes, etc.). The use of broad mirrors results in either a larger scanner wheel or fewer mirrors. The latter results in increasing'the scan angle in the far field. The scan angle is in turn limited by the necessity of keeping the primary beam from excessively spilling over the reflector edges.

The scan angle magnitude in the scanner wheel system may be controlled by the angular positioning of the wheel mirrors relative to the feed axis. For example,

the angle, FIG. 1, results in a 2/] ratio of angle of reflected energy rotation to wheel rotation, while the 45 angle, FIG. 3, results in a 1/] ratio.

From this discussion, it should be evident that careful selection of the interrelated design parameters is necessary to achieve optimum or desired overall antenna performance. In the case of the arrangement shown in FIGS. 1 and 2, feed blockage also must be considered if a large feed horn is used.

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.

' Wherefore, I claim the following:

1. An antenna system having a flat sub-reflector and a quasi-spherical main reflector with a centrally formed aperture therein, the system comprising:

a hollowed mirror wheel having its axis concentric with the center of the aperture and located behind the aperture;

means mounted adjacent the. wheel for feeding a steady flow of microwave energy to the wheel which is reflected therefrom; and

means for further reflecting the energy, reflected from the wheel, through the aperture to the main and sub-reflectors for transmission to the far field.

2. The subject matter of claim 1 wherein the mirror wheel is comprised of a cylindrical body having flat plate internal mirror components thereon.

3. The structure of claim 2 wherein the means for further reflecting energy, reflected from the wheel, comprises a stationary mirror mounted inwardly of the mirror wheel at an angular orientation relative to an axis of the cylindrical body.

4. The system of claim 3 wherein the means feeding a steady flow of microwave energy to the mirror wheel comprising a stationary feed horn which protrudes through an opening in the center of the stationary mirror.

5. The subject matter of claim 4 together with means connected to the mirror wheel for rotating the wheel and producing a unidirectional sector scan that is subsequently transmitted to the far field.

6. The system of claim 1 wherein the mirror wheel is comprised of a plurality of trapezoidal-shaped flat mirror components forming a frusto-conical hollowed body.

7. The subject matter recited in claim 6 wherein the means for further reflecting the energy, reflected from the wheel, comprises a stationary mirror mounted inwardly of the mirror wheel at an angular orientation relative to an axis of the frusto-conical hollowed body.

8. The system of claim 7 wherein the means feeding a steady flow of microwave energy to the mirror wheel comprises a stationary horn positioned adjacent the body so that the energy emitted therefrom flows in a path parallel to the axis of the frusto-conical body.

9. The subject matter defined in claim 8 together with bearing means mounted to the mirrored frustoconical body to permit the rotation thereof while the statonary horn feeds a steady flow of energy;

thereby effecting the generation of a unidirectional sector scan in the far field.

10. The subject matter defined in claim 8 together with means mounted to the stationary horn for imparting nutating motion to the horn while the mirrored frusto-conical body remains stationary thereby producing a conical scan in the far field.

Claims

1. An antenna system having a flat sub-reflector and a quasispherical main reflector with a centrally formed aperture therein, the system comprising: a hollowed mirror wheel having its axis concentric with the center of the aperture and located behind the aperture; means mounted adjacent the wheel for feeding a steady flow of microwave energy to the wheel which is reflected therefrom; and means for further reflecting the energy, reflected from the wheel, through the aperture to the main and sub-reflectors for transmission to the far field.

9. The subject matter defined in claim 8 together with bearing means mounted to the mirrored frusto-conical body to permit the rotation thereof while the stationary horn feeds a steady flow of energy; thereby effecting the generation of a unidirectional sector scan in the far field.