Abstract:
A phased array antenna consisting of like multi-element tiles whose elements are located so as to produce an irregular array when the tiles have mutually different orientations, e.g., random. The resulting irregular array reduces the effective translational period of the array elements, which in turn ameliorates grating lobes even for wide (one wavelength) effective element spacings. An antenna so designed can maintain low peak sidelobes at far higher frequencies than a conventional translational-periodic phased array antenna of the same element density.

Description:
This application is a continuation of application No. 09/815,756 filed Mar. 23, 2001 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to phased array antennas and more particularly to an antenna configuration which ameliorates grating lobes while having wide effective element spacings on the order of one wavelength. 
     2. Description of Related Art 
     Phased array antennas are well known and provide excellent electronic beam steering capabilities. However, such antennas require expensive electronics, such as phase shifters, circulators, amplifiers, etc. associated with each radiating element. To reduce manufacturing costs, antenna element support members such as tiles have recently been developed, each incorporating multiple elements. Where identical tiles are utilized, cost savings can result because such tiles can be mass produced. To further reduce antenna cost, it has become desirable to reduce the element count as much as possible while still providing the same desired aperture size; however, when element spacing exceeds one half wavelength in any regular grid of antenna elements, grating lobes appear when the beam is scanned. In general, element count can be reduced by global random thinning or aperiodic element locations, but such approaches do not lend themselves to tiling and hence do not realize the full cost savings potential of mass production. 
     SUMNMARY 
     Accordingly, it is an object of the present invention to provide an improvement in phased array antennas having wide element spacings. 
     It is yet another object of the invention to provide an antenna which maintains low peak sidelobes at far higher operating frequencies than a conventional translational-periodic phased array antenna having the same element density. 
     It is still a further object of the invention to provide a phased array antenna which substantially reduces or eliminates grating lobes while having element spacings which exceeds one half wavelength. 
     It is yet a further object of the invention to provide a tiled phased array antenna which ameliorates grating lobes for effective element spacings on the order of one wavelength. 
     The foregoing and other objects are achieved by a phased antenna array comprised of an arrangement of like contiguous tiles in the form of a regular polygon having an identical number and relative positioning of antenna elements which by a judicious choice of tile element positions combined with tile rotations result in an irregular or aperiodic array so as to reduce the effective translational period of the array elements which ameliorates grating lobes for elements having an average density of one per square wavelength, i.e., one wavelength spacing. This is achieved, in one aspect of the invention, by randomly orienting a set of square tiles having, for example, four antenna elements located thereon where two of the antenna elements are aligned with a diagonal of the respective tile, and where the other two elements are equi-distantly located on either side of the diagonal. In one tile embodiment, the first two elements are located in the region adjacent one corner of the tile while the other two elements are located in the region adjacent an opposite corner of the tile. In a second tile embodiment, one element of the four antenna elements is located in the region adjacent one corner of the tile along the diagonal while the other three elements are aligned linearly across a diagonal in a region adjacent the opposite corner of the tile. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific embodiments, while disclosing the preferred embodiments of the invention, are provided by way of illustration only inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood when the following detailed description is considered in conjunction with the accompanying drawings, which are provided by way of illustration only, and thus are not meant to be limitative of the present invention, and wherein: 
     FIG. 1 is illustrative of a rectangular four element tile arrangement utilized for implementing a square grid, and having a one wavelength spacing; 
     FIG. 2 is illustrative of a regular array of tiles, such as shown in FIG. 1, implementing a one wavelength square grid; 
     FIG. 3 is a depiction of the scan beam and the grating lobes which appear when the regularly spaced array shown in FIG. 2 is scanned, for example, 45° in azimuth; 
     FIGS. 4 a - 4   d  are illustrative of a first embodiment of a four element tile in accordance with a first embodiment of the subject invention and further illustrative of four possible 90° rotations or orientations thereof; 
     FIG. 5 is an illustration of the tile shown in FIG. 4, further depicting the details of the relative positions and mutual spacing of the antenna elements; 
     FIG. 6 is illustrative of an irregular array of elements resulting from a random orientation of a plurality of tiles shown in FIGS. 4 and 5; 
     FIG. 7 is illustrative of the resulting grating lobe amelioration achieved with a phased array such as shown in FIG. 6; 
     FIGS. 8 a - 8   d  are illustrative of a second preferred embodiment of a four element tile in accordance with the subject invention and four possible 90° rotations or orientations thereof; 
     FIG. 9 is illustrative of the relative positions and mutual spacing of the elements in the tile shown in FIG. 8; 
     FIG. 10 is illustrative of an irregular array of antenna elements resulting from a random orientation of a plurality of tiles, such as shown in FIGS. 8 and 9; 
     FIG. 11 is illustrative of the resulting grating lobe amelioration achieved with a phased array such as shown in FIG. 10; and 
     FIG. 12 is illustrative of a triangular tile; 
     FIG. 13 is illustrative of diamond shaped tile; 
     FIG. 14 is illustrative of a hexagonal tile; 
     FIG. 15 is illustrative of an arcuate shaped tile consisting of nine straight line segments; 
     FIG. 16 is illustrative of an array of diamond shaped tiles; and 
     FIGS. 17A and 17B are illustrative of two types of curvilinear arrays comprised of the arcuate tiles shown in FIG.  16 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing figures where like reference numerals refer to like parts throughout, reference is first made to FIG. 1 which is illustrative of a support member consisting of a tile  10  having the shape of a regular polygon and, more particularly, a square including four antenna elements arranged in a conventional square configuration and having a spacing of one wavelength (1λ). As shown, the tile  10  has a total size of 2λ×2λ or four square wavelengths. Since there are four elements  12  per tile  10 , the effective spacing is one wavelength. 
     A 16×16 array of tiles shown in FIG. 1 yields a square array of 1024 elements on a one wavelength (1λ) square grid as shown in FIG.  2 . When an array such as shown in FIG. 2 having regular element spacing on the order of one wavelength is scanned, for example 45° in azimuth, the array has a two dimensional far field radiation pattern (2-D FFT) as shown in FIG.  3 . It can be seen with respect to FIG. 3 that a relatively large grating lobe  14  is formed near boresight  16 , which is undesirable. 
     These grating lobes can be ameliorated, i.e., substantially reduced, if not eliminated, by arranging the radiating elements of an antenna tile array so that an irregular array is provided when combined with copies of the same tile having mutually different orientations. 
     Such an arrangement is shown, for example, in FIGS. 4-6 where a preferred embodiment of an antenna tile in accordance with the subject invention is depicted. A four element antenna tile  20  having 2λ×2λ side dimensions or four square wavelength is shown in FIGS. 4 a - 4   d  rotated counter-clockwise by 90° through four possible orientations. Further as shown, two elements  12   1  and  12   2  straddle a diagonal line  22  extending between opposite corners  24  and  26  of the tile in the region adjacent the corner  24  and being equally located on either side thereof, while the other two elements  12   3  and  12   4  are aligned with the diagonal  22  and located in the region adjacent the other corner  26  of the tile  20 . 
     Referring now to FIG. 5, depicted thereat is the tile orientation of FIG. 4 d , with the relative dimensions associated with the element spacings being shown. The distance between the two pairs of elements  12   1 ,  12   2  and  12   3 ,  12   4  comprises the hypotenuse of a right triangle having side lengths of λ/4 and thus is equal to λ/{square root over (8)}=0.3536λ. Since there are four elements per file, the effective density is one square wavelength. 
     It is to be noted that when the elements  12   1 ,  12   2 ,  12   3 ,  12   4  are collapsed to a line source in azimuth or elevation, the elements appear to be on a λ/2 grid, although the average spacing in the tile is still one per square wavelength, or one wavelength spacing. When combined with the four different allowed rotations as shown in FIG. 4, collapsing the element positions to a line source along any intercardinal axis also yields an effective sub-wavelength spacing. 
     As shown in FIG. 6, when the four different allowed tile rotations of FIGS. 4 a - 4   d  are chosen randomly, a 16×16 array of tiles  20  forms an irregular array of 1024 elements on an average one wavelength grid. When such an array is now scanned, for example, 45° in azimuth, the array shown in FIG. 6 has a 2-D FFT as shown in FIG.  7 . The grating lobe energy has been ameliorated. The grating lobe energy is not gone, but is now smeared out at a lower power level across a larger solid angle with the resulting peak of the far side lobes being 20 dB or less. 
     Considering now FIGS. 8-10, shown thereat is a second preferred embodiment of a four element tile also having side dimensions of 2λ×2λ or four square wavelength as in the first embodiment. However, the location of the four antenna elements  12   1  . . .  12   4  is now changed to one where three of the elements  12   1 ,  12   2 , and  12   3  are linearly aligned perpendicular to the diagonal  22  in the vicinity of the corner  24 , while the fourth element  12   4  is located in the vicinity of the opposite corner  26 . Moreover, two of the elements  12   2  and  12   4  are positioned along the diagonal  22 , but now have a relatively greater spacing as shown in FIG.  9 . 
     As shown in FIG. 9, the distance between elements  12   1  and  12   2  and between  12   2  and  12   3  are equal to the hypotenuse of a right triangle having sides of λ/4 and thus being equal to λ/{square root over (8)}=0.3536λ. The distance between elements  12   2  and  12   4 , however, is the hypotenuse of a right triangle having sides of λ/2 and which would be equal to 2 λ/{square root over (8)} or 0.7071λ. Since there are four elements on the tile  30 , effective spacing is one wavelength as before with respect to the first embodiment shown in FIG.  5 . 
     Again, it should be noted that when the elements  12   1 ,  12   2 ,  12   3  and  12   4  on the tile  30  are collapsed to a line source in azimuth or elevation, the elements appear to be on a one half wavelength grid, although the actual average spacing is still one wavelength. When combined with four different allowed rotations as shown in FIGS. 8 a - 8   d , where four clockwise 90° rotation are depicted, collapsing the line element positions to a line source along any intercardinal axis also yields effective sub-wavelength spacing. 
     As before, when the four different allowed 90° rotations of the tile  30  are chosen randomly, a 16×16 array of tiles results in an irregular array of 1024 elements on an average one wavelength grid as shown in FIG.  10 . When the irregular array of FIG. 10 is scanned, for example 45° in azimuth, the array has a 2-D FFT as shown in FIG.  11 . Again, the grating lobe as shown in FIG. 3 has been ameliorated, i.e., is smeared out at a lower power level across a larger solid angle as before. 
     Although what has been described and illustrated herein is a structure consisting of identical square tiles with four elements, it should be noted, that when desirable, any size tile and any desired number of elements per tile may be used, where larger numbers of elements on larger tiles would lead to a greater savings in manufacturing costs. Also, other polygonal tile shapes may be resorted to such as shown, for example, in FIGS. 12,  13 ,  14  and  15  where a triangular tile  20 - 1 , a diamond shaped tile  20 - 2  in the form of a parallelogram, a hexagonal tile  20 - 3 , and an arcuate tile  20 - 4  consisting of nine straight line segments are depicted. FIG. 16 is illustrative of an aperiodic tile arrangement utilizing two different sized diamond shaped tiles  20 - 2 ( a ) and  20 - 2 ( b ) of the type shown in. FIG.  13 . FIGS. 17A and 17B, on the other hand, are illustrative of a spiral type tile arrangement and a concentric circular tile arrangement utilizing arcuate shaped tiles shown in FIG.  15 . In each instance, an irregular array of antenna elements results which produces grating lobe amelioration. 
     FIGS. 16,  17   a , and  17   b , moreover, illustrate tiling arrangements whereby randomness in element placement comes not only from random orientation of the tiles  20 , but also from the inherent translational aperiodicity of the tiling. Also, these tiling arrangements provide more possible orientations for the tiles  20 . In FIG. 16, for example, the diamond-shaped tiles  20 - 2 ( a ) and  20 - 2 ( b ) may appear in ten different orientations, and in FIGS. 17 a  and  17   b , the arcuate tiles  20 - 4  appear in twenty-four different orientations. 
     Accordingly, the foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.