Convex Mount For Element Reduction In Phased Arrays With Restricted Scan

Abstract

Grating lobe free scanning in a phased array with sparse element spacing is obtained by restricting the maximum scan angle for elements in the array, and arranging the elements in a convex form. One convex form is a paraboloid, which may be continuous, or piecewise in nature, tiled with flat segments.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first system diagram and

FIG. 2 shows a second system diagram.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In phased-array systems, the commonly stated requirement for λ/2 spacing between elements (where λ is the operating wavelength) arises from the desire to minimize sidelobes when scanning at angles up to λ/2 radians, or 90° from the scan center, which is a line normal to the plane of the array. Sparse arrays, where the element spacing is greater than λ/2 create grating sidelobes for large scan angles. While post-processing approaches to reduce the ghosting introduced by these sidelobes exist the better ones are computationally expensive and scene dependent, making them impractical in dynamic environments such as security scanning.

In prototypical phased array applications such as the Distant Early Warning (DEW) radar system, or AEGIS AN/SPY-1 phased array radars, wide scan angles, up to 2 π steraians, are required. However, in many applications, a smaller solid angle scan field is sufficient. As an example, in security screening of individuals or objects, the scan solid angle is limited by body size or object size, and is far less than 2π steradians. Similarly, a systems designer may wish to have N phased arrays opening in parallel in order to increase throughput by a factor of N, i.e. looking at N bodies or targets in a given volume at the same time. In such a case the solid scan angle required of any given array in the system is roughly divided by N.

A top view of an embodiment of the present invention is shown in FIG. 1. Array tiles 110 form phased array 100. Tiles 110 are arranged to approximate a paraboloid 120. For each tile 110 the scan center line, shown as 120, is defined as the line normal to the plane of the tile and intersecting the tile at its center. The maximum scan angle θ_max 210 when extended as line 220 generates scan zone boundary 310 with the center of the scan zone 300 being the parabolic focus. According to the present invention the maximum scan angle θ_max is considerably less than π/2 radians, or 90° from the scan center of each tile.

Each tile 110 is comprised of a plurality of elements, commonly packaged together with their control system. In a dense array, these elements are optimally spaced at λ/2, commonly in a rectangular or hexagonal packing. According to the present invention, since the maximum scan angle θ_max 210 is now restricted, element packing may be less dense while still insuring grating lobe free scanning

For a continuous-phase phased array, the maximum element period p (spacing) free of grating lobes is p=λ(1+sin(θ_max))². It can be seen that this relationship encompasses the common limiting cases. For θ_max=π/2, p=λ/2, and for p=λ, θ_max=0. For a 2D array, the element density is reduced by a factor of 4/(1+sin(θ_max))².

The parabolic form shown in FIG. 1 represents one embodiment. The arrangement of tiles 110 must be convex, and may be piecewise-planar, consisting of flat tile segments approximating a parabola 120, as shown in FIG. 1, or other convex form as shown in FIG. 2. Examples of other useful convex forms are a circle and an ellipse. The curved form 120 may be designed to approximate any of the classic conic sections with the exception of a hyperbola; the choice of conic section for form 120 depends on how the array is fed.

In FIG. 2, a set of coplanar tiles 110 and 130 are surrounded by tiles 140 and 150 which are angled in, forming a convex surface which is symmetrical around its center point in his case, tile 115. An alternative embodiment would be a true non-segmented paraboloid or ellipsoid, with the entire array of elements formed onto a curved surface.

In an embodiment used for scanning people, the volume to be scanned may be thought of as cylindrical in nature, and antenna array 100 need form a convex shape such as a parabola 120 in two dimensions. In a system where the target volume is spherical in nature, antenna array 100 should form a convex shape in three dimensions. This shape can be a sphere, a cylinder, an ellipsoid, a paraboloid, or a piecewise-planar approximation of any of these.

The principles of the present invention pertain equally to not only continuous-phase transmit or receive arrays, but also to other modalities such as reflectarrays, transmission (lens) arrays, binary-phase arrays, and so on. As an example, in a reflectarray geometry, the convex shape is chosen to focus the feedhorn to the sweet spot of the pattern i.e. the feedhorn and the scan center are conjugate foci. An ellipsoid is the preferred shape in this case.

While the embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.

Claims

1. A phased array antenna operating at a wavelength λ comprising: a plurality of antenna elements arranged into an array, where the antenna elements are arranged to be convex in at least one direction, and spaced greater than λ/2 in the convex direction.

2. The phased array antenna of claim 1 where the array operates with a maximum scan angle of less than π/2 radians in the convex direction.

3. The phased array antenna of claim 2 where the antenna elements are arranged to be piecewise-convex in at least one direction.

4. The phased array antenna of claim 1 where the convexity approaches a parabola.

5. The phased array antenna of claim 1 where the convexity approaches an ellipse.

6. The phased array antenna of claim 1 where the convexity approaches a circle.

7. The phased array antenna of claim 1 where the antenna elements are arranged to be convex in three dimensions.

8. The phased array antenna of claim 1 where the array is an active array.

9. The phased army antenna of claim 1 where the array is a passive array.

10. The phased array antenna of claim 1 where the array is a transmissive array.

11. The phased army antenna of claim 1 where the array is a reflector array.

12. The phased array antenna of claim 1 where the array is a passive programmable reflector array.