Parameter variation with array size for broadband arrays of irregular polyomino subarrays

Abstract

IntroductionLarge phase scanned arrays have instantaneous bandwidth limited by beam displacement (squint) as frequency is varied, and that displacement can only be eliminated by introducing time delay at the subarray level. Unfortunately the use of rectangular subarrays creates a periodic grid with relatively large spacings between subarrays, and this causes the radiation of large quantization lobes. Our recent studies have shown that aperiodic grids made of polyomino subarrays are free of the quantization lobes of rectangular subarray grids and do not introduce any significant pattern distortion or additional loss. Figure 1 shows an array of 64 x 64 (4096) elements, using eight element polyomino-shaped subarrays. The basic L-octomino subarray shown in the figure is rotated and/or flipped to form eight different shapes, but needing only two basic power divider configurations. Throughout this discussion, the array has center frequency at f o , and all the data was computed at 1.3 f o , so the array is meant to operate over 60% bandwidth at u o =v o =0.5 (a 45 degree conical scan area). Figure 2 shows the projection of the 3-dimensional plots of the radiated patterns from an array of 64 x 64 (4096) elements using eight element rectangular subarrays as compared to those for the irregular subarrays. The use of irregular subarrays eliminated the well-defined quantization lobes and reduced the peak sidelobe level from approximately -11.4 to -26.3, a reduction of 14.9 dB.In our studies we have imposed several limitations that are necessary for the arrays to be practical. One is that we require the subarrays to have 2 n elements (ie 2,4,,8…) to avoid power divider loss. The second is that, for ease of fabrication, we use only one basic type of subarray per array, and then rotate that subarray through 360° by 90° increments. This last restriction gave rise to the concern that the duplication of shapes within any aperture would lead to some large residual sidelobes. Our published results [1] have shown that if the arrays are assembled with "randomly" rotated polyominos, then large peak sidelobes are not present. Still we have suspected that at some design sidelobe level this problem may still become an issue. 129 1-4244-0878-4/07/$20.00 ©2007 IEEEThe question remains: Is there some lower bound on the peak sidelobe level, and if so, what is it? The present study has been undertaken to investigate the behavior of arrays of irregularly shaped polyominos as a function of array size. In this way we expect to be able to make estimates of bandwidth, gain, peak, and average sidelobes for larger arrays. Array GeometryAlthough the presentation may include some data for an even larger array, the study described here entailed using a single array of L-shaped octominos. To investigate the size dependence issues, we looked at this single array of 64 x 64 elements, and cut off sections of the array to make five different arrays with progressively larger dimensions, as shown in Figure 3. These arrays were separately excited with -40 dB T...