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Fourier Transforms in Radar and Signal Processing_9

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Tham khảo tài liệu 'fourier transforms in radar and signal processing_9', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Array Beamforming 185 separation d wavelengths by a pseudorandom step chosen within an interval of width d 0.5 which ensures that the elements are at least half a wavelength apart. Figure 7.10 a shows the response in u space for an array of 21 elements at an average spacing of 2 3. A sector beam of width 40 degrees centered at broadside was specified. A regular array would have a pattern repetitive at an interval of 1.5 in u and this is shown by the dotted response. The irregular array repetitions are seen to degrade rapidly but the pattern that matters is that lying in the interval 1 1 in u. This part of the response leads to the actual pattern in real space shown in Figure 7.10 b . We note that the side lobes are up to about 13 dB rather poorer than for the patterns from regular arrays shown in Figures 7.6 7.8 and 7.9 though this level varies considerably with the actual set of element positions chosen. The integration interval I was chosen to be 1 1 to give the least squared error solution over the full angle range from 90 degrees to 90 degrees and its reflection about the line of the array . A second example is given in Figure 7.11 for an array of 51 elements but illustrating the effect of steering. In Figure 7.11 a b the 40-degree beam is steered to 10 degrees and again we see the rapid deterioration of the approximate repetitions in u space of the beam and a nonsymmetric side-lobe pattern though the levels are roughly comparable with those of the first array. The average separation is 0.625 wavelengths giving a repetition interval of 1.6 in u. If we steer the beam to 30 degrees Figure 7.10 c d there is a marked deterioration in the beam quality. This is because one of the repetitions falls within the interval I over which the pattern error is minimized so the part of this beam near u 1 that should be zero is reduced. At the same time the corresponding part of the wanted beam near u 1 2 should be unity so the solution tries to hold this level up. We note that