Elevation beamforming performance of a 1.75D array

1.75D arrays allow limited beam steering in elevation, which is valuable for three-dimensional (3-D) imaging within a certain field of view. 3-D imaging has important advantage in detecting and visualizing small lesions from multiple viewing angles. Recently, Tetrad Inc has manufactured 1.75D arrays using the state-of-the-art technologies. The array we have consists of 10 rows in elevation and 128 elements in azimuth, 1280 elements in total. The central frequency of the array is 5MHz and the bandwidth is 60%. We have constructed 1280 parallel transmission circuitry, 1280 to 40 multiplexing and 40 parallel receiving circuitry. Upon each transmission, 1280-element RF signals were multiplexed to 40 parallel electronic channels, amplified, sampled and stored in the PC. The elevation performance of the array was evaluated through simulations and experiments using a point target. The parameters evaluated were -6dB, -20dB beamwidths and grating lobe strength. Compared with the parameters obtained from the array when it was focused at the array broadside, the -6dB and -20dB beamwidths were increased by 3% when the array was steered 5 degrees in elevation. The -6dB and -20dB beamwidths were increased by 7%, when the array was steered 10 degrees in elevation. The grating lobe strength was increased by 5% compared with strength when the array was focused at the broadside. We conclude that the 1.75D array can be used for 3-D imaging within a 20-degree narrow angle and with acceptable performance in elevation.