______ steering is not possible with annular arrays.

______ steering is not possible with annular arrays.

• A. Mechanical
• B. Electronic
• C. Acoustic
• D. Optical

Answer: (B)

The key idea is that steering a beam electronically relies on creating a lateral phase gradient across a two-dimensional aperture. An annular array is a set of concentric rings, which is rotationally symmetric about the central axis. You can adjust delays across the rings to focus the beam at different depths, but that symmetry prevents you from tilting the beam off the axis with electronic delays alone. In other words, you can electronically focus, but you cannot electronically steer the beam to a off-axis direction using an annular array. You can still scan the area by mechanically moving the probe, but not by electronic steering.

The key idea is that steering a beam electronically relies on creating a lateral phase gradient across a two-dimensional aperture. An annular array is a set of concentric rings, which is rotationally symmetric about the central axis. You can adjust delays across the rings to focus the beam at different depths, but that symmetry prevents you from tilting the beam off the axis with electronic delays alone. In other words, you can electronically focus, but you cannot electronically steer the beam to a off-axis direction using an annular array. You can still scan the area by mechanically moving the probe, but not by electronic steering.