Which statement describes a beam that is symmetric about the beam axis?

Which statement describes a beam that is symmetric about the beam axis?

• A. Annular array transducer
• B. 1D linear array
• C. Phased array
• D. Curved array

Answer: (A)

A beam that is symmetric about its axis comes from an aperture that is rotationally symmetric. An annular array uses concentric circular rings, so the aperture is circular in all directions around the axis. When each ring is excited with similar amplitude and proper phase, the emitted field looks the same no matter which azimuth you look from, giving a beam that is symmetric around its central axis. In contrast, a linear array has a rectangular aperture, which makes the beam elongated in one plane and not the same in all directions. A curved array changes the shape of the aperture but doesn’t guarantee circular symmetry of the beam, and a phased array can be arranged in many forms and steered, which also doesn’t ensure axis symmetry by default. Hence, the annular array best describes a beam symmetric about the beam axis.

A beam that is symmetric about its axis comes from an aperture that is rotationally symmetric. An annular array uses concentric circular rings, so the aperture is circular in all directions around the axis. When each ring is excited with similar amplitude and proper phase, the emitted field looks the same no matter which azimuth you look from, giving a beam that is symmetric around its central axis.

In contrast, a linear array has a rectangular aperture, which makes the beam elongated in one plane and not the same in all directions. A curved array changes the shape of the aperture but doesn’t guarantee circular symmetry of the beam, and a phased array can be arranged in many forms and steered, which also doesn’t ensure axis symmetry by default. Hence, the annular array best describes a beam symmetric about the beam axis.