What is the region in which the beam pattern becomes stable and far-field behavior dominates called?

What is the region in which the beam pattern becomes stable and far-field behavior dominates called?

• A. Near field
• B. Far field
• C. Fraunhofer zone
• D. Shadow zone

Answer: (C)

In this part of wave propagation, the pattern you see settles into a stable shape only when you’re far enough away from the source—the region where the Fraunhofer zone applies. In the near field, the emitted waves interfere in complex ways and the pattern changes with distance, so you can’t treat the beam as a fixed angular distribution. Once you reach the far field, the wavefronts across the transducer’s aperture become essentially planar, the angular distribution becomes fixed, and the pattern is governed by the aperture shape (the beam pattern is the Fourier transform of the aperture). The field then falls roughly as 1/r, and the main lobes and nulls don’t change with distance. The boundary distance is often estimated as r > 2D^2/λ, where D is the aperture diameter and λ is the wavelength. This is why the region is called the Fraunhofer zone.

In this part of wave propagation, the pattern you see settles into a stable shape only when you’re far enough away from the source—the region where the Fraunhofer zone applies. In the near field, the emitted waves interfere in complex ways and the pattern changes with distance, so you can’t treat the beam as a fixed angular distribution. Once you reach the far field, the wavefronts across the transducer’s aperture become essentially planar, the angular distribution becomes fixed, and the pattern is governed by the aperture shape (the beam pattern is the Fourier transform of the aperture). The field then falls roughly as 1/r, and the main lobes and nulls don’t change with distance. The boundary distance is often estimated as r > 2D^2/λ, where D is the aperture diameter and λ is the wavelength. This is why the region is called the Fraunhofer zone.