Why are annular phased arrays described as multi-focus systems?

• A. They can create multiple focal zones along the scan line
• B. They have a single focal depth
• C. They require electronic steering
• D. They have no focus

Answer: (A)

Annular phased arrays are described as multi-focus systems because their concentric ring elements act like multiple separate apertures that can be weighted and timed to generate several focal regions along the beam path. Each ring contributes to the overall wavefront, and by adjusting the delays and amplitudes across the rings, you create constructive interference at specific depths. With several rings available, you can design patterns that produce a shallow focus from the inner rings and a deeper focus from the outer rings, or switch between different patterns to yield multiple focal zones along the scan line. This ability to form more than one focus at different depths is what makes annular phased arrays inherently multi-focus. Steering can be used as well, but the defining feature is the capacity to create multiple focal zones due to the multiple rings.