What is the effect of using a smaller pitch in an array?

• A. Increases grating lobes and reduces elevational resolution
• B. No effect on grating lobes or elevational resolution
• C. Reduces grating lobes and improves elevational resolution
• D. Only affects axial resolution

Answer: (C)

Pitch is the spacing between the individual elements in the array. Using a smaller pitch means the elements are closer together, which reduces the chances of grating lobes—the unwanted secondary beams that arise from periodic sampling of the wavefront when spacing is large relative to the wavelength. As the spacing gets smaller, these grating lobes are suppressed or pushed out of the region of interest. At the same time, the elevation beam becomes more focused because the aperture sampling in the elevational direction is denser, leading to a narrower elevation beam and thus better elevational (slice-thickness) resolution. Axial resolution, on the other hand, depends mainly on pulse length and is not directly affected by pitch. So, a smaller pitch reduces grating lobes and improves elevational resolution.