Since Phased arrays have discrete elements, equally spaced, they can experienced side and grating lobes. They are experienced when the element spacing is greater than a half wavelength (see details below). The spatial distortion effect causes some sidelobes to become substantially larger in amplitude, and approaching the level of the main lobe; these are called grating lobes.
Grating lobes are a special case of a sidelobe. Grating lobes are at a maximum when Î© = nÎ©/d. The main lobe is at sin Î˜ = 0. Grating lobes are undesirable for ultrasonic measurements and must be avoided. They appear for n=+/- 1,2 etc.
For d (element spacing) = 0,5 – sin Î˜ > 1, no real grating lobes, therefore, to prevent grating
d/Î© â‰¦ 1/ (1+ sinÎ˜max)
Since sin is maximized at an angle of zero, if d/Î© â‰¦ 0,5, no grating is present.
For example, for Aluminum and Steel: 5 MHz (1,2 mm Î©) implies d should be smaller than 0,6 mm for optimum results.