Effective Field Diameter

The total width of the area covered by meaningful magnetic field output. Tissue outside this diameter receives very weak field (less than 10% of peak). This tells you how large the target zone is. It's calculated as twice the distance from center where the field drops to 10% of its peak strength. Coil diameter and geometry primarily determine this value. Drive voltage doesn't significantly change it. A coil that isn't round (an oval or a rectangular winding) gets two figures, a major and a minor: the span along its longer axis and the span along its shorter one, each measured on that axis's own scan arms. The two differ by design and describe the shape of the coil's reach.