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.
Seen from above, a coil lays its field down as a footprint that is strongest over the center and fades with distance. The dashed ring is the usable edge, the distance where the field has fallen to a tenth of its peak. The effective radius reaches from the center to that edge, the effective diameter is twice it, and the shaded disc inside is the effective field area, the surface the coil usefully covers.
