The distance from center where the average field strength drops to 50% of its peak value. This boundary defines the edge of the Therapeutic Core zone. A shorter 50% falloff distance indicates a more tightly focused field. A longer distance indicates broader coverage.
The field is strongest right at the coil and weakens with distance in every direction. On the on-axis dipole model the report uses, it reaches half its peak strength at about three quarters of a coil radius from center and one tenth at about two radii. A larger coil holds the field up farther out.
