The magnetic field strength measured at the exact center point of the accessory (0mm from center). For pancake coils, this is typically the strongest point. For donut coils, center field is weaker than the ring zone. Comparing center field to peak field gives the Ring/Center Ratio used to identify coil type.
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.
