A numeric ratio comparing the peak field strength of a coil to the field strength at the center. A ratio of at least 1.15 combined with a peak sitting at least 20% off-center indicates a Donut Coil, meaning the ring is meaningfully stronger than the center. A ratio near 1.0 with a centered peak indicates a Pancake Coil, meaning the center dominates. The ring zone location varies with coil size and is determined by where the windings are concentrated rather than a fixed distance.
How the wire is wound sets where the field is strongest. Each profile is a real measured cross-section. A pancake coil is a tight flat spiral, so its field stays strong across a broad center. A donut coil leaves a hole in the middle, so its field peaks in a ring and dips at the center. A loop coil is one large turn, so its field peaks over the wire ring and eases off toward the middle and the rim. Copper dots are the wire seen end-on.
