A flat coil wound in a ring shape with an open hole through the center. It's fully encased within its housing, so the field is measured along the surface face only. The enclosed design means the field is delivered through the top or bottom face rather than through the center hole. This produces a broader, more even field distribution with stronger output near the outer ring of windings. A donut coil is identified by a peak that sits off-center on a ring (at least 20% of the way out along the scan) and a Ring/Center Ratio of at least 1.15. It carries fewer total windings than a solid pancake of the same diameter, so its peak Gauss at the face is lower and it draws more drive current to reach a given depth. That is the tradeoff for its wider, more even coverage, which suits broad target areas rather than a single focal point.
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.
