On loop coils, the magnetic field directly above the center is weaker than the field above the winding. On the 3D Field Volume chart this shows up as a dimple or crater at the top of the dome. The dip is normal for loop geometry. It doesn't mean the coil is defective. Patients positioned over the center of a loop receive less surface field than those over the winding. This matters when planning applicator positioning. The cause is geometric. A loop coil has no wire at the central axis, so the field there comes only from the surrounding ring. Pancake and donut discs don't show this dip because their windings cover the center directly.
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.
