Air core reactor includes a coil connected between first and second terminals. The coil is made of a succession of bundles of conductor (B1, B2, B3, . . . , BN) connected in series along an axis between the first terminal and the second terminal. Each bundle is made of one wire wound around the axis to form a multi-layer winding having a cross-section of N winding layers in a direction perpendicular to the axis, from a winding layer of rank 1 which is the closest to the axis to a winding layer of rank N which is the furthest from the axis. Each perpendicular winding layer includes several winding layers in the direction of the axis. The number of axial winding of the perpendicular winding layer of rank j (j=2, . . . , N) is equal or less than the number of axial winding layers of the perpendicular winding layer of rank j−1.