[en] The calculation and measurement results of magnetic field formation in the 103 cm sector-focusing cyclotron for acceleration of (H₁⁺ and D₂⁺) hydrogen and (He₃⁺² and He₄⁺²) helium ions in the energy range from 5 up to 20 Z²/A MeV are given. The cyclotron magnetic field structure is of a three-sector type. Isochromous dependencies of an average magnetic field, depending on a radius for any acceleration regime have been formed by four concentric windings. Two groups of harmonic windings for the correction of magnetic field azimuth heterogeneity are stipulated. The curves of electromagnet magnetization on the average field at final acceleration radius and dependencies of the average magnetic field on the radius, as well as the isochronous curves for charged particle beam acceleration of various energies are presented. The specifications of currents in the concentric windings are obtained. The orbital parameter calculations have proved that betatron oscillations during the acceleration are stable and the amplitude of coherent oscillations, excited in the center on the first harmonic, does not exceed the amplitude of incoherent oscillations. The calculation results are in a good agreement with the magnetic measurement data