An antenna (200) of an RFID reader based on the magnetic field, especially intended for mobile stations. The main coil (220) of the antenna is inductively coupled to the feeding source, whereby the main coil becomes galvanically isolated from the source. For this purpose, the antenna structure includes an auxiliary coil (230) and a feed element (240) in addition to the main coil. The auxiliary coil is galvanically connected to the main coil, and there is a relatively strong inductive coupling (M) between the feed element and the auxiliary coil. The feed element is coupled directly to the AC source in the reader, in which case an alternating voltage is induced in the auxiliary coil and an alternating current is generated in it and the main coil. The connection to the RFID tag in the object is provided with the magnetic field corresponding to that current. The reliability of the antenna improves in comparison to the known antennas, because the mechanical junctions that are susceptible to the deterioration of the contact are omitted.