The invention relates to a mobile field coil unit for a medical system for detecting the position of a patient, medical equipment and/or prostheses in an operating area, wherein the field coil unit is designed to emit or receive an alternating electromagnetic field. The field coil unit can be arranged movably on the patient or in an area next to the patient for operation as intended, wherein the field coil unit has an outer contour on at least one placement side, which outer contour is designed to surround an object or a part of a patient laterally in order to prevent lateral sliding between the field coil unit and the object or part of the patient. Preferably, the field coil unit is a field generator.
