The invention concerns a system for producing specific field distributions in a defined volume or region. The system comprises an array of electromagnetic field generating elements each with an integrated sensor for measuring the phase and amplitude of the current flowing in a metallic element (antenna or coil) or field in a slot based element, a measurement device to enable measurement of both phase and amplitude of the electrical signals from the sensors with sufficient dynamic range for quantification of the signal, a multi channel radio frequency power source with individually controllable amplitude and phase to excite each of the electromagnetic field generating elements, and a feedback controller enabling controlled adjustment of the amplitude and phase of the radio frequency power source based on the signals from the sensors. The system controls the outputs of a multi channel radio frequency power source such that the current in a metallic or the field in a slot based electromagnetic field generating elements provide the desired electromagnetic field values and the superposition of the fields produced by each electromagnetic field generating element produces said specific electromagnetic field distribution in said defined volume or region. As the signal picked up by each sensor is directly related to the phase and amplitude of the current or field in the associated electromagnetic field generating element and hence the electromagnetic field generated by the array elements, where the current or field measured is the sum of both the applied (from the radio frequency power source) and secondary excitations from mutual coupling and reflections hence the measured value represents the ideal excitation in the absence of mutual coupling, reflections and mismatch. The feedback controller modifies the direct excitation such that the total excitation is the ideal array excitation without coupling or mismatch. The invention implicitly corrects for the coupling and misma
