A system for producing specific field distributions in a defined volume or region 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 or the 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 superposition of the fields produced by each electromagnetic field generating element produces a desired electromagnetic field distribution in the defined volume or region.