An antenna array having a central active element and a plurality of passive elements surrounding the active element is disclosed. According to the invention, the control device provides an economy mode in which the power of the source is lower in a prescribed time interval by comparison with a normal object detection mode, means being provided to ensure switching back into the normal object detection mode in the event of a predefined object situation. An envelope circuit may be electrically coupled to the detector circuit and configured to receive the pulsing AC voltage and current magnitude signals and to produce a DC voltage envelope signal and a DC current envelope signal.