In order to produce a planar antenna array in a radar sensor with a multitude of microstrip feeder lines and a multitude of coupling slots for emitting microwave energy into open space, which achieves a favorable antenna efficiency when embodied in a planar design using LTCC technology, the invention proposes embodying the feeder lines and the coupling slots in a multilayer ceramic substrate produced by means of the LTCC thick layer technique with an upper and a lower grounded layer, as well as enclosing the feeder lines and the coupling slots with plated-through contacts from the upper grounded layer to the lower one.