The invention is a planar antenna which comprises a plurality of antenna elements positioned relative to each other in a predetermined orientation. Each of the elements is selectively electrically connectable to one or more of the other antenna elements. A plurality of switches electrically connect the plurality of antenna elements to each other. The switches are opened and closed by a signal operating at high speed enabling the antenna to act alternately in a first antenna configuration and a second antenna configuration using the same antenna structure. Doppler signals from the first and second antenna configurations are processed to determine the direction of a person or object in relation to a door.