A novel design of an S-band active array antenna with enhanced frequency selectivity properties, for radar applications, is presented. The array unit cell consists of a stacked-patch radiator, characterized by an operational bandwidth of [2.8 - 3.4] GHz. A microstrip two-pole band pass filter is connected to the antenna feeding line, improving in-band frequency selectivity. Furthermore, a stop-band region is created in correspondence of the third harmonic (X-band [8.5-10.5] GHz). This is achieved through the insertion of slots on the patch surfaces, lowering the respective resonant frequencies associated to the third harmonic. Furthermore, filter stub resonators have been properly designed in order to create stop-band region within the X-band. Good in-band matching and out-of-band suppression performances are achieved when scanning up to 60° for all the main planes (E, D, H). The Institute of Electrical and Electronics Engineers (IEEE) - Antennas and Propagation Society (APS)