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In the present work, the design of an S-band active array antenna with inherent frequency selectivity properties is described. The radiating element, based on a stacked-patch configuration, is characterized by an operational bandwidth of [2.8 -3.4] GHz. In-band frequency selectivity is performed by a microstrip two-pole band pass filter connected to the antenna feeding line. A spurious-rejection band is created within the Xband ([8.5-10.5] GHz) when scanning up to 60 degrees on the three main planes (E, H, D). This result has been achieved at the antenna level, through the insertion of slots on the patch surfaces, lowering the respective resonant frequencies associated to the third harmonic. Furthermore, filter resonators have been designed in order to obtain the desired X stop-band region.