A low-profile antenna-triplexer is realized using a substrate integrated waveguide (SIW) technology for radio altimeter/WLAN/ISM band applications. The antenna is composed of two SIW cavity resonators where a smaller cavity is nested inside the larger one. The larger cavity is excited by using two distinct microstrip feedlines, while the smaller one by a coax probe. The antenna produces three distinct resonances around 4.18, 5.2, and 5.8 GHz simultaneously employing one annular slot and two transverse slots. By exciting cavity modes (TE110/TE 120) and the patch mode (TM10) simultaneously, an average isolation of better than 23 dB is accomplished among three input ports. As compared to the conventional counterparts, the proposed geometry is simple to realize in a compact space. To validate the proposed idea, the design is experimentally tested, and the measured responses show a good agreement with the simulations. Moreover, the antenna shows a front-to-back ratio better than 19 dB and measured gain values of 6.56, 4.2, and 5.85 dBi at three resonances. The proposed design is compact, easy to fabricate, and capable to integrate with planar circuits. © 2011 IEEE.