A self-triplexing antenna employs a slot backed with a circular substrate integrated waveguide (SIW) cavity, fed by three distinct feedlines to produce three different resonant frequencies in C-Band (4 to 8 GHz). By optimizing the antenna parameters, port-to-port isolation greater than 25 dB is obtained. The antenna operates at frequency bands around 5.6, 6.64 and 6.95 GHz, with gains of 4.48, 3.7 and 4.3 dBi, respectively. Its compact planar geometry is attractive for mobile applications. © 2020 Horizon House. All rights reserved.