A novel design technique of planar triple band self-triplexing Substrate Integrated Waveguide (SIW) cavity backed slot antenna is presented. The proposed antenna uses three separate feeding ports to excite three SIW cavity modes into the proposed design. A dumbbell shaped slot antenna placed at top metallic plate is excited by these modes to radiate at corresponding resonant frequencies in C (4-8 GHz), X (8-12 GHz) and Ku (12-18 GHz) band. The behaviour of the proposed SIW cavity modes are analyzed with the help of half mode theory to predict the resonant frequencies of the proposed antenna. The fabricated prototype of the antenna resonates at 6.76 GHz, 8.62 GHz and 13.33 GHz and exhibits unidirectional radiation pattern with a gain of 3.78 dBi, 4.46 dBi, 5.6 dBi and front-to-back ratio (FTBR) of 14 dB at three resonant frequencies respectively. The proposed antenna maintains a high isolation of better than 15 dB between its three excitation ports while retaining its compact configuration which makes it attractive for high frequency on chip antenna applications. © 2021 EurAAP.