A compact half-mode substrate integrated waveguide (HMSIW) cavity-backed self-diplexing antenna is demonstrated for 5.2/5.8 GHz applications. HMSIW cavity uses one open-end rectangular slot inserted on the top cladding which divides the cavity into two unequal apertures. Each aperture produces two different resonant frequencies around 5.2 and 5.8 GHz when fed by two discrete microstrip feeds. By appropriately optimizing the antenna dimensions, an isolation of better than 25 dB is realized which aids to introduce the self-diplexing property. The proposed design is realized on a single-layered dielectric substrate with the help of low-cost printed-circuit-board process and experimental study is performed, which shows unidirectional and similar radiations at the both resonances with the respective gain values of 3.23 and 4.38 dBi. © 2019 European Association on Antennas and Propagation.