In this paper, a novel mode decoupling technique of a SIW dual mode square resonator is presented. It is then utilized to design a self diplexing radiator. A single corner cut square cavity with a triangular ring slot on top is used for dual mode excitation and mode decoupling. Both the electric and magnetic mode couplings are used simultaneously to improve the port isolation. Once the single square cavity is optimized for good port isolation, two extra TE110 mode cavities are further added to improve the isolation. Finally, 37 dB port isolation is achieved. The measured gains are 4.45 dBi and 4.20 dBi at 10.5 GHz and 11.7 GHz respectively. The proposed design can be used in a transceiver system without any circulator, switch or additional bandpass filter. © 2019 IEEE.