This letter provides a detailed explanation of a novel high-gain cross slot substrate integrated waveguide (SIW) rectangular cavity-backed antenna. The main SIW cavity is further divided into seven small linearly placed cavities at a distance of 0.9$${{\rm{\lambda }}}_{{\rm{g}}}$$. Each cavity contains a cross-slot at an equal distance from each other. The defective ground structure is placed at the bottom layer of an antenna with two metallic vias to divert the wave towards edge slots for further gain improvement. The antenna shows a measured gain of approximately 12 dBi from 11.37 to 11.78 GHz. The radiation efficiency of 92% and the front-to-back ratio of 28 dB is achieved. The proposed antenna provides a very high directive fan-like pattern which can be utilized for satellite and radar applications in the X-band.