In this article, a novel design for the bandwidth improvement of a substrate integrated waveguide (SIW) cavity-backed slot antenna (CBSA) is presented. The introduced structure utilizes an inverted L-shaped slot and unbalanced metalized vias. The proposed antenna is of low height and holds a planar geometry while sustaining lower losses and lightweight. The inverted L-shaped slot at the bottom plate is responsible for radiation. The quality factor of the SIW cavity is significantly reduced by the inverted L-shaped slot which leads to attaining a wideband response. The proposed antenna resonates at two frequencies 9.4025 GHz and 10.0075 GHz in the first band from the range 9.27 GHz to 10.13 GHz with the fractional bandwidth of 8.915% and at 12.07 GHz in the second band from the range 11.96 GHz to 12.2 GHz with fractional bandwidth of 1.9%.