In this paper, a planar substrate integrated waveguide (SIW) cavity backed slot antenna for wideband application is presented. The proposed antenna uses SIW cavity hybrid modes to realize the wideband response which are excited by long bow tie shaped slot antenna loading at the top plate of the cavity. To excite more than two hybrid modes in the proposed design, an additional SIW cavity is placed and excited through an aperture in the common sidewall between them. As a result, three hybrid modes are excited in the design which excite the slots to radiate with wideband response. The proposed antenna is fed by simple microstrip line and exhibits a bandwidth of 1.56 GHz (15.6%) covering X-band Radio navigation, Radio location applications while maintaining unidirectional radiation pattern and a gain above 4.1 dBi throughout the operating bandwidth of the antenna. The fabricated prototype of the antenna is implemented in a thin substrate (height ∼ 0.026λ0) which makes it attractive for conformal applications. © 2016 IEEE.