A novel design technique to implement dual band substrate integrated waveguide (SIW) cavity backed planar slot antenna is presented. The proposed antenna uses a combination of multiple SIW cavities of different size connected through coupling window in the common sidewall of the cavities. The cavities are loaded with long bow tie shaped slots to achieve higher bandwidth of the antenna. The placement of long bow tie slots helps to generate multiple hybrid modes which can be tuned by varying slot dimensions. First, two SIW cavities loaded with bow-tie slots are used to excite four closely placed hybrid modes in X band (8-12 GHz) to realise dual band response with a bandwidth of 600 MHz (7.2%) and 888 MHz (8.74%) and a gain of 5.3 and 4.4 dBi, respectively. The design is further extended using a set of four SIW cavities arranged as two pair of coupled SIW cavities along with bow-tie slots and placed in orthogonal orientation to implement dual band dual polarised slot antenna while maintaining its planar configuration. The proposed antenna exhibits a bandwidth of 390 MHz (3.9%) and 790 MHz (7.29%) with a gain of 6.1 and 5.4 dBi, respectively. © The Institution of Engineering and Technology 2016.