In this paper, effects of different shapes of periodic slots etched on the broad wall of a via-less substrate integrated waveguide leaky wave antennas are presented. Different shapes of the slots namely circular, elliptical, rectangular and triangular are considered. A comparative performance study based on gain, radiation efficiency and scan range of the antenna for the different shapes is studied over X-band frequency range. Several structures are fabricated. 50 ω microstrip lines are used to excite the antennas. It is observed that the rectangular shaped slot is the best choice if higher gain, radiation efficiency and wide scan range including the stopband is considered. Measured results match well with simulated results. © 2015 IEEE.