A leaky-wave antenna (LWA) on a substrate integrated waveguide (SIW) with continuous beam-scanning capabilities and improved gain is presented. A one-dimensional (1-D) longitudinal slot-array SIW antenna is used as the main radiating element. It was found that due to the capacitive effect of the slot, an open-stopband (OSB) restrains broadside radiation. Although a reduction of the bandgap can be achieved when the slots are closer to the center, the radiation performance of the LWA degrades around the broadside as the beam scans from backward to forward. Initially, the capacitive effect was mitigated, and hence, the OSB was suppressed by introducing a group of three shorting vias at the opposite side of each longitudinal slot. However, the gain of the antenna drops significantly around its upper scan angles in the forward directions. To improve the radiation performance further, the center via of each of the via group is replaced by a transverse slot. The new unit cell is then used to design a modified 1-D slot-array LWA, and the antenna is analyzed, prototyped, and measured to verify the concept.