A novel technique to increase the pattern bandwidth of substrate-integrated waveguide leaky-wave antennas (SIW LWAs) is proposed. By coupling several SIWs, it is shown that the gain at the desired angle can be kept stable across a wide-frequency band. A systematic design methodology based on a simple transverse equivalent network is presented. Practical coupled-SIW designs with gain exceeding 10 dB and pointing at 30° in the 15-GHz band are reported to validate the theory. Simulation and experimental results demonstrate an enhancement of the half-power gain bandwidth from 2.5% (380 MHz) in the single-SIW design to 9% (1360 MHz) for an LWA composed of three coupled SIWs.