An H-shaped slotted leaky-wave antenna (LWA) is proposed based on balanced composite right/left-handed (CRLH) transmission lines (TLs) to improve the backward scanning bandwidth and achieve constant gain. The proposed antenna consists of a substrate-integrated waveguide (SIW) unit cell array configured by an H-shaped slot cut in the top layer of the SIW and a patch embedded beneath the H-shaped slot. The proposed design improves the backward scanning bandwidth compared to planar SIW LWAs. The measured beam radiation is in the range from −79° to 80° over the frequency range of 7.6–12.5 GHz. Moreover, a constant gain of greater than 10 dB is achieved over the entire X-band frequency range.