A novel composite right/left-handed (CRLH) transmission line cell (TLC) based on a substrate integrated waveguide is proposed, which can be used for a leaky wave antenna for achieving large beam scanning-range/bandwidth ratio. Compared with previous CRLH TLCs, the proposed one has a narrower fast wave region where the propagation constant beta changes from -k(0) to +k(0). An X-band leaky wave antenna (LWA) has been designed and fabricated and measurements are presented. Both simulated and experimental results show that the developed LWA has a narrow fractional bandwidth of 26% (from 8.3 to 10.8 GHz) while having a continuous wide beam scanning range of -65 degrees to +54 degrees.