Spatial modulation (SM) technology is one of promising methods that can achieve excellent energy efficiency for 5G/6G wireless communications by switching massive antenna elements but only employing small number of RF chains. In this article, we will apply this technology to another type wireless antenna: leaky coaxial cable (LCX). LCX is a line shaped antenna, and mainly employed for wireless coverage in some long-shallow environments such as tunnels, underground malls due to its uniform and stable radiation characteristics along the cable. Compared with conventional monopole antenna, LCX has large number of slots on cable for transmitting or receiving wireless signal. Therefore, by opening or closing (On/Off) these slots, the total number of channel patterns will be largely increased. This property can be used for the design of SM and for the improvement of capacity of wireless system. This paper will explain this idea and show the SM capacity using On/Off the slots of LCX over environments with and without reflected waves.