Spoof Surface Plasmon (SSP) modes have been of great interest in designing low loss and single-layer transmission lines and other microwave components. These slow-wave structures have great potential to form high efficient and single layer antennas; however, their conversion to fast propagating waves has not attracted much attention so far. This paper exploits the spoof surface plasmon modes to design and experimentally verify single-layer leaky wave antennas. It is showed that by introducing a gap between the cells of SSP structures, the dispersion curve moves from the slow-wave region to the fast-wave region. The broadband mode conversion is achieved by designing a short transition from high momentum to low momentum modes. The proposed leaky wave antenna presents the high efficient radiation over the entire bandwidth. This method for designing planar and single-layer leaky wave antennas holds promises for antenna integration in integrated microwave circuits.