Abstract: The CLIC (Compact LInear Collider) uses a two beam concept where a high current drive beam provides the energy needed for acceleration of the main beam. The CLIC drive beam accelerator starts with a high current injector using a sophisticated sub-harmonic bunching system. This lecture will focus on the design of the Sub Harmonic Bunchers (SHBs) the first RF components of the injector. SHBs have 499.75 MHz resonant frequency half of the following RF structures resonant frequency inside the injector. SHBs and their RF power sources should be wide-band enough to have fast phase switching capability in about 10 ns which is needed for the CLIC combination scheme. A backward traveling wave structure has been optimized for this task. It will be shown also how to avoid asymmetrical fields inside the coupler cells and how to compensate beam loading by changing the phase velocity.