The Green's function of electromagnetic field in cholesteric liquid crystals with a pitch being large compared to the wavelength is considered. The Green's function is constructed using the solutions of uniform Maxwell equations. The case of the far zone is analysed in detail. The periodic system is distinguished from an anisotropic medium by a discontinuity of the wave vector surface and a break of beam vector surface. The forbidden zone corresponds to capture of beams with small angles of incidence forming a wave channel. Within this wave channel the Green's function asymptotics differs from 1 / r behaviour.