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A compact rectangular TEio-TEao mode converter is developed for the lower hybrid current drive (LHCD) launcher on the Experimental Advanced Superconducting Tokamak (EAST) at 4.6 GHz. The converter with pe-riodic width perturbation aims to divide the microwave power into three sub-waveguides in the poloidal direction. We present the design and numerical calculation of the mode converter. Calculations are performed on the ripple wall converter by codes based on numerical solving the coupled-mode differential equations and on the simulation of the High Frequency Structure Simulator (HFSS) package. The resulting conversion efficiency from TEIO mode to TE3o mode exceeds 95% within the bandwidth from 4.56 GHz to 4.64 GHz, and the retu loss of the oversized transducer can be considerably decreased to 0.068% by means of a capacitive button embedded in the E-plane of the waveguide.