Transmission characteristics of a new type of radial folded waveguide structure, consisting of a bifurcating metal disc with finite thickness between lower and upper radial waveguides and placed unsymmetrically in the folded portion as a general case, are investigated theoretically using a mode matching technique. A novel technique is proposed in which step junctions included in the bifurcating metal disc between lower and upper radial waveguides play an essential role in minimizing reflections caused by finite thickness of the bifurcating metal disc as well as the unsymmetry at the folded region and hence, a complete power transmission is achieved. Based on numerical computations some practical structure which is applied as a feeder for a planar radial line radiating structure of a plasma generating device is designed and perfect matching performance is demonstrated. ©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 415–423, 1999