A synthesis method for circular‐polarization radial‐line slot antennas excited by four posts inside the radial line is presented. This excitation generates a rotational mode inside the waveguide, and the slots that radiate the field can be placed arranging concentric rings. The reflected field inside the radial line can be controlled. We present the design and a prototype model for this antenna that shows practical results. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 70–72, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10675