This paper deals with a circular double‐ridged waveguide (CDRW) as a key element of traveling‐wave systems for microwave thermal processing. Approximate formulas for estimation of basic characteristics of CDRW partially filled with dielectric are derived with the use of the equivalent circuit technique. For accurate analysis, 3D FDTD models are developed and used for computation of coupling, electric field, and dissipated power in a coaxially excited five‐section CDRW applicator with three spherical loads. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1708–1713, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22539