The Bloch‐wave characteristic impedance as well as the Bloch‐wave propagation constant of a rectangular waveguide periodically loaded with a series of thin, circular, centrally located, metal posts along the direction of propagation has been investigated both theoretically and experimentally. The theoretical analysis has been made to explore the frequency dependence of the real and imaginary parts of the Bloch‐wave characteristic impedance. As a part of the analysis, the study of the &ohgr;‐&bgr; diagram has also been exercised. The experimentally obtained data show agreement with the theoretical results.