00740095IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 1, JANUARY 1999 115 Letters Comments on “Revisiting Characteristic Impedance and Its Denition of Microstrip Line with a Self-Calibrated 3-D MoM Scheme James C. Rautio Charactersitic impedance (Z0 ) is well understood for lossless homogeneous transmission lines. These include, for example, coaxial cable, stripline, and rectangular waveguide. (Note that while rectangular waveguide is dispersive and non-TEM, impedance is uniquely dened because it is homogeneous.) In these cases, for a given mode, propagating in a given direction, at a given frequency, the ratio of transverse E to transverse H is constant everywhere. The value of this ratio is the characteristic impedance of the line. An equivalentcircuit theory voltage and ……