In this paper the definition of characteristic impedance for lossless microstrip and coplanar lines has been considered. It has been shown that due to a significant value of the displacement current related to longitudinal component of the electric field, impedance definition becomes ambiguous. Such ambiguity can cause considerable errors in design procedure. This effect is especially noticeable in the coplanar lines, in opposite to microstrip ones. To confirm the validity of the applied algorithm (spectral domain approach) the propagation coefficients and characteristic impedances have been compared to values obtained from commercial software.
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