Characteristic impedance of microstrip lines.

It is shown that it is feasible to force the complex average power, P of a microstrip line to be given by the usual circuit definition: P = l(o)V / 2 where l and V are the current and voltage of the equivalent transmission line and '*' denotes complex conjugation. If this requirement is made, then the three common definitions of characteristic impedance [namely, the voltage-current, power-voltage, and power-current definitions] all become equivalent. Then the remaining arbitrariness in microstrip characteristic impedance, Z(o), stems not from the choice of definition, as sometimes argued, but from the ability to choose one of the magnitudes /l/, /V/, or /Z(o)/ for convenience, and also the phase of either l or V (but not their relative phase). This clarification should make it easier to simplify equivalent transmission lines and equivalent circuits for drivers, loads, and discontinuities.