A note on the complex Poynting vector and on the fractional current on the upper surface of a microstrip line

It is shown how the two forms and should be used when extracting reactance information from the complex Poynting vector in a microstrip problem. When the strip is narrow it is necessary to distinguish between the actual and the effective dielectric constants of the substrate in the formulation of the fields, and this leads to a determination of an axial magnetic field component with a certain fraction of the axial current diverted to the top of the strip conductor.