On the validity limits of the transmission line theory in evaluating differential-mode signals along a two-wire line above a ground plane

In this paper, the possibility of extending the validity range of the transmission line theory in evaluating differential mode signals along two-wire lines above a ground plane is discussed through analysis and simulation. It is shown that, when the above-ground height of the two wires is larger than about twice their separating distance, the calculation of the characteristic impedance of the line can be performed disregarding the presence of the ground. The presented analysis suggests that the validity of the transmission line theory in the evaluation of differential-mode signals is conditioned upon an electrically short distance between the wires, regardless of the height above the ground plane.