A high-frequency equivalent circuit for a gap source on a microstrip line

An equivalent circuit is derived for a voltage gap source on an infinite microstrip line. The circuit model is valid for all frequencies, including high frequencies where simple transmission-line theory is not accurate. The circuit model accounts for the fact that different definitions of characteristic impedance diverge at high frequency, as well as the fact that the continuous-spectrum current excited by the gap source becomes appreciable at high frequency. The continuous-spectrum current accounts for leaky modes that may get excited at high frequency, as well as the fact that radiation effects become important at high frequency.