Measuring the impedance of a transmission line
I would use a time domain reflectometer(TDR) to measure the impedance of your transmission line. It will tell you the impedance of your transmission line as well as where your transmission has changes in impedance. TDR's works by applying short square waves with a set rise time and then will measure the reflections in your transmission line. Then the TDR will use these reflections to calculate the impedance and position of the impedance changes.