The unknown impedance which is to be measured is connected at the end of the transmission line as shown in Figure below. The transmission line is excited with a source of desired frequency . From the standing wave pattern, three quantities, namely the maximum voltage , minimum voltage , and the distance of the voltage minimum from the load is measured. The ratio of and gives the VSWR on the line.

We know that at point B on the transmission line where the voltage is minimum, the impedance is real and its value is . The impedance is nothing but the transformed value of the load impedance . We can therefore obtain the unknown impedance by transforming back from point B to point A. Let the distance of the voltage minimum from the load be . Since the transformation from B to A is away from the generator, the distance BA is negative. The unknown impedance therefore is