A transmission line is terminated by an impedance that differs significantly from the characteristic impedance of the line. What impedance will be measured at the input of the line?

When a transmission line is terminated with an impedance that does not match its characteristic impedance, standing waves form along the line due to reflected energy. These standing waves cause the ratio of voltage to current—which defines impedance—to vary continuously along the length of the cable. Consequently, the impedance measured at the input end of the transmission line will not simply be the characteristic impedance, nor will it necessarily be zero or infinite. Instead, the exact impedance value depends on the electrical length of the line between the mismatch and the measurement point. This principle explains how transmission line stubs can be used as impedance matching devices. Because the input impedance changes with length when mismatched, choosing a specific length allows the operator to present a specific desired impedance to the transmitter.