There is provided a method and apparatus for determining the input impedance (ZL) of an AC transmission line used for Power Line Communications (PLC), using a transmitted data packet having a preamble portion. The method comprises, at the analog-front-end (AFE) of the AC transmission line, determining the current through the AFE and the AC transmission line during transmission of the preamble portion of the data packet and determining the voltage during transmission of the preamble portion of the data packet. The method further comprises calculating the total impedance (Zt) from the determined current and voltage. The total impedance (Zt) includes the input impedance (ZL) of the AC transmission line and the AFE impedance (ZAFE). Finally, the method includes determining the input impedance (ZL) of the AC transmission line from the total impedance (Zt).