A system (10) for monitoring PLC impedance of a phase conductor 12.1 of a high voltage overhead power transmission line (OHTL) 12 comprises a hybrid 40 comprising a first port (40), a second port (40.2) and a third port (40.3). The hybrid is configured to divide power received at the first port between the second and third ports, at least partially to isolate the second port from power received at the third port and at least partially to isolate the third port from power received at the second port. The isolation is a function of an impedance connected to the first port 40.1. The first port is connected to the phase conductor. The second port is connected to be driven by an alternating signal source (14). The second and third ports are connected to a monitoring logging device (42) for monitoring signals at the second and third ports, thereby to monitor the isolation at second and third ports and hence impedance at the first port.