The radiated emission from the power-line communications (PLC) is primarily caused by the common mode (CM) current converted from the differential mode (DM) current at the remote unbalanced elements (RUE) distributed on the power-line. The LCL measured at the outlet (outlet LCL) is often used as a measure of the unbalance despite the negative results. To resolve the issue, we have formulated the transmission line theory of the LCL and the CM current due to the mode conversion at an RUE on the power-line. We have shown that the outlet LCL underestimates the CM current at the RUE due to the CM propagation loss between the outlet and the RUE. We have also shown that the CM current measured at the modem output can be arbitrarily reduced by increasing the CM impedance of the modem without reducing the radiated emission. This is a fatal loophole of the CM current based regulations.
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