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Coupling losses caused by impedance mismatches are often underestimated and neglected by power-line communications researchers and manufacturers. Further, the variations in network topology combined with unknown cable and load impedances obscure the requirements for impedance adaptation, causing researchers and manufacturers to default to a guessed impedance level and a fixed winding ratio. Attempting to clarify said requirements, this paper approaches the transmitter-to-receiver chain using a simplified lumped-parameter model, which distinguishes between the influences of series (cable) and parallel (load) power-line impedances. Using apparent power transfer to the receiver as criterion, the influence of both the transmitter and receiver coupler winding ratios are investigated for 50- modems at frequencies below 1 MHz. Live measurements indicate that the results are useful regardless of power-line topology, yielding gains up to 10 dB.