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In this paper, the design of an impedance-adapting bidirectional transformer coupling circuit for low-voltage power-line communications is described. It is shown that transmission through the transformer is governed by series resonance of the leakage inductance and coupling capacitance. At low frequencies though, the magnetizing inductance plays an important role in filtering the power waveform. The coupling transformer, if properly designed, can equalize terminating impedances on either side for maximum power transfer. Furthermore, these equalized terminating impedances facilitate symmetrical bidirectional band-pass transmission.