Abstract: A transmission line carries power cables and twin wire communication lines. The coupling unit consists of an elongated, oil filled insulating body having metal caps closing off its opposite ends. The twin wires are brought to the low impedance terminals of an autotransformer which also has a pair of high impedance terminals and a center tap. Each of the high impedance terminals is coupled through a coupling capacitor to the high impedance terminals of a second autotransformer mounted within the second metal cap. The second autotransformer also has a center tap and also has a pair of low impedance terminals for connection to the communications equipment. The center tap of the first autotransformer is connected through a capacitor or a cathode drop arrester to the first metal cap. Overvoltage arresters are connected from the center tap to the low impedance and to the high impedance terminals of both autotransformers.