Abstract: A method and apparatus are described for coupling a high-speed data communication modem to a communication line such as a coaxial cable with minimal transmission losses when the modem is not transmitting data. The transmitter coupling apparatus comprises a transformer, first and second transistors, biasing circuits for each of the transistors and a low-pass filter. The data signal to be transmitted is applied to the first transistor which is biased for linear amplification when data are being transmitted and for saturation when data are not being transmitted. The output of the first transistor is applied to the primary winding of the transformer where it is coupled to the secondary winding which is connected in series with the central conductors of the coaxial transmission line. When saturated, the first transistor grounds the winding terminal to which it is connected.