This note presents an analysis of a travelling-wave coupler for fast wave excitation only of a transverse field parameter beam amplifier. The analysis indicates that, as expected, ‘ a dip condition ’ for a transverse field tube can be achieved ensuring almost complete transfer of the fast wave beam noise to the circuit and of the circuit signal power to the fast beam wave. This requires -that the slow and fast beam waves be sufficiently separated in velocity (i.e. the cyclotron frequency be high enough), that the circuit wave be in close synchronism with the fast beam wave and that the coupling between the circuit and the beam be loose