[en] We describe a technique to routinely measure the scattering matrix of a radio frequency (rf) current drive antenna in a magnetic fusion experiment during high power operation. This technique can be used under any conditions of phase or amplitude of excitation. It involves modulation of the phase of the rf voltage applied to each of the antenna ports by a few degrees, measurement of the complex forward and reflected voltages, detection of the phase modulation, and solution of a 2x2 matrix problem to yield the scattering matrix. Further calculation then yields the antenna close-quote s impedance matrix. Because it involves only a small modulation, this method can be used to routinely monitor matching, decoupling, or plasma loading and to provide input to tuning calculations so that the matching and phasing conditions can be maintained. We show results for the case of 400 kW Alfvgrave en wave current drive experiments on the Phaedrus-T tokamak. We also show how the results can be used to tune the antenna. copyright 1997 American Institute of Physics