Accuracy of the equivalent circuit model using a fixed beam impedance for klystron gain cavities

Most small-signal calculations of the modulation in a klystron´s gain cavity use an equivalent circuit which includes a fixed beam impedance. Comparing this calculation to the gain calculated self-consistently, we note there are appreciable errors in both the calculated amplitude and phase of the cavity´s modulation. These errors may lead to large accumulated errors in determining either the tube´s small-signal or large-signal gain. Both techniques are used in a comparison with an existing S-band klystron