Theory is presented concerning the attenuation of the original beam waves, the fast, the slow, and two synchronous waves, and the generation of noise waves if part of the beam is intercepted.To obtain compact equations matrix notation is used throughout. The matrix notation offers the additional advantage that both the behavior of the waves and their mutual correlation can be studied simultaneously. This renders the theory, which accounts both for pure interception and for partial scattering of the beam, much more general in nature. The electron beam diameter is finite.In addition to attenuation of the original and generation of noise waves it is found that two new synchronous waves related to the original fast and slow waves are generated.Experiments at 550 Mc with an Adler‐Wade tube containing a movable vane show good agreement with the theory presented, provided the forward scattering is taken into account. The experiment indicates that the theory is probably correct at least insofar as the fast wave, the slow wave, and the correlated components of the synchronous waves are concerned.