The relation between the particle dynamics of an intense relativistic electron beam and its accompanying microwave radiation has been explored. A diagnostic has been developed which makes use of the strong forward directed x‐ray radiation pattern of relativistic electrons colliding with a lowZthin target foil, to determine the transverse electron momentum of the beam as it moves along a magnetic field. The transverse momentum of the beam electrons is controlled both by a magnetic undulator, consisting of spaced iron rings similar to those used in previous cyclotron maser experiments, and by a carefully wound electromagnet consisting of alternating dipoles. The effect of relative undulator strength on the beam momentum distribution is determined, together with total microwave power as measured by a calorimeter. The results, which are interpreted with the aid of a numerical model, indicate a positive correlation between perpendicular momentum and radiated power. The spread in perpendicular velocity of the beam electrons was found to depend on the type of undulator used, the relative undulator strength and also influences the amount of radiated power.