Pulsed magnetron oscillators sometimes show an instability known as ``twinning'' in which the spectrum shows two, or more, closely spaced peaks. This phenomenon is undesirable and can lead to problems when it occurs in magnetrons used in radars. An experimental investigation of twinning is described from which conclusions are drawn about the design parameters that affect it. It has been shown that changes in factors, such as the magnetic field strength, the external match, the cathode coating, and the cathode temperature, can be used to reduce the probability of twinning or to move the point at which it occurs outside the normal operating range of the tube. Possible theoretical explanations are discussed.}
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