Low-noise microwave oven magnetrons with fast start-oscillation by azimuthally varying axial magnetic fields

A technique has been proven to significantly reduce the noise and to hasten the startup in microwave signals from dc operating oven magnetrons. The technique is based on imposition of an azimuthally varying axial magnetic field in magnetrons. For an N-cavity magnetron operating in pi-mode, the optimal number of periods of azimuthal variations in the axial magnetic field is N/2. This technique demonstrates the fastest startup and lowest start-oscillation current by prebunching the electrons. Measurements show dramatic reduction in noise of 600-800 W oven magnetrons operating dc near 2.45 GHz. The sidebands are eliminated (-30 dB) and the noise within 50 MHz from the carrier is reduced by 30 dB. Noise reduction is observed at low (up to 60 mA), medium (60-200 mA) and high currents (200-300 mA), for both aged and fresh oven magnetrons. The reduction in power and in electronic efficiency is of order of 10%, in this low-noise, fast startup operation.