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Abstract: The magnetron injection gun is able to provide a beam of high perveance to be used in high power microwave tubes. But usually this kind of gun is regarded as of the crossed-field type, and has the characteristic high noise found in erossed-field devices. This limits its use.There have been experiments that negated such views. In 1962, a C-band low noise T. W. T. used a magnetron injection gun and obtained a noise figure of 3.1dB. Again in 1965, a S-band forward wave amplifier of medium power obtained a noise figureof 3.5dB. But these devices are all limited to low or medium powers, the cathode length is quite short, i.e. the ratio of cathode length to cathode diameter is quite small. using the magnetron injection gun in a magnetron of high power whether we can obtain small enough noise figure is still a problem needing further investigation. In this paper, a discussion of chief factors that influence the noise of magnetron injection guns in a high power magnetron and adoption of a method of changing the distribution of magnetic field around the cathode in order to reduce noise are made. Experimental results of guns adopting this method are presented.