bandwidth of 1 Hertz at a frequency removed from the centre frequency by 10 MHz or more was down by 190 db [1]. This ratio, when applied to a 7.5 gigawatt SPS transmitter, would mean that only 7.5 microwatts would be radiated in a 10 KHz bandwidth. Magnetron Life The key to long life in the magnetron is the carburized thoriated tungsten filamentary type cathode. It is used in the microwave oven magnetron because it will provide good life even under the conditions of high abuse it receives in the microwave oven application. On a well filtered DC power supply, where the magnetron is allowed to keep the filament at the optimum temperature by internal feedback means, extremely long life can be predicted. There is a strong theoretical and experimental body of data that can be used to predict the life of carburized thoriated tungsten filaments, reported upon elsewhere [2], Life depends upon the reservoir of thorium in a carburized layer, and therefore the depth of that layer, and upon the outward rate of diffusion of thorium from the layer which depends upon the temperature. The SPS magnetron filament type cathode has been designed to use a 0.075 cm. diameter thoriated tungsten wire that is 40% carburized, and to operate at a temperature of I8600 Kelvin. The resulting life expectancy is 60 years - a truly phenomenal life that can be defended by both theory
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