A method for calculating the noise figure of the low-noise traveling wave tube is described.The low noise electron gun of the traveling wave tube is equivalent to a cascaded space-charge-wave impedance transformer

for which the formula for impedance transformation is relatively simple.The noise figure of the traveling wave tube can be obtained by calculating the equivalent noise impendance stage by stage.Some methods for calculating the parameters of the correlative fundamental space-chatge-wave transformer are discussed

and then the calculation procedure and talle are presented.This method is comparatively simple

its physical concept is clear

and the noise figure can be obtained merely by calculating the equivalent impedance

without using special impedance charts.The result obtained by this method is very close to that obtained by the method proposed by Buchmiller and it can satisfy the practical requirements of engineering design.