This paper presents a method of calculation of the noise figure of power travelling wave tubes which employ a conical Pierce gun to produce the electron beam. Under the assumption that the electron beam is single-velocity and one-dimensional

the transmission equation of space charge wave set up in the conical electron beam is derived and the process of noise impedance transformation is obtained. Theoretical calculations show that changing the convergence angle

the radius of curvature

the anode voltage and the length of the drift region can reduce the noise figure of a power travelling wave tube. Theoretical results presented in this paper are examined with the experimental noise figure values of a few of practical tubes.