Expressions for the shot noise generated in the input cavity of a gyro
klystron are calculated. Results are given for noise amplitude, noise
temperature, and phase noise. For a beam of uncorrelated electrons the
noise temperature scales as the beam energy. An estimate of the role
of collective effects on the noise properties is given, These might ei
ther decrease or increase the noise level. It is expected that the sho
t noise in an actual device will depend in a sensitive way on the prof
ile of magnetic field in the compression region preceding the input ca
vity.