The dielectric parameters of materials for gyrotron output windows mea
sured in the range 65-300 GHz are given. The special attention is paid
to the spread of values of the refractive index and loss tangent of d
ielectrics produced by different firms. The refractive index and loss
tangent are measured for sapphire at the cryogenic temperatures and fo
r boron nitride and silica glass at the temperatures 300-1000K.