A microwave resonator for or on a textile machine, in particular a carding, drawing or combing machine is disclosed, for connection to a measuring device for measuring the thickness and or the dampness of fibre material (FB), continuously passing through the resonator chamber (31; 331). Said microwave resonator is characterised in that at least partly at least one non-conducting dielectric (303, 307; 460; 460a, 460b) is provided in the resonator (30), the dielectric constant of which remains essentially unchanged during temperature variations which are normal during machine operation. A corresponding textile machine and a dielectric for a microwave resonator are further disclosed.