The invention relates to a spinner preparation machine with a drawing gear (1a), for drawing at least one fibre web (2), in particular, a carding, drawing (1), or combing machine with at least one microwave sensor (3; 30; 300; 3000) at the inlet or the outlet from the drawing gear (1a), for measuring the web thickness of the at least one fibre web (2). The microwave sensor (3; 30; 300; 3000) comprises at least one cavity resonator, through which the at least one fibre web (2) is to be run during the measurement. According to the invention, the spinner preparation machine has means (14; 15; 45) for the prevention of temperature-dependent deformations of the resonator walls (302, 306) of the microwave sensor (3; 30; 300; 3000) during the measurements. A cavity resonator for such a microwave sensor is also disclosed.