Different improvements of a spinning preparation machine are proposed to measure the fiber sliver thickness or the fiber sliver cross-section of at least one fiber sliver by means of one or several microwave sensors. One improvement is characterized by compressing means and special mechanical guiding elements that can be installed before, after and/or in the microwave sensor. It is also proposed to install a pair of conveyor rollers after a microwave sensor that can be designed in particular as an input roller pair of the subsequent drafting equipment. Furthermore, proposals are made for the calibration of the (at least one) microwave sensor.