Abstract: The present invention relates to an apparatus for detection of foreign bodies in materials, comprising a first antenna device for transmitting electromagnetic signals, the transmitted electromagnetic signals are in the microwave range, the signals comprise at least two signals at different frequencies, a second antenna device for receiving signals originating from the first antenna, where the received signals, at least partially, passes through the materials, device for measuring at least one parameter of the received signals for each frequency, so as to obtain parameter values, device for comparing the parameter value with the corresponding output parameter value of the transmitted signals, so as to obtain a comparison value for each of the frequencies, device for analyzing each comparison value based on a reference value, which reference value is accessible to the apparatus, and device for emitting a signal when the comparison value differs a predetermined amount from the reference value.