of the sample as well as spatial inhomogeneities within the product flow have therefore no or at most slight effect on the measuring signal since all portions of the product flow enter the measuring signal with the same weighting u2014 owing to the approximately constant field strength u2014 as evident in the examples of fig1 to 4 7 and 9 one opening 21 can be specifically arranged away from the respective intensity maximum in order to permit individual weightings of individual production portions the embodiment according to fig5 and 6 relates to the measurement of a sheet form product 31 especially a panel form or web form product for example paper fibre fleece or the like the opening 21 is slot form the slot 30 comprising slot sections 21 a 21 b 21 c 21 d the length of the slot 30 is matched to the width of the paper web 31 to measure a paper web 31 of twice as long this same measuring unit 16 can be used at the same microwave frequency by simply selecting a linear arrangement of eight instead of four resonator sections 17 whereby the length of the resonator 11 would be approximately doubled in this way the overall size of the.