Q-,M- AND D-VARIATION BY VARIABLE FLUID VOLUME

A microwave resonator for an EPR (="electron paramagnetic resonance") probehead comprising a cavity body (1) supporting an electromagnetic microwave resonance mode, at least one sample opening (2) for inserting a sample (7) in a sample container (6), at least one microwave opening for transmitting microwave radiation into the resonator, and at least, one access opening (14',14") for inserting and removing a modifier (15) in a modifier vessel (13) into or from the cavity body, is characterized in that the modifier vessel is fixed in the at least one access opening, in that the modifier is a fluid comprising attenuator fluid and/or marker fluid and/or adaptor fluid, and in that the modifier vessel has an insert opening for filling and discharging the modifier gradually into or from the cavity body. This improves performance greatly, enabling a gradual modification of specific experimental conditions without moving any mechanical parts in the cavity body, without changing other experimental conditions and for Q- and/or M- and/or D-variation.