An actuator or servo device comprises an actuator member adapted to move linearly in the motion space of an actuator housing, a microwave position detecting device for detecting the position of the actuator member in the motion space, the actuator member being capable of producing a pressure wave acting on the position detecting device on approaching the position detecting device and a high frequency microwave antenna arrangement for emitting and receiving microwaves, which are at least partially reflected by the actuator member. The actuator device includes dielectric guard cover means, arranged in front of the microwave antenna arrangement. For mechanically uncoupling the guard cover means from the microwave antenna arrangement between the guard cover means and the microwave antenna arrangement a clearance is maintained to allow deformation of the guard cover means thereinto owing to pressure waves.