Abstract: The microwave resonator includes an enclosed resonator housing and a hollow central conductor having one end fastened to a bottom of the resonator housing and extending toward a top wall of the resonator housing. The other end of the central conductors is spaced from the top wall and includes an adjustable bellows assembly disposed coaxial of a longitudinal axis of the central conductor. A non-rotating, axially movable drive shaft is disposed coaxial of the axis of the central conductor within the central conductor. One end of the drive shaft is fastened to the bellows assembly and the other end of the drive shaft is coupled to a drive means disposed in the bottom wall to cause axial movement of the drive shaft to adjust the axial length of the bellows assembly and, hence, the axial length of the central conductor to adjust the resonant frequency of the microwave resonator.