A circulator comprises a housing in two halves with a ferrite and associated magnet in a central portion of a first one of the halves, the second housing-half above the ferrite having a membrane which is deformable by operation of an adjuster upon the membrane, so as to adjust the size of an airgap between the membrane and the ferrite. The membrane is preferably the residual portion of the second housing-half following a milling operation to provide a recess, the recess accommodating the adjuster in the form of, e.g., a screw. To further enhance predictability of the performance characteristics of the circulator, the ferrite is seated in a hollow formed in the first housing-half and the securing of the ferrite to the first housing-half is by way of an adhesive applied between a lateral face of the ferrite and a corresponding lateral face of the hollow.