This is an improvement on our earlier application, PCT/GB99/00187. The device then disclosed allowed the variation of the coupling between two points in an RF circuit in a very simple way whilst maintaining the RF phase relationship and varying the relative magnitude of the RF fields. It was characterised by a simple single mechanical control of coupling value, that has negligible effect on the phase shift across the device. This was achieved by the simple rotation of the polarisation of a TE111 mode inside a cylindrical cavity. In this application, a slight frequency dependence on the angle of rotation is correctable by a relative excess of material located opposite the apertures between the coupling cavity and the accelerating cavities.