Ferromagnetic resonators

A ferromagnetic resonator comprises a non-magnetic substrate (21), a ferrimagnetic thin film element (22) formed on a major surface of the non-magnetic substrate (21), a strip line (23) formed for example on another major surface of the non-magnetic substrate (21) and electromagnetically coupled to the ferrimagnetic thin film element (22), a conductive wall (24) facing the strip line (23) and spaced therefrom by a predetermined distance, an end of the strip line (23) being connected by the conductive wall (24) to ground potential, and a bias magnetic field source (26) applying a dc magnetic field to the ferrimagnetic thin film element perpendicular to the major surface thereof. Such an arrangement enables a drastic reduction in the effective dielectric constant of the transmission lines, whereby a filter with a high centre frequency of the order of at least ten gigahertz can be achieved despite the "short" type of structure.