This invention provides an elongate microwave radiator for insertion into a living body to treat tissue at a predetermined operating frequency. The radiator defines a monopole antenna at its tip. The monopole antenna includes a dielectric material surrounding the monopole. The dielectric material is configured to act as a resonator at the predetermined operating frequency, and encompasses generally the whole of a near-field radiation emitted by the monopole. In an illustrative embodiment, the dielectric material extends from the antenna a distance determined in accordance with the wavelength of the radiation in the dielectric.