An electrosurgical instrument with a radiating tip portion having a relative permeability and/or relative permittivity that is selected to provide an electrical length for the radiating tip portion that enables effective delivery into biological tissue of microwave EM energy supplied thereto, at two or more frequencies of choice. The instrument has a radiating tip portion disposed to receive microwave EM energy from a coaxial cable, the radiating tip portion having a first effective relative permeability at a first frequency and a second effective relative permeability at a second frequency.
