A system 10 for heating a site in a patient's body includes a transformer 14 having a primary winding 14.2 and a secondary winding 14.3, the secondary winding 14.3 having at least one tap 16 to provide a patient reference and at least two sources 22 of radio frequency (RF) energy, the two sources 22 of RF energy being out of phase with each other. A group of electrodes 24 is connected to the sources 22 of RF energy provided by the secondary winding 14.3 to be simultaneously energised. The arrangement is such that there are more active electrodes 24 connected to one of the sources 22 of RF energy than the other of the sources 22 of RF energy with the energy supplied to one active electrode 24 being out of phase with the energy supplied to an adjacent active electrode 24.