A thermal increase system coupled to a cavity for containing a load, the thermal increase system comprising: a radio frequency, RF, signal source (920, 920') configured to supply an RF signal; a first electrode arranged proximate to a first cavity wall and a second electrode arranged proximate to an opposite, second cavity wall, the first and second electrodes being electrically isolated from the remaining cavity walls; a transmission path electrically coupled between the RF signal source and the first and second electrodes (940, 950); an impedance matching network (370, 400, 972, 1000, 1100) electrically coupled along the transmission path, wherein the impedance matching network comprises a network of variable passive components (1011, 1016, 1021, 1111-1114, 1016-1019, 1021, 1023), characterised in that the impedance matching network is a double-ended variable impedance matching network (1000, 1100) that comprises: first and second inputs (1001-1, 1001-2, 1101-1, 1101-2); first and second outputs (1002-1, 1002-2, 1102-1, 1102-2); a first variable impedance circuit connected between the first input and the first output, and a second variable impedance circuit connected between the second input and the second output; power detection circuitry (930, 930', 930") configured to detect reflected signal power along the transmission path; and a controller configured to modify, based on the reflected signal power, one or more values of one or more of the variable passive components of the impedance matching network to reduce the reflected signal power. a radio frequency, RF, signal source (920, 920') configured to supply an RF signal; a first electrode arranged proximate to a first cavity wall and a second electrode arranged proximate to an opposite, second cavity wall, the first and second electrodes being electrically isolated from the remaining cavity walls; a transmission path electrically coupled between the RF signal source and the first and second electrodes (940, 950); an impedance matching network (370, 400, 972, 1000, 1100) electrically coupled along the transmission path, wherein the impedance matching network comprises a network of variable passive components (1011, 1016, 1021, 1111-1114, 1016-1019, 1021, 1023), characterised in that the impedance matching network is a double-ended variable impedance matching network (1000, 1100) that comprises: first and second inputs (1001-1, 1001-2, 1101-1, 1101-2); first and second outputs (1002-1, 1002-2, 1102-1, 1102-2); a first variable impedance circuit connected between the first input and the first output, and a second variable impedance circuit connected between the second input and the second output; first and second inputs (1001-1, 1001-2, 1101-1, 1101-2); first and second outputs (1002-1, 1002-2, 1102-1, 1102-2); a first variable impedance circuit connected between the first input and the first output, and a second variable impedance circuit connected between the second input and the second output; power detection circuitry (930, 930', 930") configured to detect reflected signal power along the transmission path; and a controller configured to modify, based on the reflected signal power, one or more values of one or more of the variable passive components of the impedance matching network to reduce the reflected signal power.