A substance treatment apparatus includes an RF signal source, power detection circuitry, a controller, and a transmission path between the RF signal source and a first electrode that radiates electromagnetic energy into a chamber. The RF signal source includes a solid-state amplifier that generates an RF signal. The power detection circuitry detects reflected signal power along the transmission path. Based on the reflected signal power, the controller modifies values of variable components within an impedance matching network electrically coupled along the transmission path to adjust a magnitude of the reflected signal power. The impedance matching network may have a double-ended input connected to a balun, and a double-ended output connected to the first electrode and to a second electrode. Alternatively, the impedance matching network may have a single-ended input connected to the RF signal source, and a single-ended output connected to the first electrode. The second electrode may be grounded.