This paper presents the first studies of combined glow + microwave discharges at the TOMAS facility, in a volume of ~ 1.1 m³, and discusses microwave propagation in the plasma. The combined discharge was realized by injection of additional microwave power (0.4–1.5 kW) at 2.46 GHz into the argon plasma of the glow discharge. The studies have shown that the injection of additional microwave power allows to reduce the voltage on the glow discharge. The maximum observed voltage decreases in the combined glow + microwave discharges compared with a reference glow discharge at ≈ 220 V. The dependence between the voltage decreases and the injected microwave power is linear. This effect of the combined glow + microwave discharges provides flexibility to study particular aspects of wall conditioning techniques relevant to larger devices.