A microwave heating device includes radiating portions adapted to radiate microwaves to the heating chamber and is operated according to operational configurations that differ in frequency or in phase shift(s) between the radiated microwaves. A learning procedure is executed by sequentially operating the radiating portions in several operational configurations. Energy efficiency data are calculated for those operational configurations. An operating frequency is selected based on energy efficiency data. An operational configuration with a maximum energy efficiency at the selected operating frequency is taken as a reference. A heating procedure is executed by sequentially operating the radiating portions in operational configurations having the selected operating frequency and respective phase shift(s) chosen around the respective phase shift(s) of the reference operational configuration. The phase shift(s) of each chosen operational configuration may have a phase shift distance from the respective phase shift(s) of the reference operational configuration, such that, in the space of the phase shifts, the reference operational configuration is surrounded by the chosen operational configurations.