1. A microwave heating device, comprising:a heating chamber which is designed to receive at least one product to be heated;
a microwave generating system designed to generate microwaves and including at least two radiating portions that are adapted to radiate microwaves to the heating chamber, a radiated power being associated with the radiated microwaves and, in use, a share of radiated power being reflected by the heating chamber; and
a control unit;the microwave generating system being configured to radiate, via the at least two radiating portions, respective microwaves having a same frequency and different phases, wherein one of the radiating portions can be taken as a reference radiating portion and the microwaves radiated by the other radiating portion(s) can have respective phase shift(s) from the microwaves radiated by the reference radiating portion, the frequency value and the phase shift value(s) defining an operational configuration of the microwave generating system, an energy efficiency being associated with each operational configuration, the energy efficiency being inversely related to a ratio of reflected power over radiated power;the control unit being configured to operate the microwave generating system and to change the frequency and the phase shift(s) of the microwaves radiated by the microwave generating system;the control unit being configured to obtain data of energy efficiency, as a function of operational configurations, relating to the at least one product to be heated;the control unit being configured to run a selection procedure, in which:for each frequency in the obtained data of energy efficiency, data of energy efficiency are processed to calculate an efficiency difference and/or an efficiency standard deviation at that frequency, wherein, for the calculation of the efficiency difference, an operational configuration with a maximum energy efficiency and an operational configuration with a minimum energy efficiency at that frequency are singled out and the efficiency difference is calculated as the difference between the maximum energy efficiency and the minimum energy efficiency, and wherein, for the calculation of the efficiency standard deviation, a mean value of energy efficiency at that frequency is calculated and the standard deviation of energy efficiency at that frequency from the mean value is calculated;
an operating frequency is selected, being the frequency value that optimizes a mathematical function that includes efficiency difference and/or efficiency standard deviation at the same frequency;
a reference operational configuration is selected, being the operational configuration with the maximum energy efficiency at the selected operating frequency;
a plurality of operational configurations are chosen, each one having the selected operating frequency and respective phase shift(s) that are chosen around the corresponding phase shift(s) of the reference operational configuration;the control unit being configured to run a heating procedure, in which the microwave generating system is sequentially operated in said plurality of operational configurations chosen in the selection procedure.