An apparatus is provided for applying electromagnetic energy at a frequency or frequencies in a frequency range of 1 MHz to 100 GHz to an object in a cavity via at least one radiating element. At least one processor is configured to receive a target energy distribution, select, based on the target energy distribution, a plurality of sets of values of field-affecting parameters controllable by the apparatus. The parameters include at least one of frequency, phase, and amplitude. The apparatus provides for the control of a source coupled to the at least one radiating element to excite a respective field pattern for each selected set of values.