An apparatus for irradiating a load with UHF or microwave radiation includes a cavity for accepting a load, at least one energy feed for transmitting energy into the cavity at a plurality of UHF or microwave frequencies to irradiate the load, and a controller. The controller sets transmission durations, by setting for each of a plurality of frequencies, a respective transmission duration, repeatedly causes energy to be transmitted into the cavity at the plurality of UHF or microwave frequencies according to a duty cycle, and switches ON or OFF transmission of the energy at different frequencies in different repetitions of the duty cycle, which defines an allotted transmission time for each of the plurality of UHF or microwave frequencies. Over a plurality of repetitions of the duty cycle the energy is transmitted at each of the plurality of frequencies for the respective transmission duration.