1. A heater for thawing/warming a perishable dielectric load, the heater comprising:a heating chamber configured to hold the perishable dielectric load during thawing/warming thereof,
a transmitter unit configured to generate electromagnetic energy having predefined spectral properties,
an emitter configured to produce an electromagnetic field in the perishable dielectric load based on the electromagnetic energy (RFs) from the transmitter,
a tuning circuit configured to adjust an overall impedance of the emitter, the tuning circuit and the heating chamber so that the overall impedance matches an output impedance of the transmitter, and
a controller configured to measure the overall impedance (Z) during thawing/warming of the perishable dielectric load and repeatedly generate at least one control signal configured to cause the tuning circuit to adjust the overall impedance to match the output impedance of the transmitter, wherein the controller is configured to set an initial value of the at least one control signal based on an estimate of a volume of the perishable dielectric load.