An radio frequency heating apparatus comprising: a cavity for receiving an object to be heated; an oscillator for generating a common phase reference signal; a plurality of channels (4A,4B) for generating RF radiation to be introduced into the cavity, wherein each channel comprises: an RF frequency synthesiser; a coherent quadrature modulator for modulating an RF signal outputted by the RF frequency synthesiser; a controller for outputting control signals to the modulator for controlling phase and/or amplitude of the modulated RF signal; a power amplifier for amplifying the modulated RF signal; an antenna connected to an output of the power amplifier; a first and second coherent quadrature demodulator, each configured for performing coherent quadrature demodulation using the RF signal outputted by the RF frequency synthesiser; a directional coupler connected in between the power amplifier and the antenna, the directional coupler being connected to the first coherent quadrature demodulator and to the second coherent quadrature demodulator by a forward signal coupled path and a reverse signal coupled path, respectively, wherein the first coherent quadrature demodulator is configured to monitor an RF signal that is introduced in the cavity using the RF signal outputted by the RF frequency synthesiser and wherein the second coherent quadrature demodulator is configured to monitor an RF signal that is reflected back from the cavity using the RF signal outputted by the RF frequency synthesiser; wherein the first and second coherent quadrature demodulators are configured to provide their respective output to the controller; wherein the controller is configured to use the output from the first and second coherent quadrature demodulators to alter the operation of the channel for modifying the RF radiation that is introduced to the cavity; and wherein the common phase reference signal is used to synchronise the frequency synthesisers of each channel in terms of frequency and/or phase. a cavity for receiving an object to be heated; an oscillator for generating a common phase reference signal; a plurality of channels (4A,4B) for generating RF radiation to be introduced into the cavity, wherein each channel comprises: an RF frequency synthesiser; a coherent quadrature modulator for modulating an RF signal outputted by the RF frequency synthesiser; a controller for outputting control signals to the modulator for controlling phase and/or amplitude of the modulated RF signal; a power amplifier for amplifying the modulated RF signal; an antenna connected to an output of the power amplifier; a first and second coherent quadrature demodulator, each configured for performing coherent quadrature demodulation using the RF signal outputted by the RF frequency synthesiser; a directional coupler connected in between the power amplifier and the antenna, the directional coupler being connected to the first coherent quadrature demodulator and to the second coherent quadrature demodulator by a forward signal coupled path and a reverse signal coupled path, respectively, wherein the first coherent quadrature demodulator is configured to monitor an RF signal that is introduced in the cavity using the RF signal outputted by the RF frequency synthesiser and wherein the second coherent quadrature demodulator is configured to monitor an RF signal that is reflected back from the cavity using the RF signal outputted by the RF frequency synthesiser; an RF frequency synthesiser; a coherent quadrature modulator for modulating an RF signal outputted by the RF frequency synthesiser; a controller for outputting control signals to the modulator for controlling phase and/or amplitude of the modulated RF signal; a power amplifier for amplifying the modulated RF signal; an antenna connected to an output of the power amplifier; a first and second coherent quadrature demodulator, each configured for performing coherent quadrature demodulation using the RF signal outputted by the RF frequency synthesiser; a directional coupler connected in between the power amplifier and the antenna, the directional coupler being connected to the first coherent quadrature demodulator and to the second coherent quadrature demodulator by a forward signal coupled path and a reverse signal coupled path, respectively, wherein the first coherent quadrature demodulator is configured to monitor an RF signal that is introduced in the cavity using the RF signal outputted by the RF frequency synthesiser and wherein the second coherent quadrature demodulator is configured to monitor an RF signal that is reflected back from the cavity using the RF signal outputted by the RF frequency synthesiser; wherein the first and second coherent quadrature demodulators are configured to provide their respective output to the controller; wherein the controller is configured to use the output from the first and second coherent quadrature demodulators to alter the operation of the channel for modifying the RF radiation that is introduced to the cavity; and wherein the common phase reference signal is used to synchronise the frequency synthesisers of each channel in terms of frequency and/or phase.