Automatic impedance adjustment

An apparatus for automatically adjusting the impedance of network to match a target impedance, the apparatus comprising:

• an electrical power source configured to deliver AC electrical power to a target impedance;
• a network having an input for receiving a drive signal from the electrical power source and an output connectable to an antenna, the network including a shunt reactance between the input and the output and at least one variable impedance element;
• a phase measuring circuit arranged to measure the phase difference between the voltage of the drive signal and the current of the drive signal in the network using the voltage drop across the shunt reactance;
• a voltage measuring circuit arranged to measure the magnitude of the voltage of the drive signal and the magnitude of the voltage drop across the shunt reactance;
• and
• an automatic impedance adjustment circuit in communication with the variable impedance element, the phase measuring circuit and the voltage measuring circuit and arranged to receive the measured phase difference, measured magnitude of the voltage of the drive signal and measured magnitude of the voltage drop across the shunt reactance, to determine the impedance of the network from them and automatically to reduce the difference between the determined impedance of the network and the target impedance by outputting a control signal to adjust the impedance of the variable impedance element.