Characterisation of radio frequency heating systems in industry using a network analyser

The authors describe a rapid method for attaining the desired electrical condition which matches a loaded applicator to the tank circuit of a high power radio frequency source. First, the circuit equations of two coupled resonant circuits are derived and used to predict the optimum matching conditions with respect to various criteria. These conditions are then studied experimentally using an automatic network analyser which is coupled to the applicator/tank circuit assembly via a small launching coil. The analysis considers the interaction of this launching coil with the tank circuit only; and with both the tank circuit and the applicator. The key parameters used to identify optimum matching conditions are the values of the two return loss responses and, specifically, their interrelation, as observed on the network analyser. Finally, an example is given of matching a particular radio frequency system using the present experimental method. The results are in good agreement with those from the more accurate, but extremely elaborate, nonlinear minimisation procedure