KAERI is presently developing a rectangular RF driven deuterium ion source for both neutron source and fusion applications. For a model-based design of a reliable impedance matching network for the ion source, we extend an equivalent circuit model of the ion source system by allowing effects of an isolation transformer and a Faraday shield on the system to be reflected in our previous model (Huh et al., 2017 Fusion Eng. Des.121 337). In this paper we use the model to investigate the trends in external electrical characteristics of the ion source system (e.g., voltage and current across and through an RF antenna coil and the impedance matching network circuit components) with RF power of up to 50 kW. The model results provide criteria for the impedance matching network design and data for an assessment of the ion source performance.