A prototype antenna for the ITER ion cyclotron system, based on a resonant double loop antenna design from the 1998 Engineering Design Activity, has been built and tested. Electrical properties of the antenna were measured and were in agreement with theoretical calculations. Results of high-power experiments in vacuum showed that the antenna could operate at voltages in excess of 60 kV for multisecond pulses. The stub tuning arrangement proposed for use on ITER was demonstrated to work. These results validate the original design concept of the ITER antenna.