A High Power Prototype (HPP) of the JET ITER-Like ICRF Antenna has been built and tested in a joint effort by ORNL, PPPL, ERM/KMS, and EFDA-JET. For the first time, high power vacuum tests have been performed on an antenna with features similar to those found in the current ITER antenna design. Advances include a new matching network using internal capacitors that is insensitive to target plasma characteristics, and increased power density to minimize the amount of port space required. Three-dimensional electromagnetic modeling has been used extensively to improve the antenna design and assist in interpretation of test results. During the HPP tests, capacitor voltages greater than 45kV were achieved for short (0.05s) pulses. Voltages greater than 35kV, approximately the voltage needed to couple the full 7.2MW design power into most JET plasmas, were sustained for moderate length (~0.5s) pulses. Long (10s) pulse operation was limited by excessive heating in localized regions due to rf dissipation, a problem that will be corrected in the final antenna. In this paper test results, associated numerical modeling, and their impact on the design of the final device will be reviewed.