In this paper, we used two type of high gain antennas in external RF electromagnetic environment (EME) testing system which was the most important part in the electromagnetic environment effects (E3) system testing. The first one was S band pyramidal horn antenna had wider beam width in 2GHz to 4GHz, and the other one was parabolic antenna had higher gain in the same frequency band. From using these two different antennas in irradiation in open area test sites (OATS), we found two methods to operate this test which had invariable radiation distance or invariable radiation area to adapt different type of system under test (SUT).