A virtual ionospheric antenna to generate ELF (extremely low frequency) waves is achievable by using amplitude modulated HF (high frequency) wave to modulate the electrojet current. The results of the numerical analysis show that the radiation intensities of ELF waves produced by modulated electrojet vary with the modulation frequency. Moreover, a ratio of two radiation intensities at two frequencies is height dependent. Thus, with the aid of the numerical results, the experimental measurements on the ratios of the radiation intensities of the ELF waves can provide unambiguous information on the height of the electrojet contributing to the ELF radiation. An experiment using the HAARP heating facility has been performed in 2005. The experimental results of three independent intensity ratios (25 Hz/205 Hz, 25 Hz/325 Hz, and 25 Hz/625 Hz) are used to demonstrate this new diagnostic technique. The height of the source region, where the modulated electrojet radiates most effectively, is determined to be between 94 Km and 97 Km.