Simultaneous high-resolution spectra of the upshifted enhanced plasma line were obtained with the 4.68-MHz and 430-MHz Arecibo radars in the presence of HF transmissions at two closely spaced frequencies. The spectra obtained with the 4.68-MHz radar showed two narrow features with positive Doppler shifts equal to the two closely spaced frequencies of the HF transmission; all the spectral power was contained in those two narrow features. The spectra obtained with the 430--MHz radar showed a single narrow feature with a positive Doppler shift equal to the arithmetic mean of the same two closely spaced frequencies; the spectral power in that narrow feature contained about 3% of the total spectral power. The present results broadly confirm the tentative interpretation of earlier observations with a 46.8-MHz radar at Arecibo. The results also show the fundamental difference in the physical processes leading to the enhanced plasma line spectra observed with the two radars. ¿American Geophysical Union 1991