Wonder what the gain is on hf for that thing.
If we assume 65% efficiency (a good middle of the road number, could be slightly higher or lower, at a bet lower is more likely) it would be about 26.5 dBi on 8175 kHz, and 22.4 dBi on 5125 kHz. If the antenna is very efficient you might add as much as 2 dB to each of those.
(edit) Found this paper here : http://www.naic.edu/~astro/ao50/Arecibo_50th_Paper_Breakall_revised_Oct_23_2013.pdf which calls the gain on 8175 kHz as 25.5 dBi and the gain at 5125 kHz as 22.2 dBi. That comes out to about 52% on 8175 kHz and 62% on 5125 kHz.
(edit 2) Looking at that paper it describes the transmitter as 6x 100 kW transmitters repurposed from a former HF OTHR, and the lower freq of 5100 kHz selected because the OTHR was not designed to go below that freq. I wonder which OTHR that was? The power and frequency limits are correct for the FPS-118 (5 to 28 MHz, 100 kW per transmitter module, either 6 or 12 transmitter modules per site). However I thought the FPS-118's were not scrapped until after the new HF transmitter was added to Arecibo? I guess it could the FPS-112 transmitter, but I did not think that one did CW.
T!