I've been monitoring the HAARP transmissions from Alaska for quite a while. You can usually get a feel for what frequencies they are transmitting on by looking at the HAARP HF spectrum monitor waterfall at:
http://www.haarp.alaska.edu/data/spectrum2/www/hf.htmlLook for the bright yellow bands such as those for the chart on Feb. 12, 2012. I can't get the exact frequency from the chart but I can usually guess within 100 kHz from it. On this chart, at about 0650Z it showed a band near 4500 kHz. That's where I found the signal - 4500 kHz. They changed the frequency at 0700Z and I found the new signal at 3250 kHz.
In this case the signal consisted of one continuous steady carrier with another carrier sweeping slowly up through the other carrier repeatedly every 10 seconds or so. That particular signal is fairly easy to find. In the past, they have also transmitted a series of carriers that would last 10 seconds and then move up in frequency 5.25 kHz over and over repeatedly in the range of 2.7 to 2.9 MHz.
Of course, there were the famous 7 MHz EME experiments a few years ago. I heard the moon echoes about 70% of the time.