How does one generate a 1500 Hz ULF signal with an HF transmitter? There are a few ways you can do it, as long as you have a few hundred KW of ERP. As we've known since about the 1930s, the ionosphere is non linear....if you pump it hard enough it acts like a diode. A really BIG diode. Like any diode, the ionosphere produces sums and differences of any two signals you put into it. We don't care about the sums, but the main difference signal is very useful...and fairly easy to generate....either by using two CW transmitters separated by the desired ULF frequency....or modulating an AM transmitter at the desired ULF frequency. The ionosphere does the rest. The efficiency of this process is a little less than one percent...but that's enough to do some really interesting stuff. Stay tuned.