Low Frequency Communications Experiment
Ever since I built a receiver more than 40 years ago to pick up RF frequencies around 15 kHz (it was probably still Kc/s—kilocycles per second—back then) being used to transmit messages to submarines, I've had an interest in very low frequency (VLF) RF. Some of those transmissions were in Morse Code and used very high power transmitters and huge antennas. New digital processing techniques that allow reception of signals below the noise level have created opportunities for amateur radio operators to experiment with communications on VLF. (Digital techniques are also being used in meteor scatter communications, moon-bounce communications and, of course, HF communications.)
The South Gate Amateur Radio Club
has one of the best Websites for amateur radio news, including new RF technology news. Roger, G3XBM, reported on tests from DK7FC in Germany using 8.970 kHz. And no, that's not a typo—the decimal point is between the 8 and the 9. DK7FC transmissions from a vertical radiator 300 meters above ground by a kite provided less than 100 mW ERP from a transmitter running several hundred Watts. I wonder how hard it would be to couple a VLF signal into a grounded 2,000-foot TV broadcast tower.
The DK7FC transmissions were copied by 4X1RF in Israel, some 2,873 km distant.
Roger notes that "Extremely accurate frequency setting, very narrow bandwidth FFT-based receivers and extremely long transmission times are all required to copy these signals at any DX distance—a casual listen on VLF would result in lots of noise and disappointment—but with some effort almost anyone with a small E-field probe or loop and a PC running freely available software can detect these signals."
Roger's report, 8.970 kHz - DK7FC's latest tests
has links to articles on VLF, including receiving equipment and other information. By the way, G3XBM has a permit to operate his own transmitter on 8.970 kHz.
Before anyone looking for wireless broadband spectrum gets excited, no, you can't transmit TV at 9 kHz, nor can you stream video at that frequency.